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Register a new userHard X-ray colours of Neutron Star and Black Hole Low Mass X-ray Binaries with INTEGRAL
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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, model 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.
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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.
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.
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.
Early results from the INTEGRAL Core Program, for a sample of eight persistently bright neutron star low mass X-ray binaries in the energy range from 5 keV to 200 keV are presented. It is shown that INTEGRAL efficiently detects sources and that spectra may be obtained up to several hundreds of keV by combining data from three of the four INTEGRAL instruments: JEM-X, IBIS and SPI. For the source GX 17+2 it is shown that the spectrum extends well above 100 keV with a flattening of the spectrum above 30 keV. This might suggest a non-thermal comptonisation emission, but uncertainties in the current data reduction and background determination do not allow firm conclusions to be drawn.
The characteristics of black-hole X-ray binaries can be used to obtain information about their evolutionary history and the process of black-hole formation. In this paper I focus on systems with donor masses lower than the inferred black-hole masses. Current models for the evolution of hydrogen-rich, massive stars and of helium stars losing mass in a wind cannot explain the current sample of black-hole mass measurements. Assuming that the radial evolution of mass-losing massive stars is at least qualitatively accurate, I show that the properties of the BH companions lead to constraints on the masses of black-hole progenitors (at most twice the black-hole mass) and on the strength of winds in helium stars (fractional amount of mass lost smaller than about 50%). Constraints on common-envelope evolution are also derived.
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