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Register a new userSRG/ART-XC, Swift, NICER and NuSTAR study of different states of the transient X-ray pulsar MAXI J0903-531
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The results of the broadband spectral and timing study of the recently discovered transient X-ray pulsar MAXI J0903-531 in a wide range of luminosities differing by a factor of ~30 are reported. The observed X-ray spectrum in both states can be described as a classical pulsar-like spectrum consisting of the power-law with the high-energy cutoff. We argue that absence of the spectrum transformation to the two-hump structure expected at low fluxes points to a relatively weak magnetic field of the neutron star below (2-3)$times10^{12}$ G. This estimate is consistent with other indirect constraints and non-detection of any absorption features which can be interpreted as a cyclotron absorption line. Timing analysis of the NuSTAR data revealed only slight variations of a single-peaked pulse profile of the source as a function of the energy band and mass accretion rate. In both intensity states the pulsed fraction increases from 40% to roughly 80% with the energy. Finally we were also able to obtain the orbital solution for the binary system using data from the Fermi/GBM, NICER and NuSTAR instruments.
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We report results of the first broadband observation of the transient X-ray pulsar GRO J1008-57 performed in the quiescent state. Observations were conducted quasi-simultaneously with the Mikhail Pavlinsky ART-XC telescope on board SRG and NuSTAR right before the beginning of a Type I outburst. GRO J1008-57 was detected in the state with the lowest observed luminosity around several $times 10^{34}$ erg s$^{-1}$ and consequently accreting from the cold disk. Timing analysis allowed to significantly detect pulsations during this state for the first time. The observed pulsed fraction of about 20% is, however, almost three times lower than in brighter states when the accretion proceeds through the standard disk. We traced the evolution of the broadband spectrum of the source on a scale of three orders of magnitude in luminosity and found that at the lowest luminosities the spectrum transforms into the double-hump structure similarly to other X-ray pulsars accreting at low luminosities (X Persei, GX 304-1, A0535+262) reinforcing conclusion that this spectral shape is typical for these objects.
Context: During the ongoing all-sky survey, the Mikhail Pavlinsky ART-XC telescope on board the SRG observatory should discover new X-ray sources, many of which can be transient. Here we report on the discovery and multiwavelength follow-up of a peculiar X-ray source SRGA J043520.9+552226=SRGe J043523.3+552234 - the high-energy counterpart of the optical transient AT2019wey. Aims: Thanks to its sensitivity and the survey strategy, the Mikhail Pavlinsky ART-XC telescope uncovers poorly studied weak transient populations. Using a synergy with current public optical surveys, we are aiming at revealing the nature of these transients to study its parent populations. The SRGA J043520.9+552226 is the first transient detected by ART-XC which has a bright optical counterpart suitable for further studies. Methods: We have used available public X-ray and optical data and observations with SRG, INTEGRAL, NuSTAR, NICER and ground-based telescopes to investigate the source spectral energy distributions at different phases of the outburst. Results: Based on X-ray spectral and timing properties derived from space observations, optical spectroscopy and photometry obtained with the 2.5-m and RC600 CMO SAI MSU telescopes, we propose the source to be a black hole in a low-mass close X-ray binary system.
We present results for the first observed outburst from the transient X-ray binary source MAXI J0637-430. This study is based on eight observations from the Nuclear Spectroscopic Telescope Array (NuSTAR) and six observations from the Neil Gehrels Swift Observatory X-Ray Telescope (Swift/XRT) collected from 2019 November 19 to 2020 April 26 as the 3-79 keV source flux declined from 8.2e-10 to 1.4e-12 erg/cm^2/s. We see the source transition from a soft state with a strong disk-blackbody component to a hard state dominated by a power-law or thermal Comptonization component. NuSTAR provides the first reported coverage of MAXI J0637-430 above 10 keV, and these broadband spectra show that a two-component model does not provide an adequate description of the soft state spectrum. As such, we test whether blackbody emission from the plunging region could explain the excess emission. As an alternative, we test a reflection model that includes a physical Comptonization continuum. Finally, we also invoke a spectral component based on reflection of blackbody returning radiation due to the bending of light by the strong gravity of the black hole. We discuss the physical implications of each scenario and demonstrate the value of constraining the source distance.
We present a first catalog of sources detected by the Mikhail Pavlinsky ART-XC telescope aboard the SRG observatory in the 4-12 keV energy band during its on-going all-sky survey. The catalog comprises 867 sources detected on the combined map of the first two 6-month scans of the sky (Dec. 2019 - Dec. 2020) - ART-XC sky surveys 1 and 2, or ARTSS12. The achieved sensitivity to point sources varies between ~5x10-12 erg/s/cm2 near the Ecliptic plane and better than 10-12 erg/s/cm2 (4-12 keV) near the Ecliptic poles, and the typical localization accuracy is ~15 arcsec. Among the 750 sources of known or suspected origin in the catalog, 56% are extragalactic (mostly active galactic nuclei (AGN) and clusters of galaxies) and the rest are Galactic (mostly cataclysmic variables (CVs) and low- and high-mass X-ray binaries). For 116 sources ART-XC has detected X-rays for the first time. Although the majority of these (~80) are expected to be spurious (for the adopted detection threshold), there can be a significant number of newly discovered astrophysical objects. We have started a program of optical follow-up observations of the new and previously unidentified X-ray sources, which has already led to the identification of several AGN and CVs. With the SRG all-sky survey planned to continue for a total of 4 years, we can expect the ART-XC survey in the 4-12 keV band to significantly surpass the previous surveys carried out in similar (medium X-ray) energy bands in terms of the combination of angular resolution, sensitivity, and sky coverage.
ART-XC (Astronomical Roentgen Telescope - X-ray Concentrator) is the hard X-ray instrument with grazing incidence imaging optics on board the Spektr-Roentgen-Gamma (SRG) observatory. The SRG observatory is the flagship astrophysical mission of the Russian Federal Space Program, which was successively launched into orbit around the second Lagrangian point (L2) of the Earth-Sun system with a Proton rocket from the Baikonur cosmodrome on 13 July 2019. The ART-XC telescope will provide the first ever true imaging all-sky survey performed with grazing incidence optics in the 4-30 keV energy band and will obtain the deepest and sharpest map of the sky in the energy range of 4-12 keV. Observations performed during the early calibration and performance verification phase as well as during the on-going all-sky survey that started on 12 Dec. 2019 have demonstrated that the in-flight characteristics of the ART-XC telescope are very close to expectations based on the results of ground calibrations. Upon completion of its 4-year all-sky survey, ART-XC is expected to detect ~5000 sources (~3000 active galactic nuclei, including heavily obscured ones, several hundred clusters of galaxies, ~1000 cataclysmic variables and other Galactic sources), and to provide a high-quality map of the Galactic background emission in the 4-12 keV energy band. ART-XC is also well suited for discovering transient X-ray sources. In this paper, we describe the telescope, results of its ground calibrations, major aspects of the mission, the in-flight performance of ART-XC and first scientific results.
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