Swift J164449.3+573451 (AKA GRB110328A): A new candidate SFXT?
ATel #3242; J. A. Kennea (PSU), P. Romano (INAF-IASF Palermo), H. A. Krimm (CRESST/GSFC/NASA), J. Cummings (NASA/UMBC), D. N. Burrows (PSU), G. Tagliaferri (INAF-OAB), P. A. Evans (U Leicester), S. D. Barthelmy (NASA/GSFC), A. A. Breeveld (UCL-MSSL), J. M. Gelbord (PSU)
on 28 Mar 2011; 21:12 UT
Distributed as an Instant Email Notice Transients
Credential Certification: Jamie A. Kennea (firstname.lastname@example.org)
Subjects: X-ray, Transient
At 12:57 UT on 2011-Mar-28 Swift/BAT triggered on a newly discovered transient source, initially thought to be a new GRB, GRB110328A (Cummings et al., GCN #11823). A second BAT trigger on this source at 13:40UT on 2011-Mar-28, cast doubt on the GRB nature of this source (Barthelmy et al., GCN #11824), and it is likely that the triggering source is a hard galactic X-ray transient, which was named Swift J164449.3+573451. The most accurate position for this transient is RA, Dec = 251.20787, +57.58334 which is equivalent to:
RA (J2000): 16h 44m 49.89s
Dec (J2000): +57d 35' 00.0"
with an uncertainty of 1.7 arcsec (radius, 90% confidence), reported by Osborne et al., GCN #11826. This location does not correspond with any known cataloged object, and examination of a 6.5ks ROSAT PSPC observation taken on 1990-Jun-01 shows no detection of the source, from which we extrapolate an 3-sigma upper limit of 2.5 x 10^-13 erg/s/cm-2 in the XRT 0.3-10 keV band.
Examining the first two orbits of Swift/XRT data, we observe the following. In the first orbit the source is initially detected at a rate of ~10 XRT count/s, rising to ~50 c/s at the end of the first orbit. During this period the spectrum can be fit by an absorbed power-law model with a photon index of 1.66 +/- 0.04. The source also showed enhanced absorption over the expected galactic value of 1.7x10^20 cm^-2, with an additional absorption component of 7.4 +/- 0.4 x 10^21 cm^-2. The average flux, uncorrected for absorption, was 9.5 x 10^-10 erg/s/cm^2 (0.3-10 keV).
In the second orbit the source had become much fainter (~1 c/s), and much softer, with a photon index of 2.97 +/- 0.35. The average flux in the 2nd orbit, uncorrected for absorption, was 3.3 x 10^-11 erg/s/cm^2 (0.3-10 keV).
We searched for emission at the outburst location in the BAT hard X-ray transient monitor in the 15-50 keV band. During the first trigger interval (1208s from 2011-Mar-28 12:57:50 UT), the source was detected at 0.022 +/- 0.004 ct/s/cm^2 (100 mCrab), fading somewhat by the second interval (1134s from 13:22 UT) to 0.019 +/- 0.002 ct/s/cm^2. The source was not in the BAT field of view for roughly 21 hours before the trigger, last appearing in the BAT FOV at 2011-Mar-27 16:05:28 UT. The source was not detected in this pointing.
When integrating the four days prior to the trigger (Mar 24-27) we see a 4.2-sigma excess (0.0028 +/- 0.0007 ct/s/cm2 or 13 mCrab) and the light curve on shorter time scales shows variation. There was no detection before Mar 24. This suggests that the source was active in hard X rays for several days before the trigger.
We note that the overall behavior is not consistent with a GRB nature. However, there are striking similarities with what was observed during the 2008 March 19 outburst of the supergiant fast X-ray transient (SFXT) IGR J16479-4514 (Romano et al 2008, ApJ, 680, L137) starting from the double triggering of the BAT. The shape of the IGR J16479-4514 light curve was also rising to a peak that reached about 60 c/s at the end of the first orbit followed by a much dimmer, 0.5 c/s, second orbit. The photon index was 0.98+/-0.07 and 2.6+/-0.7 in the first and second orbit, respectively, with absorption in excess of the expected Galactic value.
Given these similarities, the large dynamic range of ~50 shown in the light curve of Swift J164449.3+573451, and a record of hard X-ray activity preceding the BAT triggers, we suggest that this may be a new SFXT candidate, despite the location off the Galactic plane. We also note that despite the low extinction towards this object (E(B-V) = 0.019, Schlegel et al., 1998), there is no detection of an optical counterpart by UVOT with an 3 sigma upper limit of v>20. Levan et al. (GCN #11825) also report an upper limit of r>22.2.
Further observations at all wavelengths are strongly encouraged.