Chandra Detection of Three Enigmatic X-ray Transients: GRS 1741.9-2853,
ATel #1013; M. P. Muno (Caltech), R. Wijnands (Amsterdam), Q. D. Wang (UMass Amherst), S. Park, W. N. Brandt (Penn State), F. E. Bauer (Columbia), and Z. Wang (McGill)
on 27 Feb 2007; 00:07 UT
Distributed as an Instant Email Notice Transients
Credential Certification: M. Muno (mmuno@srl.caltech.edu)
Subjects: X-ray, Binary, Neutron Star
The Chandra X-ray observatory ACIS-I observed the locations of
two transient low-mass X-ray binaries, GRS 1741.9-2853 (see
ATel #1005, #1006, #1008) and GRO J1744-28, and the unidentified
very faint X-ray transient XMM J174457-2850.3 starting on 2007
Feb 22 at 03:29:33, for 39 ks. All three sources were detected.
GRS 1741.9-2853 was detected at 17h 45m 2.34s, -28o 54' 49.9" (J2000;
0.5" uncertainty), consistent with Muno et al. (2003, ApJ, 598,
474). Although it was located 5.5' off-axis from the aimpoint, it was
bright enough that photon pileup affected the total number of counts
recorded. We measured 24,417 photons within 90% of the PSF, but
examining the number of counts obtained in an annulus excluding the
inner 70% of the PSF, we estimate that the true count rate was 30%
higher. We applied the standard model for ACIS pile-up, and we find
the spectrum is consistent with a Gamma=0.9+/-0.1 power law absorbed
by N_H=9.0+/-0.5e22 cm^-2. The observed 2-8 keV flux (corrected for
pile-up) was 3e-11 erg cm^-2 s^-1, and the estimated intrinsic 2-8 keV
luminosity for D=8 kpc was 4e35 erg s^-1. This source was probably
identified by both Swift and INTEGRAL (as IGR J1745-2853) in the last
two weeks. We note that in the formal 2' error circle of IGR
J17453-2853 (ATel 1005) Chandra detects no sources, where sources
brighter than 2e-14 erg cm^-2 s^-1 (2-8 keV, Gamma=1.5 power law)
would be detected in 50% of trials. This further supports the Swift
identification of IGR J17453-2853 with GRS 1741.9-2853 (ATel #1008). It
has been active at this level several times over the past 7 years
(e.g., Muno et al. 2003, ApJ, 598, 474).
GRO J1744-28 was detected at 17h 44m 33.09s, -28o 44' 27.2" (J2000;
0.5" uncertainty) at 6.7' from the aimpoint. We detected 602 photons
from the source. The spectrum could be modeled as a Gamma=2.3+/-0.4
power law absorbed by N_H=6.5+/-0.1e22 cm^-2. The observed 2-8 keV
flux was 4.1e-13 erg cm^-2 s^-1, and the intrinsic 2-8 keV luminosity
for D=8 kpc was 6.0e33 erg s^-1. This is about a factor of three
brighter than was observed on 2001 July 18 (Wijnands & Wang 2002, ApJ,
568, L93), indicating some enhanced activity from this source.
The very-faint X-ray transient XMM J174457-2850.3 was detected at 17h
44m 57.46s, -28o 50' 21.0" (J2000; 0.5" uncertainty) at 2.0' from the
aimpoint. We detected 158 photons from the source. The spectrum could
be modeled with an absorbed power law with Gamma = 0.6+/-0.6 and a N_H
of 5+/-3e22 cm^-2. The absorbed 2-8 keV flux was 1.3e-13 erg cm^-2
s^-1 and the 2-8 keV luminosity was 1.3e33 erg s^-1 for D=8 kpc. This
is intermediate between its luminosity in full outburst (4.5e-12 erg
cm^-2 s^-1; e.g., Sakano et al. 2005; Wijnands et al. 2006) and
quiescence (<2e-14 erg cm^-2 s^-1; Sakano et al. 2005). Therefore,
this source also shows enhanced X-ray activity.