SPI observations of positron annihilation radiation from the 4th galactic quadrant: Spectroscopy
V. Lonjou
The status of the analysis of the electron/positron annihilation radiation performed by INTEGRAL/SPI in the 4th GALACTIC QUADRANT is reported. We use data from the first two Galactic Center Deep Exposures (GCDE) and from the Galactic Plane Scans (GPS). The analysis presented here is focused on the spectroscopic aspects of the electron/positron annihilation radiation. Background substraction and model fitting methods are described, and the parameters of the 511 keV line (flux, energy, and width) are deduced.
paper_final.tex
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%SPI observations of positron annihilation radiation from the 4th galactic quadrant: Spectroscopy
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%paper for the 5th INTEGRAL workshop proceedings
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%Lonjou v le 03/03/2004 
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\ProvidesFile{esapub.tex}
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\documentclass[a4paper,twocolumn]{esapub} % European paper
%\documentclass[letterpaper,twocolumn]{esapub} % American paper
%\usepackage{times}
\usepackage{natbib}
\usepackage{epsfig}
\usepackage{graphicx}
\title{SPI observations of positron annihilation radiation from the 4th galactic quadrant: Spectroscopy}
\author{V. Lonjou}
\author{G. Weidenspointner\footnote{ESA Fellow}}
\author{J. Kn{\"o}dlseder}
\author{P. Jean}
\author{M. Allain}
\author{P. von Ballmoos}
\author{M.J. Harris}
\author{J.P. Roques}
\author{G.K. Skinner}
\affil{Centre d'{\'E}tude Spatiale des Rayonnements, BP 4346, 31028
Toulouse, France} 
\author{B.J. Teegarden}
\author{N. Gehrels}
\affil{NASA/GSFC, Code 661, Greenbelt, MD 20771, USA}
\author{N. Guessoum}
\affil{American University of Sharjah, Physics Department, Sharjah, UAE}
\author{C. Chapuis}
\author{Ph. Durouchoux}
\affil{CEA Saclay, 91191 Gif-sur-Yvette, France}
\author{E. Cisana}
\author{M. Valsesia}
\affil{IASF, CNR, 20133 Milan, Italy and Universita degli studi di Pavia, 27100 Pavia, Italy}
\newcommand{\btx}{\textsc{Bib}\TeX}
\newcommand{\filename}{esapub}

\begin{document}

\keywords{gamma-ray observations; positron annihilation line; Galactic
center region}

\maketitle


%%%%%%%%%%%%%%%%
%   abstract   %
%%%%%%%%%%%%%%%%
\begin{abstract}The status of the analysis of the electron/positron annihilation radiation performed by INTEGRAL/SPI in the 4th GALACTIC QUADRANT is reported. We use data from the first two Galactic Center Deep Exposures (GCDE) and from the Galactic Plane Scans (GPS). The analysis presented here is focused on the spectroscopic aspects of the electron/positron annihilation radiation. Background substraction and model fitting methods are described, and the parameters of the 511 keV line (flux, energy, and width) are deduced.
\end{abstract}
%--VU--



%%%%%%%%%%%%%%%%%%%%
%   introduction   %
%%%%%%%%%%%%%%%%%%%%

\section{Introduction}
\label{intro}


The 511 keV line from the Galactic Center (GC) has been observed since the early seventies by a large number of balloon and satellite borne experiments. The first detection was reported in 1972 \citep{johnson72} but at an energy of 426$\pm$26 keV, making the identification of the electron/positron annihilation line uncertain. In 1977, it was observed with a high energy-resolution Germanium spectrometer establishing the line at 511 keV  with a width of only a few keV \citep{leventhal78, albernhe81}.
During the eighties variability in the measured flux was reported after a series of measurements by balloon-borne germanium detectors. The fluctuating results were interpreted as the signature of a compact source of annihilation radiation at the GC \citep{leventhal91}.
Yet during the early nineties this interpretation came into question, since neither eight years of SMM data \citep{share90} nor the revised data of the HEAO-3 Ge detectors \citep{mahoney94} showed evidence for variability in the 511 keV flux. 
Later, during the nineties, CGRO's Oriented Scintillation Spectrometer Experiment (OSSE) measured steady fluxes from the galactic bulge and disk component. The separation was achieved using data from the OSSE, SMM and TGRS experiments \citep{purcell97, milne01}. 
A possible additional component at positive latitude was attributed to an annihilation ``fountain'' in the GC \citep{dermer_skibo97}. Recently, SPI also provided its first results \citep{jean03a, knodlseder03}, giving constraints both on morphology and spectrocopy. A summary of the key parameters measured by high resolution spectrometers is given in Figure~\ref{fig:comparaison}.

We report here the measurement of the Galatic 511~keV gamma ray line emission after one year of INTEGRAL observations, focusing on the spectroscopy aspects. The morphological analysis of the 511 keV emission, using the same data, is presented in \citet{weidenspointner04c}. Both spectroscopy and morphology results were presented during this meeting in the context of previous results (\citet{jean04}). Their consequences on the properties of the annihiliation medium were exposed by \citet{guessoum04}.


%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
%   instrument and data analysis   %
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
\section{Instrument and Data Analysis}
\label{data_analysis}

%descrition de SPI
The Spectrometer for INTEGRAL (SPI) is one of the two main instruments on board the ESA INTEGRAL mission. A detailed description of SPI can be found in \citet{vedrenne03}. SPI is a coded-mask telescope with a fully-coded field of view of $16^\circ$ that uses a 19 pixel high-resolution Ge detector array resulting in an effective area of about 75 cm$^2$ at~511 keV. The energy resolution of the whole camera is 2.10$\pm$0.02 keV at 511 keV and varies between 2 and 8 keV over the full energy range (20 -- 8000 keV). 

%description du jeu de donn�es
The data analyzed in this work were accumulated during the first year's GCDE and GPS, executed as part of INTEGRAL's guaranteed time observations. Both the GCDE and the GPS observations are part of the so-called core program observations of the INTEGRAL observatory, which are proprietary to the INTEGRAL Science Working Team (ISWT) and the instrument teams for one year \citep{winkler01}. A total exposure time of 3475 ks has been obtained during two different periods March-April 2003 and September-October 2003. The GCDE consists of three rectangular pointing grids covering Galactic longitudes $|l| \le 30^\circ$ and Galactic latitudes $|b| \le 10^\circ$, and a fourth grid covering the same longitude range but $|b| \le 20^\circ$ in latitude. The GPS consists of pointings, following a saw-tooth pattern, along the Galactic plane within $|b| \le 6.4^\circ$. Details of the core program observing strategy can be found in \citet{winkler01}.

%pr�paration des donn�es:
For each pointing and each of the 19 detectors, the SPI events data were gain corrected and binned into 0.25 keV wide bins \citep{knodlseder04}. The gain correction was performed for each orbit to account for long-term drifts that arise from temperatures variations and degradation of the detectors \citep{lonjou04}. 

%Background modelling:
Background modelling is a crucial point in this analysis because the instrumental line is roughly a hundred time stronger than the astrophysical one. We have modelled the background with two components (see Figure~\ref{fig:bck}): one for the line and one for the continuum \citep{jean03b, teegarden04, jean04}. 
The continuum component describes the temporal variation of the continuum level under the line which is considered to be flat. The �line� component contains the background line and the differences between the real continuum and the flat model.
We used the SPI background line identifications \citep{weidenspointner03}, the instrumental background simulations employing the MGGPOD Monte Carlo suite \citep{weidenspointner04a, weidenspointner04b} and others published informations on detectors to identify the main contributors to the 511~keV line and continuum around it. The contributions of the different isotopes (prompt and delayed) have been fitted to the data assuming that their production rate scales with the rate of the Germanium saturated events. Therefore, templates describing the temporal variations of the background components are generated.

 
%figure bck
\begin{figure}
	\includegraphics[height=7cm,width=8cm]{bck511keV.eps}
	\caption{The instrumental background line at 511 keV. It is modelled with two components : one for the line and one for the continuum.}
	\label{fig:bck}
\end{figure}


%%%%%%%%%%%%%%%%%%%%%
%   Model Fitting   %
%%%%%%%%%%%%%%%%%%%%%
\section{Model Fitting, background model and morphology}
\label{modelfitting}
Using model fitting techniques \citep{knodlseder04}, we derived constraints on the morphology of the 511 keV line emission and to test our background models. We assume a 2D Gaussian centered on the Galactic Center for the spatial distribution, and we fit the flux in a 5 keV wide bin (508.5-513.5 keV). During the fit we can free some parameters in order to match the background model to the data. We have used four different fit types: fix (fixed background model), global (1 free parameter for the entire observation), orbit (1 free parameter per revolutio), dete (1 free parameter per detector).
%An equivalent behavior (see Figure~\ref{fig:model_fit}) of the different fit types would indicate that the background model is correct.
The similarity of the results obtained with this four background fitting options (see Figure~\ref{fig:model_fit}) indicates the high quality of our background model. However, there remains room for improvement, thus this work continues.
A FWHM=$8^{+3 \circ}_{-2}$ is deduced with a 2$\sigma$ confidence level is found for the Gaussian. The effect of a disk component has been tested but was founded  to be smaller than the effect of the size of the bulge component. For more details on the morphological aspects see \citet{weidenspointner04c}.
%figure model fit
\begin{figure}
	\includegraphics[height=6cm,width=8cm]{model_fit.eps}
	\caption{Confidence level as a function of the spatial distribution. The $\delta$ likelihood follows a $\chi2$ distribution, thus the confidence level is the square root of the $\delta$ likelihood because there is one degree of freedom.}
	\label{fig:model_fit}
\end{figure}
%spectre issu du model fit
\begin{figure}
	\includegraphics[height=6cm,width=8cm]{spectre.eps}
	\caption{511 keV flux obtained using a model of the spatial distribution
	 consisting of a Gaussian centered on the Galactic Center with a FWHM of $8^{\circ}$.}
	\label{fig:spectre}
\end{figure}



%%%%%%%%%%%%%%%
%   Results   %
%%%%%%%%%%%%%%%
\section{Results}
\label{results}
We used model fitting techniques in order to extract the spectrum. In this case, we fit the same 2D Gaussian spatial model to the data in each 1 keV energy bin in the vicinity of the 511 keV line. We free one global parameter per energy bin in order to correct for detector degradation effects and to reduce systematic errors. As an example, the resulting photon spectrum obtained using a $8^{\circ}$ FWHM Gaussian for the spatial distribution (our best fitting model distribution) is shown in Figure~\ref{fig:spectre}. The spectrum has been fitted with a Gaussian to derive the flux, energy and the width of the line. The fit was performed with different models for the continuum under the line and for several widths of the spatial distributions. The results of this analysis are summarized in Figure~\ref{fig:line_features}.
%resultat des fits
\begin{figure}
	\includegraphics[height=10cm,width=8cm]{line_features.eps}
	\caption{Flux, Position and Width of the 511 keV line from 
the Galactic Center as a function of the size of the spatial distribution.}
	\label{fig:line_features}
\end{figure}
Thus the uncertainties quoted for these results include the statistical uncertainties and the uncertainties on the spatial distribution. The width of the astrophysical line is the measured
width after deconvolving the instrumental width (FWHM$=2.10^{+0.02}_{-0.02}$~keV). We obtained a flux of $0.96^{+0.21}_{-0.14} \times 10^{-3}$ ph~cm$^{2}$~s$^{-1}$, a line centroid of $511.02^{+0.08}_{-0.09}$~keV and a width of the astrophysical line of $2.67^{+0.3}_{-0.33}$~keV. The flux value is strongly dependent on the size of the emission, but the position and width of the line are relatively stable. The best $\chi^2$ is obtained with a step function component in the background under the line.
Formally the continuum level to the left of the 511~keV line (see Figure ~\ref{fig:spectre}) is significantly higher than to its right. 
%/OU/ An excess of $0.9^{+0.42}_{-0.25} \times 10^{-6}$ph.cm$^2$.s$^{-1}$.keV is detected in the energy range 498-511 keV.
%This can be interpreted as being due to positronium continuum emission.
This is an indication of positronium continuum emission, but at this stage of the analysis we are not in a position to quote a definitive value for the positronium continuum flux and the positronium fraction.

%Nonetheless, because of systematic uncertainties in the substraction of the Galactic continuum emission we refrain from quoting a value for the positronium continuum flux and the positronium fraction.
% Concerning the flux value, this analysis is the only one which take into account the error on the spatial distribution.




\begin{figure}
	\includegraphics[height=10cm,width=8cm]{comparaison.eps}
	\caption{Flux, position and width of the GC 511 keV line measured by
	hight resolution spectrometers.}
	\label{fig:comparaison}
\end{figure}



%%%%%%%%%%%%%%%%%%
%   Conclusion   %
%%%%%%%%%%%%%%%%%%
\section{Conclusion}
\label{conclusion}
The analysis of SPI data that have been recorded during the first year INTEGRAL GCDE have provided constraints on the shape of the 511 keV line from the Galactic Center region. We derived a flux of  $0.96^{+0.21}_{-0.14} \times 10^{-3}$~ph~cm$^{-2}$~s$^{-1}$, a width of $2.67^{+0.3}_{-0.33}$~keV (FWHM) and a line centroid of $511.02^{+0.08}_{-0.09}$~keV. 
In general the results are consistent with earlier measurements (see Figure~\ref{fig:comparaison}), yet the measurement of the line width is formally inconsistent with the TGRS measurement on a 1 $\sigma$ confidence level. Such a significance for the energy and the width had never been obtained. One should note that the HEAO3 results are given for a point source at GC and that the GRIS results don�t take into account the uncertainties on the spatial distribution, so a direct comparison is not possible.
Our analysis indicates the presence of the positronium continuum, but we cannot deduce any value for the positronium fraction because of systematic uncertainties.
These results can be used in order to determine and to understand the properties of the annihilation medium \citep{guessoum04}. We are now refining this work by improving the background modeling and including much data.



%%%%%%%%%%%%%%%%%%%%%%%
%   acknowledgments   %
%%%%%%%%%%%%%%%%%%%%%%%
\section*{Acknowledgments}

Based on observations with INTEGRAL, an ESA project with instruments and science data centre funded by ESA member states (especially the PI countries: Denmark, France, Germany, Italy, Switzerland, Spain), Czech Republic and Poland, and with the participation of Russia and the USA.



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\begin{comment}
The positron annihilation radiation was first detected through its
hallmark, a gamma-ray line at 511~keV, in balloon observations of the
Galactic center region in the 1970s and has been the focus of
intense scrutiny by a large number of balloon and satellite borne
experiments ever since \citep[see reviews by,
e.g.,][]{lingenfelter_ramaty89, harris97}. 
%
Despite this remendous effort, the origin of the positrons is still
far from being understood.
%
A large variety of positron sources and production mechanisms have
been advanced over the years \citep[see e.g.\ discussions
in][]{chan_lingenfelter93, ramaty_lingenfelter94}. Among the more
promising source candidates are the nucleosynthesis products from
supernovae; more recently hypernovae/GRBs 
\citep{casse04} and the annihilation or decay of light dark matter particles
\citep{boehm03, hooper_wang04} have been revisited. Another intriguing
candidate source of positrons are compact objects.

Investigations of the morphology of the annihilation radiation promise
to provide valuable clues for identifying the source(s) of positrons
in our Galaxy, despite the fact that positons may travel from their
source before annihilating. The annihilation radiation was mapped for
the first time with the OSSE instrument on board the Compton Gamma-Ray
Observatory \citep[e.g.][]{purcell97, milne01}. At least two extended
spatial components, a (possibly dominant) extended bulge and a disk, are
required to account for the observed annihilation emission. 
%The important bulge-to-disk ratio is only poorly constrained.
Early OSSE maps showed tantalizig evidence for third emission
component located about $8^\circ$ above the Galactic center
\citep{purcell97}, however, the final analysis did not corroborate its
existence \citep{milne01}.

{\bf PvB et al.\ paper}

{\bf refer to earlier SPI results and Vincent paper}



Observations of annihilation radiation are notoriously
difficult because of the strong instrumental 511~keV background
line. In addition, interpretation of results is challenging because
the annihilation sites may be different from the location of the
positron sources if the positrons have a sufficient lifetime to escape
from their source and spread into the interstellar medium (ISM) before
annihilating. The numerous observations of annihilation radiation made
with balloon and satellite borne instruments in the 1970s and 1980s
culminated in a two-component model: steady, extended 511~keV (narrow)
line and ortho-positronium continuum emission from the Galactic bulge
and disk, and superimposed variable 511~keV line emission from one or
more compact sources near the Galactic center
\citep{lingenfelter_ramaty89}. Since then new observations advanced
our knowledge on the extended component, but failed to confirm a
significant contribution from point sources.

The annihilation radiation was mapped for the first time with the OSSE
instrument on board the Compton Gamma-Ray Observatory (CGRO). OSSE had
an aperture of $3.8^\circ \times 11.4^\circ$ FWHM and a moderate
energy resolution of 45~keV at 511~keV. At least two extended spatial
components, a dominant, extended bulge and a disk, are required to
account for the observed annihilation emission. The important
bulge-to-disk ratio is only poorly constrained. In addition, there is
weak evidence for a tantalizing enhancement of extended emission
centered about $8^\circ$ above the GC \cite{milne_00}. When combined
with these extended components, a single GC point source can at best
contribute 10\% of the total emission \cite{milne_priv_com}. The
Transient Gamma-Ray Spectrometer (TGRS) on board the WIND mission
performed the most detailed spectroscopy of the emission from the GC
region to date \cite{harris_98}. The 511~keV line was found to be
narrow ($\sim 1.8$~keV FWHM) and centered at its rest energy. The
positronium fraction was determined to be $0.94 \pm 0.4$, which
favours annihilation in a warm neutral or ionized ISM and disfavours
annihilation in dust grains, a hot ISM, or cold molecular clouds.

Because of the poor spatial resolution of most gamma-ray experiments the
question whether part of the 511~keV line emission from the GC 
region is due to point sources had to be addressed indirectly
by searching for time variability.
%In the context of the Galactic annihilation radiation the question
%whether part of the 511~keV line emission is due to point sources is
%usually considered synonymous to the question whether there exists
%variable 511~keV line emission. 
In the 1970s and 1980s the 511~keV line emission from the GC region
has been observed by many balloon and satellite borne instruments with
rather small fields-of-view. These measurements suggested that the
511~keV line flux decreased significantly between fall 1979 and spring
1980, and increased again at the end of the 1980s (see e.g.\
\cite{tueller92,harris_97} for details). However, the wide
field-of-view ($\sim 130^\circ$ FWHM) instrument on board the Solar
Maximum Mission (SMM) did not detect significant year to year
variation between 1980 and 1989 \cite{share_90}. The monitoring effort
of the GC region was carried on with the narrow field-of-view
instrument OSSE during 1991--2001. No evidence for variability was
found \cite{harris_97,milne_priv_com}. The lack of variability seen by
SMM and particularly OSSE does not rule out variability seen at
earlier times, since it is possible that the putative point source(s)
went quiet.
%
%Evidence for variable annihilation radiation, albeit broadened
%and red-shifted and hence indicating a variable positron source rather
%than a variable 511~keV line source, was obtained with the imaging
%(arcminute resolution) SIGMA instrument. 
Evidence for variable annihilation radiation from compact objects in
the form of transient broadened and red-shifted line features was
obtained with the imaging (arcminute resolution) SIGMA instrument on
board the GRANAT space observatory. However, no contemporaneous
(narrow) 511~keV line emission was observed, rendering these
transients variable positron sources rather than soucres of variable
511~keV line emission.
%
Of particular interest are outbursts of the black hole candidate
1E~1740.7-2942, located less than $1^\circ$ from the GC, whose
transient spectrum suggested the presence of a strong, broad line
centered near 400~keV. Two of these outbursts were simultaneously
observed by SIGMA and the CGRO instruments OSSE and BATSE. In both
cases the CGRO instruments did not confirm the annihilation feature
claimed by SIGMA; the disagreement has not yet been resolved (see
\cite{harris_97} for details). Therefore no satisfactory and
conclusive answer can be given to the question whether there is
variable 511~keV line emission from the GC region.

%{\bf ??? skip because ``same'' as first paragraph in next section ???}.
%The INTEGRAL observatory is better suited for studying the Galactic
%positron annihilation radiation that any previous gamma-ray mission.  The
%Ge spectrometer SPI, with its unprecedented combination of high
%spectral resolution and moderate angular resolution, is designed to
%study spatially extended line emission from the Galaxy. The imager
%IBIS will be particularly useful for searching for (transient)
%positron sources, which may exhibit 511~keV line and/or broadened and
%shifted annihilation line emission. {\bf rather positron sources than
%511~keV line sources because of red-shift}.  {\bf TBD: more detail?,
%numbers for resolution?, references - AO-1, or something more recent
%???}


\end{comment}

astro-ph0411414.extracted_bib
\begin{thebibliography}{}
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%
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%et~al., 2003, astro-ph/0309686
%
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%Paul, J., et~al., 2004, ApJ 602, L17
%
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