MAXI/GSC detection of the first X-ray burst from UCXB 4U 1822-000

ATel #8769; K. Asai, T. Mihara, M. Serino, M. Matsuoka (RIKEN), M. Nakajima, H. Negoro (Nihon U.), H. Tomida, S. Nakahira, M. Ishikawa, Y. E. Nakagawa (JAXA), M. Sugizaki, M. Shidatsu, T. Takagi, J. Sugimoto (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.), K. Tanaka, 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) report on behalf of the MAXI team:
on 4 Mar 2016; 10:33 UT
Credential Certification: Tatehiro Mihara (mihara@crab.riken.jp)

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At 15:36:42 on 2016 March 1 (UT), MAXI/GSC detected a bright X-ray emission from 4U 1822-000. It is a candidate of an ultra compact X-ray binary (UCXB) (Sect. 3.5 in Nelemans et al. MNRAS 370 255, 2006). Positive detection in 4-10 keV lasted for 15 s in the 3 s binned light curve. The average 2-10 keV flux in the 15 s was 250 +- 80 mCrab (error in 90 % confidence hereafter). Assuming that the peak flux was 500 +-250 mCrab and it corresponds to the Eddington luminosity of 1.4 solar mass, the distance was calculated to be 13 +- 4 kpc. The X-ray spectrum can be fitted by a single black body component with kT= 1.6 +- 0.6 keV (90% confidence) without absorption. The temperature of the blackbody does not change significantly even after subtracting the persistent flux. Spectral softening during the X-ray burst was not observed clearly. The MAXI flux upper limits of the previous and the next scans were 100 mCrab in 5 sigma level. The non-detections are consistent with the persistent flux as cited below. The temperature and the duration are typical for an X-ray burst. Thus the event is considered as an X-ray burst, which is historically the first detection from 4U 1822-000. It results in that the compact object of 4U 1822-000 is a neutron star.
In a long-term view, the intensity of 4U 1822-000 had stayed at about 40 mCrab in 2009-2012. The average flux between MJD 55058- 56350 was 34 mCrab. The spectrum cannot be represented by either a single blackbody or a power-law. When it is fitted with the two component model (disk-blackbody + blackbody), the temperatures are 1.5 +- 0.2 keV and 2.1 +-0.3 keV, respectively. The spectrum indicates that it is in the soft state.
After that, from about MJD 56350 (2013 February) to 56850 (2014 July), the flux monotonically decreased down to about 20 mCrab. Since then it is in the lower luminosity. The average flux between MJD 56850- 57450 was 15 mCrab. Again, the spectrum cannot be represented by either a single blackbody or a power-law. It can be fitted with the two component model (disk-blackbody + blackbody), and the temperatures are 1.2 +- 0.4 keV and 2.0 (-0.4, +1.2) keV, respectively The spectrum indicates that it is still in the soft state. The source always looks non-detection with Swift/BAT (15-50 keV). It is consistent that the hard X-ray flux in the soft state with this MAXI flux is too weak to be detected with BAT.
The X-ray burst usually occurs in the hard state (the "island" state), but also sometimes does in the lower-luminosity soft state (the "lower banana" state). We may assume that the source flux has been in the lower end in the soft state in the recent 1.5 years. Since the state transition is known to occur at 2 - 4 % of the Eddington luminosity (Ledd). By evaluating that 15 mCrab is larger than the 2 % of the Ledd, the distance is limited to be > 11 kpc. It is consistent with the distance above.
The light curve is shown at the following page;
http://maxi.riken.jp/top/index.php?cid=1&jname=J1825-000