MAXI/GSC detection of a faint X-ray transient MAXI J1957+032

ATel #7504; H. Negoro (Nihon U.), M. Serino, T. Mihara (RIKEN), S. Nakahira (JAXA), S. Ueno, H. Tomida, M. Kimura, M. Ishikawa, Y. E. Nakagawa (JAXA), M. Sugizaki, M. Shidatsu, J. Sugimoto, T. Takagi, M. Matsuoka (RIKEN), N. Kawai, T. Yoshii, Y. Tachibana (Tokyo Tech), A. Yoshida, T. Sakamoto, Y. Kawakubo, H. Ohtsuki (AGU), H. Tsunemi, R. Imatani (Osaka U.), M. Nakajima, T. Masumitsu, K. Tanaka (Nihon U.), Y. Ueda, T. Kawamuro, T. Hori (Kyoto U.), Y. Tsuboi, S. Kanetou (Chuo U.), M. Yamauchi, D. Itoh (Miyazaki U.), K. Yamaoka (Nagoya U.), M. Morii (ISM)
on 14 May 2015; 05:48 UT
Credential Certification: Hitoshi Negoro (negoro@phys.cst.nihon-u.ac.jp)

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On 2015 May 11 (MJD 57153), MAXI/GSC detected a faint X-ray transient at the position (l, b) = (43.6, -13.1). From the data obtained from 13:59 on May 11 to 17:45 on May 12, we obtain the source position at

(R.A., Dec) = (299.356 deg, 3.293 deg) = (19 57 25, +03 17 34) (J2000)

with a statistical 90% C.L. elliptical error region with long and short radii of 0.32 deg and 0.29 deg, respectively. The roll angle of long axis from the north direction is 18.0 deg counterclockwise. There is an additional systematic uncertainty of 0.1 deg (90% containment radius). The X-ray flux averaged over the 12 scans was 8.0 +- 3.0 mCrab (4-10keV, 1 sigma error).

The one-day averaged count rates in the 2-4 keV and 4-10 keV bands are 0.012+/-0.005 c/s/cm² (~7 mCrab) and 0.011+/-0.005 c/s/cm² (~10 mCrab) on May 11, and 0.020+/-0.005 c/s/cm² and 0.011+/-0.006 c/s/cm² on May 12, respectively. On May 13, the MAXI/GSC did not detect any significant flux from the source.

A simple power-law or blackbody fit to the energy spectrum obtained from 13:59 on May 11 to 5:25 on May 13 provides the photon index of 1.7+/-0.4 or the blackbody temperature of kT = 1.3+/-0.3 keV, respectively.

There are no known bright source within the error region, and one known X-ray source, 1RXS J195656.8+030428, lies at 15 arcmin apart from the center of the region. The nature of this transient source is unknown. Judging from the timescales of the time variability and the spectrum, this enhancement might be due to an outburst of a distant low-mass X-ray binary or an AGN (Blazar) flare, such like 2FGL J1931.1+0938 (Negoro et al. ATel. #5943, Kennea et al. ATel. #5946) and BZB J0244-5819 (Nakahira et al. ATel. #6012), but no catalogued Blazars are within the error region.

The followup observations are encouraged.

MAXI public archive for MAXI J1957+032