MAXI/GSC detection of an X-ray enhancement of MAXI J0243-582 (= BL Lac object BZB J0244-5819)

ATel #6012; S. Nakahira, S. Ueno, H. Tomida, M. Kimura, M. Ishikawa, Y. E. Nakagawa (JAXA), N. Kawai, R. Usui, T. Yoshii, Y. Tachibana (Tokyo Tech), T. Sakamoto, A. Yoshida, Y. Nakano, Y. Kawakubo, H. Ohtsuki (AGU), T. Mihara, M. Sugizaki, M. Serino, M. Morii, J. Sugimoto, T. Takagi, A. Yoshikawa, M. Matsuoka (RIKEN), H. Tsunemi, M. Sasaki, D. Uchida (Osaka U.), H. Negoro, M. Nakajima, H. Sakakibara, K. Fukushima, T. Onodera, K. Suzuki (Nihon U.), Y. Ueda, M. Shidatsu, T. Kawamuro, T. Hori (Kyoto U.), Y. Tsuboi, M. Higa (Chuo U.), M. Yamauchi, K. Yoshidome, Y. Ogawa, H. Yamada, Y. Morooka (Miyazaki U.), K. Yamaoka (Nagoya U.) on behalf of the MAXI team
on 26 Mar 2014; 07:39 UT
Credential Certification: Satoshi NAKAHIRA (nakahira@crab.riken.jp)

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The MAXI/GSC nova alert system triggered on an X-ray transient source at UT 2014-03-24T19:24:10 on time scale of 4 days. Its position is (R.A., Dec)= (40.84 deg, -58.20 deg) (J2000) with a 1-sigma statistical error of 0.125 deg and an additional systematic error of 0.09 deg.
This position is consistent with that of MAXI J0243-582 in the MAXI/GSC 37-Month catalog (Hiroi et al., ApJS, 207, 36, 2013).

We proposed a Swift ToO observation with 4-point tiling to cover the MAXI error circle with the Swift XRT. The observation was performed from 2014-03-25T15:18:02 with 200 sec exposure for each tiling. In the Swift XRT image, we find a bright point source at (RA, Dec)= (41.16707 deg, -58.33188 deg) which is equivalent to:

RA(J2000) = 2h 44m 40.10s
Dec(J2000) = -58d 19m 54.8s

with an estimated error of 2.3 arcseconds radius (90% c.l.). This position lies 1.54 arcseconds from the NED position of the BL Lac object BZB J0244-5819.
We therefore suggest that the trigger source is an X-ray flare of BZB J0244-5819 (=MAXI J0243-582).

The 4.5 year MAXI light curve (2-10 keV) shows long term variation peaking at ~2 mCrab (around MJD 2012 November), and it shows a brightening in the last ~ten days, which can be viewed at
http://maxi.riken.jp/mxondem/2014032508124503d6c9d0b06b5a3/index.html

The Swift/XRT spectrum can be modeled with a power-law function with a photon index of 2.05±0.3, and the 0.3-10 keV X-ray flux corrected for the Galactic absorption is 5.5×10^-11 erg cm^-2 s^-1.

We thank the Swift team for performing the observation.