MAXI detection of a highly energetic X-ray burst from 4U 1850-08
ATel #7500; H. Negoro, K. Suzuki, T. Masumitsu, T. Namba (Nihon U.), S. Nakahira (JAXA), M. Serino (RIKEN), S. Ueno, H. Tomida, M. Kimura, M. Ishikawa, Y. E. Nakagawa (JAXA), T. Mihara, 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, 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 13 May 2015; 03:26 UT
Credential Certification: Hitoshi Negoro (negoro@phys.cst.nihon-u.ac.jp)
Subjects: X-ray, Binary, Globular Cluster, Neutron Star, Transient
Referred to by ATel #: 16600
At 9:27:50 UT on 2015 May 9, the MAXI/GSC nova alert system triggered a bright X-ray burst from the ultracompact X-ray binary 4U 1850-08 (4U 1850-086) which lies inside the error circle with 4 arc-minute radius. The effective-area-corrected 2-10 keV X-ray light curve shows amplitude variations of ~ 50% with a decay trend during 40 sec scan transit, whereas the 10-20 keV X-ray curve does not show any significant variations, implying that the spectrum becomes hard as the burst progresses.
The energy spectrum during the scan transit is well described by an absorbed blackbody model with the temperature of kT = 2.44 +/- 0.07 keV and the hydrogen column density fixed to 0.39x1022 (see Sidoli et al. 2001, A&A, 368, 451). The observed 2-10 keV X-ray flux averaged over the scan transit is (4.57+/-0.18)x10-8 erg/s/cm2 (~1.9 Crab), and the bolometric blackbody flux is (7.64+/-0.50)x10-8 erg/s/cm2. The peak bolometric flux at the beginning of the scan transit can be estimated to be (10.3 +/- 3.5)x10-8 erg/s/cm2 if we assume no spectral variation. The corresponding bolometric peak luminosity is (5.8+/-2.0)x1038 erg/s, which is over the Eddington luminosity for a 1.4 solar mass neutron star (e.g., Kuulkers et al. 2003, A&A, 399, 663), for the 6.9 kpc distance source in the globular cluster NGC 6712 (in't Zand et al. ATel #5972 for comparison). We note that the peak flux here is the maximum during the scan transit, and not always the true peak of the burst.
The MAXI/SSC also detected a faint count excess (0.72+/0.31 c/s/cm2 in the 0.7-7 keV band) in the scan transit at 9:10 on 2015 May 09. Furthermore, the GSC detected a soft count excess (0.13+/-0.03 c/s/cm2 in the 2-4 keV band and 0.08+/-0.03 c/s/cm2) in the following scan transit at 11:00, implying that the burst lasted more than 6600 sec.
These unique properties of the unusually energetic and long burst are very similar to what Swift observed on 2014 March 10 (in't Zand et al. ATel #5972). MAXI also detected such a very bright (~ 3 Crab at 4-10 keV) burst from the source on 2011 November 9. These properties that Swift and MAXI observed may make this ultracompact X-ray binary distinct from other X-ray bursters.
MAXI public archive for 4U 1850-08
