Swift J16444+57: dramatic drop in intensity for the past month
ATel #4398; B. Sbarufatti (INAF-OAB/PSU), D. N. Burrows (PSU), N. Gehrels (GFSC), J. A. Kennea (PSU)
on 19 Sep 2012; 19:15 UT
Credential Certification: Boris Sbarufatti (boris.sbarufatti@brera.inaf.it)
Subjects: X-ray, Transient
The Swift XRT monitoring of tidal disruption event Swift J1644+57, discovered by Swift on 2011 March 28 has shown a dramatic decrease in the X-ray count rate starting the 5th of August, with the source dropping from a count rate of ~ 1.0E-2 counts/s (5.1E-13 erg cm^-2 s^-1) to a rate not detectable with the 1 ks daily exposure used since March 2012 for our monitoring program.
In order to assess the state of the source we have increased the length of our daily observations, first to 3 ks/day and then to 10 ks/day beginning on September 14th. We report here results from the last 5.5 days of observations, with a total exposure of 45.9 ks. Using a source region with radius 23.6 arcsec centered on the source position, and estimating the background in a composite region consisting of 1.5E5 square arcsec selected avoiding any detectable source, we obtain a 99% confidence level upper limit (computed using Bayesian statistics, see Kraft, Burrows, and Nousek 1991, ApJ) of 4.3E-4 counts/s (2.1E-14 erg cm^-2 s^-1). This indicates that the source has now faded by at least a factor of 25, making this the deepest dip we have seen in the light curve since 2011 March 30. This dip has now lasted over a month.
Since the historical XRT lightcurve shows several episodes with the source suddenly fading and then returning at the same rate level observed before the dip, Swift XRT will keep performing "sentinel observations" of 1 ks each day to check for any rebrightening.
