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Chandra confirmation of transient X-ray activity from CXOGC J174540.0-290005 north of the Galactic Center

ATel #5095; C. Heinke (U. Alberta), D. Haggard (Northwestern/CIERA), F. Baganoff (MIT), G. Ponti (MPE), N. Rea (CSIC-IEEC), F. Yusef-Zadeh (Northwestern/CIERA), D. Roberts (Northwestern/Adler), on behalf of a larger collaboration
on 1 Jun 2013; 04:17 UT
Credential Certification: Daryl Haggard (dhaggard@northwestern.edu)

Subjects: X-ray, Transient

We have performed a Chandra monitoring observation of the Galactic Center for 23.8 ks, with the ACIS-S/HETG configuration and a 1/2 subarray mode, on May 25, 2013. We are able to pinpoint the location of the active transient 20" north of Sgr A*, in addition to detecting ongoing activity from the new magnetar SGR J1745-29 (ATels #5009, #5011, #5020, #5032, #5037, #5046, #5053; Kennea et al. 2013; Mori et al. 2013; Shannon et al. 2013), and apparently quiescent emission from Sgr A* (unabsorbed Lx[2-10keV] ~ 3x10^33 ergs/s). The active transient, recently reported by NuSTAR (2013 May 18-19; Atel #5073) and Swift (2013 May 15-19; ATel #5074), is easily detected in this observation in both the zeroth-order and gratings data.

We adopt the ATCA position for the magnetar reported by Shannon & Johnson (2013)

RA = 17:45:40.16, Dec = -29:00:29.82 (uncertainties 0.022", 0.09" respectively)

and perform a boresite correction to derive an accurate position for the transient

RA = 17:45:40.07, Dec = -29:00:05.8 (uncertainty 0.1")

We can firmly associate this source with the known LMXB CXOGC J174540.0-290005, which has a previously reported Chandra position of 17:45:40.06, -29:00:05.5 (uncertainty 0.6"; Muno et al. 2005; Degenaar et al. 2012).

We extract and fit spectra from the zeroth-order data, and from the +1,-1 MEG and HEG orders, of the transient LMXB. We fit the data with an absorbed, dust-scattered powerlaw in XSPEC, using the model PILEUP*TBABS*PEGPWRLW*SCATTER to address substantial pileup in the zeroth-order data. The SCATTER model (Predehl et al. 2003) allows for optically thick dust scattering.

We find that the transient has clearly brightened between May 15-19 (using values reported in Atel #5074) to May 25, increasing from Lx[2-10 keV] ~ 1.8x10^35 ergs/s to 3.4x10^35 ergs/s (assuming a distance of 8.0 kpc). Our fitted photon index is 1.7+/- 0.3, and we measure N_H=1.6+/-0.2x10^23 cm^-2. This is uncertain due to the pileup model (we fix the alpha parameter in the XSPEC pileup model to 0.5, following Davis (2001)) and possible changes in the N_H model, so it is unclear whether the photon index is significantly harder than the ~2.4 reported by Degenaar et al. in Atel #5074. Although spectral softening during outburst decay has been seen in a number of both NS and BH systems (e.g. Armas Padilla et al. 2013), spectral hardening during the early rise has been more rarely observed, e.g. in Ter 5 X-3 (Atel #4249).

Further multiwavelength observations of CXOGC J174540.0-290005's outburst are strongly encouraged.

References:
Armas Padilla, M., et al. 2013, MNRAS, 428, 3083
Davis, J.E. 2001, ApJ, 562, 575
Degenaar et al. 2012, A&A, 545, 49
Kennea et al. 2013, accepted to ApJ letters, arXiv:1305.2128
Mori et al. 2013, accepted to ApJ letters, arXiv:1305.1945
Muno et al. 2005, ApJ 622, L113
Predehl et al. 2003, AN, 324, 73
Shannon & Johnson 2013, submitted to MNRAS, arXiv:1305.3036