A New Highly Variable X-ray Source at z ~ 1.5 in the Chandra Deep Field-South Survey

ATel #6625; Bin Luo (Penn State), W. N. Brandt (Penn State), Franz Bauer (Pontificia Universidad Catolica de Chile)
on 26 Oct 2014; 21:54 UT
Credential Certification: Bin Luo (bul119@psu.edu)

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We discovered a new highly variable Chandra source in the ongoing Chandra Deep Field-South (CDF-S) survey. The J2000 source position is RA=53.094681 and DEC=-27.694768, with a positional uncertainty ~0.6". It was not detected in the previous 4 Ms of Chandra exposure (1999-2010), yet it was detected in the newly obtained 1.06 Ms of Chandra exposure (17 observations between 2014-06-09 and 2014-10-17) with ~85 counts in the 0.5-2 keV band. Compared to the upper limit from the previous 4 Ms exposure, the 0.5-2 keV count rate has increased by a factor of >13.9. The source is ~6.4' away from the average Chandra aim point. It was not present in the 3 Ms XMM-Newton CDF-S catalog either (Ranalli et al. 2013).

The X-ray source is very soft, with no detection in the 2-8 keV band. The effective power-law photon index is thus constrained to be Gamma>2.1. We cannot constrain any count rate variability between the individual 2014 observations, largely due to the limited number of counts. Breaking these 2014 observations into two segments (before and after 2014-09-26) suggests a ~15% decrease in count rate, although this value is still within the uncertainty of the count rate measurements. At a redshift of 1.51 (see below), the 0.5-2 keV luminosity is ~1.3E43 erg/s.

The X-ray source has an optical counterpart ~0.4" away (RA_op=53.0946848, DEC_op=-27.6946506), with AB magnitudes R~24.0, z~23.3, and Ks~21.7 (Hsu et al. 2014). This counterpart has a relatively reliable photometric redshift of 1.51 (Hsu et al. 2014). We caution that there is a brighter optical source nearby (1.9" away, RA_op2=53.094074, Dec_op2=-27.694773, R~21.5) that might contaminate low-resolution follow-up observations.

A comparison of the counterpart in the MUSYC R-band image (Cardamone et al. 2010) and a VIMOS 550s R-band pre-image (PI Pentericci, program ID 194.A-2003A), obtained at 2014 Oct 01 08:20:09.6 UT with 0.66" seeing and at 1.03 airmass, shows no obvious change in magnitude. Given that the latter image is concurrent with the X-ray outburst, there appears to be no change in R-band which could be associated with this soft X-ray rise.

The nature of this X-ray source remains unclear. However, given its non-detection in the previous 4 Ms of Chandra exposure over the last ~14 years, the factor of >13.9 increase in flux, and the lack of significant flux variability over the last ~4 months (observed frame), it could be associated with a stellar tidal disruption event or an AGN that varies strongly on long timescales. The chance of finding one tidal disruption event in the currently 5 Ms CDF-S survey is broadly consistent with theoretical expectations (e.g., Wang & Merritt 2004) or the upper limit constrained from the 1 Ms CDF-S and 2 Ms Chandra Deep Field-North (CDF-N) data (Luo et al. 2008). The new CDF-S survey will continue monitoring this source with Chandra till the end of 2014. Observations in the optical/UV would be helpful to constrain its nature.