Support ATel At Patreon

[ Previous | Next | ADS ]

MAXI/GSC detection of a long-term increasing trend of the X-ray flux from SS Cygni

ATel #13744; H. Negoro (Nihon U.),T. Mihara (RIKEN), K. Kobayashi, M. Nakajima, M. Aoki, R. Takagi, K, Asakura, K, Seino, S. Mokumoto (Nihon U.), C. Guo, Y. Zhou, T. Tamagawa, M. Matsuoka (RIKEN), T. Sakamoto, M. Serino, S. Sugita, H. Nishida, A. Yoshida (AGU), Y. Tsuboi, W. Iwakiri, R. Sasaki, H. Kawai, Y. Okamoto, S. Kitakoga (Chuo U.), M. Shidatsu (Ehime U.), N. Kawai, R. Adachi, M. Niwano (Tokyo Tech), S. Nakahira, Y. Sugawara, S. Ueno, H. Tomida, M. Ishikawa, M. Tominaga, T. Nagatsuka (JAXA), Y. Ueda, S. Yamada, S. Ogawa, K. Setoguchi, T. Yoshitake (Kyoto U.), H. Tsunemi (Osaka U.), M. Yamauchi, K. Kurogi, K. Miike (Miyazaki U.), T. Kawamuro (NAOJ), K. Yamaoka (Nagoya U.), Y. Kawakubo (LSU), M. Sugizaki (NAOC) report on behalf of the MAXI team
on 20 May 2020; 05:25 UT
Credential Certification: Hitoshi Negoro (negoro@phys.cst.nihon-u.ac.jp)

Subjects: X-ray, Cataclysmic Variable, Nova

From MAXI/GSC data of the typical dwarf nova SS Cygni, we find that the average X-ray flux has been continuously increasing since around MJD 58400, 2018 October (e.g., MAXI X-ray-star ML No.672). The 2-20 keV flux reaches about 30 mCrab at the beginning of this May, which is the highest flux MAXI has ever observed since 2009 August. We note that the average X-ray flux until 2016 July 31 is about 2.6 mCrab (Hori et al. 2018, ApJS, 235, a7).

A linear fit to the MAXI light curve since MJD 58400 gives the 2-20 keV X-ray flux of 9.5+/-0.2E-4 x (MJD - 58400) photons/cm^2/s, roughly equivalent to 2.96E-2 x (MJD - 58400) mCrab. Large time variations on timescales of weeks to a month are also recognized. Using a visual light curve obtained from the AAVSO web site (https://www.aavso.org), we find that when the optical flux is high, the X-ray flux is low as reported by Wheatley, Mauche, and Mattei (2003, MNRAS, 345, 49). On the other hand, when the optical flux is low, the X-ray flux is always much higher than that observed in the optically low period in 2009-2018, which characterizes this longterm increase in the X-ray flux. We also point out a gradual increase of the baseline V magnitude from about 12 mag to 11 mag since around MJD 58400.

A power-law fit to a GSC spectrum obtained from 2020 April 27 to May 06 gives a photon index of 1.6 +/- 0.1 and the 2-20 keV flux is 5.3E-10 erg/cm^2/s. The corresponding luminosity is 8.3E-32 erg/s for the distance of 114 pc. This flux is about 2.6 times higher than the peak flux in a previous nova reported by Wheatley et al. Such high fluxes in the previous nova were, however, only observed in less than three days at the beginning and end of the nova. On the other hand, the current active X-ray state continuously lasts between novae, 20-30 days. Together with the baseline increase in the optical flux, this strongly implies continuous, high mass accretion between novae.

We encourage multi-wavelength observations to understand the emission process in this active X-ray state of the one of the best studied dwarf nova, SS Cygni.
Finally, we would like to express our thanks to AAVSO members who provide very interesting data.

MAXI data for SS Cygni