Swift Observations of ASASSN-19yt

ATel #13190; Aru Beri (IISER Mohali), Poshak Gandhi, Mayukh Pahari, Christian Knigge (Univ. Southampton)
on 14 Oct 2019; 18:25 UT
Credential Certification: Aru Beri (aru.beri8@gmail.com)

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A new transient (ASASSN-19yt) was detected on 2019-10-07 during the ongoing All Sky Automated Survey for SuperNovae (ASAS-SN, Shappee et al. 2014, ApJ, 788, 48). The source was located to be at position RA = 06h 18m 01.45s, Dec = +22d 22m 29.1s (J2000). It showed a continuous rise in the optical brightness rising from a limiting g mag of >~17 to a peak g=15 (see ATel~13176). The quiescent mag of the source could be g~20.8, as inferred from a potential Pan-STARRS1 counterpart (ATel 13176), implying an optical rise of ~6 mags.

Interestingly, as pointed out in the same ATel, an X-ray source is known to exist at this position, with historical Chandra and NuSTAR hard band fluxes in the range of ~(1-5)x10^{-13} erg/s/cm2, depending upon exact energy range of interest. It is, therefore, plausible that ASASSN-19yt could be an X-ray binary that is now rising from quiescence. In order to test this, we carried out Swift observations to search for any corresponding X-ray outburst.

Our Swift observations took place between 2019-10-13 3:05:54 and 2019-10-13 04:53:52 (UT) with a total exposure time of the 1500s. The XRT data were collected in the photon counting (PC) mode. We extracted the XRT spectrum using a circular region of 20 pixels centered on the optical position. The background spectrum was also extracted using a circular region in the vicinity of the source. We found that the XRT spectrum showed a net (background-subtracted) count rate of ~ 1.5e-03 c/s which translates to an X-ray flux F = (1.2+/- 0.7) x 10^-13 erg/cm^2/s in the 2-10 keV band, assuming Gamma=1.7. This value of X-ray flux is consistent with the values obtained with historical Chandra and NuSTAR observations.

Swift/UVOT observations revealed the uvw2 magnitude (Vega system) to be 16.53 +/- 0.07 (stat) +/- 0.03 (sys). Furthermore, recent AAVSO and ASASSN light curves show the source optical brightness to have begun to decline, already within just a few days of the peak.

According to the disc instability models, an optical precursor from the outer disc is expected leading up to an X-ray outburst (see Russell et al. 2018, ApJ,852, 90). We see no obvious X-ray rise within 1 week of the optical peak. This is not usual for typical X-ray binary transients. However, by comparisons with the likely position of the source on the quiescent 2-10 keV X-ray flux versus V-band optical magnitude plane for CVs (Mukai 2017,PASP, 129, 976), we speculate this source to be a magnetic CV. The exact nature of this source and the brief outburst remains unknown at present. Exotic scenarios such as a brief micro-lensing event cannot be ruled out. Optical spectroscopy would be important to shed further light on it if it can be caught quickly.

We thank the Swift team for rapid approval and observations. This work is supported by STFC, The Royal Society and a UGC-UKIERI Phase 3 Thematic Partnership.