[vsnet-outburst 25164] SS Cyg: fast variability at the fading part of an outburst

[Keisuke Isogai]

SS Cyg: fast variability at the fading part of an outburst

As posted in ATel, SS Cyg is now brighter than usual in X-rays.
According to Nicer data that we took, this object shows violent and fast
variability in X-rays on timescales of seconds.  The X-ray flux becomes
higher and the fast variability is amplified now.  This may be QPOs (?)
and UV light curves showed short-term variability with the period of
less than 10 sec in its past outbursts.  It seems interesting to check
if it shows rapid variations in the optical band as well.
The current magnitude is ~10 mag.  Fast photometric observations
with the exposure time of less than 10 sec are encouraged.
Filtered observations are preferable.
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> The Astronomer's Telegramhttp://www.astronomerstelegram.org
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> ATEL #13744							     ATEL #13744
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> Title:	MAXI/GSC detection of a long-term increasing trend of the X-ray
> 		flux from SS Cygni
> Author:	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
> Queries:	negoro.hitoshi at nihon-u.ac.jp
> Posted:	20 May 2020; 05:25 UT
> 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:http://maxi.riken.jp/pubdata/v7l/J2142+435/index.html
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> Password Certification: Hitoshi Negoro (negoro at phys.cst.nihon-u.ac.jp)
> http://www.astronomerstelegram.org/?read=13744
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