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Register a new userOn the Physical Nature of the Source of Ultraluminous X-ray Pulsations
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Authors
G. Ter-Kazarian
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To reconcile the observed unusual high luminosity of NuSTAR X-ray pulsations from M82X-2 with the most extreme violation of the Eddington limit, and in view that the persistent X-ray radiation from M82X-2 almost precludes the possibility of common pulsars, we tackle the problem by the implications of {em microscopic theory of black hole} (MTBH). The preceding developments of MTBH are proved to be quite fruitful for the physics of ultra-high energy (UHE) cosmic-rays. Namely, replacing a central singularity by the infrastructures inside event horizon, subject to certain rules, MTBH explains the origin of ZeV-neutrinos which are of vital interest for the source of UHE-particles. The M82X-2 is assumed to be a spinning intermediate mass black hole resided in final stage of growth. As a corollary, the thermal blackbody X-ray emission arisen due to the rotational kinetic energy of black hole escapes from event horizon through the vista to outside world that detected as ultraluminous X-ray pulsations. The M82X-2 indeed releases $sim 99.6%$ of its pulsed radiative energy predominantly in the X-ray bandpass $0.3-30$ keV. We derive a pulse profile and give a quantitative account of energetics and orbital parameters of the semi-detached X-ray binary containing a primary accretor M82X-2 of inferred mass $Msimeq 138.5-226,M_{odot}$ and secondary massive, $M_{2}> 48.3- 64.9,M_{odot}$, O/B-type donor star with radius of $R> 22.1- 25.7,R_{odot}$, respectively. We compute the torque added to M82X-2 per unit mass of accreted matter which yields the measured spin-up rate.
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We report the detection of coherent pulsations from the ultraluminous X-ray source NGC 7793 P13. The ~0.42s nearly sinusoidal pulsations were initially discovered in broadband X-ray observations using XMM-Newton and NuSTAR taken in 2016. We subsequently also found pulsations in archival XMM-Newton data taken in 2013 and 2014. The significant (>>5 sigma) detection of coherent pulsations demonstrates that the compact object in P13 is a neutron star with an observed peak luminosity of ~1e40 erg/s (assuming isotropy), well above the Eddington limit for a 1.4 M_sun accretor. This makes P13 the second ultraluminous X-ray source known to be powered by an accreting neutron star. The pulse period varies between epochs, with a slow but persistent spin up over the 2013-2016 period. This spin-up indicates a magnetic field of B ~ 1.5e12 G, typical of many accreting pulsars. The most likely explanation for the extreme luminosity is a high degree of beaming, however this is difficult to reconcile with the sinusoidal pulse profile.
We report the detection of weak pulsations from the archetypal ultraluminous X-ray source (ULX) NGC 1313 X-2. Acceleration searches reveal sinusoidal pulsations in segments of two out of six new deep observations of this object, with a period of $sim$ 1.5 s and a pulsed fraction of $sim$ 5%. We use Monte Carlo simulations to demonstrate that the individual detections are unlikely to originate in false Poisson noise detections given their very close frequencies; their strong similarity to other pulsations detected from ULXs also argues they are real. The presence of a large bubble nebula surrounding NGC 1313 X-2 implies an age of order 1 Myr for the accreting phase of the ULX, which implies that the neutron stars magnetic field has not been suppressed over time by accreted material, nor has the neutron star collapsed into a black hole, despite an average energy output into the nebula two orders of magnitude above Eddington. This argues that most of the accreted material has been expelled over the lifetime of the ULX, favouring physical models including strong winds and/or jets for neutron star ULXs.
The X-ray spectrum of the nearest ultraluminous X-ray source, M33 X-8, obtained by Suzaku during 2010 January 11 -- 13, was closely analyzed to examine its nature. It is, by far, the only data with the highest signal statistic in 0.4 -- 10 keV range. Despite being able to reproduce the X-ray spectrum, Comptonization of the disk photons failed to give a physically meaningful solution. A modified version of the multi-color disk model, in which the dependence of the disk temperature on the radius is described as r^(-p) with p being a free parameter, can also approximate the spectrum. From this model, the innermost disk temperature and bolometric luminosity were obtained as T_in = 2.00-0.05+0.06 keV and L_disk = 1.36 x 10^39 (cos i)^(-1) ergs/s, respectively, where i is the disk inclination. A small temperature gradient of p = 0.535-0.005+0.004, together with the high disk temperature, is regarded as the signatures of the slim accretion disk model, suggesting that M33 X-8 was accreting at high mass accretion rate. With a correction factor for the slim disk taken into account, the innermost disk radius, R_in =81.9-6.5+5.9 (cos i)^(-0.5) km, corresponds to the black hole mass of M sim 10 M_sun (cos i)^(-0.5). Accordingly, the bolometric disk luminosity is estimated to be about 80 (cos i)^(-0.5)% of the Eddington limit. A numerically calculated slim disk spectrum was found to reach a similar result. Thus, the extremely super-Eddington luminosity is not required to explain the nature of M33 X-8. This conclusion is utilized to argue for the existence of intermediate mass black holes with M > 100 M_sun radiating at the sub/trans-Eddington luminosity, among ultraluminous X-ray sources with L_disk > 10^(40) ergs/s.
We present nearly simultaneous NuSTAR and XMM-Newton observations of the nearby (832 kpc) ultraluminous X-ray source (ULX) M33 X-8. M33 X-8 has a 0.3-10 keV luminosity of LX ~ 1.4 x 10^39 erg/s, near the boundary of the ultraluminous classification, making it an important source for understanding the link between typical Galactic X-ray binaries and ULXs. Past studies have shown that the 0.3-10 keV spectrum of X-8 can be characterized using an advection-dominated accretion disk model. We find that when fitting to our NuSTAR and XMM-Newton observations, an additional high-energy (>10 keV) Comptonization component is required, which allows us to rule out single advection-dominated disk and classical sub-Eddington models. With our new constraints, we analyze XMM-Newton data taken over the last 17 years to show that small (~30%) variations in the 0.3-10 keV flux of M33 X-8 result in spectral changes similar to those observed for other ULXs. The two most likely phenomenological scenarios suggested by the data are degenerate in terms of constraining the nature of the accreting compact object (i.e., black hole versus neutron star). We further present a search for pulsations using our suite of data; however, no clear pulsations are detected. Future observations designed to observe M33 X-8 at different flux levels across the full 0.3-30 keV range would significantly improve our constraints on the nature of this important source.
Ultraluminous X-ray sources are considered amongst the most extremely accreting objects in the local Universe. The recent discoveries of pulsating neutron stars in ULXs strengthened the scenario of highly super-Eddington accretion mechanisms on stellar mass compact objects. In this work, we present the first long-term light curve of the source NGC 4559 X7 using all the available Swift, XMM-Newton, Chandra and NuSTAR data. Thanks to the high quality 2019 XMM-Newton and NuSTAR observations, we investigated in an unprecedented way the spectral and temporal properties of NGC 4559 X7. The source displayed flux variations of up to an order of magnitude and an unusual flaring activity. We modelled the spectra from NGC 4559 X7 with a combination of two thermal components, testing also the addition of a further high energy cut-off powerlaw. We observed a spectral hardening associated with a luminosity increase during the flares, and a spectral softening in the epochs far from the flares. Narrow absorption and emission lines were also found in the RGS spectra, suggesting the presence of an outflow. Furthermore, we measured hard and (weak) soft lags with magnitudes of a few hundreds of seconds whose origin is possibly be due to the accretion flow. We interpret the source properties in terms of a super-Eddington accretion scenario assuming the compact object is either a light stellar mass black hole or a neutron star.
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