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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

Referred to by ATel #: 1014, 1058

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.