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Unusual hard X-ray decrease in Swift J1753.5-0127 observed by MAXI and Swift

ATel #7196; T. Onodera, H. Negoro, (Nihon U.), S. Nakahira, S. Ueno, H. Tomida, M. Kimura, M. Ishikawa, Y. E. Nakagawa (JAXA), T. Mihara, M. Sugizaki, M. Morii, M. Serino, J. Sugimoto, T. Takagi, A. Yoshikawa, M. Matsuoka (RIKEN), N. Kawai, Y. Tachibana, T. Yoshii (Tokyo Tech), A. Yoshida, T. Sakamoto, Y. Kawakubo, H. Ohtsuki (AGU), H. Tsunemi, D. Uchida (Osaka U.), M. Nakajima, K. Fukushima, 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.) report on behalf of the MAXI team
on 10 Mar 2015; 12:30 UT
Credential Certification: Hitoshi Negoro (negoro@phys.cst.nihon-u.ac.jp)

Subjects: X-ray, Black Hole

Referred to by ATel #: 7216, 7697

Since the middle of February (~ MJD 57070), MAXI-GSC has detected a gradual decrease in hard X-rays from the black hole candidate Swift J1753.5-0127, which can be clearly seen in the Swift-BAT Hard X-ray Transient monitor light curve (http://swift.gsfc.nasa.gov/results/transients/SWIFTJ1753.5-0127/).

In contrast to previous 4 episodes around MJD 55172 (ATel #2341), 55319, 55620, and 56040 (ATel #4056), when the source showed possible hard-to-soft state transitions (Yoshikawa et al. 2015, PASJ 67, 11), this time the MAXI-GSC 2-4 keV X-ray flux does not show any increase especially since MJD 57081.

We then requested Swift XRT ToO observations and found that the 0.6-10 keV energy spectra obtained on March 5, 6, 9 could be roughly represented by an absorbed power-law with a photon index of 4.4 and the column density of 4-5e21 atoms/cm2. The absorbed 0.6-10 keV fluxes of March 5, 6, 9 were 1.19e-9 erg/cm2/s, 1.15e-9 erg/cm2/s, and 1.15e-9 erg/cm2/s, respectively. The 4-10 keV fluxes were 4.1e-11 erg/cm2/s, 4.3e-11 erg/cm2/s, and 3.9e-11 erg/cm2/s, respectively, which were about 1/10 of an usual flux level. Some Comptonization models give better fits to the spectra, and (disk)blackbody temperatures as a seed photon source are about 0.2 keV. The fluxes of the cold disk component are comparable to what was obtained in the hard state, suggesting that only the hard power-law component is decreasing.

A further Swift ToO observation is planned, and multiwavelength observations, especially radio observations for the detection of a jet, are encouraged.

Thanks to the Swift team to observe the source immediately and successfully.

Swift J1753.5-0127 data in MAXI public data archive