State transitions in the early stage of the outburst from the new X-ray transient MAXI J1305-704

ATel #4035; F. Suwa, H. Negoro (Nihon U.), S. Nakahira (JAXA), M. Serino, T. Mihara (RIKEN), N. Kawai (Tokyo Tech), S. Ueno, H. Tomida, M. Ishikawa (JAXA), M. Sugizaki, T. Yamamoto, M. Matsuoka (RIKEN), M. Morii, R. Usui (Tokyo Tech), A. Yoshida (AGU), H. Tsunemi, M. Kimura (Osaka U.), M. Nakajima, M. Asada, H. Sakakibara, N. Serita (Nihon U.), Y. Ueda, K. Hiroi, M. Shidatsu, R. Sato (Kyoto U.), Y. Tsuboi, M. Higa (Chuo U.) M. Yamauchi, Y. Nishimura, T. Hanayama (Miyazaki U.), K. Yamaoka (Waseda U.) report on behalf of the MAXI team:
on 12 Apr 2012; 06:04 UT
Credential Certification: Hitoshi Negoro (negoro@phys.cst.nihon-u.ac.jp)

Tweet

We further report results from MAXI observations of the new X-ray transient MAXI J1305-704 (Sato et al. #4024). Recent MAXI/GSC data show that the one-day averaged 2-20 keV flux almost linearly increased from around April 5, 2012 (MJD=56022) to April 10, and started decreasing on April 11. The peak flux on April 10 was about 50 mCrab in the 4-10 keV band.

A preliminary spectral analysis, avoiding contamination from the nearby persistent source 4U 1254-690, shows that energy spectra obtained with MAXI/GSC are roughly described by single power-law functions with photon indices of 1.8 +/- 0.2 on April 7 and 8 (average of the 2 days), 1.9 +/- 0.2 on April 9, and 2.4 +/- 0.2 on April 10 where the column density is fixed to 2e21 cm^-2 (Kennea et al. #4034).

We have also found that the X-ray fluxes, close to the time of the Swift ToO observations reported by Greiner et al. (#4030) and Kennea et al. (#4034), were relatively low, and a power-law fit to an energy spectrum obtained at three scan transits from 16:38 to 19:43 on April 10 gives a photon index of 2.2 +/- 0.4, consistent with that obtained by Kennea et al. (#4034). If these data with low fluxes are excluded, the spectrum on April 10 can be represented by a power-law with a photon index of 2.7 +/- 0.3, or by a disk blackbody model with a temperature of 1.4 +/- 0.2 keV.

On April 11, the 4-10 keV flux decreased, but the 2-4 keV flux still remained.

These spectral changes imply that the source is undergoing a hard-to-soft state transition through the intermediate state on April 9 and 10. These features are consistent with a picture that the source is a black hole candidate (#4030, #4034). Other possibilities, e.g. low-mass neutron star binary, however, cannot be excluded. Further follow-up observations are strongly recommended. MAXI public data archives of the source can be found at http://maxi.riken.jp/top/index.php?cid=000000000001&jname=J1306-704 .

MAXI home page