MAXI/GSC detection of peculiar soft X-ray enhancement probably from SLX 1735-269

ATel #10115; S. Sugita (Tokyo Tech), H. Negoro, (Nihon U.), J. Sugimoto, T. Mihara, M. Serino (RIKEN), S. Ueno, H. Tomida, S. Nakahira, M. Ishikawa, Y. Sugawara (JAXA), M. Sugizaki, W. Iwakiri, M. Shidatsu, T. Takagi, M. Matsuoka (RIKEN), N. Kawai, N. Isobe, T. Yoshii, Y. Tachibana, Y. Ono, T. Fujiwara, S. Harita, Y. Muraki (Tokyo Tech), M. Nakajima, T. Masumitsu, K. Tanaka, T. Kawase, A. Sakamaki (Nihon U.), A. Yoshida, T. Sakamoto, Y. Kawakubo, Y. Kitaoka (AGU), H. Tsunemi, R. Shomura (Osaka U.), Y. Ueda, T. Kawamuro, T. Hori, S. Oda, A. Tanimoto (Kyoto U.), Y. Tsuboi, Y. Nakamura, R. Sasaki (Chuo U.), M. Yamauchi, K. Furuya (Miyazaki U.), K. Yamaoka (Nagoya U.) report on behalf of the MAXI team
on 23 Feb 2017; 07:43 UT
Credential Certification: Hitoshi Negoro (negoro@phys.cst.nihon-u.ac.jp)

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At 15:26 UT on 2017 February 20, the MAXI/GSC nova-alert system triggered on a short, soft X-ray burst-like event in the 2-4 keV band. Assuming that the source flux was constant over the transit, we obtain the source position at
(R.A., Dec) = (264.605 deg, -27.043 deg) = (17 38 25, -27 02 34) (J2000)
with a statistical 90% C.L. elliptical error region with long and short radii of 0.45 deg and 0.39 deg, respectively. The roll angle of the long axis from the north direction is 71.0 deg counterclockwise. There is an additional systematic uncertainty of 0.5 deg (90% containment radius). The neutron star binary SLX 1735-269 is 3.5 arc-min from the above position, and well within the error region. The X-ray flux averaged over the scan was 86 +/- 24 mCrab (2.0-4.0keV, 1 sigma error). Weak soft X-ray enhancement was also recognized (but not significant) at the previous scan transit before 92 min, and no significant flux was recognized at the next scan transit after 92 min. The duration of this event is not clear due to limited X-ray photon counts at these scan transits.

The enhancement is mostly recognized below 4 keV, and if we fit the GSC spectrum with an absorbed blackbody model with n_H = 1.47e22 cm^-2 fixed (David et al. 1997, A&A, 322, 229) assuming the source to be SLX 1735-269, we obtain a temperature of 0.67 +/- 0.13 keV. Poor statistics do not allow us to constrain the size of the emission region. But if the temperature is fixed to the above value, we obtain the radius of a spherical emission region of 22 +/- 4 km, assuming the distance of 6 kpc (Molkov et al. 2005, 434, 1069; Galloway et al. 2008, ApJS, 179, 360), suggesting the photospheric radius-expansion burst of this source.

SLX 1735-269 exhibited "giant" X-ray bursts (Gallow et al. 2008), and high energetic bursts were also observed by MAXI (e.g., Negoro et al. ATel. #4622; Serino et al. 2016, PASJ, 68, S95). This burst, however, shows the spectrum much softer than before and different from usual X-ray bursts. This may indicate that the enhancement does not come from SLX 1735-269. The soft energy spectrum is reminiscent of those of MAXI J0158-157 (Morii et al. 2013, ApJ, 779, 118), and MAXI J1501-026 (Nakahira et al. ATel. #7954). Thus, the nature of this enhancement is of great interest.

Flash Report for a soft X-ray flare from SLX 1735-269