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Swift/BAT detection of an SGR-like flare from near Sgr A*

ATel #5009; J. A. Kennea (PSU), H. Krimm, S. Barthelmy, N. Gehrels, C. Markwardt, J. Cummings, F. Marshall (GSFC), T. Sakamoto (AGU), N. Degenaar, M. T. Reynolds, J. M. Miller (Michigan), C. Kouveliotou (MSFC)
on 26 Apr 2013; 02:48 UT
Credential Certification: Jamie A. Kennea (kennea@astro.psu.edu)

Subjects: X-ray, Black Hole, Soft Gamma-ray Repeater, Transient

Referred to by ATel #: 5011, 5032, 5033, 5035, 5037, 5046, 5070, 5074, 5095, 5124, 5222, 5254, 5332, 5847, 7023

Degenaar et al (ATEL #5006) recently reported elevated X-ray emission from Sgr A* detected in daily monitoring by Swift XRT, which has persisted for at least a day (Dwelly and Ponti, ATEL #5008). We report here on observations of Sgr A* triggered by the detection of a short flare by Swift BAT on April 25, 2013 at 19:15:25UT (Barthelmy et al., GCN #14443).

Using the data set from T-240 to T+303 sec, we report on updated analysis of BAT trigger #554491. The BAT ground-calculated position is RA, Dec = 266.388, -28.982, which is

RA(J2000) = 17h 45m 33.3s
Dec(J2000) = -28d 58m 55s

with an uncertainty of 2.1 arcmin, (radius, sys+stat, 90% containment). The partial coding was 100%. Sgr A* lies 2.1 arcmin from the center of the BAT error circle, marginally outside it.

The light curve shows a single peak of duration 0.032 sec, visible in the energy bands from 15-50 keV. There is no sign of any further emission. The time-averaged spectrum from T+0.00 to T+0.03 sec fit by a simple power-law model shows a power law index of 3.0 +- 0.4 (chi squared 89.4 for 57 d.o.f.). Using this model, the calculated fluence in the 15-150 keV band is 5.0 +/- 1.3 x 10^-9 erg/cm2. The 1-sec peak photon flux measured from T-0.48 sec in the 15-150 keV band is 0.3 +- 0.1 ph/cm2/sec.

Formally, the best fit model to the time-averaged spectrum is a single blackbody. The fit shows a blackbody temperature of 11.3 +/- 0.9 keV (chi squared 44.2 for 58 d.o.f.). A cut-off power law model gives a cutoff energy of 46.0 +/- 4.0 keV (chi squared 66 for 56 d.o.f.), but is unable to constrain the spectral index. All the quoted errors are at the 90% confidence level.

XRT began observing 19 minutes after the BAT trigger, and was on-target for a total of 265s. During this time, an X-ray source was detected at the following UVOT-enhanced position: RA/Dec (J2000) = 266.41745, -29.0079, equivalent to:

RA(J2000) = 17h 45m 40.19s
Dec (J2000) = -29d 00m 28.4s

with an estimated uncertainty of 2.8 arcsec radius (90% confidence). Sgr A* lies 2.0 arcsec from this position, inside the error circle. No other X-ray point sources are detected. The flux of this source is consistent with that reported in ATEL #5006, continuing to be elevated above the average Sgr A* quiescent X-ray flux.

Although Sgr A* has been shown to flare in X-rays repeatedly (e.g. Degenaar et al., 2013), continued elevated X-ray emission is unprecedented, leading to speculation that this and the BAT flare may be related to an interaction with the G2 cloud (Gillessen et al., 2012, 2013). However, the short timescale of the flare is difficult to reconcile given the mass of Sgr A*. Based on the BAT data we suggest the flare may be a new soft-gamma repeater (SGR) near Sgr A*. This is based on (1) the short duration (30 msec) of the flare, (2) the persistent emission from this region recently detected by XRT (Degenaar et al., ATel #5006; Dwelly & Ponti, ATel #5008), (3) the blackbody spectrum and energy cutoff at 46 keV and 4) the high density of magnetar candidates (30% of the total population) in a longitude range of 30 degrees from the center along the plane towards the Scutum galactic arm. We note at this time we have been unable to detect any periodicity in the XRT data in the typical SGR range (2-12 s).

If this is a new SGR, the angular separation between it and Sgr A* would have to be relatively small (conservatively <4 arcsec). Alternatively the X-ray counterpart of the BAT flare could be below the level of XRT detectability.

We encourage follow-up of Sgr A* during this period of high X-ray emission. In particular we recommend observations by Chandra, to unambiguously determine if the enhanced emission is from Sgr A* or from an unresolved transient.

Swift monitoring of Sgr A* is on-going. Results of this monitoring can be seen online at the following web page: http://swift-sgra.com/