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Register a new userUltraluminous X-ray sources as neutrino pulsars
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Added by
Alexander A. Mushtukov
Publication date
2018
fields
Physics
and research's language is
English
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The classical limit on the accretion luminosity of a neutron star is given by the Eddington luminosity. The advanced models of accretion onto magnetized neutron stars account for the appearance of magnetically confined accretion columns and allow the accretion luminosity to be higher than the Eddington value by a factor of tens. However, the recent discovery of pulsations from ultraluminous X-ray source (ULX) in NGC 5907 demonstrates that the accretion luminosity can exceed the Eddington value up to by a factor of 500. We propose a model explaining observational properties of ULX-1 in NGC 5907 without any ad hoc assumptions. We show that the accretion column at extreme luminosity becomes advective. Enormous energy release within a small geometrical volume and advection result in very high temperatures at the bottom of accretion column, which demand to account for the energy losses due to neutrino emission which can be even more effective than the radiation energy losses. We show that the total luminosity at the mass accretion rates above $10^{21},{rm g,s^{-1}}$ is dominated by the neutrino emission similarly to the case of core-collapse supernovae. We argue that the accretion rate measurements based on detected photon luminosity in case of bright ULXs powered by neutron stars can be largely underestimated due to intense neutrino emission. The recently discovered pulsating ULX-1 in galaxy NGC 5907 with photon luminosity of $sim 10^{41},{rm erg,s^{-1}}$ is expected to be even brighter in neutrinos and is thus the first known Neutrino Pulsar.
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We review observations of ultraluminous X-ray sources (ULXs). X-ray spectroscopic and timing studies of ULXs suggest a new accretion state distinct from those seen in Galactic stellar-mass black hole binaries. The detection of coherent pulsations indicates the presence of neutron-star accretors in three ULXs and therefore apparently super-Eddington luminosities. Optical and X-ray line profiles of ULXs and the properties of associated radio and optical nebulae suggest that ULXs produce powerful outflows, also indicative of super-Eddington accretion. We discuss models of super-Eddington accretion and their relation to the observed behaviors of ULXs. We review the evidence for intermediate mass black holes in ULXs. We consider the implications of ULXs for super-Eddington accretion in active galactic nuclei, heating of the early universe, and the origin of the black hole binary recently detected via gravitational waves.
Ultraluminous X-ray sources (ULXs) are a population of extragalactic objects whose luminosity exceeds the Eddington limit for a 10 Msun black hole (BH). Their properties have been widely interpreted in terms of accreting stellar-mass or intermediate-mass BHs. However at least three neutron stars (NSs) have been recently identified in ULXs through the discovery of periodic pulsations. Motivated by these findings we studied the spectral properties of a sample of bright ULXs using a simple continuum model which was extensively used to fit the X-ray spectra of accreting magnetic NSs in the Galaxy. We found that such a model, consisting of a power-law with a high-energy exponential cut-off, fits very well most of the ULX spectra analyzed here, at a level comparable to that of models involving an accreting BH. On these grounds alone we suggest that other non-pulsating ULXs may host NSs. We found also that above 2 keV the spectrum of known pulsating ULXs is harder than that of the majority of the other ULXs of the sample, with only IC 342 X-1 and Ho IX X-1 displaying spectra of comparable hardness. We thus suggest that these two ULXs may host an accreting NS and encourage searches for periodic pulsations in the flux.
Although ultra-luminous X-ray sources (ULX) are important for astrophysics due to their extreme apparent super-Eddington luminosities, their nature is still poorly known. Theoretical and observational studies suggest that ULXs could be a diversified group of objects composed of low-mass X-ray binaries, high-mass X-ray binaries and marginally also systems containing intermediate-mass black holes, which is supported by their presence in a variety of environments. Observational data on the ULX donors could significantly boost our understanding of these systems, but only a few were detected. There are several candidates, mostly red supergiants (RSGs), but surveys are typically biased toward luminous near-infrared objects. Nevertheless, it is worth exploring if RSGs can be members of ULX binaries. In such systems matter accreted onto the compact body would have to be provided by the stellar wind of the companion, since a Roche-lobe overflow could be unstable for relevant mass-ratios. Here we present a comprehensive study of the evolution and population of wind-fed ULXs and provide a theoretical support for the link between RSGs and ULXs. Our estimated upper limit on contribution of wind-fed ULX to the overall ULX population is $sim75$--$96%$ for young ($<100$ Myr) star forming environments, $sim 49$--$87%$ for prolonged constant star formation (e.g., disk of Milky Way), and $lesssim1%$ for environments in which star formation ceased long time ($>2$ Gyr) ago. We show also that some wind-fed ULXs (up to $6%$) may evolve into merging double compact objects (DCOs), but typical systems are not viable progenitors of such binaries because of their large separations. We demonstrate that, the exclusion of wind-fed ULXs from population studies of ULXs, might have lead to systematical errors in their conclusions.
The aim of the present paper is to investigate a possible contribution of the rotation-powered pulsars and pulsar wind nebulae to the population of ultraluminous X-ray sources (ULXs). We first develop an analytical model for the evolution of the distribution function of pulsars over the spin period and find both the steady-state and the time-dependent solutions. Using the recent results on the X-ray efficiency dependence on pulsar characteristic age, we then compute the X-ray luminosity function (XLF) of rotation-powered pulsars. In a general case it has a broken power-law shape with a high luminosity cutoff, which depends on the distributions of the birth spin period and the magnetic field. Using the observed XLF of sources in the nearby galaxies and the condition that the pulsar XLF does not exceed that, we find the allowed region for the parameters describing the birth period distribution. We find that the mean pulsar period should be greater than 10-40 ms. These results are consistent with the constraints obtained from the X-ray luminosity of core-collapse supernovae. We estimate that the contribution of the rotation-powered pulsars to the ULX population is at a level exceeding 3 per cent. For a wide birth period distribution, this fraction grows with luminosity and above 10E40 erg/s pulsars can dominate the ULX population.
We study properties of luminous X-ray pulsars using a simplified model of the accretion column. The maximal possible luminosity is calculated as a function of the neutron star (NS) magnetic field and spin period. It is shown that the luminosity can reach values of the order of $10^{40},{rm erg/s}$ for the magnetar-like magnetic field ($Bgtrsim 10^{14},{rm G}$) and long spin periods ($Pgtrsim 1.5,{rm s}$). The relative narrowness of an area of feasible NS parameters which are able to provide higher luminosities leads to the conclusion that $Lsimeq 10^{40},,{rm erg/s}$ is a good estimate for the limiting accretion luminosity of a NS. Because this luminosity coincides with the cut-off observed in the high mass X-ray binaries luminosity function which otherwise does not show any features at lower luminosities, we can conclude that a substantial part of ultra-luminous X-ray sources are accreting neutron stars in binary systems.
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