An Overabundance of Transient X-ray Binaries within 1 pc of the Galactic Center
M. P. Muno
\altaffilmark{1,2} E. Pfahl
\altaffilmark{3} F. K. Baganoff
\altaffilmark{4} W. N. Brandt
\altaffilmark{5} %G. P. Garmire
\altaffilmark{6} A. Ghez
\altaffilmark{1} J. Lu
\altaffilmark{1} and M. R. Morris\altaffilmark{1} % and A. Loeb
\altaffilmark{4} %G. R. Ricker
\altaffilmark{5}
During five years of Chandra observations, we have identified seven X-ray transients located within 23 pc of \sgrastar. These sources each vary in luminosity by more than a factor of 10, and have peak X-ray luminosities greater than $5 \times 10^{33}$ \ergs, which strongly suggests that they are accreting black holes or neutron stars. The peak luminosities of the transients are intermediate between those typically considered outburst and quiescence for X-ray binaries. Remarkably four of these transients lie within only 1 pc of \sgrastar. This implies that, compared to the numbers of similar systems located between 1 and 23 pc, transients are over-abundant by a factor of $\approx 20$ per unit stellar mass within 1 pc of \sgrastar. It is likely that the excess transient X-ray sources are low-mass X-ray binaries that were produced, as in the cores of globular clusters, by three-body interactions between binary star systems and either black holes or neutron stars that have been concentrated in the central parsec through dynamical friction. Alternatively, they could be high-mass X-ray binaries that formed among the young stars that are present in the central parsec.
transients_apj.tex
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\newcommand{\etal}{\mbox{et al.}}
\newcommand{\ergcms}{erg cm$^{-2}$ s$^{-1}$}
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\newcommand{\degree}{$^\circ$}
\newcommand{\msun}{$M_{\odot}$}
\newcommand{\pc}{\,{\rm pc}}
\newcommand{\yr}{\,{\rm yr}}
\newcommand{\gyr}{\,{\rm Gyr}}
\newcommand{\emsun}{\,M_\odot}
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\newcommand{\chandra}{{\it Chandra}}
\newcommand{\rosat}{{\it ROSAT}}
\newcommand{\granat}{{\it GRANAT}}
\newcommand{\asca}{{\it ASCA}}
\newcommand{\einstein}{{\it Einstein}}
\newcommand{\bepposax}{{\it BeppoSAX}}
\newcommand{\xmm}{{\it XMM-Newton}}
\newcommand{\integral}{{\it INTEGRAL}}
\newcommand{\rxte}{{\it RXTE}}

\newcommand{\sgrastar}{\mbox{Sgr A$^*$}}

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\shortauthors{Muno \etal}
\shorttitle{X-ray Transients at the Galactic Center}

\submitted{} %Submitted to \apj\ Letters
%\received{2002 November 25}
\begin{document}
\title{An Overabundance of Transient X-ray Binaries within 1 pc of 
the Galactic Center}
\author{M. P. Muno,\altaffilmark{1,2} E. Pfahl,\altaffilmark{3}
F. K. Baganoff,\altaffilmark{4} 
W. N. Brandt, \altaffilmark{5}
%G. P. Garmire,\altaffilmark{6}
A. Ghez,\altaffilmark{1}
J. Lu,\altaffilmark{1}
and M. R. Morris\altaffilmark{1} 
% and A. Loeb,\altaffilmark{4}
%G. R. Ricker,\altaffilmark{5}
}

\altaffiltext{1}{Department of Physics and Astronomy, University of California,
Los Angeles, CA 90095; mmuno@astro.ucla.edu}
\altaffiltext{2}{Hubble Fellow}
\altaffiltext{3}{Chandra Fellow, Department of Astronomy, University of 
Virginia, Charlottesville, VA 22904}
\altaffiltext{4}{Kavli Institute for Astrophysics and Space Research,
Massachusetts Institute of Technology, Cambridge, MA 02139}
\altaffiltext{5}{Department of Astronomy and Astrophysics, 
The Pennsylvania State University, University Park, PA 16802}
%\altaffiltext{4}{Center for Astrophysics, Cambridge, MA, 02138}

\begin{abstract}
During five years of Chandra observations, we have identified 
seven X-ray transients located within 23 pc of \sgrastar.
These sources each vary in luminosity by more than a factor of 10, and 
have peak X-ray luminosities greater than $5 \times 10^{33}$ \ergs, 
which strongly suggests that they are accreting black holes or 
neutron stars. The peak luminosities of the transients are intermediate 
between those typically considered 
outburst and quiescence for X-ray binaries. Remarkably four of these 
transients lie within only 1 pc of \sgrastar. This implies that, compared 
to the numbers of similar systems located between 1 and 23 pc, 
transients are over-abundant by a factor of $\approx 20$ per unit stellar 
mass within 1 pc of \sgrastar.
It is likely that the excess transient X-ray sources are
low-mass X-ray binaries that were produced, as in the cores of globular 
clusters, by three-body
interactions between binary star systems and either black holes or 
neutron stars that have been concentrated in the central parsec through 
dynamical friction. Alternatively, they could be high-mass X-ray
binaries that formed among the young stars that are present in the 
central parsec.
\end{abstract}

\keywords{Galaxy: center --- stellar dynamics --- X-rays: binaries}

\section{Introduction}

\chandra\ has observed the inner 20 pc of the Galaxy repeatedly for 
the past five years, primarily to monitor the super-massive black hole
\sgrastar\ \citep{bag03}. However, the combination of the repeated 
observations, the 
exquisite sensitivity of \chandra, and the high stellar density in the field
also is ideal for detecting and studying transient X-ray sources. 
The total mass of stars in a pencil beam enclosing the inner 20~pc of the 
Galaxy is $\sim 10^{8}$~\msun, or $\sim 0.1$\% of the Galactic stellar 
mass. Therefore, these observations sample a huge number of stars, and are 
likely to reveal rare X-ray sources such as accreting black holes and neutron 
stars. Moreover, while traditional all-sky 
X-ray monitors detect sources as faint as $\simeq 10^{36}$~\ergs\ 
near the Galactic center \citep[e.g.,][]{lev96,jag97},
even short \chandra\ observations are sensitive to sources three orders of 
magnitude fainter. 
As a result, these observations can be used to study a little-observed 
regime of accretion for X-ray binaries between outburst 
($L_{\rm X} > 10^{36}$~\ergs) and quiescence ($L_{\rm X} < 10^{33}$~\ergs;
see Sakano \etal\ 2004).

The distribution of X-ray binaries in the Galactic 
center could also yield insight into the interaction between the
central super-massive black hole and the surrounding dense concentration of
stars. First, tens of thousands of stellar-mass black holes and
neutron stars are likely to have settled dynamically into the central 
parsec of the Galaxy over the past few Gyr \citep{mor93,lee95,meg00}. 
Three-body interactions 
between this concentration of compact objects and background binary stars can 
form X-ray binaries (E. Pfahl \& A. Loeb, in prep.), in a similar 
manner to that by which 
low-mass X-ray binaries (LMXBs) form in globular 
clusters \citep[e.g.,][]{rpr00}.
Second, several dozen stars only 7~Myr old have been discovered $< 1$~pc 
from \sgrastar\ \citep[e.g.,][]{kra95}, and high-mass X-ray 
binaries (HMXBs) may have formed among this young stellar population. 
Therefore, we have searched the \chandra\ observations of \sgrastar\ for 
transient X-ray sources
that are likely to be accreting black holes and neutron stars. 

\section{Observations and Results} 

The region within 10\arcmin\ of \sgrastar\ has been observed sixteen
times between 1999--2004 with the Advanced CCD Imaging Spectrometer 
imaging array (ACIS-I) aboard the {\it Chandra X-ray Observatory} 
\citep{wei02}. The first twelve observations are listed 
in Table~2 of \citet{mun03a}. The remaining four were taken on 2003 June 19 
for 25 ks (sequence 3549), 2004 July 5--7 for 99 ks (sequences 4683 and 4684), 
and 2004 August 28 for 5 ks (sequence 5630).
We reduced the data and searched for X-ray sources using the techniques 
explained in \citet{mun03a}.
% 2003 Jun 19 18:28:55 & 3549 & 24,791 & 266.42092 & --29.0105 & 347 \\
% 2004 Jul 05 22:33:11 & 4683 & 49,524 & 266.41605 & --29.0124 & 286 \\
% 2004 Jul 06 22:29:57 & 4684 & 49,527 & 266.41597 & --29.0124 & 285 \\
% 2004 Aug 28 12:03:59 & 5630 &  5,106 & 266.41477 & --29.0121 & 271 \\

We used three criteria to select transient X-ray sources that were most likely 
to be accreting black holes and neutron stars near the Galactic center
\citep[compare, e.g.,][]{cam98}. 
First, in order to avoid including foreground sources, we selected those
sources with X-ray spectra that were absorbed by a column of gas and dust
similar to or greater than that toward \sgrastar, which is equivalent to 
$5\times10^{22}$ cm$^{-2}$ of hydrogen \citep{bag03}. This was implemented
by requiring that the soft color, defined as in \citet{mun03a}
using the 0.5--2.0 and 2.0--3.3 keV energy bands, was larger than --0.175. 

Second, we selected only those sources with peak luminosities  
$\ga 5\times10^{33}$~\ergs, because the fainter sources are 
most likely cataclysmic variables \citep[][]{ver97}. 
For a distance of 8 kpc \citep{rei99} and a 
$\Gamma = 1.5$ power law spectrum, our 
threshold corresponds to fluxes greater than $2 \times 10^{-5}$ 
photon cm$^{-2}$ s$^{-1}$ (2--8 keV). The 
flux limit at which a source 
can be detected 
at the 3-$\sigma$
 level within 10\arcmin\ of the aim-point of a 5~ks 
observation is 
%

\begin{inlinefigure}%[th]
\centerline{\psfig{file=sgrabox.ps,width=\linewidth}}
\caption{
Chandra images of the 1\arcmin\ by 1\arcmin\ field around \sgrastar, taken
at several epochs. Four of the transient X-ray sources are present in this 
image, and their locations are indicated schematically by circles. 
}
\label{fig:ximg}
\end{inlinefigure}

\noindent
%
$ 5\times10^{-6}$ photon cm$^{-2}$ s$^{-1}$ 
\citep[Fig. 7 in][]{mun03a}, so we are uniformly sensitive to 
sources this bright located within 23 pc in projection from \sgrastar.

Finally, in order to select accreting sources with large transient outbursts,
we required the flux from the source to vary by at least a factor of 10. This 
eliminates bright, steady X-ray sources such as young pulsars 
and Wolf-Rayet and O stars in binaries.\footnote{
One known 
cataclysmic variable would pass our selection criteria --- 
GK Per exhibits X-ray outbursts lasting 45 days that brighten by a factor 
of $\approx 10$ to peak luminosities of $1\times10^{34}$~\ergs\ 
\citep[e.g.,][]{krw79}. None of the outbursts from our sources appears 
similar to that of GK Per, as they each either last longer than a year, or 
brighten by more than a factor of 100.
}

Seven sources satisfied our selection criteria, and are listed in 
Table~\ref{tab:sources}. Images displaying the four 
transients closest to \sgrastar\ are displayed in Figure~\ref{fig:ximg}.

\subsection{Properties of the Transients}

We have examined the light curves of each of the transients
using the techniques described in \citet{mun04b}.
We display the flux history of the sources in 
Figure~\ref{fig:lcurves}. We have omitted CXOGC J174502.3--285450 
(GRS 1741.9--2853) because this source is described in Muno, 
Baganoff, \& Arabadjis (2003)\nocite{mun03b}.
In that paper, we describe a thermonuclear X-ray burst 
lasting $\approx 50$~s that identifies the source as a neutron-star 
LMXB. No similar, short bursts were observed from the other transients.

We searched for periodic variability as described in \citet{mun03c}. 
We find no evidence for signals with periods between 10--5000~s, with 
typical upper limits on their rms amplitudes ranging from 6\% for 
CXOGC J174554.3--285454\footnote{The upper limit on the amplitude of 
the 172~s signal reported by \citet{por04} is 4\% rms.} 
to 26\% for CXOGC J174538.0--290022. Most signals at longer periods are
produced by slow, random variations in the intensities of each source. 
However, a signal and its 
harmonic are detected with a period of $7.9$~h in the 2004 July 
observations of CXOGC~J174540.0--290031. The modulation is highly significant
(chance probability $< 10^{-10}$), and resembles  
eclipses that recur at the orbital period. Unfortunately, the
relevant individual observations were less than 14~h long, so this 
result is not
yet secure.

%
\begin{deluxetable}{lccc}
\tablecolumns{4}
\tablewidth{0pc}
\tablecaption{Transient X-ray Sources in the inner 10\arcmin\ of the 
Galaxy\label{tab:sources}}
\tablehead{
\colhead{Source} & %\multicolumn{2}{c}{Position (J2000)} & 
\colhead{Offset} & 
\colhead{Min $L_{\rm X}$} & \colhead{Max $L_{\rm X}$} \\
\colhead{(CXOGC J)} & % RA & DEC & 
(arcmin) & 
\multicolumn{2}{c}{2--8 keV (\ergs)}
} 
\startdata
174502.3--285450 & % 266.2598 & --28.9139 & 
9.98 & $<7\times10^{31}$ & $1.5\times10^{36}$ \\
174535.5--290124 & % 266.3982 & --29.0234 & 
1.35 & $<9\times10^{30}$ & $3.3\times10^{34}$ \\
174538.0--290022 & % 266.4086 & --29.0062 & 
0.44 & \phantom{$<$}$1.2\times10^{33}$ & $2.6\times10^{34}$ \\
174540.0--290005 & % 266.4170 & --29.0016 & 
0.37 & $<4\times10^{31}$ & $3.4\times10^{34}$\\
174540.0--290031 & % 266.4168 & --29.0086 & 
0.05 & $<2\times10^{31}$ & $8.5\times10^{34}$\\
174541.0--290014 & % 266.4208 & --29.0040 & 
0.31 & $<8\times10^{31}$ & $4.8\times10^{33}$ \\
174554.3--285454 & % 266.4769 & --28.9153 & 
6.38 & $<2\times10^{31}$ & $6.2\times10^{34}$
\enddata
\tablecomments{
We report 
$L_{\rm X}$ for the 2--8 keV band, because the inferred flux at lower 
energies depends heavily upon the assumed absorption column toward each 
source. The bolometric luminosities should be a factor
of 3--10 larger. Three sources are 
not listed in the catalog of \citep{mun03a}, so we give their positions
here: 
CXOGC J174554.3--285454: $\alpha = 266.4769$, $\delta = -28.9153$ 
(XMMU J174554.4--285456 in Porquet \etal\ 2004);
CXOGC J174540.0--290005: $\alpha = 266.4170$, $\delta = -29.0016$;
and CXOGC J174540.0--290031: $\alpha = 266.4168$, $\delta = -29.0086$.
}
\end{deluxetable}

%\noindent
%

Spectral results for most of the transients can be found in 
\citet{mun03b}, \citet{mun04b}, and \citet{por04}.
The spectra are diverse, but are consistent with the 
variety observed from known X-ray binaries. Otherwise, the spectra
do not yield any clues as to the natures of the 
mass donors or compact objects.

%The spectra from all of the sources can be modeled with a power
%law as a function of energy that is absorbed by interstellar gas and dust. 
%The spectral slopes vary greatly, with photon indices
%from $\Gamma = 0 - 4$, both when comparing the ensemble of 
%sources and when observing the evolution of outbursts from individual sources.
%The absorption columns are often larger than the average value 
%toward the Galactic center ($N_{\rm H} = 6\times10^{22}$ cm $^{-2}$), and
%vary throughout the outbursts of several 
%sources, which suggests that $\ga 10^{22}$ cm$^{-2}$ of absorbing 
%material may be local to some systems. These characteristics 
%are consistent with the diversity in spectral properties of known X-ray
%binaries, but otherwise do not yield any clues as to the natures of the 
%mass donors or compact objects. 

To constrain better the natures of the mass donors, we have searched for 
infrared counterparts to the four transient sources within 1\arcmin\ of 
\sgrastar\ using archival NICMOS observations and engineering observations 
taken with the Keck AO laser guide star system. % (Fig.~\ref{fig:irimg}). 
We find that three of the 
four sources have potential infrared counterparts with $K \approx 15-17$, 
although the high density of stars makes it likely that these are random
associations. We find that there is a $\approx 50$\% chance that three 
of four randomly-placed, 0\farcs3 circles would contain a star with $K<16$. No 
observations of comparable depth are available for the sources located 
1--10\arcmin\ from \sgrastar.



Finally, VLA observations have revealed a radio outburst coincident with
the appearance in X-rays of CXOGC~J174540.0--290031, with a peak intensity
of 100 
mJy at 1 GHz (G. Bowers, F. Yusef-Zadeh, \& D. Roberts, in prep.). The 
large radio outburst suggests that this source is more likely to contain 
a black hole than a neutron star 
primary \citep[e.g.,][]{fk01}.

\subsection{Spatial Distribution of the Transients}

Remarkably, four of the seven transient X-ray sources are located 
within 0\farcm45 of \sgrastar, which corresponds to a projected distance 
of $<1$~pc. This concentration of transients near \sgrastar\
is significant even when one takes into account the $R^{-2}$ increase
in stellar density approaching
%

\begin{inlinefigure}
\centerline{\psfig{file=lightcurves.ps,width=0.9\linewidth}}
\caption{
The long-term flux history of the six transient sources not displayed 
in \citet{mun03b}.
}
\label{fig:lcurves}
\end{inlinefigure}

\noindent
%
\sgrastar. 
Using 
the models of \citet{lzm02}, we find that the stellar mass within 
a sphere of 1~pc radius centered on \sgrastar\ is 
$2\times10^6$ \msun, whereas the mass in the sphere between 1 and 
23~pc (10\arcmin) is $3\times10^7$ \msun. This implies that there is 
a factor of 20 overabundance of bright transients within 1 pc of 
\sgrastar. 

We have computed the significance of this excess using the Fisher 
Exact Probability test. As a null hypothesis, we take the observed 
positions of Galactic center X-ray sources \citep{mun03a}, 
and assume that the 7 transients should be randomly distributed 
among these. For the combined set of observations, above the 
completeness limit ($F_{\rm X} = 5\times10^{-7}$ \phcms) we find 
24 sources within 1 pc of \sgrastar, and 464 sources between 
1 and 23 pc. The probability 
that $\ge 4$ of the 7 transients would randomly lie within 1 pc of 
\sgrastar\ is $2\times10^{-4}$.%\footnote{Repeating the above calculation
%considering sources inside and outside 3 pc of \sgrastar\ yields a factor
%of 10 over-abundance that has a $7.7\times10^{-4}$ chance of occurring
%randomly.}

%Assuming that the number of transients follows the Poisson 
%distribution, the probability that the rate of transients per unit
%mass is constant over the inner 23 pc is $<5\times10^{-5}$.

%% for 3.6 pc, the mass is 0.7-1 \times 10^{7}, the specific frequency is
%% (5-7) \times 10^{-7} \msun$^{-1}$, and the over-abundance is 10--40

\section{Discussion}

These transients are unusual because they were observed
at luminosities intermediate between those considered outburst and 
quiescence for X-ray binaries \citep{cam98,mer00}. 
Indeed, the class of faint X-ray transients discussed by 
%\citet{zand01} and 
\citet{king00} have peak luminosities that are 
$\ga 10$ times larger than those in our sample.
X-ray transients are typically identified using all-sky monitors
such as those on \rxte\ and \bepposax, which detect sources with fluxes 
$\ga 10^{-10}$~\ergcms\ \citep{lev96,jag97}. As a result, all-sky
monitors can detect transients similar to those in Table~\ref{tab:sources} 
out to a distance of only 600~pc. Within this distance, there are few known 
accreting black holes or neutron stars. 
Therefore, continued observations of 
the Galactic center will yield insight into the duty cycles of 
X-ray binaries accreting at low average rates of $\la 10^{-12}$
\msun\ yr$^{-1}$ \citep[cf.][]{sak04}.

%% what are the inferred mass accretion rates that I'm talking about?
%% how does this compare to systems, in, e.g., Belcynski \& Taam?
%% Chakrabarty \& Morgan 1998

Unfortunately, the relative lack of observations of X-ray binaries with 
$L_{\rm X} = 10^{33} - 10^{35}$~\ergs\ makes it difficult to establish
conclusively the natures of these transients. The fact that they are 
transient suggests that the systems are either LMXBs with unstable accretion
disks, or HMXBs in which either the mass-loss rate from the secondary is highly
variable or the compact primary has an eccentric orbit about
its companion. The lack of counterparts with $K < 15$ suggests that
the transients are either HMXBs with companions fainter than a B2IV star 
\citep{weg94,weg00}, or LMXBs. Finally, the 
candidate 7.9~h period in CXOGC~J174540.0--290031 is too short to accommodate
anything other than an LMXB,\footnote{The radius of a Roche-lobe-
filling mass donor would be $\approx 0.8 R_\odot$, 
which rules out a high-mass star (see Frank, King, \& Raine 1995, eq. 4.10).} 
while its bright radio outburst suggests that it 
contains a black hole \citep[e.g.,][]{fk01}.

The excess of transient X-ray sources within 1 pc of \sgrastar\ 
brings to mind the fact that, 
relative to their numbers in the rest of the Galaxy, LMXBs
are also a factor of $\sim 100$ over-abundant in globular clusters
\citep{katz75,cla75}. In the cores of globular clusters, three-body 
interactions produce this excess of LMXBs 
\citep[e.g., Rasio \etal\ 2000;][]{pool03}. 
We propose that the same process occurs near Sgr A$^*$, an idea
described fully in an upcoming paper by E. Pfahl \& A. Loeb.  Below is
an estimate of the number of dynamically-formed LMXBs in the
central parsec.

There may be $\ga$$10^4$ neutron stars and stellar-mass black holes
within 1\,pc of Sgr A$^*$, most driven there by dynamical friction
(Morris 1993; Miralda-Escud{\' e} \& Gould 2000).  We suppose that the
number densities of both neutron stars and black holes in the central
parsec are $n_x(r) = 10^3\pc^{-3}\,(r/{\rm pc})^{-2}$, roughly
$\simeq$1\% of the total stellar density (Genzel et al. 2003).  Some
of these compact objects may acquire a stellar companion following a
binary-single exchange encounter.  Binaries certainly populate the
central parsec, although their statistics are unknown.  We speculate
that $\sim$10\% of the stars are in binaries for $r > 0.1\pc$, and
assume a binary density of $n_b(r) = 10^4\pc^{-3}\,(r/{\rm pc})^{-2}$.  In
the central 0.1\,pc, stellar speeds of $\ga$$200 {\rm km~s}^{-1}$ 
make ionization
likely within $\sim$1\,Gyr for systems with initial semimajor axes of
$a \ga 0.1\au$.  We further assume for simplicity that the average
binary semimajor axis and systemic mass, $\langle a \rangle$ and
$\langle M_b \rangle$, are independent of $r$.  The issues of
dynamical evolution and survival of binaries will be more fully
discussed in E.\ Pfahl \& A.\ Loeb (in prep.).

For a given compact object of mass $M_x$ at radius $r < 1\pc$, the
exchange rate is $\gamma \sim n_b\Sigma \sigma$, where $\Sigma \simeq
2\pi \langle a \rangle G(\langle M_b \rangle + M_x)/\sigma^2$ is the
characteristic exchange cross section, $\sigma(r) = (GM_*/3r)^{1/2}$
is the one-dimensional velocity dispersion, and $M_* \simeq 3.7\times
10^6\emsun$ is the mass of Sgr A$^*$.  Numerically we find
%
\begin{equation}
\gamma \sim 10^{-12}\yr^{-1}
\left(\frac{\langle a \rangle}{0.1\au}\right)
\left(\frac{\langle M_b \rangle + M_x}{M_\odot}\right)
\left(\frac{r}{{\rm pc}}\right)^{-3/2}~.
\end{equation}
%
The rate density in the central parsec is $n_x \gamma \propto
r^{-7/2}$, while $n_x\gamma \propto r^{-4}$ in the roughly isothermal
region at $r \ga 2$--3\,pc.  This profile is much steeper than the
stellar density, which could partly explain the
overabundance of transients near Sgr A$^*$.  Volume integration of
$n_x \gamma$ over $r = 0.1$--1\,pc yields
%
\begin{equation}
\Gamma \sim 10^{-7}\yr^{-1} 
\left(\frac{\langle a \rangle}{0.1\au}\right)
\left(\frac{\langle M_b \rangle + M_x}{M_\odot}\right)~.
\end{equation}
%
For black holes, we expect $\langle M_b \rangle + M_x \simeq 10\emsun$
and $\Gamma_{\rm BH} \sim 10^{-6}\yr^{-1}$, while for neutron stars
$\Gamma_{\rm NS} \sim \text{few}\times 10^{-7}\yr^{-1}$.  Within the
central parsec, the dynamical friction time scale is $\sim$$10\,{\rm
Gyr}\,(M_x/M_\odot)^{-1}(r/{\rm pc})^{1/2}$.  This dynamical friction 
should limit the lifetime of an LMXB to $\sim$1\,Gyr.  Therefore,
perhaps 100--1000 neutron stars and black holes in the central parsec
have stellar companions.  Nearly all of these should be
luminous X-ray sources at some stage, in many cases driven by the
expansion of the star as it leaves the main sequence.  Thus, if $\la 1$\% 
of dynamically-formed LMXBs are in outburst each year, they 
can easily account for the overabundance of transients
in the central parsec.


In addition, since a concentration of young stars is observed within
the central parsec \citep[e.g.,][]{kra95}, some of the X-ray 
transients within 1 pc of \sgrastar\ could be HMXBs that formed recently.
The observed young, emission-line stars are thought to be the 
brightest few percent of $\sim 10^4$ stars that formed 
only $\approx 3-7$~Myr ago \citep{kra95}. However, the relative lack 
of older, K and M supergiants suggests that 
star formation was not sustained on time scales of tens of Myr. Since
this is the lifetime of a HMXB, any excess of such systems should be 
$\la 7$~Myr old. 

Predicting the numbers of HMXBs that have been produced in the 
central parsec is difficult, because the strong
tidal forces produced by \sgrastar\ makes it unlikely
that stars can form through the same mechanisms as in the Galactic disk
\citep{mor93}.  Instead, it has been suggested that the young 
stars in the central parsec were formed either {\it in situ} either after the 
collision of dense molecular clouds or in a massive disk around 
\sgrastar\ \citep[see, e.g.,][]{gen03,lb03,ml04}, or as a cluster at larger 
radii that settled dynamically into the central parsec 
\citep[e.g.,][]{ger01, pmg03, km03}. A crude estimate can be obtained
by assuming that HMXBs form with a comparable 
efficiency as in the Galactic disk, in which case $\sim 10$\% of 
binaries that initally contain a massive star should evolve into 
wind-accreting X-ray binaries (e.g., Pfahl, Rappaport, \& 
Podsiadlowski 2002)\nocite{prp02}. However, in $7$~Myr only stars more 
massive than $\simeq 30$~\msun\ should 
collapse to form compact objects. If we assume 
an initial mass function $dN/dM \propto M^{-2}$ and a 
minimum mass of 1~\msun\ \citep{kra95}, 
$\sim 3$\% of the $10^4$ stars will have $M > 30 M_{\odot}$. Therefore, 
$\sim 30$ HMXBs could have formed in the past 7~Myr. If these HMXBs are 
$\ga 10$ times more active than dynamically-formed
LMXBs, they could contribute to the transient population in the central parsec.

If the young stars in the central parsec were carried there by an in-falling
cluster, then a few HMXBs could have been formed through binary-single
interactions in the cluster cores. From eq.~(1) above, 
in a cluster with $n \sim 10^6$ pc$^{-3}$ \citep[e.g.,][]{km03}, 
a standard binary fraction of 0.5, and 
$\sigma \sim 10$ km s$^{-1}$, the interaction rate between a black hole and
binaries with $\langle a \rangle \sim 0.1$~AU will 
be $\gamma \sim 10^{-10}$ yr$^{-1}$. 
Assuming that there are initially $\sim 10^5$ stars in the cluster and 
$\sim 3$\% of them are black holes, then, in 7~Myr, $\sim$10 black 
hole binaries should form via three-body interactions. Thus,
HMXBs that were dynamically formed in an in-falling cluster 
could contribute to the number of transients in the central parsec.

Further progress can be made using several approaches. If variable infrared
or radio counterparts to these X-ray transients can be identified, it 
would enable us to determine whether the observed transients 
near \sgrastar\ are LMXBs or HMXBs. \chandra\ observations of 
similar star-forming environments, such as young, dense clusters, could 
establish how many HMXBs are formed along with the young stars. Finally, 
theoretical population synthesis studies could determine how efficiently 
HMXBs (not to mention their progenitor stars) can form in the extreme 
environment near \sgrastar.

%% in a disk: Levin, Y. \& Beloborodov, A. M. 2003, \apj, 590, L33
%% colliding clouds: Genzel, R., Pichon, C., Eckart, A., Gerhard, O. E., 
%%  \& Ott, T. 2000, \mnras, 317, 348

%% what is the time scale for such a disk to dissipate?
%% What is the mass turn-off at the current age of the cluster? Would a B
%% star remain bound??
%% $t_{ms} \approx 10^{7.2} m_{10}^{-1.9}$ yr (Meynet \etal\ 1994)
%% note that AGB stars suggest 100 Myr population Krabbe; Blum; Genzel
%% Krabbe, A. \etal\ 1995, \apj, 447, L95: relative lack of KM supergiants 
%% suggests that star formation did not occur with 10^7 - 10^8 years!
 
%% Portegies Zwart, S. F., McMillan, S. L. W., \& Gerhard, O. 2003, \apj, 593,
%% 352: starting with 6e4 solar mass, must core collapse (and so form hard
%% binaries, falls in from 5 pc to 1 pc in <1Myr. not really close enough.
%% Kim, S. S. \& Morris, M. 2003, \apj, 597, 312 echo this, and state that
%% the cluster must be 10^6 \msun\ and formed about 5 pc away. 70\% of the 
%% central 1\% of the stars end up in the central pc. too many He I stars.
%% Hansen, B. M. S. \& Milosavljevi\'{c}, M. 2003, \apj, 593, L77 state 
%% that a 4000 \msun\ IMBH could finish the job.
%% Kim, S. S., Figer, D. F. \& Morris, M. 2004, \apj, 607, L123
%% state a 10^4 solar mass black hole is needed, which 
%% is larger than one would expect
%% check Gerhard, 0. 2001
%% summary: clusters live 10-50 Myr, and if they sink, it is within a few Myr


%% what do I know about collision time scales in the clusters?

%% note that Kim, Morris, \& Lee suggest that a SFR of 0.01 \msun\ yr$^{-1}$
%% is in contrast with a rate of 0.3-0.6 \msun\ yr$^{-1}$ from G\"{u}sten 1989
%% based on UV continuum, and 0.05 \msun\ yr$^{-1}$ from IR stars. However,
%% Figer et al. 2004 find agreement with the general SFR within a factor of 
%% two, and with the few hundredths of \msun\ yr$^{-1}$ fed in from CMZ.
%% I should really avoid the issue of the star formation rate. . .

\acknowledgments
We thank A. Loeb and G. Bellanger for helpful discussions, 
and H. Tananbaum for providing a discretionary \chandra\ observation 
to monitor one of the transients. MPM was supported by NASA and the Hubble 
Fellowship program through grant number HST-HF-01164.01-A, and EP 
by NASA and the Chandra 
Postdoctoral Fellowship program through grant number PF2-30024. WNB was 
supported by NSF CAREER Award 9983783.

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\end{document}



%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
%% extra information

%
%\begin{inlinefigure}
%\centerline{\psfig{file=spectra.ps,width=\linewidth}}
%\caption{
%X-ray spectra of six of the transients, taken 
%over a range of luminosities. The black, blue, and red lines,
%represent spectra taken as the fluxes decrease. 
%A spectrum of the seventh source, GRS 1741.9--2853, is displayed in 
%\citet{mun03b}. These spectra are among the first to 
%be obtained for transient X-ray binaries with 
%$L_{\rm} = 10^{33} - 10^{35}$~\ergs.}
%\label{fig:spec}
%\end{inlinefigure}
%
%\noindent
%


%
\begin{deluxetable*}{lcccccccccc}
\tablecolumns{11}
\tablewidth{0pc}
\tablecaption{Transient X-ray Sources in the inner 10\arcmin\ of the 
Galaxy\label{tab:sources2}}
\tablehead{
\colhead{} & \colhead{} & \multicolumn{3}{c}{$L_{\rm min}$ Epoch}
& \multicolumn{3}{c}{$L_{\rm med}$ Epoch}
& \multicolumn{3}{c}{$L_{\rm max}$ Epoch} \\
\colhead{Source} & \colhead{Offset} & 
\colhead{$L_{X}$} & \colhead{$N_{\rm H}$} & \colhead{$\Gamma$} &
\colhead{$L_{X}$} & \colhead{$N_{\rm H}$} & \colhead{$\Gamma$} &
\colhead{$L_{X}$} & \colhead{$N_{\rm H}$} & \colhead{$\Gamma$}\\
\colhead{(CXOGC J)} & (arcmin) & 
\colhead{(erg s$^{-1}$)} & \colhead{($10^{22}$ cm$^{-2}$)} & \colhead{} &
\colhead{(erg s$^{-1}$)} & \colhead{($10^{22}$ cm$^{-2}$)} & \colhead{} &
\colhead{(erg s$^{-1}$)} & \colhead{($10^{22}$ cm$^{-2}$)} & \colhead{} \\
} 
\startdata
174502.3--285450 & 9.98 & 
$<9\times10^{31}$ & \nodata & \nodata &
\nodata & \nodata & \nodata &
$1.5\times10^{36}$ & $9.7^{+0.2}_{-0.2}$ & $1.88^{+0.04}_{-0.04}$ \\
%
174554.3--285454 & 6.38 & 
$<2\times10^{31}$ & \nodata & \nodata &
\nodata & \nodata & \nodata &
$6.2\times10^{34}$& $8.7^{+0.3}_{-0.2}$ & $2.8^{+0.1}_{-0.1}$ \\
%
174535.5--290124 & 1.35 & 
$<9\times10^{30}$ & \nodata & \nodata &
$4\times10^{33}$ & $15^{+2}_{-2}$ & $2.3^{+0.6}_{-0.3}$ &
$3.3\times10^{34}$ & $11^{+2}_{-1}$ & $0.5^{+0.4}_{-0.2}$ \\
%
174538.0--290022 & 0.44 & 
$1.2\times10^{33}$ & $8$ & $2.2^{+3}_{-4}$ &
$\times10^{33}$ & $19^{+7}_{-4}$ & $6^{+2}_{-1}$ &
$2.6\times10^{34}$ & $22^{+6}_{-3}$ & $8^{+2}_{-1}$ \\
%
174540.0--290005 & 0.37 & 
$<4\times10^{31}$ & \nodata & \nodata &
\nodata & \nodata & \nodata &
$3.4\times10^{34}$ & $11^{+1}_{-2}$ & $1.6^{+0.2}_{-0.4}$\\
%
174541.0--290014 & 0.31 & 
$<8\times10^{31}$ & \nodata & \nodata &
$ \times 10^{34}$ & $27^{+5}_{-4}$ & $4^{+1}_{-1}$ &
$4.8\times10^{33}$ & $9.3^{+2}_{-5}$ & $0.0^{+0.5}_{-0.7}$ \\
%
174540.0--290031 & 0.05 & 
$<2\times10^{31}$ & \nodata & \nodata &
$1.6 \times 10^{34}$ & $19^{+1}_{-3}$ & $-0.6^{+0.3}_{-0.5}$ &
$8.5\times10^{34}$ & $12^{+3}_{-4}$ & $1.7^{+0.6}_{-1.0}$ \\
\enddata
\tablecomments{
We report 
$L_{\rm X}$ only for the 2--8 keV band, because the inferred flux at lower 
energies depends heavily upon the assumed absorption column toward each 
source. It is likely that the bolometric luminosities are a factor
of 3--10 larger than those in this table. 
Three sources are 
not listed in the catalog of \citep{mun03a}, so we give their positions
here: 
CXOGC J174554.3--285454: $\alpha = 266.4769$, $\delta = -28.9153$ 
(XMMU J174554.4--285456 in Porquet \etal\ 2004);
CXOGC J174540.0--290005: $\alpha = 266.4170$, $\delta = -29.0016$;
and CXOGC J174540.0--290031: $\alpha = 266.4168$, $\delta = -29.0086$.
}
\end{deluxetable*}


\begin{figure}
\centerline{\psfig{file=nicmos.ps,width=0.8\linewidth}}
\caption{
NICMOS images at 2.22 $\mu$m of fields containing each of the X-ray sources 
within the central arcminute around the Galactic center. The 0\farcs3 
uncertainty on the position of each source is indicated with the circles.
The faintest stars 
in these images have $K\approx 17$. Faint stars are found within the 
uncertainties on the positions of each sources, but the high density of stars
suggests that these are random interlopers.
}
\label{fig:irimg}
\end{figure}

\begin{figure}[t]
\centerline{\psfig{file=obs_lightcurves.ps,width=0.8\linewidth}}
\caption{
Extra figure. The long-term and short-term flux history of six of the 
transient sources.
The seventh, GRS 1741.9-2853, is displayed in \citet{mun03b}.
}
\end{figure}



{\bf CXOGC J174502.3--285450 $=$ GRS 1741.9--2853:} The detailed properties
of this source were reported by \citet{mun03b}. The source produces 
thermonuclear X-ray bursts, which demonstrate that it is a neutron star LMXB.
For comparison with the other sources in our sample, its spectrum is 
consistent with a power law of photon index $\Gamma = 1.7$ absorbed by a 
column density of $8.09^{+0.07}_{-0.07} \times 10^{22}$ cm$^{-2}$ of gas 
and dust. In quiescence, too few counts are obtained to derive its 
spectral properties.

{\bf CXOGC J174554.3--285454 $=$ XMMU J174554.4--285456:} This source
was first detected by \xmm\ on 2002 October 3 \citep{por04}, and exhibited 
another outburst in July 2004. Otherwise, the source has remained below the 
\chandra\ and \xmm\ detection thresholds, which suggests that the outbursts
last for only a couple months. In outburst, we find that its spectrum is 
consistent with a $\Gamma = 2.80^{+0.09}_{-0.08}$ power law absorbed by 
$8.7^{+0.3}_{-0.2} \times 10^{22}$ cm$^{-2}$ of interstellar gas and dust.
We find no evidence for periodic variability, with a 90\% upper limit of 
6\% rms at frequencies between $2\times10^{-4}$ and 0.1~Hz.
%We did not detect
%the candidate 172 second, 6\% rms amplitude periodicity during our 
%\chandra\ observations, with an upper limit to its rms amplitude of 4\%.

{\bf CXOGC J174535.5--290124:} This source is found within the error circle
the neutron star LMXB AX J1745.6--2901 \citep{mae96,ks96}. 
However, the eclipses that are observed 
in the X-ray emission from AX J1745.6--2901 at its 8.4 hours orbital 
period are not observed in the \chandra\ source. This is unlikely to be 
a result of the low flux level at which we observed CXOGC J174535.5--290124, 
because eclipses often become deeper as LMXBs that are viewed 
edge-on enter quiescence \citep[e.g.,][]{nhb02}. Therefore, these 
sources are not likely to be associated. 

As the intensity of the source increases, the spectrum hardens from a 
$\Gamma = 2.3^{+0.6}_{-0.3}$ power law when $F_{\rm X} = 5\times 
10^{-14}$~\ergcms\ (2--8 keV), to $\Gamma = 1.6^{+0.4}_{-0.3}$ when 
$F_{\rm X} = 1.4\times 10^{-13}$~\ergcms, to $\Gamma = 0.5^{+0.4}_{-0.2}$ when 
$F_{\rm X} = 2.2\times 10^{-12}$~\ergcms. The inferred absorbing column 
remains constant near $1.3\times10^{23}$~cm$^{-2}$. No pulsataions are 
detected with upper limits as strict as 16\% rms in 2003 June.

{\bf CXOGC J174540.0--290005:} This is the first report of this source, 
which was detected in 2003 June. Its spectrum in outburst is consistent with
a $\Gamma = 1.6^{+0.2}_{-0.4}$ power law absorbed by 
$N_{\rm H} = 1.1^{+0.1}_{-0.2} \times 10^{23}$~cm$^{-2}$ of gas and dust. 
In quiescence, the spectrum extracted from the position of the source is 
consistent with that of the background diffuse emission. No pulsations are 
detected with an upper limit of 14\% rms.

{\bf CXOGC J174540.0--290031:} This source was first identified in \chandra\
observations in 2004 July, although in retrospect it was also active during
\xmm\ observations in 2004 March (G. Bellanger \etal, in preparation). The 
source also is detected in VLA images
throughout 2004, with a peak radio flux of 100 mJy at 1.4~GHz (D. Roberts
\etal, in preparation). The large
radio luminosity suggests that the source is more likely to contain a 
black hole, altough it is unprecedented for a source
this faint in X-rays to produce a bright radio outburst 
\citep[e.g.,][]{fk01}.

The spectrum of the source was hard in 2004 July, with rising number of
photons as a function of energy ($\Gamma = -0.6^{+0.3}_{-0.5}$) and 
a large absorption that is probably mostly intrinsic 
($N_{\rm H} = 1.9^{+0.1}_{-0.3} \times 10^{23}$~cm$^{-2}$). During the short,
5ks observation on 2004 August 28,
the source was a factor of two brighter, had a softer spectrum 
($\Gamma \approx 0.7$), and appeared less heavily absorbed 
($N_{\rm H} \approx 1\times10^{23}$ cm$^{-2}$). In quiescence, the 
the spectrum extracted from the position of the source is 
consistent with that of the background diffuse emission. 

A signal and its 
harmonic are detected with a period of $\approx 8$~h in the 2004 July 
observations, and could represent the orbital period. However, given that the
individual observations were less than 14 h long, this result should be 
confirmed with further observations. No pulsations are 
detected at shorter periods with an upper limit of 13\% rms.

{\bf CXOGC J174541.0--290014:} Aside from spending two years below th
detection threshold between 2000 and 2001, this source has been persistently
bright and its basic properties are listed in \citep{mun04b}. We find that
the spectrum is hard and heavily absorbed in outburst at 
$F_{\rm X} \ga 2\times10^{-13}$~\ergcms,
with $N_{\rm H} = 1.8^{+0.1}_{-0.1} \times 10^{23}$~cm$^{-2}$ and 
$\Gamma = 2.1^{+0.3}_{-0.2}$. The source softens and becomes more absorbed as
its flux declines to $F_{rm X} = 7\times10^{14}$~\ergcms, at which time
$N_{\rm H} = 2.7^{+0.5}_{-0.4} \times 10^{23}$~cm$^{-2}$ and 
$\Gamma = 4^{+1}_{-1}$. No periodic signal is detected, with an upper limit
of 13\% rms.

{\bf CXOGC J174538.0--290022:} This source has been persistently bright, and
is listed in \citep{mun04b}, but its 2--8~keV luminosity has monotonically 
decreased by a factor of 20 over the past 5 years. Unlike the other sources, 
its spectrum is steep, and is best described by thermal plasma models. At 
its maximum intensity, the spectrum was 
consistent with a $kT = 0.7^{+0.1}_{-0.1}$ keV plasma absorbed by 
$N_{\rm H} = 2.2^{+0.6}_{-0.3} \times 10^{23}$~cm$^{-1}$ of gas and dust. 
At faintest, the source was consistent with a $kT = 1.9^{+0.2}_{-0.3}$~keV
plasma absorbed by $N_{\rm H} = 8^{+1}_{-1} \times 10^{23}$~cm$^{-2}$. No 
pulsations were detected, with an upper limit of 26\% rms. 

BH LMXB:
Hynes, R. I. \etal\ astro-ph/0407405
Kong A. K. H., McClintock, J. E., Garcia, M. R., Murray, S.S., \& Barret, D.
  2002, \apj, 570, 277

NS LMXB:
Rutledge, R. E., Bildsten, L., Brown, E. F., Pavlov, G. G., \&
  Zavlin, V. E. 2001, \apj, 551, 921
Campana, S., Israel, G. L., Stella, L., Gastaldello, F., \& Mereghetti, S.
  2004, \apj, 601, 474

eclispes: Nowak, M. A., Heinz, S., \& Begelman, M. C. 2002, \apj, 573, 778

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%


 The inner parcecs of the 
Galaxy are distinct from the Galactic bulge in that star formation 
has clearly occurred recently, evidence of which is most dramatically
found in three massive star clusters that are only a few Myr old: 
the Arches and Quintuplet located
30 pc in projection from the Galactic center, and IRS 16 within the 
central parsec. IRS 16 in particular must have formed under unusual 
conditions. If its member stars formed {\it in situ}, then 
they must have done so out of an extremely dense molecular cloud, and
under remarkably strong tidal forces. Alternatively, it could have 
began its life several parsecs from \sgrastar\ as a cluster similar to 
the Arches and Quintuplet, and settled dynamically
into the Galactic center. Ultimately, similarly massive stars that have
formed near the Galactic center throughout its history should leave behind
stellar-mass black holes, which may have settled into the central parsec
to create a cluster of tens of thousands of compact objects.



The implications of this overabundance depend upon the ages of these 
X-ray binaries, which in turn can be determined by the natures of the 
mass donors. Systems in which the compact object accretes from the wind
of a high-mass, early-type star will necessarily be younger than 
the lifetime of of the companion, $5\times10^7$ years. 
Systems in which the mass donors are low-mass companions that over-fill 
their gravitational potentials will typically be much older 
systems, often $>10^9$ years. 

Therefore, we have attempted to identify
candidate infrared counterparts. Early-type donors will typically have 
$K < 18$, whereas low-mass donors can be as faint as $K>21$. 
For the four sources within 0.5\arcmin\ of \sgrastar\, we searched archival 
images at 1.60 and 2.22 $\mu$m that were taken with the 
NICMOS camera aboard the {\it Hubble Space Telescope} over the course
of 1997--1998. Each of the four X-ray transients had infrared counterparts 
within the nominal 0\farcs3 uncertainties on their positions, with 
magntidues ranging from $K = 11-17$. However, the density
of stars is so high that the matches are likely to be random interlopers. 


The properties of the X-ray emission often provide some clues in this regard. 
The X-ray spectra are rather poor indicators of the 
natures of the sources, as those of well-studied low- and high-mass X-ray 
binaries are strongly dependent on the nature of the compact object, the 
orbital periods, and the observed inclinations. Temporal variability 
provides more secure indications of the natures of X-ray binaries.
For instance, the detection of pulsations indicate that the central
compact object is a highly-magnetized neutron star, and the vast majority 
of these are young systems with high-mass donors. None of the transients
we observed exhibit such pulsations, with upper limits on their amplitudes
of 10--30\%. The detection of thermonuclear bursts would indicate that the 
accretor is a weakly-magnetized neutron star, and most of these have 
low-mass donors. The only burst detected was from GRS 1741.9-2853.
Finally, the detection of periodic modulations in the X-ray emission due
to the orbital period would indicate the sytems contains a low-mass companion,
as high-mass systems have orbital periods of days to years, which is 
significantly longer than most individual observations. In the July 2004 
observation of CXOGC~J174540.0--290031, we do detect 
modulations with a time scale of 9.3 hours and an amplitude of 30\% rms.
However, because the individual observations
are only 14 h long, we can not establish that these variations are strictly
periodic.

The three transients that lie outside of the inner few parsecs 
have been detected previously by other instruments. Two of these have 
been identified as neutron-star low mass X-ray binaries by the presence
of thermonuclear X-ray bursts. CXOGC J174502.3--285449 is the transient 
GRS 1741.9-2853 \citep{mun03b}. CXOGC J174535.5--290124 lies within the 
25\arcsec\ of the source AX J1745.6--2901, although the eclipses that should 
be observed in the X-ray emission from this source at its 8.4 hours orbital 
period are not observed in the \chandra\ counterpart. The third source,
CXOGC J174554.3--285454, is XMMU J174554.4--285456, which 
was first active in October 2002. We did not detect
the candidate 172 second, 6\% rms amplitude periodicity during our 
\chandra\ observations, with an upper limit to its rms amplitude of 4\%.
Therefore, the nature of this third source remains elusive.

What is the consequence of forming that many massive stars continuously 
at the Galactic center?

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
%% star formation information

between 10^3-10^4 msun of stars have formed in the last 2-7 Myr
  so the rate is on order 5e-4 to 5e-3 msun yr^-1
within 1.5'' of Sgr A*, the stars have a featureless specturm, which could
  result from stripping of red-giants, or the formation of young or recycled
  stars
between 1-10'' is an old population with a spinkling of young stars only 5-8
  Myr old.
Genzel, R. et al. 2003, \apj, 594, 812

Where are ``squeezars'' important?

Figer, D. F., Rich, R. M., Kim, S. S., Morris, M., \& Serabyn, E. 
  astro-ph/030975
0.02 Msun yr^{-1} over 100 pc.

Blum, R. D., Ram\'{i}rez, S. V., Sellgren, K., \& Olsen, K. 2003
  only 25\% of the stars in the central 5 pc are older than 5 Gyr. 
  For 1e7 solar masses the average rate is ~5e-4 solar masses per year.

  factor of 2-10 more star formation in the past 100 Myr, based on AGB 
  and super-giant stars

  4e-6 msun yr$^{-1}$ within the CMZ (200 pc), 8e-4 msun yr$^{-1}$ in GC (5pc)
  radius of 2pc: average 6e-5 Msun yr$^{-1}$, peak 2.4e-4. 


Levin, Y \& Boloborodov, A. M. 2003, \apj, 590, L33
 forming stars from a disk around the black hole. Most of the fragmentation
 takes place at 10^4-10^5 GM/c^2 < 0.03 pc (Goodman 2003).
 density is 2.4e9(M/3e6)(R/0.2 pc)^-3 cm^-3

Portegies Zwart, S. F., McMillan, \& Gerhard (2003)
 clusters within 5 pc will spiral into the Galactic center, 64k stars
Portegies Zwart, S. F., McMillan, S. L. W., \& Baumgardt, H. 2004, in 
  ``The Formation and Evolution of Massive Young Star Clusters'', ASP 
  Conference Series, eds. H.J.G.L.M Lamers, A. Nota, \& L. J. Smith 
 16\% of stars are given a companion. binary fraction increases by a factor of
 two as the clusters evolve (total number may not change?)

Do Kim and Portegies Zwart agree? I think it is a question of outlook.
Kim, S. S. \& Morris, M. 2003, \apj, 597, 312 
  state that a 10^5 solar mass cluster must be born dense and within <5 pc 
  of Sgr A*.

Kim, S. S., Figer, D. F., \& Morris, M. 2004, 
  10^5 msun cluster. IMBH comprising 20\% of the cluster mass is 
  needed or else the stars spread out between 0.8-3.5 pc. However, the
  number of stars delivered to the inner parsec only differs by a factor
  of three with or without a black hole. Sheesh. Big Deal.
see also Hansen, B. M. S. \& Milosavljevi\'{c}, M. 2003, \apj, 593, L77

Nayakshin, Cuadra, \& Sunyaev (2003)

Eric, how large is the core radius that you are assuming? Mark gets 
  0.05-0.25 pc.
How old are Be stars?

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
%% fields covering the transients
174540.0-290005 GC-A (wide)
174540.0-290031 CALCENTER on 09/03/2002; only in 160W
174541.0-290014 20arc on 9/03/2002; earlier observation is the wide camera
                if anything, the star is fainter in 2002. 
174538.0-290022 GC-D (wide)

60arc and NEARCENTER contain no counterparts


%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
%% the following is junk



Additional clues as to the natures of the sources:

CXOGC~J174535.5--290124, CXOGC J174540.0--290031, and CXOGC J174541.0--290014 
have very hard spectra,
suggesting that the X-ray emitting regions are partially-absorbed by material
local to the system. 
CXOGC~J174535.5--290124 and CXOGC J174541.0--290014 becomes softer as they
fade, which has been seen from HMXBs. However, CXOGC J174541.0--290014 may 
be the counterpart to the neutron-star LMXB AX Jetc. However, there is no 
evidence for recurring 6.35 h eclipses in its light curve, whereas other 
quiescent LMXBs do exhibit full eclipses.

CXOGC J174502.3--285450 and 174540.0--290005 
have spectra typical of low-mass X-ray binaries that are consistent 
with $\Gamma = 1.7$ power laws.
CXOGC J174502.3--285450 is a known neutron-star LMXB, GRS 1741.9--2853.
CXOGC J174540.0--290005 softens as it fades, and at minimum is consistent with
background plasma. 

CXOGC J174554.3--285454 has a rather steep spectrum, consistent with a 
$\Gamma = 2.8$ power law or a $kT = 4$ keV plasma. The spectrum steepened 
when compared to October 2002, when it was brighter.

CXOGC J174538.0--290022 stands out because the source is 
bright ($L_{\rm X} \approx 4 \times 10^{34}$~\ergs) and cool (consistent
with emission from a $kT = 0.7$~keV plasma) in 1999, and steadily
fades and becomes hotter for the next 5 years. 

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
%% the following are K magnitudes of OB stars
Visual Magnitudes
Wegner, W. 2000, \mnras, 319, 771
		O5	B0	B9.5
Class IV-V. 	-4.4	-2.9	-0.1
Class III	-4.35	-3.35	-1.00
Class Iab-Ib-II	-4.95	-3.9	-1.9
Class Ia	-5.2	-5.35	-5.70
Class IV-V, e	-5.65	-3.45	-0.70
			B2	B9.5
Class III, Be		-4.1	-1.15
	
Intrinsic Colors
Wegner, W. 1994, \mnras, 270, 229
	K-V			H-V			M-V
	O5	B0	B9	O5	B0	B9	O5	B0	B9
Ia	0.9	0.61	0.01	0.87	0.59	0.02	0.98	0.67	0.02
Iab-II	0.87	0.59	0.01	0.83	0.57	0.01	0.94	0.66	0.01
giant	0.88	0.69	0.09	0.80	0.60	0.08	0.96	0.76	0.10
dwarf	0.89	0.76	0.17	0.81	0.69	0.16	1.02	0.86	0.19

distance modulus is -5*alog10(8.e3/10.0) = -14.51

Extinction law:
Reike, G. H., \& Lebofsky, M. J.
Ak/Av = 0.112
Ah/Av = 0.175

Predehl, P, \& Schmitt, J.H.M.M. 1995, \aap, 293, 889
N_H = 1.97e21 * Av
for N_H = 5e22, Av = 25

Tan \& Draine use Ak=3.5, implying Ah=5.5. (I get Ak=2.8, Ah= 4.4)

Supergiant systems earlier than B2 will have K < 14.7
Be systems could be as faint as K=17.5. ouch. not readily observed 
with NICMOS.


%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
%% the following are measurements of LMXBs, HMXBs, and CVs

Cen X-4 
Rutledge, R. E., Bildsten, L., Brown, E. F., Pavlov, G. G., \&
  Zavlin, V. E. 2001, \apj, 551, 921
Einstein	1980 Jul 28	2-3e32	0.5-4.5
EXOSAT		1986 Feb 21	4-11e32	0.5-4.5
ASCA		1994 Feb 27	4(3)e32	0.5-10
ROSAT		1995 Aug16-18	7e31	0.1-2.4
ASCA		1997 Feb 4-5	3(2)e32	0.5-10
SAX		1999 Feb 9	4.7e32	0.5-10
Chandra		2000 Jun 23	1.7e32	0.5-10

Campana, S., Israel, G. L., Stella, L., Gastaldello, F., \& Mereghetti, S.
  2004, \apj, 601, 474
XMM		2001 Aug20-21	3.9e32	0.5-10	with hr, 45% rms variability


Aql X-1
Campana, S. \& Stella, L. 2003, \apj, 597, 474
Chandra		2000 Nov 28	7.7
Chandra		2001 Feb 19	1.8
Chandra		2001 Mar 23	3.0
SAX		2001 Apr 14	1.9e-12
Chandra		2001 Apr 20	2.4


V404 Cyg
Hynes, R. I. et al. astro-ph/0407405
Chandra 2003 Jul 28-29: variability from 1e32 to 2e33 on time scales of ~hour
Kong A. K. H., McClintock, J. E., Garcia, M. R., Murray, S.S., \& Barret, D.
  2002, \apj, 570, 277
ROSAT		1992 Nov	8.1e33	0.1-2.4
ROSAT		1992 Nov	1.1e33	0.7-2.4
ASCA		1994 May	1.2e33	1-10
SAX		1996 Sep	1.0e33	1-10
Chandra		2000 Apr 26	3-8e33	0.3-7

A0620-00
Kong
ROSAT		1992 Mar	4e30	0.4-1.4
ASCA		1994 Mar	<8e30	0.5-10
Chandra 	2000 Feb 29	2.1e30	0.5-7.0

GRO J1655-40
ASCA		1996 Mar	3e32	0.5-10
Chandra		2000 July 1	2.4e31	0.5-7

XTE J1550-546
Chandra		2000 Aug-Sep	6.7e32	0.5-7


EXO 0748-676	
1993 Mar,May	1.5e-10 0.5-10
Parmar, A. N., White, N. E., Giommi, P. and Gottwald, M. 1986, \apj, 308, 199
  Einstein	1980 May 22	1.2e34	0.2-3.5 (10 kpc)	6e-13 
  4\% residual flux in eclipse from ADC.
Thomas, B., Corbet, R., Smale, A. P., Asai, K., \& Dotani, T. 1997, \apj, 
  480, L21
  ASCA spectrum Gamma=0.61, Ecut=9keV, plus some soft component. absorption 
  increases by a factor of 100 in dips. 1-15e-10 erg cm^-2 s^-1 (1-10) 
  1.5e-10 here.

X 1822-731
Parmar, A. N., Oosterbroek, T., Del Sordo, S., Segreto, A., Santangelo, A., 
  Dal Fiume, D., \& Orlandini, M. 2000, \aap, 356, 175
Observed fluxes are 5e-10 erg cm^-2 s^-1 (1-10 keV). For a distance of 2.5 kpc,
  this equals 4e35. De-absorbed, Lx~1e36 erg/s. Lx~2e37 based on standard
  Lx/Lopt. Line at 6.6 keV with EW 150 eV. alpha=-0.8 Ecut=5.7 keV, 1.3keV BB
Iaria, R., Di Salvo, T., Burderi, L, \& Robba, N. R. 2001, \apj, 557, 24
  Beppo SAX 54 eV iron at 6.5 keV. Edge as well?
  Flux 1.55 e-9 erg cm^-2 s^-1 de-absorbed, 0.1-100 keV
Heinz, S. \& Nowak, M. 
  ASCA+XTE spectrum: 
  THICK: nH=8e20 plus nH=5e22, Gamma=0.8, Ecut=12 power law, 6.5 keV Fe with
    300 eV EW
  THIN: 1.4 keV bb, Gamma=0.04, Ecut=8 keV power law, 
    6.4 keV iron with 101 eV EW, 1 keV raymond smith.

2S 0921-63
Kallman
Observed 2-10 keV fluxes are 
ASCA 5.4
Chandra 3.6
XMM 5.9e-11 erg cm^-2 s^-1
That Lx is ~3e35 erg/s for 7 kpc
They argue Lx ~ 1e36 erg/s (7 kpc) in order to accomodate oxygen lines

See Campana, S. Colpi, M., Mereghetti, S., Stella, L., \& Tavani, M. 1998
  Aarev, 8, 279. Few observations between 1e33-1e35 erg/s

4U 2129+47
Nowak, M. A., Heinz, S., \& Begelman, M. C. 2002, \apj, 573, 778
in outburst, the eclipse is 0.2 in phase, and 75\% depth
in quiescence 0.08 in binary phase, eclipse is total.
also a sinusoidal modulation with half-amplitude 0.012/0.037; 23\% rms.
2-8 keV flux 0.5e-13 erg cm^-2 s^-1, about 10-25\% of 0.5-8 keV flux. 
distance 4-6 kpc based on F star coincident with source (donor is expected
to be a K or M star). Lx = (1-2)e32 erg/s (2-8 keV).

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
%% HMXBs

4U 0115+63
Campana, S., Gastaldello, F., Stella, L, Israel, G. L., Colpi, M., Pizzolato, F., Orlandini, M., \& D. Dal Fiume 2001, \apj, 561, 924
outburst from 2e34 to 5e36 erg/s in <15 h. quiescence of 2e33 erg/s.
Gamma=1 spectrum cut off above 10 keV

RX J0146.9+6121
Mereghetti, S., Tiengo, A., Israel, G. L., \& Stella, L. 2000, \aap, 354, 567
quiescent flux of 5e34 erg/s, outburst to 1e37 erg/s

LS I+61 303
Greiner, J. \& Rau, A. 2001, \aap, 375, 145
XTE March 1996, varied between 8-21e-12 erg cm^-2 s^-1 (2-10 keV), 
  Gamma = 1.5-2.5 power law
Leahy, D. A., Harrison, F. A., \& Yoshida, A. 1997, \apj, 475, 823
ASCA Feb 1994, 4-6e-12 erg cm^-2 s^-2 (2-10), 
  Gamma = 1.7-1.8 power law

V0332+53
Campana, S., Stella, L., Israel, G. L., Moretti, A., Parmar, A. N., \& Orlandini, M. 2002, \apj, 580, 389
Chandra	2001 Jan 4	5e32 erg/s (0.5-10)
faintest in quiescence ever!
Unger, S. J., Norton, A. J., Coe, M. J., \& Lehto, H. J. 1992, \mnras, 256, 725
EXOSAT 1983-84 2e-10 (1-10) erg cm^-2 s^-1 (de-absorbed) 
???
Temna Nov 15 1983 peak 3.2e-9 erg cm^-2 s^-1 (2-30)
Had three outbursts in the early 80s, and one in 89. Did it then became quiet 
or was there simply poor coverage?

SAX J2103.5+4545

X Per 
Coburn, W., Heindl, W. A., Gruber, D. E., Rothschild, R. E., Staubert, R., 
  Wilms, J. \& Kreykenbohm, I. 2001, \aph, 552, 738
 Persistently at 4e34 erg/s (3-100 keV) 1998-1999 RXTE
Di Salvo, T., Burderi, L., Robba, N. R. \& Guainazzi, M. 1998, \apj, 509, 897
  Beppo SAX 2.4e34 erg/s in 0.1-200 keV (half is 1-10 keV)
Robba, N. R., \& Warwick, R. S. 1989, \apj, 346, 469
  EXOSAT 2e-10 erg cm^-2 s^-1 2-10 keV 1983-1986.

RX J0440.9+4431/BSD 24-491
3e34 erg/s (3-30 keV) Reig, P. \& Roche, P. 1999, \mnras, 306, 100
similarly faint. Gamma = 1.5, Ecut=4.5, NH=4.1

1H 0521+373/BD +37 1160
Polcaro, V F., et al. 1990, \aap, 231, 354
 Gamma = 1.5 power law at high energies. Detections at 
 upper limit of 0.39 muJy 2-25 1985 March 6 EXOSAT
 1.2 muJy 2-6 keV UHURU, 4e33 erg/s
 0.6 muJy HEAO1-A1 (position doesn't really match) 
 distance 1.7 kpc. as bright as 4e33 erg/s, fainter than 1e33 erg/s

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
%% CVs

U Gem hard flux increases in outburst
Mason Cordova \& Swank 1979
SS Cyg hard flux increases by a factor of 4, then decreases
BV Pup
V426 Cyg
ven der Woerd, Heise, \& Bateson 1986
XY Ari outburst with RXTE, factor of 4 increase

Sources in All-Sky monitor catalog
BY Cam  190 
SS Cyg	75  quiescence: kT=20 keV and Fx=2e-10 (2-30 keV)
	    outbust: kT=8 keV, and Fx=3e-11
	    transition: kT intermediate, Fx up to 7e-10 lasts 5-10 days
	5e32 erg/s at brightest
  Wheatley, P. J., Mauche, C. W., \& Mattei, J. A. 2003, \mnras, 345, 49
EF Eri	94  also 2e32 erg/s when bright
AM Her	90  bright state 3e32 erg/s, faint is down by a factor of 300

EX Hya	105 ordinarily 3e32 erg/s, brightens by factor of 10 for 2-3 days 
  Hellier, C., Kemp, J., Naylor, T., Bateson, F. M., Jones, A., Overbeek, D., 
    Stubbings, R., \& Mukai, K. 2000, \mnras, 313, 703
 See also TV Col, V1223 Sgr, XY Ari, YY Dra
  Szkody, P. et al. 2002, AJ, 123, 413 on DO Dra -- absorption increase to 
    1.5e22, flux increase by 12. 

For XY Ari, flux increase by 24 for a few 
    days. Hellier, C. Mukai, K, & Beardnore, A. P. 1997, MNRAS, 292, 397

 Additional similar sources: HT Cam, GZ Cnc, NSV 10934, CW Mon
  Kato, T. et al. 2003, \pasj, 55, 489; Kato, T., Dubovsky, P. A., Stubbings, 
   R., Simonsen, M., Yamaoka, H., Nelson, P., Monard, B., Peaece, A., \& 
   Garradd, G. 2002, \aap, 396, 929
VW Hyi	65
GK Per	340-470(!) King, A. R., Ricketts, M. J., \& Warwick R. S. 1979, 
   \mnras, 187, 77
  saw a 45 day, 6.5e33 erg/s 
  outburst; EXOSAT and GINGA saw the same. factor of 10 increase in 
  flux. IR flux increased by 1 magnitude. possibly the most luminous CV.
  Watson, M. G., King, A. R., \& Osborne, J. 1985, \mnras, 212, 917
Hellier, C., Mukai, K., \& Osborne, J. P. 1998, \mnras, 297, 526
  5e-10 erg cm^-2 s^-1

Weight, A., Evans, A., Naylor, T., Wood, J. H., & Bode, M. F.
 Journal: R.A.S. MONTHLY NOTICES V.266, NO. 3/FEB1, P. 761, 1994
340 pc from K counterpart. 470 pc from light echo

XY Ari;  Hellier, C. Mukai, K, & Beardnore, A. P. 1997, MNRAS, 292, 397
 5 day outburst, 10x less luminous. GK Per has a longer orbital period

U Gem	81

V603 Aql, varies by a factor of 2 in the ROSAT band
RS Oph, GK Per, V841 Oph, and RR Pic. 5 out of 11 out of 81 novae
Orio, M., Covington, J., \& \"{O}gelman, H. 2001, \aap, 373, 542 
peak luminosity < few 1e33 erg/s

YZ Cnc decreases by a factor of ~10 during ourburst, which lasts a few days
Verbunt, F., Wheatley, P. J., \& Mattei, J. A. 1999, \aap, 346, 146

Novae since 1980:
382 Vel 1999 
 assumed distance 2 kpc
 Mukai, K. \& Ishida, M. 2001, \apj, 551, 1024
 day	Nh	kT	Fe E	Fe EW	Lx
 20.5	10e22	10	6.63	130	4.5e34
 31	8	4	6.3	190	7.9e34
 35	6	4	6.4	220	7.2
 50	3	3	7.1	220	7.6
 59	2	2			4.9		

U Sco 1999
  14 kpc, 1.4e21 cm^2 kT=0.22-0.52 keV with BeppoSAX

Nova V723 Cas
 Kamath, U. S. \& Ashok, N. M. 1999, \aass, 136, 107
 J 6.4-8.1; H 6-7.6; K 5.5-7.1 D=2.4-7.6 kpc

1974 Cyg 1992
 Krautter, J., \"{O}gelman, H., Starrfield, S., Wichmann, R., \& Pfeffermann, E.
  1996, \apj, 456, 788
 Nh declines from 2e22 to 3e21 cm^-2 from day 63 to 653
 hard tail increases by a factor of 10 then declines over the course of a year
 shell expands by 1500 km/s. distance ~2kpc
Balman, S., Krautter, J., \& \"{O}gelman, H. 1998, \apj, 499, 395
 day	nH	kT	Fx
 63	14e21	10	4e-12
 91	11	5	4.6
 97	7	1.8	5.2
 147	5.5	0.9	18
 255	2	1.0	5
 261	2	0.9	5
 291	2	0.9	3
 612	2	1	.4
 624	2	1	.5
 635	2	1	.5
 653	2	1	.5

4157 Sgr 1992a
  no detection after 19 months 
351 Pup 1991
 Orio, M., Balman, S., Della Valle, M., Gallagher, J., \& \"{O}gelman, H.
  1996, \apj, 466, 410
 16 months after outburst, D=4.7 kpc, 3e-12 erg cm^-2 s^-1 (deabs 0.2-2.4 keV), 
   kT=1.1 keV nH =5.5(4)e21; 8e33 erg/s

838 Her 1991
 Harrison, T. E. \& Stringfellow, G. S. 1994, \apj, 437, 827
 IR photometry from 3-190 days. L,K have secondary peak at 30-40 days, then 
 falls off as t^-4 and t^-6 
 J 4.92-15.4; H 4.6-14.6; K=4.1,14.5, L=3.5,10.5
 O'Brien, T. J., Lloyd, H. M., \& Bode, M. F. 1994, \mnras, 271, 155
 5 days after outburst: either kT=10, Nh=3.2, fx=.54e-11 (0.01-2.4 keV)
                        or     kT=0.7, Nh=17.9, fx=8.5e-11 
 take d=2-14 kpc(!)1 year later, it decreased by >40 times
868 Cen 1991
4160 Sgr 1990
3890 Sgr 1990
443 Sct 1989
745 Sco 1989
394 CrA 1987
QV Vul 1987
827 Her 1987
OS And 1986
Sgr 1986
QU Vul 1984
PW Vul 1984
4092 Sgr 1984
GQ Mus 1983
 Shanley, L., \"{O}gelman, H., Gallagher, J. S., Orio, M., \& Krautter, J 1995, 
  \appj, 438, L95
 observed with ROSAT in 1992 and 1993. shut off by 1993. all photons < 0.7 keV
MU Ser 1983
RS Oph 1985 RN
4121 Srg 1983
SS LMi 1980

Matteucci, F., Renda, A., Pipino, A., \& Della Valle, M. 2003, \aap, 405, 23
Nova rate in the Galaxy is 20-40 per year. 

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
%% diffuse emission. Is it a jet?

brightening of the diffuse emission is from 2-3 counts per pixel to 
6-10 counts per pixel, over an area of about 10 pixels. so before I get
25 counts total, and now I get 80 counts total. 
s = 80/sqrt(25) = 16
sf = (80-25)/sqrt(80) = 10 

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
%% monitoring observations

RXTE/ASM: 10 mcrab in daily averages and uncrowded fields; 
3e-10 erg cm^-2 s^-1
At least 7 transient LMXBs within 3 kpc. Threshold is 3e35 erg/s.
To detect at 1e34 erg/s, need a source at <530 pc.
\bibitem[Levine \etal(1996)]{lev96} Levine, A. M., Bradt, H., Cui, W.,
	Jernigan, J. G., Morgan, E. H., Remillard, R., Shirey, R. E., \&
	Smith, D. A. 1996, \apj, 469, L33

BeppoSAX WFC, few milliCrab per day.
Threshold at 3 kpc is 1e35 erg/s, so it could detect a 1e34 erg/s source at 
1 kpc.
Jager, R. \etal\ 1997, \aaps, 125, 557

in 't Zand et al. report SAX J1711.6-3808 as a faint transient

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