MAXI/GSC detection of a long-term increase in the X-ray flux from AT 2018wey

ATel #13948; H. Negoro, M. Nakajima, M. Aoki, K. Kobayashi, R. Takagi, K, Asakura, K, Seino (Nihon U.), T. Mihara, 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 16 Aug 2020; 14:04 UT
Credential Certification: Hitoshi Negoro (negoro@phys.cst.nihon-u.ac.jp)

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The optical transient AT 2019wey (ATLAS19bcxp) was discovered by ATLAS on 2019 December 7 (MJD 58824) (Tonny et al. 2019, TNSTR, 2553). SRG/ART-XC also discovered the source in X-rays on March 18 (MJD 58926) and named the source SRGA J043520.9+552226 and SRGE J043523.3+552234 (Mereminskiy et al. ATel. #13571). The source is suggested or proposed to be a hostless transient/possible supernova (Mereminskiy et al.), a BL Lac type object (Lyapin et al. ATel. #13576), or an accreting binary (Yao ATel #13932). Yao et al. reported on X-ray brightening from Swift/XRT and NICER monitoring observations from April 12 (MJD 58951) to August 8 (MJD 59069).

We investigate long-term X-ray activity of this source using MAXI/GSC data. Due to the ISS orbit precession of about 72 days, the source region was often unobservable for ~12 days or observed with only degraded cameras for ~ 28 days, resulting in regular data gaps. Here, we list the average source fluxes during the following periods between the data gaps.

 
----------------------------------------------------------- 
     Observation          2-10 keV Flux       Significance 	 
     Period (MJD)    [10^-3 c/cm^2/s] (mCrab)   [sigma] 
----------------------------------------------------------- 
58500.03 - 58752.18    0.37 +/- 0.58  ( 0.2)      0.6  
58778.34 - 58795.86   -1.18 +/- 1.38  (-0.5)    (-0.9) 
58805.85 - 58823.53    1.62 +/- 1.25  ( 0.7)      1.3 
58847.45 - 58866.59    4.59 +/- 1.35  ( 2.1)      3.4 
58878.52 - 58896.27    2.99 +/- 1.30  ( 1.3)      2.3 
58923.92 - 58969.05    2.86 +/- 0.92  ( 1.3)      3.1 
58996.76 - 59011.09    4.57 +/- 1.66  ( 2.0)      2.8 
59024.05 - 59042.97   12.37 +/- 1.37  ( 5.5)      9.0 
59070.54 - 59072.96   31.80 +/- 5.2   (14.2)      6.0 
----------------------------------------------------------- 

The GSC data show no significant enhancement before the optical discovery (for the result of no detection until 2016 July 31, see Hori et al. 2018, ApJS, 235, 7). From MJD 58847.45 to 58866.59, however, a significant (~3.4 sigma level) increase is recognized before the X-ray detection by SRG. The rise profile from MJD 58805.85 to 59072.96 is roughly represented by a constant and an exponential function with a time constant of about 30 days. There are no apparent spectral variations in the count ratios of the 2-4 keV and 4-10 keV bands.

The optical brightening might start between 10:30 on 2019 Dec. 5 (MJD 58822.4375) and 09:51 on Dec. 7 (MJD 58824.4104) (Tonny et al. 2019). During the period, MAXI observed the source region from 11:50 on Dec. 5 to 11:03 on Dec. 6 at intervals of about 92 min except for 16:27 to 22:40 on Dec. 5, and we found one possible flaring event at 00:13 on Dec. 6. The 2-4 keV and 4-10 keV fluxes are -0.008 (+0.011, -0.007) c/cm^2/s, and 0.092 (+0.054, -0.044) c/cm^2/s, respectively. Because of the detection at the edge of cameras, this might be due to particle events. More careful analysis is needed.

We highly recommend to monitor this interesting source in multiwavelengths.

MAXI Observations of AT 2019wey