In recent years, the formation and evolution of rapidly accreting supermassive stars has received significant attention in the hope of better understanding the origin of high redshift quasars. It is often taken for granted that once formed, these supermassive stars will encounter the general relativistic radial instability and collapse to form massive black holes. Here, we present the first ever general relativistic hydrodynamical simulations of the collapse of rapidly accreting supermassive stars. We find that black hole formation is in many cases prevented by nuclear burning due to the long timescales of the collapse of these stars ($10^6$ s). Consequently, this is a novel astrophysical site for hot CNO burning and hydrogen burning via proton captures. For Pop III accreting supermassive stars, we find that only stars with very high (100 $\rm{M_\odot}/$yr) or low (0.1 $\rm{M_\odot}/$yr) accretion rates can form black holes, with models in between undergoing energetic thermonuclear pulsations. The final fate of these pulsating models may be to undergo subsequent pulsations or explosions or to collapse to black holes. For metal rich accreting supermassive stars ($Z\geq 0.1 \rm{Z_\odot}$), we do not find any black hole formation, with some models undergoing extremely energetic explosions ($10^{55}$ ergs). Our results invite further study on the formation of massive black holes from rapidly accreting supermassive stars which have reached the general relativistic radial instability.
Nova explosions are caused by global thermonuclear runaways triggered in the surface layers of accreting white dwarfs. It has been predicted that localised thermonuclear bursts on white dwarfs can also take place, similar to Type I X-ray bursts observed in accreting neutron stars. Unexplained rapid bursts from the binary system TV Columbae, in which mass is accreted onto a moderately-strong magnetised white dwarf from a low-mass companion, have been observed on several occasions in the past $\approx40$ years. During these bursts the optical/UV luminosity increases by a factor of $>3$ in less t…
In low-mass X-ray binaries, the accretion of stellar material onto a neutron star can fuel unstable thermonuclear flashes known as Type I X-ray bursts. Simulating these events using computational models can provide valuable information about the nature of the accreting system. One-dimensional (1D) astrophysics codes with large nuclear reaction networks are the current state-of-the-art for simulating X-ray bursts. These codes can track the evolution of isotopes through thousands of nuclear reaction pathways, to predict the released nuclear energy and final composition of the ashes. In this thes…
Possible manifestations of accreting magnetars are discussed. It is shown that the four ultra-luminous X-ray pulsars can be understood in the accreting low magnetic field magnetar scenario. The NGC300 ULX1 pulsar may have a higher dipole magnetic field than other sources. General constraint on their mass accretion rate confirmed their super-Eddington nature. Lower limits on their beaming factor are obtained. They do not seem to have strong beaming. The duty cycle of the ULX burst state can also be constrained by their timing observations. ULX pulsars may be in accretion equilibrium in the long…
Accreting millisecond X-ray pulsars (AMXPs) show burst oscillations during thermonuclear explosions of the accreted plasma which are markedly different from those observed in non-pulsating low mass X-ray binaries. The AMXP XTE J1814-338 is known for having burst oscillations that are phase locked (constant phase difference) and coincident with the accretion powered pulsations during all its thermonuclear bursts but the last one. In this work we use a coherent timing analysis to investigate this phenomenon in more detail and with higher time resolution than was done in the past. We confirm that…
The millisecond pulsar MAXI J1816-195 was recently discovered in an outburst by MAXI in 2022 May. We study different properties of the pulsar using data from NuSTAR and NICER observations. The unstable burning of accreted material on the surface of neutron stars induces thermonuclear (Type-I) bursts. Several such thermonuclear bursts have been detected by MAXI J1816-195 during its outburst. We investigate the evolution of the burst profile with flux and energy using NuSTAR and NICER observations. During the NuSTAR observation, a total of four bursts were detected from the source. The duration …
We report on the Neutron Star Interior Composition Explorer (NICER) monitoring campaign of the 468 Hz accreting millisecond X-ray pulsar IGR J17379-3747. From a detailed spectral and timing analysis of the coherent pulsations we find that they show a strong energy dependence, with soft thermal emission lagging about 640 microseconds behind the hard, Comptonized emission. Additionally, we observe uncommonly large pulse fractions, with measured amplitudes in excess of 20% sinusoidal fractional amplitude across the NICER passband and fluctuations of up to ~70%. Based on a phase-resolved spectral …
In this paper we present a coherent timing analysis of the 401 Hz pulsations of the accreting millisecond X-ray pulsar SAX J1808.4-3658 during its 2019 outburst. Using observations collected with the Neutron Star Interior Composition Explorer (NICER), we establish the pulsar spin frequency and orbital phase during its latest epoch. We find that the 2019 outburst shows a pronounced evolution in pulse phase over the course of the outburst. These phase shifts are found to correlate with the source flux, and are interpreted in terms of hot-spot drift on the stellar surface, driven by changes in th…
Two \textit{NuSTAR} observations of the luminous X-ray pulsar LMC X-4 in October and November 2015 captured several bright accretion flares from this source, which has a long history of stable pulse and superorbital behavior. We present a timing analysis of these data in which we detect a rapid pulse turn-on in association with the accretion flares, during which the source reaches super-Eddington luminosities. Pulsations, which are normally seen from this source, are found to only occur for approximately one hour before and during the bright flares. Beyond one hour before and after the flares,…
Many distinct classes of high-energy variability have been observed in astrophysical sources, on a range of timescales. The widest range (spanning microseconds-decades) is found in accreting, stellar-mass compact objects, including neutron stars and black holes. Neutron stars are of particular observational interest, as they exhibit surface effects giving rise to phenomena (thermonuclear bursts and pulsations) not seen in black holes. Here we briefly review the present understanding of thermonuclear (type-I) X-ray bursts. These events are powered by an extensive chain of nuclear reactions, whi…
The excess of the rate of type I X-ray bursts over that expected when the matter fallen between bursts completely burns out in a thermonuclear explosion is explained in terms of the model of a spreading layer of matter coming from the accretion disk over the neutron star surface. Such excess is observed in bursters with a high persistent luminosity, $4\times 10^{36}\ \mbox{erg s}^{-1}\leq L_{X}\leq 2\times 10^{37}\ \mbox{erg s}^{-1}$. In this model the accreting matter settles to the stellar surface mainly in two high-latitude ring zones. Despite the subsequent spreading of matter over the ent…
Millisecond spinning, low magnetic field neutron stars are believed to attain their fast rotation in a 0.1-1 Gyr-long phase during which they accrete matter endowed with angular momentum from a low-mass companion star. Despite extensive searches, coherent periodicities originating from accreting neutron star magnetospheres have been detected only at X-ray energies and in ~10% of the presently known systems. Here we report the detection of optical and ultraviolet coherent pulsations at the X-ray period of the transient low mass X-ray binary system SAX J1808.4-3658, during an accretion outburst …
Runaway thermonuclear burning of a layer of accumulated fuel on the surface of a compact star provides a brief but intense display of stellar nuclear processes. For neutron stars accreting from a binary companion, these events manifest as thermonuclear (type-I) X-ray bursts, and recur on typical timescales of hours to days. We measured the burst rate as a function of accretion rate, from seven neutron stars with known spin rates, using a burst sample accumulated over several decades. At the highest accretion rates, the burst rate is lower for faster spinning stars. The observations imply that …
Detection of low-luminosity X-ray pulsations from the accreting millisecond pulsar IGR J17511-3057: An ever-thinning thread between bright accretion and sub-luminous states
INAF–Osservatorio Astronomico di Brera（意大利国家天体物理研究所布雷拉天文台）；INAF-Osservatorio Astronomico di Roma（意大利国家天体物理研究所罗马天文台）；Institute of Space Sciences (ICE, CSIC)（空间科学研究所（加泰罗尼亚高等研究理事会））；Institut d’Estudis Espacials de Catalunya (IEEC)（加泰罗尼亚空间研究院）；INAF, Istituto di Astrofisica Spaziale e Fisica Cosmica（意大利国家天体物理研究所空间天体物理与宇宙学研究所）；ASI - Agenzia Spaziale Italiana（意大利航天局）
Impact of Accretion Assumptions on Pulse Profile Modelling of Superburst Oscillations in 4U 1636-536
Anton Pannekoek Institute for Astronomy, University of Amsterdam（阿姆斯特丹大学安东·潘内科克天文研究所）；Independent researcher（独立研究者）；IRAP, CNRS（法国国家科学研究中心天体物理学研究所）；Université de Toulouse, CNES, UPS-OMP（图卢兹大学，法国国家空间研究中心，观测宇宙学研究所）；Center for Relativistic Astrophysics, School of Physics, Georgia Institute of Technology（佐治亚理工学院物理学院相对论天体物理中心）；European Space Agency, ESTEC（欧洲空间局欧洲空间研究与技术中心）；Columbia Astrophysics Laboratory, Columbia University（哥伦比亚大学哥伦比亚天体物理实验室）；Institut für Astronomie und Astrophysik, Kepler Center for Astro and Particle Physic（天文与天体物理研究所，开普勒天体物理与粒子物理中心）
A semi-coherent search for optical pulsations from Scorpius X-1
INAF – Osservatorio Astronomico di Roma（意大利国家天体物理研究所罗马天文台）；INAF – Osservatorio Astronomico di Brera（意大利国家天体物理研究所布雷拉天文台）；INFN, Sezione di Roma（意大利国家核物理研究所罗马分部）；Università di Roma ‘La Sapienza’（罗马第一大学）；University of Oxford（牛津大学）；Fundación Galileo Galilei – INAF（伽利略基金会-意大利国家天体物理研究所）
In the last 20 years our understanding of the millisecond pulsar (MSP) population changed dramatically. Thanks to RXTE, we discovered that neutron stars in LMXBs spins at 200-750 Hz frequencies, and indirectly confirmed the recycling scenario, according to which neutron stars are spun up to ms periods during the LMXB-phase. In the meantime, the continuous discovery of rotation-powered MSPs in binary systems in the radio and gamma-ray band (mainly with the Fermi LAT) allowed us to classify these sources into two "spiders" populations, depending on the mass of their companion stars: Black Widow,…
The superslow pulsation X-ray pulsars in high mass X-ray binaries
在高质量X射线双星（HMXB）中存在一类特殊的X射线双星，它们的$P_{\rm spin}>1000$ s脉动非常慢。我们用积分观测研究了这些超低脉动中子星双星在硬X射线波段的时间和光谱性质。长期监测观测发现了两个源的自旋周期演化：4U 2206+54（$P_{\rm spin}\sim 5560$ s with $\dot{P}_{\rm spin}\sim 4.9\times 10^{-7}$ s s $^{-1}$）的自旋下降趋势和2S 0114+65（$P_{\rm spin}\sim 9600$ s with $\dot{P}_{\rm spin}\sim -1\times 10^{-6}$ s $^{-1}$）的长期自旋上升趋势。一个Be X射线瞬变，SXP 1062（$P_{\rm spin}\sim 1062$ s），也显示了一个快速的自旋下降率$\dot{P}_{\rm spin}\sim 3\times 10^{-6}$ s $^{-1}$在爆发。在标准的X射线双星演化模型中，除非中子星具有更强的表面磁场（$B>10^{14}$ G），否则这些超低速脉动中子星是不可能产生的。超低速自旋周期的物理起源仍然不清楚。讨论了超低速脉动X射线暴的可能成因和演化通道。超低速脉动X射线双星可能是较年轻的X射线双星系统，仍处于最终平衡态之前的快速演化阶段。或者，它们可能是一类新…
Supermassive black holes might form by direct collapse, with a supermassive star (SMS) as progenitor. In this scenario, the SMS accretes at > 0.1 Msun/yr until it collapses into a massive black hole seed due to the general-relativistic (GR) instability. However, the exact mass at which the collapse occurs is not known, as existing numerical simulations give divergent results. Here, this problem is addressed analytically, which allows for ab initio, reliable determination of the onset point of the GR instability, for given hydrostatic structures. We apply the relativistic equation of radial pul…
We report the detection of an infrared burst lagging a thermonuclear Type I X-ray burst from the accreting neutron star 4U 1728-34 (GX 354-0). Observations were performed simultaneously with XMM-Newton (0.7-12 keV), NuSTAR (3-79 keV) and HAWK-I@VLT (2.2$μ$m). We measure a lag of $4.75 \pm 0.5$ s between the peaks of the emission in the two bands. Due to the length of the lag and the shape of the IR burst, we found that the most plausible cause for such a large delay is reprocessing of the Type I burst X-rays by the companion star. The inferred distance between the neutron star and the companio…
We present a comprehensive X-ray study of the neutron star low-mass X-ray binary IGR J17062-6143, which has been accreting at low luminosities since its discovery in $2006$. Analysing NuSTAR, XMM-Newton and Swift observations, we investigate the very faint nature of this source through three approaches: modelling the relativistic reflection spectrum to constrain the accretion geometry, performing high-resolution X-ray spectroscopy to search for an outflow, and searching for the recently reported millisecond X-ray pulsations. We find a strongly truncated accretion disk at $77^{+22}_{-18}$ gravi…
We present the discovery of 528.6 Hz pulsations in the new X-ray transient MAXI J1816-195. Using NICER, we observed the first recorded transient outburst from the neutron star low-mass X-ray binary MAXI J1816-195 over a period of 28 days. From a timing analysis of the 528.6 Hz pulsations, we find that the binary system is well described as a circular orbit with an orbital period of 4.8 hours and a projected semi-major axis of 0.26 light-seconds for the pulsar, which constrains the mass of the donor star to $0.10-0.55 M_\odot$. Additionally, we observed 15 thermonuclear X-ray bursts showing a g…
We present a new method of matching observations of Type I (thermonuclear) X-ray bursts with models, comparing the predictions of a semi-analytic ignition model with X-ray observations of the accretion-powered millisecond pulsar SAX J1808.4--3658 in outburst. We used a Bayesian analysis approach to marginalise over the parameters of interest and determine parameters such as fuel composition, distance/anisotropy factors, neutron star mass and neutron star radius. Our study includes a treatment of the system inclination effects, inferring that the rotation axis of the system is inclined $\left(6…
The detection of coherent X-ray pulsations at ~314 Hz (3.2 ms) classifies MAXI J1957+032 as a fast-rotating, accreting neutron star. We present the temporal and spectral analysis performed using NICER observations collected during the latest outburst of the source. Doppler modulation of the X-ray pulsation revealed the ultra-compact nature of the binary system characterised by an orbital period of ~1 hour and a projected semi-major axis of 14 lt-ms. The neutron star binary mass function suggests a minimum donor mass of 1.7e-2 Msun, assuming a neutron star mass of 1.4 Msun and a binary inclinat…
The study of transiently accreting neutron stars provides a powerful means to elucidate the properties of neutron star crusts. We present extensive numerical simulations of the evolution of the neutron star in the transient low-mass X-ray binary MAXI J0556--332. We model nearly twenty observations obtained during the quiescence phases after four different outbursts of the source in the past decade, considering the heating of the star during accretion by the deep crustal heating mechanism complemented by some shallow heating source. We show that cooling data are consistent with a single source …
