INTEGRAL/IBIS and Swift/XRT follow up softening of MAXI J1828-249

ATel #5492; R. Krivonos (1, 2), S. Tsygankov (3, 4,1) (1: IKI, Moscow; 2: University of California, Berkeley; 3: FINCA; 4: University of Oulu)
on 21 Oct 2013; 06:08 UT
Credential Certification: Roman Krivonos (krivonos@iki.rssi.ru)

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The INTEGRAL observatory continues to monitor the Galactic Center region where the new transient source MAXI J1828-249 was recently discovered (Nakahira et al, ATEL #5474). Two serial data sets have been taken in the context of the on-going Galactic Latitude Scans program. Hard X-ray flux from the source is steadily going down and by the time of the last observations (Oct 18 @ 17:55 - Oct 20 @ 10:55, 2013, UTC) reached level of 34.5+\-1.3 mCrab in 20-40 keV, 35.5+/-2.1 mCrab in 40-80 keV, and 36.2+/-4.7 mCrab in 80-120 keV, which is ~5% less compared to the previous data set taken between Oct 17 at 04:32 and Oct 18 at 00:26, 2013 (UTC).

The source spectrum above 20 keV as seen by INTEGRAL/ISGRI could be well fitted by a power-law with the photon index of ~2 being unchanged during the time periods mentioned above.

The source behavior at energies below 10 keV is well captured by Swift/XRT observations. For the joint analysis we selected two XRT data sets nearly simultaneous with our INTEGRAL observations: OBS ID 00032997002 taken on Oct 17 at 17:15, and OBS ID 00032997005 started on Oct 20 at 00:52, 2013 (UTC). Best fit model for both XRT observations consists of an absorbed (wabs; nH=0.25) disk blackbody (diskbb; Tin=0.7 keV) and power-law (photon index=2). The power-law component slope is very well consistent with that obtained from the corresponding INTEGRAL/ISGRI data. The only noticeable change in the XRT spectrum is the raising of the disk blackbody normalization leading to the 0.5-10 keV flux increase from (6.5+/-0.1)x10^-9 to (8.8+/-0.1)x10^-9 ergs s^-1cm^-2 (90% confidence, uncorrected for the absorption).

Thus, nearly simultaneous Swift/XRT and INTEGRAL observations of MAXI J1828-249 make it possible to constrain soft blackbody and hard power-law spectral components, demonstrating gradual softening of the source X-ray emission, as suggested by Kennea et al. (ATEL #5479) and Negoro et al. (ATEL #5483). The joint XRT and INTEGRAL spectral fit is shown in the link below.

Continue of the source monitoring in both soft and hard X-ray bands is very desirable.

Joint Swift/XRT and INTEGRAL/IBIS spectral fit