MAXI/GSC and Swift/BAT detect a new hard X-ray transient MAXI J1836-194

ATel #3611; H. Negoro, M. Nakajima (Nihon U.), S. Nakahira (RIKEN), M. Morii (Tokyo Tech), H. A. Krimm (CRESST/GSFC/USRA), D. M. Palmer (LANL), J. A. Kennea (PSU), T. Mihara, M. Sugizaki, M. Serino, T. Yamamoto, T. Sootome, M. Matsuoka (RIKEN), S. Ueno, H. Tomida, M. Kohama, M. Ishikawa (JAXA), N. Kawai, Sugimori, R. Usui, T. Toizumi (Tokyo Tech), A. Yoshida, K. Yamaoka (AGU), H. Tsunemi, M. Kimura, H. Kitayama (Osaka U.), F. Suwa, M. Asada, H. Sakakibara (Nihon U.), Y. Ueda, K. Hiroi, M. Shidatsu (Kyoto U.), Y. Tsuboi, T. Matsumura, K. Yamazaki (Chuo U.) report on behalf of the MAXI and Swift teams
on 30 Aug 2011; 19:29 UT
Distributed as an Instant Email Notice Transients
Credential Certification: Hitoshi Negoro (negoro@phys.cst.nihon-u.ac.jp)

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On 2011 August 30, MAXI/GSC detected a previously unknown transient source at the position of (RA, Dec) = (279.12 -19.41; J2000) with a systematic uncertainty of 0.2 deg. We name this source MAXI J1836-194. Preliminary analysis of MAXI/GSC data shows that the source has brightened since the beginning of 2011 August 29, and the 4-10 keV fluxes on August 30 reached approximately 25 mCrab.

Swift/BAT also detects this transient source in the 15-50 keV band. The best BAT position is (RA, Dec) = (278.959, -19.385; J2000) with a 90% error radius of 3 arc minutes. The source was first detected on 2011 August 29 (MJD 55802) at a rate of 0.006 +/- 0.0007 ct/s/cm^2 (~30 mCrab). The source remained at a comparable rate on Aug. 30. It was not detectable on Aug. 28 (3-sigma upper limit of 0.0025 ct/s/cm^2). It was detected on-board Swift in a 1576-second observation starting at UT 10:14:56 on Aug. 30 at 5-sigma significance, which was below the threshold for triggering.

Follow-up target of opportunity observations of MAXI J1836-194 by Swift are currently being performed and will be reported on in a later ATEL. Follow-up observations at multi-wavelengths are strongly encouraged.