NICER follow up of the new X-ray transient IGR J17533-2928

ATel #14442; A. Sanna (Univ. Cagliari), D. Altamirano (Univ. of Southampton), L. Burderi, A. Riggio (Univ. Cagliari), T. Di Salvo (Univ. Palermo), M. Ng (MIT), J. Chenevez (DTU Space), J. F. Steiner (CfA)
on 6 Mar 2021; 11:25 UT
Distributed as an Instant Email Notice Transients
Credential Certification: Andrea Sanna (andrea.sanna@dsf.unica.it)

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Following the announcement of the new X-ray transient IGR J17533-2928 discovered by INTEGRAL (ATels #14425, #14430), we started observing the source with the Neutron Star Interior Composition Explorer (NICER) starting from 2020 February 28 at 13:40 UT, for a total exposure of approximately 11.3 ksec.

The average source count rate measured by NICER was 5.9 cts/sec (0.5-10 keV). The 0.5-10 keV power spectrum does not present significant periodic signals in the frequency range 5E-3Hz - 2.5E3 Hz. We detected a 4.2 sigma (single trial) quasi-periodic oscillation at 28 ± mHz. The QPO is sharp (FWHM: 9 ± 3 mHz), with a 0.5-10 keV fractional rms amplitude of 16±2 %.

The continuum X-ray spectrum extracted from the NICER data in the 0.5-10 keV energy range is well described (red. χ² 1.23 for 556 d.o.f.) by an absorbed blackbody and a thermally comptonized continuum (bbody+nthcomp in XSPEC). The absorption column density derived from the spectral fit is N_H = 1.7(2)E21 cm^-2. The blackbody component is described by a temperature kT=0.82(2) keV while the thermally comptonized component is characterized by a photon index Gamma=1.1(2) produced by an electron population with temperature kTe=4.1(4) keV and a black-body seed photon temperature kTb=0.13(3) keV. The absorbed 0.5-10 keV source flux is 3.9(3)E-11 erg cm^-2 s^-1. There is no evidence of a spectral Fe line feature in the energy range 6.4-7 keV.

At the moment, the temporal and spectral properties of IGR J17533-2928 do not allow us to unambiguously identify the nature of the compact object in the binary X-ray binary system. Therefore, further X-rays, as well as multi-wavelength observations, are strongly encouraged.

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.