Swift detects an X-ray and optical outburst from a Galactic source probably associated with 1RXH J173523.7-354013

ATel #1528; G. L. Israel (INAF-OA Roma), H. A. Krimm (CRESST/GSFC/USRA) N. Rea (U Amsterdam), D. Perez (U Leicester/U Jean), D. N. Burrows (PSU), A. P. Beardmore (U Leicester), S. Campana (INAF-OA Milan), S. Covino (INAF-OA Milan), J. Cummings (GSFC/UMBC), A. De Luca (INAF-IASF),P. Esposito (INAF-IASF), P. A. Evans (U Leicester), D. Gotz (CEA-Saclay), J. E. Hill (CRESST/GSFC/USRA), S. T. Holland (CRESST/USRA/GSFC), J. A. Kennea (PSU), C. B. Markwardt (CRESST/GSFC/UMCP), G. Novara (U Pavia), P. T. O'Brien (U Leicester), J. P. Osborne (U Leicester), K. L. Page (U Leicester), P. Roming (PSU), T. Sakamoto (CRESST/GSFC/UMBC), G. K. Skinner (CRESST/GSFC/UMBC), and A. Tiengo (INAF-IASF)
on 15 May 2008; 22:22 UT
Credential Certification: GianLuca Israel (gianluca@mporzio.astro.it)

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The Swift team and collaborators report on the detection of an outburst from a previously unknown Galactic transient consistent in position with the ROSAT source 1RXH J173523.7-354013.

At 10:32:37 UT on May 14, 2008, the Swift Burst Alert Telescope (BAT) triggered (trigger=311603) and located an X-ray source (Krimm et al. GCN #7713; Baumgartner et al. GCN #7727).

The time profile of the burst in the 15-150 keV energy range is consistent with a single peak, with very similar rise and decay times, emerging from background for about 100 s (10:31:59 - 10:33:39 UTC). The BAT spectrum was very soft, with no signal detected above 35 keV. A fit with a power law in the 14-40 keV range yields a reduced chi^2 of 1.50, with a very steep photon index of 4.4 ±0.4 (at 90% c.l.). A better fit (reduced chi^2 of 1.05) is obtained with a blackbody model. This yields a temperature of 2.5+/-0.3 keV and a 14-40 keV flux of about 3.7x10^-9 erg cm^-2 s^-1. Extrapolation of the best fit blackbody model yields a 2-10 keV unabsorbed flux of 1.2x10^-8 erg cm^-2 s^-1.

Following the BAT detection, Swift observed the source with XRT on 2008 May 14th, starting from UT 10:35:07 (about 150s after the BAT trigger). The first set of data was 90s long, taken in WT (Windowed Timing) mode, during which we detected a bright source, nominally about 90ct/s (in the 2-10 keV range; a factor of 100-1000 brighter than the level detected by ROSAT), displaying a rapid fading. The WT spectrum (about 6000 photons) is well fitted (reduced chi^2 of 1.14) by an absorbed ((8+\-1) 10^21 cm^-2) blackbody with kT=1.87+\-0.07 keV and with a flux of 8 x 10^-9 erg cm^-2 s^-1 (in the 2-10 keV band).

The second set of data was carried out in PC (Photon Counting) mode and amounts to 2060s starting at UT11:52:55 . Only one bright source is present within the BAT positional uncertainty region. We find an astrometrically corrected X-ray position (using the XRT-UVOT alignment and matching UVOT field sources to the USNO-B1 catalogue) of:

RA (J2000): 17h 35m 23.75s

Dec (J2000): -35d 40' 16.2"

with an uncertainty of 1.7 arcsec (radius, 90% confidence). This is 2.5" from the ROSAT HRI position of 1RXH J173523.7-354013, consistent with being the same source. The source is detected at a count rate of about 1.0 ct/s and displayed a decaying trend. The corresponding PC spectrum is fitted well (reduced chi^2 of 1.08) by an absorbed, (9.5 +\- 1.5) 10^21 cm^-2, blackbody with a characteristic temperature of kT=0.66+\-0.03 keV. The absorbed flux is 3.4 x 10^-11 erg cm^-2 s^-1 (in the 2-10 keV).

A search for pulsations gave negative results: we inferred a 3-sigma upper limit (to the semi-amplitude of any sinusoidal signal) of 20-30% (for periods in the 10s-3.5ms range) and 30-50% (periods in the 100s-5s) for the XRT data in the WT- and PC-mode datasets, respectively.

UVOT detected a transient optical source close to the X-ray source position:

RA(J2000.0): 17h 35m 23.75s

Dec(J2000.0) = -35d 40' 16.1"

with an uncertainty of 0.56 arcsec (radius, 90% confidence). The magnitude is white = 18.62 +/- 0.09 (1-sigma, statistical) in a 77 s exposure starting 154s after the BAT trigger. This source faded by approximately 2.3 mag over the next 6000 s.

Based on the above results, we are not yet able to unambiguously assess the nature of the X-ray and optical outburst among possible different scenarios. These range from a X-ray (super)burst from a low mass X-ray binary (currently preferred) to a rare magnetar outburst. In the latter case, the UVOT detection would represent the first prompt emission ever detected in the optical band for this class of objects.

Further Swift observations are scheduled.