Discovery of a fast X-ray transient in the Chandra Deep Field-South survey

ATel #6541; Bin Luo (Penn State), Niel Brandt (Penn State), Franz Bauer (Pontificia Universidad Católica de Chile)
on 5 Oct 2014; 02:10 UT
Distributed as an Instant Email Notice Transients
Credential Certification: Bin Luo (bul119@psu.edu)

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We discovered a fast X-ray transient in one of the new Chandra Deep Field-South (CDF-S) observations (obsid 16454, ~50 ks exposure). The J2000 source position is RA=53.161661 and DEC=-27.859467, with a positional uncertainty ~0.5". It appears as a new X-ray source with ~110 counts in the 0.5-8 keV band, and it was not present in the previous ~4.5 Ms of exposure with Chandra. Fitting the X-ray spectrum with a simple power law yields Gamma=1.43+/-0.28.

Inspection of the Chandra light curve indicates that the bulk of detected photons arrived over a time span of ~1 ks. The rise time is ~70-160 s, and the decay time appears exponential with net rate going as (t/100.)^(-1.6) (e-folding time ~100 s). No further detection of the source is found. The transient occurred on 2014 Oct 01 07:04:37 UT. It is unlikely due to any previously known Chandra instrumental effect based on several tests performed.

The source is ~4' away from the CDF-S aim point, and it is within the 3D-HST coverage. The closest 3D-HST counterpart is ~0.4" away, with AB magnitudes R~30, z~28, and Ks~26, and a low-quality photometric redshift of ~0.53 (Skelton et al. 2014). There is another possible counterpart ~0.6" away, which is even fainter. The next closet 3D-HST source is 1.9" away. At a redshift of 0.53, the peak X-ray luminosity is ~1.3E45 erg/s (count rate ~0.1 cts/s).

We searched through the ESO archive for additional optical constraints. A VIMOS pre-image (PI Pentericci, program ID 194.A-2003A) overlaps with the position of the X-ray flare. The 550s R-band image was obtained at 2014 Oct 01 08:20:09.6 UT with 0.66" seeing and at 1.03 airmass. After aligning the X-ray and R-band images to 0.1", no object is detected at the location of the X-ray flare, with an approximate limit of R>~25.5 (3sigma, 0.5" radius aperture).

The nature of this transient remains unclear. Observations of any supernova or afterglow emission in optical, radio, etc. would be helpful.