Insight-HXMT observations of MAXI J1348-630

ATel #12470; Chen Y.-P.(IHEP), Ma X.(IHEP), Huang Y.(IHEP), Ge M.-Y.(IHEP), Tao L.(IHEP), Qu J.-L.(IHEP), Zhang S.(IHEP), Zhang S.-N. (IHEP), HXMT-Collaboration
on 4 Feb 2019; 11:24 UT
Credential Certification: Yu-Peng Chen (chenyp@ihep.ac.cn)

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Insight-HXMT observed the newly discovered X-ray transient MAXI J1348-630 (ATel #12425, #12430, #12434, #12441, #12447, #12448, #12456, #12457, #12469) for a total of about 160 ksec between 2019 Jan. 27 and Feb. 2. Based on these Insight-HXMT observations, we performed timing and spectral analyses, and report the detections of QPO and spectral evolutions during the outburst.

The source has exhibited flux variability on time scale of as short as a few seconds, in a broad energy band covered by the three instruments of Insight-HXMT: LE (1-10 keV), ME (5-30 keV) and HE (25-250 keV). The broad-band spectra are well fitted by a model consisting of an absorbed cut-off power-law and a diskbb, in which the column density is estimated to be ~0.45+/-0.1 E22 cm-2. The spectra are dominated by a hard power-law component with a cutoff energy decreasing from 122+/-3 keV to tens keV. The temperature of the soft X-ray component diskbb decreases smoothly from 0.78+/-0.03 keV to 0.67+/-0.02 keV and the flux in 15-100 keV increases gradually from 4E-8 erg cm-2 s-1 to 8E-8 erg cm-2 s-1.

The average Fourier power spectrum of its lightcurve on Jan. 30th can be described by broad Lorentzian components plus a variable QPO feature of a maximum centroid frequency of 0.71 +/- 0.01 Hz, which has a FWHM of 0.16 +/- 0.04 Hz and a fractional rms amplitude of about 5% in the energy band of 25-100 keV. The QPO feature is generally visible across a bandwidth of 1-100 keV and has a relatively weak dependence on energy. However, the QPO centroid frequency evolved significantly from 0.2 Hz to about 0.7 Hz as its flux increases. These properties suggest that the source is currently staying at a typical hard or intermediate hard spectral state of a black hole X-ray binary. Insight-HXMT will continue to observe the source until it is too faint or becomes invisible to Insight-HXMT .