MAXI/GSC and Swift/BAT detection of an outburst from the Be/X-ray binary pulsar H 1145-619

ATel #7215; T. Mihara (RIKEN), H. Negoro (Nihon U.), H. A. Krimm (CRESST/GSFC/USRA), M. Nakajima (Nihon U.), M. Sugizaki, J. Sugimoto (RIKEN), S. Ueno, H. Tomida, S. Nakahira, M. Kimura, M. Ishikawa, Y. E. Nakagawa (JAXA), M. Morii, M. Serino, T. Takagi, A. Yoshikawa, M. Matsuoka (RIKEN), N. Kawai, T. Yoshii, Y. Tachibana (Tokyo Tech), A. Yoshida, T. Sakamoto, Y. Kawakubo, H. Ohtsuki (AGU), H. Tsunemi, D. Uchida (Osaka U.), K. Fukushima, T. Onodera, K. Suzuki, M. Fujita, F. Honda, T. Namba (Nihon U.), Y. Ueda, M. Shidatsu, T. Kawamuro, T. Hori (Kyoto U.), Y. Tsuboi, A. Kawagoe (Chuo U.), M. Yamauchi, Y. Morooka, D. Itoh (Miyazaki U.), K. Yamaoka (Nagoya U.)
on 12 Mar 2015; 21:52 UT
Credential Certification: Tatehiro Mihara (mihara@crab.riken.jp)

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MAXI/GSC and Swift/BAT detected an outburst of the Be/X-ray binary pulsar H 1145-619 (= 4U 1145-619, V801 Cen) after 18 years of quiescence. It is 15.6 arcmin away from another pulsar, 1E1145.1-6141 (= 3A 1145-616, V830 Cen ) which shows rather persistent X-ray emission of about 15 mCrab in the 2-20 keV band. X-ray enhancement from this region was first detected by the MAXI/GSC nova alert system on 2015 February 28 (MJD 57081). On 2015 March 7, it became clear that the X-ray emission from this region was shifted to the position of H 1145-619 in the GSC image accumulated over the last 4 days, and we sent an alert to the MAXI mailing list. The peak of the outburst was on 2015 March 4 (57085), on which the flux reached 60 +/- 5 mCrab in the 2-20 keV. The outburst from H 1145-619 was first detected in the Swift/BAT transient monitor on 2015 February 28 (MJD 57081) at a level of 0.003 +/- 0.001 ct s^-1 cm^-2 (15-50 keV) or ~15 mCrab. The flux peaked on 2015 March 4 at 0.01 +/- 0.001 ct s^-1 cm^-2 (15-50 keV) or ~50 mCrab and has since declined to a barely detectable level of < 10 mCrab.
The GSC and Swift/BAT light curves can be found at
http://maxi.riken.jp/top/index.php?cid=1&jname=J1148-622
and
http://swift.gsfc.nasa.gov/results/transients/weak/H1145-619/
which show that the flux increased simultaneously.
The follow-up observation was carried out by Swift on 2015 March 11 4:45-10:15. The position obtained with UVOT was (RA,dec)=(177.00083, -62.20676) with Error radius 2.0" (90% confidence). The results confirmed that the present outburst emerged from H 1145-619, not 1E1145.1-6141 nor a new transient. The observed XRT flux in the 0.3-10 keV band was (6.5 +/- 0.7) x 10^-11 erg s^-1 cm^-2 ( = 3 mCrab).
H 1145-619 had been as bright as 100~200 mCrab in 1975-1979, 1982-1985, and 1994 (Stevens et al. 1997). The spectrum by HEAO-1 (MED and HED) are powerlaw with photon index of 1.5 (White et al. 1980). Then, the last outburst occurred on 1996 September 29 (Corbet and Remillard 1996). Since then it has been in the quiescent state. In 1997 October, at two orbital periods later than the last outburst, the RXTE (PCA and HEXTE) observed H 1145-619 with the flux of 15 mCrab, which means the quiescent state.
Although the current outburst has almost decayed, an optical spectroscopic observation of the H alpha emission line is encouraged. Also, we need to watch out for the next periastron passage after one orbital period (186.5 day, Warwick et al. 1985), since the last outburst was followed by a small short outburst with the 70% of the peak flux. This will happen approximately 2015 September 6.