NICER detection of renewed activity from 4U 1730-22
ATel #15945; G. C. Mancuso (IAR, FCAGLP), D. Altamirano (Univ. of Southampton), P. M. Bult (RIVM), A. Sanna (Univ. of Cagliari), T. Guver (Istanbul Univ.), S. Guillot (IRAP-CNRS)
on 15 Mar 2023; 04:31 UT
Credential Certification: Giulio Cesare Mancuso (gmancuso@iar.unlp.edu.ar)
Subjects: X-ray, Binary, Neutron Star
MAXI/GSC detected a new outburst of the historic neutron star low-mass X-ray binary 4U 1730-22 on 7 June 2021 (ATel#14683). The last time the source has been detected in an outburst was 50 years ago. NICER monitored the source for a period of around 80 days, after which the source appeared to be transitioning to quiescence. The lack of pointed observations prevented us from confirming this result. Renewed activity in the direction of 4U 1730-22 was reported on 13 February 2022 by the MAXI Nova alert system. The source remained bright for 200 days, after which it transitioned to a low-luminosity hard state. In the last NICER monitoring observation, performed on 9 October 2022, the 0.5-10 keV background-subtracted count rate was ~75 cts/s. Solar constraints prevented further NICER monitoring.
We report on the first NICER ToO observation of 4U 1730-22 after the source emerged from the NICER Sun-constraint window. On 16 February 2023, NICER collected ~3.2 ks of pointed observations divided into two orbits. The source is clearly detected at ~300 cts/s and ~340 cts/s (0.5-10 keV) in the 1st and 2nd orbit, respectively, indicating a renewal of its accretion activity. The 16 sec time resolution 3.8-6.8 keV / 2.0-3.8 keV count rate ratio is 0.22-0.26, which is consistent with the soft spectral state of the source (Bult et al. 2022; ApJ, 940, 1).
We performed spectral analysis in the 0.5-10 keV range and found that the averaged energy spectrum was well described by an absorbed disc blackbody plus a power-law model. We estimate a disc blackbody temperature of kT = 0.80 +/- 0.03 keV and a power-law photon index Gamma = 2.08 +/- 0.05. We took into account the interstellar medium absorption using the TBabs model with the abundances of Anders & Grevesse (1989). We measured an N_H = 0.48 +/- 0.01 x 10^{22} cm^{-2}. The derived unabsorbed 0.5-10 keV flux of the source is 1.56 +/- 0.03 x 10^{-9} erg/cm^{2}/s.
NICER is a 0.2-12 keV X-ray telescope operating on the International Space
Station. The NICER mission and portions of the NICER science team activities
are funded by NASA.
