Further analysis of MAXI J0758-456 and possible association with 1RXS J080114.6-462324

ATel #8993; T. Masumitsu, H. Negoro (Nihon U.), N. Kawai (Tokyo Tech), T. Mihara, J. Sugimoto, M. Shidatsu, W. Iwakiri (RIKEN), S. Nakahira, S. Ueno, H. Tomida, M. Ishikawa, Y. E. Nakagawa, Y. Sugawara (JAXA), M. Sugizaki, M. Serino, T. Takagi, M. Matsuoka (RIKEN), M. Arimoto, T. Yoshii, Y. Tachibana, Y. Ono, T. Fujiwara (Tokyo Tech), A. Yoshida, T. Sakamoto, Y. Kawakubo, H. Ohtsuki (AGU), H. Tsunemi, R. Imatani (Osaka U.), M. Nakajima, K. Tanaka (Nihon U.), Y. Ueda, T. Kawamuro, T. Hori, 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 27 Apr 2016; 11:07 UT
Credential Certification: Hitoshi Negoro (negoro@phys.cst.nihon-u.ac.jp)

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We report further analysis results of MAXI/GSC data for a short soft X-ray transient MAXI J0758-456 (Nakagawa et al. ATel #8983, Kennea et al. #8988). Using 4-10 keV GSC data during the burst-like activity, we obtained the following source position and error regions by unbinned maximum-likelifood fits.
For a constant flux source,
(R.A., Dec) = (119.652 deg, -45.704 deg) = (07 58 36, -45 42 15) (J2000)
with a 90% C.L. statistical error of 0.32 deg and an additional systematic uncertainty of 0.1 deg (90% containment radius),
and for a variable source, a rectangular error box for the transient source with the following corners:
(R.A., Dec) = (118.679 deg, -44.450 deg) = (07 54 42, -44 27 00) (J2000)
(R.A., Dec) = (119.513 deg, -44.211 deg) = (07 58 03, -44 12 39) (J2000)
(R.A., Dec) = (120.485 deg, -46.604 deg) = (08 01 56, -46 36 13) (J2000)
(R.A., Dec) = (119.605 deg, -46.826 deg) = (07 58 25, -46 49 33) (J2000)

The best position for a constant source flux was different from the previously reported position (#8983) by 5.4 arcmin, which was simply determined from a peak position in a smoothed 4-10 keV image.

As reported by Kennea et al. from Swift/XRT observations (#8988), no bright source was present in the former error region, but a known cataclysmic variable source, 1RXS J080114.6-462324, was detected in the latter rectangular error region.

We reanalyzed the GSC data assuming that the burst-like activity originates from 1RXS J080114.6-462324, although there is a possibility that an unknown source exhibited a burst.

The 2-10 keV X-ray count rates increased at almost the same time (< 2-3 sec) that the source entered the field of view of a GSC camera, and the light curve simply showed an exponential decay with a time constant of 10.5 +/- 2.0 sec after the correction of a triangular collimator angular response. The peak 2-10 keV count flux at the beginning of the MAXI observation was 4.2 +/- 1.3 counts/s/cm2, which was obtained by the exponential function fit to the above effective-area corrected light curve (http://maxi.riken.jp/atel/maxij0758_lc_2-10kev.jpg). The peak count rate roughly corresponds to 1.4 Crab which is about 1000 times higher flux than that of the Swift follow-up observation. The corresponding 2-10 keV luminosity is about 9e34 erg/s for a 150 pc distance source (e.g, Masetti et al. 2010, A&A, 519, A96).

An energy spectrum during the burst-like activity can be roughly represented by a blackbody model with a temperature of 1.09 +/- 0.15 keV, or a power-law model with a photon index of 2.5 +/- 0.4 (both without an interstellar absorption model). We also note that the source showed a significant spectral softening during the flux decay.

MAXI public archival data of MAXI J0758-456