Three Fast X-ray Transients Discovered from Chandra Archival Data

ATel #13171; Dacheng Lin(UNH), Jimmy Irwin (UA), Edo Berger (CfA)
on 8 Oct 2019; 03:11 UT
Distributed as an Instant Email Notice Transients
Credential Certification: Dacheng Lin (dacheng.lin@gmail.com)

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Here we report our discovery of three fast X-ray transients (FXTs) from the search over the Chandra archival data. These FXTs were all serendipitously detected and have many properties (especially a light curve of a fast rise to a plateau lasting 1-2 ks followed by a steep decay) similar to CDF-S XT2, which was argued to be powered by a new-born magnetar formed in a binary neutron star merger. Below are the results of our preliminary analysis. CXO J234503.4-423841 or XRT 170831 was detected in a 74 ks Chandra observation (ObsID 20635) on Aug 31, 2017 at the position of R.A.=23:45:03.44, Decl.=-42:38:41.7. The source was not detected until about 70 ks into the observation when the X-ray flux rose within a few tens of seconds (the rising phase was not detected) by a factor of about 400 to about 1.0e-12 erg/s/cm^2. The flux remained at that level for ~1.1 ks before decaying approximately as t^-1.3 for another 4.6 ks. The observation stopped before the decay ended. The total flare spectrum can be fitted with an absorbed powerlaw of photon index 2.0+-0.5 and absorption column density of (0.8+-0.6)e22 cm^-2 (errors are at the 90% confidence level), assuming zero redshift. The source was covered in the WFC3 Infrared Spectroscopic Parallel Survey, and we identified a candidate host galaxy that appears blue (F606W: 24.92 AB mag, F814: 25.20 AB mag, F110W: 24.93 AB mag, F160W: 24.69 AB mag) and is highly elongated. The source is a little offset from the nucleus and from the galaxy's plane. CXO J050706.7-315210 or XRT 030510 was detected in a 46 ks Chandra observation (ObsID 4062) on May 10, 2003 at the position of R.A.=05:07:06.76, Decl.=-31:52:10.8. The source was not detected until about 23 ks into the observation when the X-ray flux rose within a few seconds by a factor of about 300 to about 2.0e-12 erg/s/cm^2. The flux remained at that level for 1.7 ks and decayed approximately as t^-2.0. The flare lasted about 14 ks. The total flare spectrum can be fitted with a powerlaw of photon index of 1.8 with weak absorption. There is some evidence for softening of the X-ray spectrum from photon index of 1.6 at the peak to photon index of 2.1 during the decay. The field around the source was covered in the Dark Energy Survey, showing no counterpart (limiting AB magnitudes are 24.6 in g, 24.0 in r, 24.0 in i, 23.2 in z, 21.7 in Y). The field around the source was covered in the VISTA Hemisphere Survey without a counterpart detected either (the limiting magnitudes are 20.4 in J and 18.4 in Ks). CXO J010344.5-214845 or XRT 110919 was detected in a 92 ks Chandra observation (ObsID 13454) on September 19, 2011 at the position of R.A.=01:03:44.59, Decl.=-21:48:45.9. The source was not detected in the first 1 ks of the observation. Then the flux increased by at least a factor of 100 (compared with the observation after the flare) to about 3e-13 erg/s/cm^2. The X-ray flux remained at that level for 2.4 ks, and then decayed approximately as t^-2.9. There is evidence for softening of the X-ray spectrum from photon index of 1.5 at the peak to 3.0 during the decay. The field around the source was covered in the Dark Energy Survey, without counterparts found (limiting AB magnitudes are: 24.1 in g, 24.0 in r, 23.2 in i, 23.0 in z, 21.6 in Y). The field around the source was covered in the VISTA Hemisphere Survey without counterparts detected either (the limiting magnitudes are 20.6 in J and 18.5 in Ks). The search over gamma-ray bursts from various observatories (Fermi, Swift and INTEGRAL) found no detection of gamma-ray bursts associated with these FXTs.