MAXI/GSC and Swift/BAT detections of a new X-ray outburst from MAXI J0911-655/Swift J0911.9-6452

ATel #8872; M. Serino (RIKEN), K, Tanaka, H. Negoro (Nihon U.), H. A. Krimm (CRESST/GSFC/USRA), J. A. Kennea (PSU), P. Romano (INAF-IASFPA), H. Tomida, S. Nakahira, M. Ishikawa, Y. E. Nakagawa (JAXA), T. Mihara, M. Sugizaki, M. Shidatsu, T. Takagi, J. Sugimoto, M. Matsuoka (RIKEN), N. Kawai, 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, T. Masumitsu (Nihon U.), Y. Ueda, T. Kawamuro, T. Hori, A. Tanimoto (Kyoto U.), Y. Tsuboi, S. Kanetou, Y. Nakamura, R. Sasaki (Chuo U.), M. Yamauchi, D. Itoh, K. Furuya (Miyazaki U.), K. Yamaoka (Nagoya U.), M. Morii (ISM)
on 26 Mar 2016; 02:14 UT
Distributed as an Instant Email Notice Transients
Credential Certification: Hitoshi Negoro (negoro@phys.cst.nihon-u.ac.jp)

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The MAXI/GSC nova-alert system triggered on a faint uncatalogued X-ray transient source at 05:30 UT on 2016 February 19 (MJD 57437). Assuming that the source flux was constant over the transits, we obtain the source position at
(R.A., Dec) = (137.760 deg, -65.574 deg) = (09 11 02, -65 34 26) (J2000)
with a 90% C.L. statistical error of 0.52 deg and an additional systematic uncertainty of 0.1 deg (90% containment radius). We tentatively name the source MAXI J0911-655.

The long term GSC X-ray light curve at the region shows that an increase in X-ray flux has been recognized since around February 10 (MJD 57428). The 4-10 keV flux averaged over 6 scan transits on February 17 (MJD 57435) was 14.0 +/- 5.0 mCrab (1 sigma error). After that, the flux is stable at 10-15 mCrab.

The source Swift J0911.9-6452 was first detected in the BAT transient monitor on 29 February 2016 (MJD 57447). It had peaked at about 0.0045 +/- 0.001 ct/s/cm^2 (~20 mCrab) in the 15-50 keV band on 25 Feb 2016 and since that time has been detected in the BAT at an average rate of ~0.0025 ct/s/cm^2 (~10 mCrab). Archival results show weak detection back to 14 Feb 2016. The best BAT position is
(R.A., Dec) = (137.975 deg, -64.881 deg) = (09 11 54.0, -64 52 51.6) (J2000).

From these observational properties, these sources, MAXI J0911-655 and Swift J0911.9-6452, are thought to be the same. The angular distance of the two best positions is relatively large (~ 0.088 deg), but this may be due to a large uncertainty of the MAXI position. The BAT position is consistent with the globular cluster NGC 2808, which harbors 16 X-ray sources (Servillat et al. 2008, A&A 490, 641). The observed flux of 10 mCrab corresponds to a luminosity of ~ 2.6e36 erg/s for a source in the cluster at a distance of 9.6 kpc, suggesting that the source is an X-ray binary hosting a low-mass star and a neutron star or a black hole (LMXB). Servillat et al. (2008) pointed out there is one likely neutron star-quiescent LMXB (Chandra Source 1 = XMM-Newton source C2). The present outburst may be due to this source.

Swift observations are planned.