Further brightening of the X-ray transient MAXI J1535-571, suggesting the presence of a black hole

ATel #10708; H. Negoro. T. Kawase (Nihon U.), M. Sugizaki (RIKEN), S. Ueno, H. Tomida, Y. Sugawara, N. Isobe, M. Ishikawa, R. Shimomukai (JAXA), T. Mihara, M. Serino, S. Nakahira W. Iwakiri, M. Shidatsu, M. Matsuoka (RIKEN), N. Kawai, S. Sugita, T. Yoshii, Y. Tachibana, S. Harita, Y. Muraki, K. Morita (Tokyo Tech), A. Yoshida, T. Sakamoto, Y. Kawakubo, Y. Kitaoka, T. Hashimoto (AGU), H. Tsunemi, T. Yoneyama (Osaka U.), M. Nakajima, A. Sakamaki (Nihon U.), Y. Ueda, T. Hori, A. Tanimoto, S. Oda (Kyoto U.), Y. Tsuboi, Y. Nakamura, R. Sasaki, H. Kawai (Chuo U.), M. Yamauchi, C. Hanyu, K, Hidaka (Miyazaki U.), T. Kawamuro (NAOJ), K. Yamaoka (Nagoya U.)
on 5 Sep 2017; 16:23 UT
Credential Certification: Hitoshi Negoro (negoro@phys.cst.nihon-u.ac.jp)

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The new X-ray transient MAXI J1535-571 currently in outburst was independently discovered by Swift (GCN #21788, ATel #10700) and MAXI (ATel #10699) on 2017 September 2, and the optical followup observation successfully detected an uncatalogued counterpart of the source on September 3 (ATel #10702).

MAXI/GSC data show that the 2-20 keV X-ray flux has been almost linearly increasing since the beginning of September 2, which triggered the MAXI Nova-Alert systems (Negoro et al. 2016, PASJ, 68, S1) at 14:40 and 23:55 on September 2 (ATel #10699).

The GSC spectrum obtained in 7 scans from 01:20 to 10:36 UT on September 5 is well represented by an absorbed power-law model with nH of (3.4 +/- 0.9)e22 and a photon index of 1.84 +/- 0.11. The absorption column density nH is consistent with that obtained in the Swift observation (ATel #10700), but the photon index is steeper (softer) than before. The unabsorbed 2-10 keV flux is 1.13e-8 erg/s/cm2.

The extrapolated, unabsorbed 1-60 keV flux is 3.1e-8 erg/s/cm2, and the corresponding luminosity for a 8 kpc distance source is 2.4e38 erg/s, which exceeds the Eddington luminosity of a neutron star with 1.4 solar mass for pure hydrogen accretion gas. The observed absorption column density, larger than the total column density to the direction (1.4-1.5)e22 calculated by https://heasarc.gsfc.nasa.gov/cgi-bin/Tools/w3nh/w3nh.pl, suggests that the source is located in a deep field of the galactic disk, and/or that the source is obscured by the accretion (disk) gas or dense interstellar gas. We also note that the distance to Cir X-1 (apart from MAXI J1535-571 is 1.98 deg, and nH = 2e22 cm-2) is ~ 9.4 kpc (Heinz et al. 2015, ApJ, 806, 265), and that of MAXI J1543-563 (1.36 deg apart, and nH = 1.4e22 cm-2) is > 8.5 kpc (Stiele et al. 2012, MNRAS, 422, 679).

Furthermore, rapid X-ray variability with no obvious periodicity is clearly seen in GSC 40 sec light curves (also see GCN #21792). All these observed properties strongly suggest that the source is a low-mass X-ray binary containing a black hole, currently, in the hard state.

Continuous, multi-wavelength observations of this bright X-ray nova are highly encouraged to reveal the nature of the source, and to detect a relativistic jet and quasi-periodic oscillations during the hard-to-soft state transition (e.g., Fender et al. 2004, MNRAS, 355, 1105) expected to occur within a week.

MAXI data for MAXI J1535-571