The recent discovery of pulsating ultra-luminous X-ray sources (ULXs) shows that the apparent luminosity of accreting neutron stars (NSs) can exceed the Eddington luminosity by a factor of hundreds. The relation between the actual and apparent luminosity is a key ingredient in theoretical models of ULXs but it is still under debate. A typical feature of the discovered pulsating ULXs is a large pulsed fraction (PF). Using Monte Carlo simulations, we consider a simple geometry of accretion flow and test the possibility of simultaneous presence of a large luminosity amplification due the geometrical beaming and a high PF. We argue that these factors largely exclude each other and only a negligible fraction of strongly beamed ULX pulsars can show PF above 10 per cent. Discrepancy between this conclusion and current observations indicate that pulsating ULXs are not strongly beamed and their apparent luminosity is close to the actual one.
The presence or lack of eclipses in the X-ray lightcurves of ultraluminous X-ray sources (ULXs) can be directly linked to the accreting system geometry. In the case where the compact object is stellar mass and radiates isotropically, we should expect eclipses by a main-sequence to sub-giant secondary star on the recurrence timescale of hours to days. X-ray lightcurves are now available for large numbers of ULXs as a result of the latest XMM-Newton catalogue. We determine the amount of fractional variability that should be injected into an otherwise featureless lightcurve for a given set of sys…
How Beaming Shapes the Demographics of Ultraluminous X-ray Sources?
School of Astronomy and Space Science, Nanjing University（南京大学天文与空间科学学院）；Key Laboratory of Modern Astronomy and Astrophysics (Nanjing University), Ministry of Education（教育部现代天文与天体物理重点实验室（南京大学））
The discovery of pulsating ultra-luminous X-ray sources (PULX) suggests that neutron stars are presumably common within the ultra-luminous X-ray source (ULX) population though the majority of the population members are currently lacking pulsations. These systems are likely to host neutron stars accreting mass at super-Eddington (super-critical) rates from their massive companion in high-mass X-ray binaries. Taking into account the spherization of the accretion flow in the super-critical regime, the beaming of X-ray emission, and the reduction of the scattering cross-section in a strong magneti…
Extreme accretion in X-ray pulsars (XRPs) results in radiation-driven outflows launched from the inner parts of the accretion disc. The outflows affect the apparent luminosity of the XRPs and their pulsations through the geometrical beaming. We model processes of geometrical beaming and pulse formation using Monte Carlo simulations. We confirm our earlier statement that strong amplification of luminosity due to the collimation of X-ray photons is inconsistent with a large pulsed fraction. Accounting for relativistic aberration due to possibly high outflow velocity ($\sim 0.2c$) does not affect…
Ultra-luminous X-ray sources (ULXs) are off-nuclear point sources in nearby galaxies with luminosities well exceeding the Eddington limit for stellar-mass objects. It has been recognized after the discovery of pulsating ULXs (PULXs) that a fraction of these sources could be accreting neutron stars in high-mass X-ray binaries (HMXBs) though the majority of ULXs are lacking in coherent pulsations. The earliest stage of some HMXBs may harbor rapidly rotating neutron stars propelling out the matter transferred by the massive companion. The spin-down power transferred by the neutron-star magnetosph…
A Phase-resolved View of Millihertz Quasi-periodic Oscillations in the Ultraluminous X-ray Source M51 ULX-7: Evidence for a Magnetically Truncated Disk and Geometrical Beaming
Ultra-luminous X-ray sources (ULXs) are those X-ray sources located away from the centre of their host galaxy with luminosities exceeding the Eddington limit of a stellar-mass black hole ($L_X>10^{39}\;{\rm erg\,s}^{-1}$). The discovery of X-ray pulsations in some of these objects (e.g. M82~X-2) suggests that a certain fraction of the ULX population may have a neutron star accretor. We present systematic modelling of low- and intermediate-mass X-ray binaries (LMXBs and IMXBs; donor-star mass range $0.92$--$8.0$~M$_{\odot}$ and neutron-star accretors) to explain the formation of this sub-popula…
Ultra-luminous X-ray sources (ULXs) are a class of extra-galactic, point-like, off-nuclear X-ray sources with X-ray luminosity from $\sim 10^{39}$~erg s$^{-1}$ to $10^{41}$~erg s$^{-1}$. We investigated the temporal and broadband X-ray spectral properties of the ULX M81~X--6 using simultaneous \textit{Suzaku} and \textit{NuSTAR} observations. To understand the nature of the source, we searched for pulsating signals from the source using the \textit{NuSTAR} observation. However, we failed to identify any strong pulsating signals from the source. Alternatively, the broadband spectral modelling w…
Motivated by the recent discoveries that six Ultraluminous X-ray Sources (ULXs) are powered by highly super-Eddington X-ray pulsars, we searched for additional pulsating ULX (PULX) candidates by identifying sources that exhibit long-term flux variability of at least an order of magnitude (a common feature seen in the 6 known PULXs, which may potentially be related to transitions to the propeller regime). Expanding on previous studies, we used the available fluxes from XMM-Newton, Swift and Chandra, along with carefully computed upper limits in cases of a non-detection, to construct long-term l…
The hunt for new pulsating ultraluminous X-ray sources: a clustering approach
Department of Electronics, Information and Bioengineering, Politecnico di Milano（电子工程与信息学院，米兰理工学院）；INAF--Osservatorio Astronomico di Roma（意大利国家天文物理研究所罗马天文台）；Dipartimento di Fisica, Università degli Studi di Roma ``Tor Vergata''（罗马Tor Vergata大学物理系）；INAF, Istituto di Astrofisica Spaziale e Fisica Cosmica（意大利国家天文物理研究所空间天体物理与宇宙物理研究所）；Institut d'Estudis Espacials de Catalunya (IEEC)（加泰罗尼亚空间研究机构）；Institute of Space Sciences (ICE, CSIC)（空间科学研究所（ICE, CSIC））；Ciela – Montreal Institute for Astrophysical Data Analysis and Machine Learning（蒙特利尔天体物理数据与机器学习研究所）
We report here the discovery of NGC 7793 ULX-4, a new transient ultraluminous X-ray source (ULX) in NGC 7793, a spiral galaxy already well known for harbouring several ULXs. This new source underwent an outburst in 2012, when it was detected by \textit{XMM-Newton} and the \textit{Swift} X-ray telescope. The outburst reached a peak luminosity of 3.4$\times 10^{39}$ erg\ s$^{-1}$ and lasted for about 8 months, after which the source went below a luminosity of $10^{37}$ erg\ s$^{-1}$; previous \textit{Chandra} observations constrain the low-state luminosity below $\sim$ 2$\times 10^{36}$ erg\ s$^…
We have obtained near-simultaneous Swift/XRT imaging and Gemini GMOS spectroscopy for the ultraluminous X-ray source (ULX) NGC~300 ULX-1 (formerly designated SN~2010da). The observed X-ray emission is consistent with an inhomogeneous wind that partially obscures a central, bright inner accretion disk. We simultaneously fit eleven 0.3-10 keV spectra obtained over a $\sim$1 year time period (2016 April to 2017 July) using the same partial covering model, and find that although the covering fraction varies significantly (from 78% to consistent with 0%), the unabsorbed luminosity remains essential…
Neutrinos from hidden ultraluminous X-ray sources in the Galaxy
Instituto Argentino de Radioastronomía (CCT La Plata, CONICET（阿根廷射电天文研究所）；CICPBA（拉普拉塔国立大学天文与地球物理学院）；UNLP)（马德普拉塔物理研究所）；Facultad de Ciencias Astronómicas y Geofísicas, Universidad Nacional de La Plata（马德普拉塔国立大学理学院物理系）；Instituto de Investigaciones Físicas de Mar del Plata (IFIMAR – CONICET)；Departamento de Física, Facultad de Ciencias Exactas y Naturales, Universidad Nacional de Mar del Plata
M82 X-2 is the first pulsating ultraluminous X-ray source (PULX) discovered. The luminosity of these extreme pulsars, if isotropic, implies an extreme mass transfer rate. An alternative is to assume a much lower mass transfer rate, but with an apparent luminosity boosted by geometrical beaming. Only an independent measurement of the mass transfer rate can help discriminate between these two scenarios. In this Paper, we follow the orbit of the neutron star for seven years, measure the decay of the orbit ($\dot{P}_{orb}/{P}_{orb}\approx-8\cdot10^{-6}\mathrm{yr}^{-1}$), and argue that this orbita…
Pulsating ultraluminous X-ray sources (PULXs) are accreting pulsars with apparent X-ray luminosity exceeding $10^{39}\, \rm erg\ s^{-1}$. We perform Monte-Carlo simulations to investigate whether high collimation effect (or strong beaming effect) is dominant in the presence of accretion outflows, for the fan beam emission of the accretion column of the neutron stars in PULXs. We show that the three nearby PULXs (RX J0209.6$-$7427, Swift J0243.6+6124 and SMC X-3), namely the three musketeers here, have their main pulsed emission not strongly collimated even if strong outflows exist. This conclu…
