The X-ray Spectral Change and Light Curve of the TDE Candidate AT2019pev/ZTF19abvgxrq
ATel #13238; Hui-Hsuan Chung, Albert K. H. Kong, and Kwan-Lok Li (National Tsing Hua University, Taiwan)
on 29 Oct 2019; 15:22 UT
Credential Certification: K. L. Li (lilirayhk@gmail.com)
Subjects: X-ray, Transient, Tidal Disruption Event
We analyze thirteen of the Swift-XRT data for the TDE candidate AT2019pec/ZTF19abvgxrq (ATel #13127, ATel #13132, ATel #13135, ATel #13163, ATel #13170, ATel #13213) detected between late-September and late-October in 2019. The spectra and light curves are extracted with the online Swift-XRT data product generator. We model the spectra by fitting an absorbed power-law model, an absorbed disk blackbody model, and an absorbed power-law plus a disk blackbody component. All the extracted spectra (except for 27th October) have been grouped by at least 20 counts per spectral bin for chi2 statistics.
The first observation on 24th September shows a soft-dominated spectrum, fitted better with a fixed Galactic column density and a disk blackbody Tin of 0.12 +/- 0.006 keV. The model flux is 3.2e-12 ergs/cm^2/s (0.3-5.0 keV) or a model luminosity of 8.0e43 ergs/s assumed at a redshift of 0.096.
On 29th September, the source is captured at its peak flux of 1.4e-11 ergs/cm^2/s (0.3-5.0 keV), fitted with an absorbed power-law (photon index=3.0 +/- 0.2) plus a disk blackbody spectrum (Tin=0.16 +/- 0.02 keV). One day later, the source shows a similar spectrum with a photon index of 3.0 +/- 0.2 and temperature of 0.15 +/- 0.06 keV. The flux slightly dropped to 1.2e-11 ergs/cm^2/s. These two observations show evident variability.
The following observations starting from 3rd October have indicated that the spectra are dominated by power-law. The source's photon indices range from 2.4 +/- 0.18 to 3.0 +/- 0.25, with flux varying between 3.3e-12 ergs/cm^2/s and 6.3e-12 ergs/cm^2/s. In some cases, we fixed the absorption to Galactic column density to obtain a better reduced chi2 value. The last observation on 27th October has the shortest exposure time, leading to a spectrum with the lowest counts. We instead binned the spectrum with at least 10 counts and fit it with an absorbed power-law model, obtaining a photon index of 3.0 +/- 0.61. We also note that the Swift spectrum on 15th October has a photon index of 3.0 +/- 0.2, which is close to the photon index obtained from the Chandra LETG X-ray observation (ATel #13213) on 14/15 October.
We investigated and plotted the Swift-XRT light curve by setting a bin length of 500 seconds. The light curve suggests the observations from 29th to 30th in September show evident variability, while the following observations show slight variability. We also examined the flux evolution over all the observations and the time dependence of the flux decay after the peak observation on 29th September. We found the flux decay is proportional to t^(-0.89) instead of a typical TDE profile of t^(-5/3).
