The hard X-ray spectrum of MAXI J1820+070 observed by Swift/BAT
ATel #11427; Melania Del Santo, Alberto Segreto (INAF/IASF-Palermo)
on 16 Mar 2018; 08:38 UT
Credential Certification: Melania Del Santo (melania@ifc.inaf.it)
Subjects: X-ray, Binary, Black Hole, Transient
We have analysed the Swift/BAT data of the new transient MAXI J1820+070
(Atel #11399,#11400, #11403, #11404, #11406, #11418, #11420, #11421, #11423,#11424, #11425, #11425)
collected from 2018-03-07 00:41:07 until 2018-03-15 06:20:58.
The hard X-ray flux of MAXI J1820+070 was rapidly increasing:
on March 15th it reached 1 Crab, 1.5 Crab and 1.8 Crab in the energy ranges
15-40 KeV and 40-80 keV and 80-150 keV, respectively.
Despite the strong flux variation the source show a constant hardness ratio.
The 15-200 keV spectrum of MAXI J1820+070, averaged in the time interval MJD 58190.004-58192.264 (about 42 ks exposure), is well fit by the thermal Comptonisation model NTHCOMP (reduced Chi^2=1.07(172)).
It results in a photon index Gamma=1.54+/-0.01 and an electron temperature kTe=65+/-7 keV
that are typical parameters of the hard spectral state observed in black hole binaries.
