Swift J164449.3+573451/GRB 110328A: Continued Swift Monitoring

ATel #3250; 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), G. Israel (INAF-OAR), P. Esposito (INAF-OAC), T. Sakamoto (GSFC/UMBC), M. M. Chester (PSU), S. T. Holland (CRESST/USRA/GSFC), C. Wolf (PSU), N. Gehrels (NASA/GSFC)
on 30 Mar 2011; 19:25 UT
Distributed as an Instant Email Notice Transients
Credential Certification: Jamie A. Kennea (kennea@astro.psu.edu)

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We report on continued monitoring of the new X-ray transient Swift J164449.3+573451/GRB 110328A (e.g. Kennea et al, ATEL #3242) by Swift. During 2011-Mar-29, Swift J164449.3+573451/GRB 110328A brightened significantly, triggering the Swift/BAT twice, at 18:26:25.19UT and 19:57:45.19UT respectively (see Sakamoto et al., GCN Circ #11842 for more details on the BAT characteristics).

The XRT light-curve shows significant variability, and is characterized by periods of bright flares, with the XRT measured brightness varying with a dynamic range of ~400 (~0.2 XRT c/s to ~80 XRT c/s). These flaring episodes last of the order of 2-3 x 10^4 seconds, with many flares occurring within these time periods. In the XRT light-curve, which currently covers from 2011-Mar-28 12:57UT to 2011-Mar-30 13:46UT, ~2 days, we see evidence for 5 major flaring outbursts, the timing of these outbursts does not appear to be periodic. These outburst events are seen in both XRT and BAT light-curves, which are well correlated. The underlying light-curve shows no evidence for a fading trend.

The source shows hardness changes that are correlated with the source brightness, with a correlation coefficient of r=0.71 when comparing the XRT count rate to the ratio of the 1.5-10 keV and 0.3-1.5 keV bands.

A Fourier analysis of the first set of WT data (1.3 ks, MJD 55648.559-55648.946) was performed and no significant periodicity was found. The 3-sigma upper limits on the pulsed fraction, defined as semi-amplitude of the modulation divided by the mean source count rate, computed according to Israel & Stella 1996,ApJ, 468, 369 for a sinusoidal signal is 12% for periods between 3.5ms to 3s (we could not set meaningful upper limits on the pulsed fraction above 3s because of substantial red noise). Further timing analysis is ongoing.

Summed UVOT observations of the source reveal no detection of an optical counterpart in any filter. The deepest images obtained are in the UVOT white and u filters; co-added images give the following upper limits: white (11.3ks) > 23.4 mag; u (23.6ks) > 22.9 mag. Photometry was performed using the UVOT photometric system (Poole et al. 2008, MNRAS, 383, 627).

The source has a very atypical light-curve if it is a GRB. MAXI reports detection of this transient ~4 hours before the initial BAT trigger (Kimura et al., ATEL #3244), suggesting that the source was rising from quiescence at the time of the initial BAT trigger. This behavior is much more typical of a Galactic X-ray transient or some AGN flares. For example in the case of MAXI J1659-152, MAXI detected the rising flux ~5.5 hours before the source had reached sufficient brightness to trigger BAT (Negoro et al., ATEL #2873), although we note that an apparent associated optical counterpart with a measured redshift of z=0.35 (Levan et al., GCN Circ #11833, Thoene et al., GCN Circ #11834), and the detection of a bright radio source at a location consistent with this optical afterglow, are consistent with the transient being extragalactic in origin. We also note that the properties of the X-ray light-curve are becoming less consistent with those of an SFXT, which this source was previously suggested to be (Kennea et al., ATEL #3242), and this combined with the lack of a bright optical companion and the high galactic latitude, make the SFXT hypothesis less likely.

Monitoring observations of this source by Swift are on-going.