Detection of X-ray reflection in MAXI J1816-195 with the NuSTAR

ATel #15470; Jaiverdhan Chauhan, Anne Lohfink (Montana State University), Priya Bharali (MGGAC, Mahe, Puducherry, India), Manoj Mandal (Midnapore City College, India), Paul Draghis (University of Michigan), Sabyasachi Pal (Midnapore City College, India), Andrea Sanna (University of Cagliari)
on 25 Jun 2022; 00:51 UT
Distributed as an Instant Email Notice Transients
Credential Certification: Jaiverdhan Chauhan (jaiverdhan.chauhan@montana.edu)

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On 2022 June 7, an uncatalogued X-ray transient was detected by MAXI (ATel #15418), and its location was then determined by Swift (ATel #15421). NICER has detected 528 Hz pulsations and a thermonuclear burst, leading to the source being classified as an accretion-powered millisecond pulsar (ATel #15425). Further, NICER also measured the binary orbital period of the source to be 17402.27 s (ATel #15431). MAXI J1816-195 was not detected in the radio frequency band (6.86 and 8.45 GHz) with the Yamaguchi Interferometer (ATel #15437). The source has not been detected in the optical and NIR band (ATel #15427, #15445, #15458) but a probable IR counterpart was reported in ATel #15467. NuSTAR observed MAXI J1816-195 on 2022 June 23 beginning at 10:31:52.000 (UTC) for a total exposure time of 40 ks. The source was detected at the location of RA (J2000) = 18h 16m 52.1256s, Dec (J2000) = –19d 37m 58.727s. The average background-subtracted NuSTAR count rate was 97+/-2 cts/s (both detectors combined). We detected the 528 Hz pulsation in the NuSTAR data and we also observed two thermonuclear bursts, where the maximum count rate of the bursts was ~2656 cts/s (background-subtracted). A preliminary spectral analysis revealed a spectrum describable by a disk blackbody and cut-off power-law with X-ray reflection features. We include low-energy Swift/XRT data (ATel #15467), and model our initial spectra with the blackbody accretion disk model and the self-consistent ionized reflection model. We found an absorption column of NH=2.65+/-0.13 x 10^22 cm^-2, a disk temperature of 0.49+/-0.03 keV, a photon index of 1.76+/-0.07, and a high energy cut-off of 32+/-4 keV. We calculated the unabsorbed total X-ray flux and disk flux to be ~3.5e-9 erg/s/cm^2 and ~5.3e-10 erg/s/cm^2, respectively, in the energy band 0.5 – 65.0 keV. For our initial fitting the reduced chi2/dof = 1.06/1714. We thank the NuSTAR SOC Team for making these observations possible.