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Register a new userDetectability of neutron star -- white dwarf coalescences by eROSITA and ART-XC
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Coalescences of neutron stars and white dwarfs are relatively frequent phenomena, outnumbering other types of compact object mergers (neutron stars and black holes without involving white dwarfs) altogether. Such event potentially can produce not only optical, but also an X-ray burst. Transient source CDF-S XT2 [1] can be an example of this type of events as suggested by [2]. In this note we estimate the rate of these transients in the field of view of X-ray instruments on-board Spectrum-RG satellite. We demonstrate that during four years of the survey program several thousand of events related to neutron star - white dwarf mergers might appear in the field of view of eROSITA. Collimation of X-ray emission can reduce this number. Smaller, but comparable number of transients is expected in the case of ART-XC telescope. However, due to relatively short duration - $lesssim 10^4$ s, - mostly such transients might be visible just in one scan of telescopes ($sim 40$ s), and so only a few photons are expected to be detected which makes definite identification without additional information very problematic.
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We report the discovery of three previously unknown cataclysmic variables in the data of the first year of the all-sky X-ray survey by the SRG orbital observatory. The sources were selected due to their brightness in the 4--12 keV band in the data of the Mikhail Pavlinsky ART-XC telescope. They are also detected by the eROSITA telescope, which provided accurate localizations and spectral data for broad-band spectral analysis. All three objects had been previously known as X-ray sources from the ROSAT all-sky survey and XMM-Newton slew survey, but their nature remained unknown. The X-ray spectra obtained by eROSITA and ART-XC are consistent with optically thin thermal emission with a temperature kT>~15 keV for SRGAJ194638.9+704552 and SRGAJ225412.8+690658 and kT>~5 keV for SRGAJ204547.8+672642. This, together with the inferred high X-ray luminosities ($2times 10^{32}$-$3times 10^{33}$ erg s$^{-1}$), strongly suggests that all three sources are CVs. We have obtained optical photometry and spectroscopy for these objects using the AZT-33IK 1.6-m telescope of the Sayan Observatory. The optical properties confirm the CV nature of the objects. We conclude that SRGAJ194638.9+704552 is an intermediate polar, SRGAJ204547.8+672642 is most likely a polar or an intermediate polar, and SRGAJ225412.8+690658 can be either a magnetic or a non-magnetic CV. We also measured an orbital period of 2.98~hours for SRGAJ204547.8+672642, based on TESS data. Three out of the planned eight SRG all-sky surveys have now been completed. We expect to find plenty of new CVs during the survey and to continue our optical follow-up program.
VLBI and JVLA observations revealed that GW170817 involved a narrow jet ($ theta_j approx 4^circ $) that dominated the afterglow peak at our viewing angle, $ theta_{rm obs} approx 20^circ $. This implies that at the time of the afterglow peak, the observed signal behaved like an afterglow of a top-hat jet seen at $ theta_{rm obs} gg theta_j $, and it can be modeled by analytic expressions that describe such jets. We use a set of numerical simulations to calibrate these analytic relations and obtain generic equations for the peak time and flux of such an afterglow as seen from various observing angles. Using the calibrated equations and the estimated parameters of GW170817, we estimate the detectability of afterglows from future double neutron star mergers during the Advanced LIGO/Virgo observation run O3. GW170817 took place at a relatively low-density environment. Afterglows of similar events will be detectable only at small viewing angles, $ theta_{rm obs} lesssim 20^circ $, and only $sim 20% $ of the GW detections of these events will be accompanied by a detectable afterglow. At higher densities, more typical to sGRB sites, up to $ 70% $ of the GW detections are expected to be followed by a detectable afterglow, typically at $ theta_{rm obs} sim 30^circ $. We also provide the latest time one should expect an afterglow detection. We find that for typical parameters, if the jet emission had not been detected within about a year after the merger, it is unlikely to be ever detected.
We investigate the effects of mass transfer and gravitational wave (GW) radiation on the orbital evolution of contact neutron-star-white-dwarf (NS-WD) binaries, and the detectability of these binaries by space GW detectors (e.g., Laser Interferometer Space Antenna, LISA; Taiji; Tianqin). A NS-WD binary becomes contact when the WD component fills its Roche lobe, at which the GW frequency ranges from ~0.0023 to 0.72 Hz for WD with masses ~0.05-1.4 Msun. We find that some high-mass NS-WD binaries may undergo direct coalescence after unstable mass transfer. However, the majority of NS-WD binaries can avoid direct coalescence because mass transfer after contact can lead to a reversal of the orbital evolution. Our model can well interpret the orbital evolution of the ultra-compact X-ray source 4U 1820--30. For a 4-year observation of 4U 1820--30, the expected signal-to-noise-ratio (SNR) in GW characteristic strain is ~11.0/10.4/2.2 (LISA/Taiji/Tianqin). The evolution of GW frequencies of NS-WD binaries depends on the WD masses. NS-WD binaries with masses larger than 4U 1820--30 are expected to be detected with significantly larger SNRs. For a (1.4+0.5) Msun NS-WD binary close to contact, the expected SNR for a one week observation is ~27/40/28 (LISA/Taiji/Tianqin). For NS-WD binaries with masses of (1.4+>~1.1) Msun, the significant change of GW frequencies and amplitudes can be measured, and thus it is possible to determine the binary evolution stage. At distances up to the edge of the Galaxy (~100 kpc), high-mass NS-WD binaries will be still detectable with SNR>~1.
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.
We report on the results of a 4-year timing campaign of PSR~J2222$-0137$, a 2.44-day binary pulsar with a massive white dwarf (WD) companion, with the Nanc{c}ay, Effelsberg and Lovell radio telescopes. Using the Shapiro delay for this system, we find a pulsar mass $m_{p}=1.76,pm,0.06,M_odot$ and a WD mass $m_{c},=,1.293,pm,0.025, M_odot$. We also measure the rate of advance of periastron for this system, which is marginally consistent with the GR prediction for these masses. The short lifetime of the massive WD progenitor star led to a rapid X-ray binary phase with little ($< , 10^{-2} , M_odot$) mass accretion onto the neutron star (NS); hence, the current pulsar mass is, within uncertainties, its birth mass; the largest measured to date. We discuss the discrepancy with previous mass measurements for this system; we conclude that the measurements presented here are likely to be more accurate. Finally, we highlight the usefulness of this system for testing alternative theories of gravity by tightly constraining the presence of dipolar radiation. This is of particular importance for certain aspects of strong-field gravity, like spontaneous scalarization, since the mass of PSR~J2222$-0137$ puts that system into a poorly tested parameter range.
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