NuSTAR detection of a transient in outburst north of Sgr A*

ATel #5073; Francois Dufour, Victoria M. Kaspi (McGill University), Eric V. Gotthelf (Columbia University), Frederick K. Baganoff (MIT), Fiona A. Harrison (Caltech)
on 20 May 2013; 06:48 UT
Distributed as an Instant Email Notice Transients
Credential Certification: Victoria Kaspi (vkaspi@physics.mcgill.ca)

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Weekly NuSTAR follow-up of the new magnetar in the Galactic Center (SGR J1745-29; ATels #5020, #5046) on 2013 May 18-19 (observation start time UT 17:36:07, 81.9 ks elapsed time, 39.1 ks exposure) has revealed new activity from a second, nearby source.

An imaging analysis of the NuSTAR data yields a position of the new source 25.7" north of SGR J1745-29 (with a circular error of 6.9" radius, 68% confidence; above 7.1 keV). This position assumes the dead reckoning coordinates of the NuSTARDAS pipeline. Under the assumption of no rotational offset, this gives an absolute position of 17h45m40.5s -29d00'03.2" with a circular statistical error of 6.0" radius, 68% confidence. Correcting this position for the observed offset of SGR J1745-29's position gives an absolute position of 17h45m40.2s -29d00'04.7" with a circular statistical+systematics error of 13.1" radius, 90% confidence.

A three color image (red:3.0-6.2 keV, green:6.2-7.1 keV, blue:7.1-80.0 keV; smoothed with a 17.5" boxcar) is available at http://www.hep.physics.mcgill.ca/~dufourf/sgra_transient_regions_with_chandra.tiff , with the wavdetect source regions for the same image color (68% confidence) and a white region for the position of SGR J1745-29 according to Rea et al. 2013 (ATel #5032; with 7.1" radius combined systematics, 90% confidence). The yellow circle shows the corrected position of this transient, with 90% statistical+systematic errors.

The above position is consistent with that of the transient source CXOGC J174540.0-290025, which had an outburst in 2005 (Muno et al. 2005, ApJ, 622, L113) and again in 2006 (Degenaar & Wijnands 2009, A&A, 495, 547). We tentatively associate this NuSTAR transient with this source.

Preliminary spectral analysis gives a 2-25 keV unabsorbed flux of ~4.3E-11 erg/cm2/s (above the flux of SGR J1745-29, as observed on 2013 May 11-12), with a spectrum much harder than that of either SgrA or SGR J1745-29. This implies a 2-25 keV luminosity of 3.6E35 erg/s at a distance of 8 kpc.

The light curve shows that the count rate in a 1' radius aperture around SGR J1745-29 rises from 0.95 counts/s/module to 1.15 counts/s/module over the 81.9 ks span of the observation. The May 11-12 observation had a count rate of 0.55 counts/s/module in the same aperture. A preliminary Fourier analysis of counts from a 12.6" radius aperture around the new source reveals no astrophysically relevant periodic signal.

Further X-ray, NIR, and radio observations of the Galactic Center are encouraged to unambiguously identify this transient.