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تسجيل مستخدم جديدHard state neutron star and black hole X-ray binaries in the radio:X-ray luminosity plane
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Motivated by the large body of literature around the phenomenological properties of accreting black hole (BH) and neutron star (NS) X-ray binaries in the radio:X-ray luminosity plane, we carry out a comparative regression analysis on 36 BHs and 41 NSs in hard X-ray states, with data over 7 dex in X-ray luminosity for both. The BHs follow a radio to X-ray (logarithmic) luminosity relation with slope $beta=0.59pm0.02$, consistent with the NSs slope ($beta=0.44^{+0.05}_{-0.04}$) within 2.5$sigma$. The best-fitting intercept for the BHs significantly exceeds that for the NSs, cementing BHs as more radio loud, by a factor $sim$22. This discrepancy can not be fully accounted for by the mass or bolometric correction gap, nor by the NS boundary layer contribution to the X-rays, and is likely to reflect physical differences in the accretion flow efficiency, or the jet powering mechanism. Once importance sampling is implemented to account for the different luminosity distributions, the slopes of the non-pulsating and pulsating NS subsamples are formally inconsistent ($>3sigma$), unless the transitional millisecond pulsars (whose incoherent radio emission mechanism is not firmly established) are excluded from the analysis. We confirm the lack of a robust partitioning of the BH data set into separate luminosity tracks.
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In this chapter, I present the main X-ray observational characteristics of black-hole binaries and low magnetic field neutron-star binaries, concentrating on what can be considered similarities or differences, with particular emphasis on their fast-timing behaviour.
Neutron star X-ray binaries emit a compact, optically thick, relativistic radio jet during low-luminosity, usually hard states, as Galactic black-hole X-ray binaries do. When radio emission is bright, a hard power-law tail without evidence for an exp
onential cutoff is observed in most systems. We have developed a jet model that explains many spectral and timing properties of black-hole binaries in the states where a jet is present. Our goal is to investigate whether our jet model can reproduce the hard tail, with the correct range of photon index and the absence of a high-energy cutoff, in neutron-star X-ray binaries. We have performed Monte Carlo simulations of the Compton upscattering of soft, accretion-disk or boundary layer photons, in the jet and computed the emergent energy spectra, as well as the time lag of hard photons with respect to softer ones as a function of Fourier frequency. We demonstrate that our jet model explains the observed power-law distribution with photon index in the range 1.8-3. With an appropriate choice of the parameters, the cutoff expected from Comptonization is shifted to energies above ~300 keV, producing a pure power law without any evidence for a rollover, in agreement with the observations. Our results reinforce the idea that the link between the outflow (jet) and inflow (disk) in X-ray binaries does not depend on the nature of the compact object, but on the process of accretion. Furthermore, we address the differences of jets in black-hole and neutron-star X-ray binaries and predict that the break frequency in the spectral energy distribution of neutron-star X-ray binaries, as a class, will be lower than that of black-hole binaries.
The X-ray spectra of Low Mass X-ray Binaries (LMXB) can change on short time-scales, making it difficult to follow their spectral characteristics in detail through model fitting. Colour-colour (C-C) diagrams are therefore often used as alternative, m
odel independent, tools to study the spectral variability of these sources. The INTEGRAL mission, with its high sensitivity, large field of view and good angular resolution, is well suited to study the hard X-ray properties of LMXBs. In particular the ISGRI imager on board of INTEGRAL allows the regular monitoring of the sources in the less frequently studied domain above 20 keV. In this proceeding, C-C diagrams have been made with data from the INTEGRAL public archive; a search is made for systematic differences in the C-C diagrams between black hole candidates (BH) and neutron stars (NS) in LMXBs using a moments analysis method.
Here we study the rapid X-ray variability (using XMM-Newton observations) of three neutron-star low-mass X-ray binaries (1RXS J180408.9-342058, EXO 1745-248, and IGR J18245-2452) during their recently proposed very hard spectral state (Parikh et al.
2017). All our systems exhibit a strong to very strong noise component in their power density spectra (rms amplitudes ranging from 34% to 102%) with very low characteristic frequencies (as low as 0.01 Hz). These properties are more extreme than what is commonly observed in the canonical hard state of neutron-star low-mass X-ray binaries observed at X-ray luminosities similar to those we observe from our sources. This suggests that indeed the very hard state is a distinct spectral-timing state from the hard state, although we argue that the variability behaviour of IGR J18245-2452 is very extreme and possibly this source was in a very unusual state. We also compare our results with the rapid X-ray variability of the accreting millisecond X-ray pulsars IGR J00291+5934 and Swift J0911.9-6452 (also using XMM-Newton data) for which previously similar variability phenomena were observed. Although their energy spectra (as observed using the Swift X-ray telescope) were not necessarily as hard (i.e., for Swift J0911.9-6452) as for our other three sources, we conclude that likely both sources were also in very similar state during their XMM-Newton observations. This suggest that different sources that are found in this new state might exhibit different spectral hardness and one has to study both the spectral as well as rapid variability to identify this unusual state.
We report on unusually very hard spectral states in three confirmed neutron-star low-mass X-ray binaries (1RXS J180408.9-342058, EXO 1745-248, and IGR J18245-2452) at a luminosity between ~ 10^{36-37} erg s^{-1}. When fitting the Swift X-ray spectra
(0.5 - 10 keV) in those states with an absorbed power-law model, we found photon indices of Gamma ~ 1, significantly lower than the Gamma = 1.5 - 2.0 typically seen when such systems are in their so called hard state. For individual sources very hard spectra were already previously identified but here we show for the first time that likely our sources were in a distinct spectral state (i.e., different from the hard state) when they exhibited such very hard spectra. It is unclear how such very hard spectra can be formed; if the emission mechanism is similar to that operating in their hard states (i.e., up-scattering of soft photons due to hot electrons) then the electrons should have higher temperatures or a higher optical depth in the very hard state compared to those observed in the hard state. By using our obtained Gamma as a tracer for the spectral evolution with luminosity, we have compared our results with those obtained by Wijnands et al. (2015). We confirm their general results in that also our sample of sources follow the same track as the other neutron star systems, although we do not find that the accreting millisecond pulsars are systematically harder than the non-pulsating systems.
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