  During hard X-ray observations of the Norma spiral arm region by the Nuclear
Spectroscopic Telescope Array (NuSTAR) in 2013 February, a new transient
source, NuSTAR J163433-4738.7, was detected at a significance level of 8-sigma
in the 3-10 keV bandpass. The source is consistent with having a constant
NuSTAR count rate over a period of 40 ks and is also detected simultaneously by
Swift at lower significance. The source is not significantly detected by
NuSTAR, Swift, or Chandra in the days before or weeks after the discovery of
the transient, indicating that the strong X-ray activity lasted for between
~0.5 and 1.5 days. Near-IR imaging observations were carried out before and
after the X-ray activity, but we are not able to identify the counterpart. The
combined NuSTAR and Swift energy spectrum is consistent with a power-law with a
photon index of Gamma = 4.1(+1.5)(-1.0) (90% confidence errors), a blackbody
with kT = 1.2+/-0.3 keV, or a bremsstrahlung model with kT = 3.0(+2.1)(-1.2)
keV. The reduced-chi2 values for the three models are not significantly
different, ranging from 1.23 to 1.44 for 8 degrees of freedom. The spectrum is
strongly absorbed with NH = 2.8(+2.3)(-1.4)e23 cm-2, 9(+15)(-7)e22 cm-2, and
1.7(+1.7)(-0.9)e23 cm-2, for the power-law, blackbody, and bremsstrahlung
models, respectively. Although the high column density could be due to material
local to the source, it is consistent with absorption from interstellar
material along the line of sight at a distance of 11 kpc, which would indicate
an X-ray luminosity >1e34 erg/s. Although we do not reach a definitive
determination of the nature of NuSTAR J163433-4738.7, we suggest that it may be
an unusually bright active binary or a magnetar.
