MAXI/GSC detection of weak X-ray brightening from Be/X-ray binary pulsar 4U 0728-25 and revised orbital period
ATel #9820; M. Nakajima (Nihon U.), T. Mihara, W. Iwakiri (RIKEN), S. Nakahira, S. Ueno, H. Tomida, M. Ishikawa, Y. Sugawara (JAXA), Y. E. Nakagawa (JAMSTEC), M. Sugizaki, M. Serino, M. Shidatsu, J. Sugimoto, T. Takagi, M. Matsuoka (RIKEN), N. Kawai, N. Isobe, S. Sugita, T. Yoshii, Y. Tachibana, Y. Ono, T. Fujiwara, S. Harita, Y. Muraki (Tokyo Tech), A. Yoshida, T. Sakamoto, Y. Kawakubo, Y. Kitaoka (AGU), H. Tsunemi, R. Shomura (Osaka U.), H. Negoro, K. Tanaka, T. Masumitsu, T. Kawase (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.)
on 2 Dec 2016; 11:56 UT
Credential Certification: Motoki Nakajima (firstname.lastname@example.org)
Subjects: X-ray, Binary, Neutron Star, Transient, Pulsar
Referred to by ATel #: 9823
We report on the detection of the outburst from Be/X-ray binary pulsar 4U 0728-25 observed by MAXI/GSC.
In addition, we have revised the binary orbital period which was previously defined by Corbet et al. (1997).
The weak X-ray flux brightening was detected by the MAXI/GSC nova-search system (Negoro et al. 2016) on 2016 November 29 (MJD 57721). The 4-10 keV flux reached 0.0170 +- 0.0059 photons/s/cm2 (15 +- 5 mCrab) on 2016 November 28 (MJD 57720). Before this weak brightening, the nova-search system had already found a faint X-ray flux enhancement from the source on 2016 June 7 (MJD 57546) and 2016 September 18 (MJD 57649). The each peak flux in the 4-10 keV energy band of the previous brightening is ~20 mCrab, and the outbursts lasted for only a few days. (These detection were not circulated to the MAXI alert mail.) The intervals between each brightening are ~103 days and ~72 days respectively. These intervals are comparable with as several times as the orbital period of 34.46 +- 0.12 days (Corbet et al. 1997). Using the orbital parameters (T0=50365.5(MJD); Corbet et al. 1997), the orbital phase of the detected peaks are determined to be 0.35-0.45 which does not match with the expected outburst peak phase. Thus we attempted to revise the orbital (outburst) period using a period search techniques. A new ephemeris derived from the RXTE/ASM and the MAXI/GSC data spanning the past 21 years predicts the X-ray outburst peaks as:
Toutburst = MJD 50365.5 + n * (34.5446 +- 0.0153)
where n is the number of orbit since the outburst in October 1996. Using a new ephemeris, it is turned out that the date of the triggered event by the MAXI/GSC nova-search system is 0.93 in the orbital phase. It nicely matches with the outburst expected phase.