Ultraluminous X‐ray sources: Three exciting years

Bachetti, Matteo

doi:10.1002/asna.201612312

Cited by 24 publications

(25 citation statements)

References 85 publications

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“…In this work, we have illustrated the results of MAD simulations with the rather extreme values of a * = 0 and a * = 0.9: we find that the low-spin models are a better approximation to ULX behaviour, with a spectral turnover between 1 and 10 keV. MAD models with a * = 0.9 predict too much emission above 10 keV (regardless of accre-tion rate and inclination), an energy band where observed ULX spectra drop much more steeply (e.g., Bachetti 2016;Bachetti et al 2013;Rana et al 2015;Walton et al 2014Walton et al , 2013. Clearly, further work needs to be done to produce a grid of simulations over the full range of spins and mass accretion rates, but our first results are encouraging.…”

Section: X-ray Spectramentioning

confidence: 67%

Spectra of black hole accretion models of ultraluminous X-ray sources

Narayan

Sądowski

Soria

2017

Monthly Notices of the Royal Astronomical Society

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We present general relativistic radiation MHD simulations of super-Eddington accretion on a 10M black hole. We consider a range of mass accretion rates, black hole spins, and magnetic field configurations. We compute the spectra and images of the models as a function of viewing angle, and compare them with the observed properties of ultraluminous X-ray sources (ULXs). The models easily produce apparent luminosities in excess of 10 40 erg s −1 for pole-on observers. However, the angle-integrated radiative luminosities rarely exceed 2.5 × 10 39 erg s −1 even for mass accretion rates of tens of Eddington. The systems are thus radiatively inefficient, though they are energetically efficient when the energy output in winds and jets is also counted. The simulated models reproduce the main empirical types of spectra -disk-like, supersoft, soft, hard -observed in ULXs. The magnetic field configuration, whether MAD ("magnetically arrested disk") or SANE ("standard and normal evolution"), has a strong effect on the results. In SANE models, the X-ray spectral hardness is almost independent of accretion rate, but decreases steeply with increasing inclination. MAD models with non-spinning black holes produce significantly softer spectra at higher values ofṀ , even at low inclinations. MAD models with rapidly spinning black holes are quite different from all other models. They are radiatively efficient (efficiency factor ∼ 10 − 20%), super-efficient when the mechanical energy output is also included (70%), and produce hard blazar-like spectra. In all models, the emission shows strong geometrical beaming, which disagrees with the more isotropic illumination favoured by observations of ULX bubbles.

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Section: X-ray Spectramentioning

confidence: 67%

Spectra of black hole accretion models of ultraluminous X-ray sources

Narayan

Sądowski

Soria

2017

Monthly Notices of the Royal Astronomical Society

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“…The class of ultraluminous X-ray sources (ULX) have Lx ∼ 10 39 − 10 42 erg s −1 (Swartz et al 2004), and it is thought that some of these are powered by pulsars, stellarmass black holes, and possibly intermediate-mass black holes for the most luminous of them (Mezcua et al 2013;Earnshaw et al 2016;Mezcua et al 2016;Bachetti 2016). The pulsars and stellar-mass black holes are associated with highmass star formation, and hence with molecular clouds.…”

Section: Discussionmentioning

confidence: 99%

Non-equilibrium chemistry and destruction of CO by X-ray flares

Mackey

Walch

Seifried

et al. 2019

Monthly Notices of the Royal Astronomical Society

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Sources of X-rays such as active galactic nuclei and X-ray binaries are often variable by orders of magnitude in luminosity over timescales of years. During and after these flares the surrounding gas is out of chemical and thermal equilibrium. We introduce a new implementation of X-ray radiative transfer coupled to a time-dependent chemical network for use in 3D magnetohydrodynamical simulations. A static fractal molecular cloud is irradiated with X-rays of different intensity, and the chemical and thermal evolution of the cloud are studied. For a simulated 10 5 M fractal cloud, an X-ray flux < 0.01 erg cm −2 s −1 allows the cloud to remain molecular, whereas most of the CO and H 2 are destroyed for a flux of ≥ 1 erg cm −2 s −1 . The effects of an X-ray flare, which suddenly increases the X-ray flux by 10 5 × are then studied. A cloud exposed to a bright flare has 99% of its CO destroyed in 10-20 years, whereas it takes > 10 3 years for 99% of the H 2 to be destroyed. CO is primarily destroyed by locally generated far-UV emission from collisions between non-thermal electrons and H 2 ; He + only becomes an important destruction agent when the CO abundance is already very small. After the flare is over, CO re-forms and approaches its equilibrium abundance after 10 3 -10 5 years. This implies that molecular clouds close to Sgr A in the Galactic Centre may still be out of chemical equilibrium, and we predict the existence of clouds near flaring X-ray sources in which CO has been mostly destroyed but H is fully molecular.

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“…Nevertheless, ultra-luminous X-ray sources (ULXs) are X-ray sources with luminosity above the Eddington limit cannot be explained by the conventional idea of normal stellar mass black hole. Therefore, the basic nature of ultraluminous X-ray pulsars are [26,27], as of now, remains unsolved [28]. However, different models have been suggested for the ULX-pulsars including magnetic field of different strength, but none of them are conventional one.…”

Section: Arxiv:191109546v1 [Gr-qc] 20 Nov 2019mentioning

confidence: 99%

“…The plot depicts the energy conditions for the inequalities Eqs (26)(27)(28)(29). as a function of the radial coordinate r. The curves for null energy condition (NEC), weak energy condition (WEC), strong energy condition (SEC), dominant energy condition (DEC) are shown in this figures, for the compact star Vela X-1.…”

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confidence: 99%

Analytical model of dark energy stars

2020

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In this article we study the structure and stability of compact astrophysical objects which are ruled by the dark energy equation of state (EoS). The existence of dark energy is important for explaining the current accelerated expansion of the universe. Exact solutions to Einstein field equations (EFE) have been found by considering particularized metric potential, Finch and Skea ansatz [1]. The obtained solutions are relevant to the explanation of compact fluid sphere. Further, we have observed at the junction interface, the interior solution is matched with the Schwarzschild's exterior vacuum solution. Based on that, we have noticed the obtained solutions are well in agreement with the observed maximum mass bound of ≈ 2M , namely, PSR J1416-2230, Vela X-1, 4U 1608-52, Her X-1 and PSR J1903+327, whose predictable masses and radii are not compatible with the standard neutron star models. Also, the stability of the stellar configuration has been discussed briefly, by considering the energy conditions, surface redshift, compactness, mass-radius relation in terms of the state parameter (ω). Finally, we demonstrate that the features so obtained are physically acceptable and consistent with the observed/reported data [2,3]. Thus, the present dark energy equation of state appears talented regarding the presence of several exotic astrophysical matters.

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Ultraluminous X‐ray sources: Three exciting years

Cited by 24 publications

References 85 publications

Spectra of black hole accretion models of ultraluminous X-ray sources

Spectra of black hole accretion models of ultraluminous X-ray sources

Non-equilibrium chemistry and destruction of CO by X-ray flares

Analytical model of dark energy stars

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