ASAS-SN discovery of a Galactic transient on the rise ASASSN-19yt (AT 2019sgf)

ATel #13176; E. Aydi (MSU), K. Z. Stanek, C. S. Kochanek, Z. S. Way (OSU), B. J. Shappee (UoHawaii), J. Strader, L. Chomiuk, A. Kawash, K. V. Sokolovsky (MSU)
on 8 Oct 2019; 22:28 UT
Credential Certification: Elias Aydi (eaydi@saao.ac.za)

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During the ongoing All Sky Automated Survey for SuperNovae (ASAS-SN, Shappee et al. 2014, ApJ, 788, 48), using data from the quadruple 14-cm "Brutus" telescope in Haleakala, Hawaii, we detect a new transient source, possibly a classical nova on the rise or a dwarf nova outburst:

 
Object        RA (J2000)   DEC (J2000)   Gal l (deg)   Gal b (deg)  Disc. UT Date   Disc. V mag  
ASASSN-19yt   06:18:01.45  22:22:29.06   189.311       3.104        2019-10-07.55   g = 15.7 

The transient is still rising in brightness with the latest measurement on 2019-10-08.57 is at g = 15.0. No previous outbursts were recorded in ASAS-SN. There is a source catalogued by PS1 (g=20.8; g-r = 0.6) and Gaia (G=20, no parallax measurement) 0.4 arcsec away from the discovery position. Most interestingly, there is an X-ray source at the position of ASASSN-19yt detected by XMM-Newton and Chandra. The XMM-Newton measurements show that the majority of the photon counts are in the harder X-rays (2-12 keV). The Chandra X-ray flux from the progenitor system is ~ 5.22 x 10^-13 erg/cm^2/s implying a luminosity of ~ 6.2 x 10^33 erg/s at 10 kpc. The same source has been also detected by NuSTAR (Zhang et al. 2018, ApJ, 859, 141) with an observed flux F = (1.3 +\- 0.6) x 10^-13 erg/cm^2/s in the 2-10 keV range and F = (1.4 +\- 0.8) x 10^-13 erg/cm^2/s in the 10-40 keV range, at NH = 2.0 x 10^21 cm^-2. The Galactic reddening maps of Schlafly et al. (2011, ApJ, 737, 103) indicates moderately high extinction towards the source with Av ~ 5. This is 5 times higher than the Av implied by the NH derived from NuSTAR observations by Zhang et al. (2018). Based on the hard X-ray emission observed during quiescence we speculate that the progenitor system could possibly be a magnetic CV or an X-ray binary.

Follow up observations are encouraged in all bands. The latest ASAS-SN light curve can be found here.