Swift/XRT spectral analysis of MAXI J1807+132: possibility of a faint low mass X-ray binary

ATel #10222; M. Shidatsu (RIKEN), Y. Tachibana, T. Yoshii (Tokyo Tech), H. Negoro (Nihon U.), K. Makishima (RIKEN), T. Kawamuro, Y. Ueda (Kyoto U.), S. Nakahira, S. Ueno, H. Tomida, M. Ishikawa, Y. Sugawara (JAXA), T. Mihara, M. Sugizaki, M. Serino, W. Iwakiri, J. Sugimoto, T. Takagi, M. Matsuoka (RIKEN), N. Kawai, N. Isobe, S. Sugita, Y. Ono, T. Fujiwara, S. Harita, Y. Muraki (Tokyo Tech), A. Yoshida, T. Sakamoto, Y. Kawakubo, Y. Kitaoka (AGU), H. Tsunemi, R. Shomura, T. Yoneyama (Osaka U.), M. Nakajima, K. Tanaka, T. Masumitsu, T. Kawase, A. Sakamaki (Nihon U.), T. Hori, A. Tanimoto, S. Oda (Kyoto U.), Y. Tsuboi, Y. Nakamura, R. Sasaki (Chuo U.), M. Yamauchi, K. Furuya (Miyazaki U.), Y. E. Nakagawa (JAMSTEC), K. Yamaoka (Nagoya U.)
on 29 Mar 2017; 06:12 UT
Credential Certification: Megumi Shidatsu (shidatsu@kusastro.kyoto-u.ac.jp)

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We analyzed the Swift/XRT spectrum of MAXI J1807+132 obtained on March 27 in the Windowed Timing mode (Kennea et al., ATel # 10216) and found that the source could be a faint low mass X-ray binary, as suggested from optical spectroscopy (Munoz-Darias et al., ATel #10221). The spectrum can be fit with a single absorbed power-law model, as already reported by Kennea et al. (ATel #10216), but seems to have a weak hump at ~1 keV and a hard tail above ~3 keV. This spectral profile is reminiscent of that of the LMXB Aql X-1 in faint phases (Sakurai et al. 2014). The overall spectral shape is also described with a combination of a disk blackbody with a inner temperature of 0.5 keV (or a blackbody with a temperature of 0.3 keV) and a power-law component with a photon index of ~1.8. Both two components are assumed to be absorbed with a cold matter with N_H = 1.0e21 cm-2, which corresponds to the total galactic column in the direction of the source. If MAXI J1807+132 had the same unabsorbed 0.8-100 keV luminosity as those of "Obs 5" or "Obs 6" in Sakurai et al. 2014 (5e35 erg/s or 1e34 erg/s), the distance is calculated as ~8 kpc or ~1 kpc, respectively. The estimated photon index can be converted to the luminosity of ~1e35 ergs/s, by using the relation between luminosity versus the photon index for neutron star low mass X-ray binaries (Wijnands et al. 2015). This luminosity and the estimated 0.5-10 keV unabsorbed flux, 4e-11 erg/cm^2/sec give a similar distance, ~5 kpc. We note, however, that the possibility of black hole X-ray binary located at larger distances is not ruled out at this stage, due to the low statistics of the data.