NuSTAR and Chandra observations of the bright X-ray transient in SDSS J143359.16+400636.0
ATel #13510; Murray Brightman (Caltech), Anna Ho (Caltech)
on 20 Feb 2020; 19:09 UT
Credential Certification: Murray Brightman (murray@srl.caltech.edu)
Subjects: X-ray, AGN, Black Hole, Quasar, Transient, Tidal Disruption Event
We report on NuSTAR and Chandra observations of the bright X-ray transient in the galaxy SDSS J143359.16+400636.0. The X-ray source was first detected by SRG/eROSITA (ATel #13494) on 2019-12-27, and more recently by Swift/XRT (ATel #13464) on 2020-02-05 where previously no X-ray source had been detected.
We obtained a NuSTAR DDT observation, which took place on 2020-02-13. The source was detected in the ~50 ks observation with a count rate of 0.01 ct/s in the 3--79 keV band in each detector, and remained above background up to ~15 keV.
The source was also serendipitously observed by Chandra on 2020-02-16 while observing the radio-loud Ic-BL SN2020bvc (PI Ho, ATel #13509) at an off-axis angle of ~8'. The source was detected in the ~10 ks observation with a count rate of 0.15 ct/s in the 0.5--8 keV band in the ACIS-S detector.
We jointly fitted the NuSTAR and Chandra spectra which are well described by an absorbed power-law over the 0.5--15 keV range, with NH=8+/-7 x10^20 cm^-2 and Gamma=2.9+/-0.15. The 2--10 keV flux is 1.1x10^-12 erg cm^-2 s^-1, which corresponds to a luminosity of 3x10^43 ergs s^-1 at z=0.09, the spectroscopic redshift reported in ATel #13507.
The source was suggested to be associated with the optical transient ZTF19acymzwg and to be a TDE candidate (ATel #13494). However, the optical transient was reported to be significantly off-center from the host galaxy nucleus by more than 1â, and may have been a supernova (ATel #13507). Furthermore in ATel #13507, the Keck/LRIS spectrum was reported to show strong narrow emission lines with AGN line ratios (see also ATel #13500).
However, the X-ray spectrum of the source is very soft for an AGN, with Gamma~3 compared to the canonical ~2. Furthermore, for AGN, the bright X-ray emission should be accompanied by bright UV emission, as predicted by the tight relationship between the flux densities at 2 keV and 2500A (e.g. Lusso et al. 2016). The restframe monochromatic flux at 2 keV is 2x10^-30 erg s^-1 cm^-2 Hz^-1, whereas the flux density in the Swift/UVOT filter UVW1, with the central wavelength of 2600A, is 3x10^-28 erg s^-1 cm^-2 Hz^-1, which is ~2 orders of magnitude fainter than expected. Therefore the AGN interpretation of this event is not a straightforward one.
We thank the NuSTAR PI, Fiona Harrison, for approving the NuSTAR DDT request, and the NuSTAR SOC for carrying out the observation. We also acknowledge the use of the Chandra DDT observation, and thank the CXC for carrying it out.