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Swift/XRT sees MAXI J1535-571 brightening and softening

ATel #10731; J. A. Kennea (PSU)
on 11 Sep 2017; 13:35 UT
Credential Certification: Jamie A. Kennea (kennea@astro.psu.edu)

Subjects: X-ray, Binary, Black Hole, Transient

Referred to by ATel #: 10733, 10734, 10761, 10768

Swift took a Target-of-Opportunity observation of MAXI J1535-571, with a 1ks exposure in Windowed Timing (WT) mode, beginning at 06:34UT on September 11th, 2017. After correcting for pile-up, we find that the spectrum is well fit by an absorbed power-law model with a photon index of 1.95 +/- 0.02, with a 0.5-10 keV flux, corrected for absorption, of 9.5 +/- 0.1 x 10-8 erg/s/cm-2. A power-law fit adequately fit the data, and addition of a thermal component did not improve the fit.

Comparatively, a 5ks WT mode observation taken on September 8th, 2017 shows a photon index of 1.57 +/- 0.02, and a flux of 4.2 +/- 0.1 x 10-8 erg/s/cm-2.

Therefore in the Swift XRT energy range, MAXI J1535-571 continues to brighten (the current count rate is 805 +/- 7 c/s) but it is getting spectrally softer, corroborating the suggestion by Nakahira et al (ATEL #10729) that MAXI J1536-571 is undergoing a state change. However, as of yet, the spectrum in the XRT energy range remains non-thermal, although high absorption of 2.4 x 1022 cm-2 may be hiding a soft thermal component.