We have identified seven ultraluminous X-ray sources (ULXs) which are coincident with globular cluster candidates (GCs) associated with M87. ULXs in the old GC environment represent a new population of ULXs, and ones likely to be black holes. In this study we perform detailed X-ray spectroscopic followup to seven GC ULXs across a wealth of archival Chandra observations and long time baseline of 16 years. This study brings the total known sample of GC ULXs to 17. Two of these sources show variability in their X-ray luminosity of an order of magnitude over many years, and one of these sources shows intra-observational variability on the scale of hours. While the X-ray spectra of the majority of globular cluster ULXs are best fit by single component models, one of the sources studied in this paper is the second GC ULX to be best fit by a two component model. We compare this new sample of GC ULXs to the previously studied sample, and compare the X-ray and optical properties counterparts across the samples. We find that the clusters that host ULXs in M87 have metallicities from g-z=1.01 to g-z=1.70. The best-fit power-law indices of the X-ray spectra range from Gamma=1.37-2.21, and the best fit inner black-body disk temperatures range from kT=0.56-1.90 keV.
We have identified three ultraluminous X-ray sources (ULXs) hosted by globular clusters (GCs) within NGC 1316's stellar system. These discoveries bring the total number of known ULXs in GCs up to 20. We find that the X-ray spectra of the three new sources do not deviate from the established pattern of spectral behaviour of the other known GC ULXs. The consistency of the X-ray spectral behaviour for these sources points to multiple paths of formation and evolution mechanisms for these rare and unique sources. Using the now larger sample of GC ULXs, we compare the optical properties of the entir…
A number of ultraluminous X-ray sources (ULXs) are physically associated with extragalactic globular clusters (GCs). We undertake a systematic X-ray analysis of eight of the brightest of these sources. We fit the spectra of the GC ULXs to single power law and single disk models. We find that the data never require that any of the sources change between a disk and a power law across successive observations. The GC ULXs best fit by a single disk show a bimodal distribution: they either have temperatures well below 0.5 keV, or variable temperatures ranging above 0.5 keV up to 2~keV. The GC ULXs w…
Ultraluminous X-ray Sources (ULXs) in globular clusters are low mass X-ray binaries that achieve high X-ray luminosities through a currently uncertain accretion mechanism. Using archival Chandra and Hubble Space Telescope observations, we perform a volume-limited search ($\lesssim$ 70 Mpc) of 21 of the most massive ($>10^{11.5} M_\odot$) early-type galaxies to identify ULXs hosted by globular cluster (GC) candidates. We find a total of 34 ULX candidates above the expected background within 5 times the effective radius of each galaxy, with 10 of these ($\sim29.4\%$) potentially hosted by a GC. …
Ultraluminous pulsars are a definite proof that persistent super-Eddington accretion occurs in nature. They support the scenario according to which most Ultraluminous X-ray Sources (ULXs) are super-Eddington accretors of stellar mass rather than sub-Eddington intermediate mass black holes. An important prediction of theories of supercritical accretion is the existence of powerful outflows of moderately ionized gas at mildly relativistic speeds. In practice, the spectral resolution of X-ray gratings such as RGS onboard XMM-Newton is required to resolve their observational signatures in ULXs. Us…
A Newly Identified Transient Ultraluminous X-ray Source in M31
California Institute of Technology（加州理工学院）；University of Illinois at Urbana-Champaign（伊利诺伊大学厄巴纳-香槟分校）；Université de Strasbourg（斯特拉斯堡大学）；CNRS（法国国家科学研究中心）；Observatoire astronomique de Strasbourg（斯特拉斯堡天文台）；University of Hertfordshire（赫特福德大学）；University of Southampton（南安普顿大学）
A comparative study of occurrence rates and nature of Ultraluminous X-ray sources in spiral and elliptical galaxies
Manipal Centre for Natural Sciences, Manipal Academy of Higher Education（马尼帕尔高等教育部马尼帕尔自然科学中心）；Space Astronomy Group, U R Rao Satellite Center, ISITE Campus（印度空间研究组织乌尔·拉奥卫星中心空间天文学组）
NGC 4472 is home to five ultraluminous X-ray sources hosted by globular clusters. These sources have been suggested as good black hole candidates in extragalactic globular clusters$-$ a highly sought after population that may provide observational information regarding the progenitors of merging black hole binaries. In this body of work, we present X-ray and optical follow up to one of these sources, CXOUJ1229410+075744 (GCU1). We find no evidence of [OIII] optical emission in GCU1 which indicates a lack of significant evidence for super-Eddington outflows, unlike what is seen in a handful of …
We report the discovery of a new ultraluminous X-ray source (ULX) 2XMM J125048.6+410743 within the spiral galaxy M94. The source has been observed by ROSAT, Chandra, and XMM-Newton on several occasions, exhibiting as a highly variable persistent source or a recurrent transient with a flux variation factor of >100, a high duty cycle (at least ~70%), and a peak luminosity of Lx ~ 2X10^{39} erg/s (0.2-10 keV, absorbed). In the brightest observation, the source is similar to typical low-luminosity ULXs, with the spectrum showing a high-energy cutoff but harder than that from a standard accretion d…
We combined Chandra ACIS observations of the globular cluster 47 Tucanae (hereafter, 47 Tuc) from 2000, 2002, and 2014-15 to create a deeper X-ray source list, and study some of the faint radio millisecond pulsars (MSPs) present in this cluster. We have detected 370 X-ray sources within the half-mass radius (2$'$.79) of the cluster, 81 of which are newly identified, by including new data and using improved source detection techniques. The majority of the newly identified sources are in the crowded core region, indicating cluster membership. We associate five of the new X-ray sources with chrom…
Using the Chandra Source Catalog 2.0 and a newly compiled catalogue of galaxies in the local Universe, we deliver a census of ultraluminous X-ray source (ULX) populations in nearby galaxies. We find 629 ULX candidates in 309 galaxies with distance smaller than 40\,Mpc. The foreground/background contamination is ${\sim}20\%$. The ULX populations in bona-fide star-forming galaxies scale on average with star-formation rate (SFR) and stellar mass ($M_\star$) such that the number of ULXs per galaxy is $0.45^{+0.06}_{-0.09}\times\frac{\rm SFR}{\rm M_\odot\,yr^{-1}}{+}3.3^{+3.8}_{-3.2}\times\frac{M_\…
We derive the probability for a newly formed binary black hole (BBH) to undergo an eccentric gravitational wave (GW) merger during binary-single interactions inside a stellar cluster. By integrating over the hardening interactions such a BBH must undergo before ejection, we find that the observable rate of BBH mergers with eccentricity $>0.1$ at $10\ \rm{Hz}$ relative to the rate of circular mergers can be as high as $\sim 5\%$ for a typical globular cluster (GC). This further suggests that BBH mergers forming through GW captures in binary-single interactions, eccentric or not, are likely to c…
Motivated by the newly discovery of a spin period 1.83\,ms pulsar J1835$-$3259B in the globular cluster (GC) NGC~6652, we analyze the gamma-ray data obtained with the Large Area Telescope (LAT) onboard {\it Fermi Gamma-ray Space Telescope (Fermi)} for the GC and detect the pulsations of this millisecond pulsar (MSP) at a 5.4$σ$ confidence level (the weighted H-test value is $\sim 41$). From timing analysis of the data, a pulse profile that is similar to the radio one is established. We thus consider that we have detected the \gr\ emission of the MSP, and discuss the implications. Based on our …
We report on a Chandra Director's Discretionary Time observation of the globular cluster Terzan 5, carried out ~7 weeks after the cessation of the 2010 outburst of the newly discovered transiently accreting 11 Hz X-ray pulsar. We detect a thermal spectrum that can be fitted with a neutron star atmosphere model with a temperature for an observer at infinity of kT~100 eV, and a quiescent thermal bolometric luminosity of Lq~2E33 erg/s for an assumed distance of 5.5 kpc. The thermal emission is elevated above the quiescent base level measured in 2003 and 2009, i.e., prior to the recent accretion o…
We analyse 55 ks of Chandra X-ray observations of the Galactic globular cluster M13. Using the latest radio timing positions of six known millisecond pulsars (MSPs) in M13 from Wang et al. (2020), we detect confident X-ray counterparts to five of the six MSPs at X-ray luminosities of $L_X$(0.3-8 keV)$\sim 3 \times 10^{30} - 10^{31}~{\rm erg~s^{-1}}$, including the newly discovered PSR J1641+3627F. There are limited X-ray counts at the position of PSR J1641+3627A, for which we obtain an upper limit $L_X<1.3 \times 10^{30}~{\rm erg~s^{-1}}$. We analyse X-ray spectra of all six MSPs, which are we…
The nature of ultraluminous X-ray sources (ULXs) is not yet fully understood but it can be done in the most efficient way by analyzing its optical emission and environment. We present preliminary results of photometric and spectral studies of some newly identified ULX optical counterparts. Based on the X-ray catalog of ULX candidates by Liu 2011 we have unequivocally identified in the optical range more than 100 sources, for 49 of them we carried out a long slit spectroscopy. According to these results, the majority of the most brightest candidates (m$_V < 22^m$) turned out to be background ob…
A Chandra X-ray study of millisecond pulsars in the globular cluster Omega Centauri: a correlation between spider pulsar companion mass and X-ray luminosity
Based on the recently completed {\it Chandra}/ACIS survey of X-ray point sources in nearby galaxies, we study the X-ray luminosity functions (XLFs) for X-ray point sources in different types of galaxies and the statistical properties of ultraluminous X-ray sources (ULXs). Uniform procedures are developed to compute the detection threshold, to estimate the foreground/background contamination, and to calculate the XLFs for individual galaxies and groups of galaxies, resulting in an XLF library for 343 galaxies of different types. With the large number of surveyed galaxies, we have studied the XL…
We observed the globular cluster NGC 6652 with Chandra for 47.5 ks, detecting six known X-ray sources, as well as five previously undetected X-ray sources. Source A (XB 1832-330) is a well-known bright low-mass X-ray binary (LXMB). The second brightest source, B, has a spectrum that fits well to either a power-law model (Gamma ~ 1.3) or an absorbed hot gas emission model (kT ~ 34 keV). Its unabsorbed 0.5-10 keV luminosity (L_X = 1.6+-0.1*10^34 erg/s) is suggestive of a neutron star primary; however, Source B exhibits unusual variability for a LMXB, varying by over an order of magnitude on time…
We present a systematic study of periodic X-ray sources in the massive globular cluster 47 Tuc, utilizing deep archival Chandra observations that resolve the cluster core and recently available eROSITA observations that cover the cluster outskirt. By applying the Gregory-Loredo algorithm, we detect 20 periodic signals among 18 X-ray sources, ranging between 205-95731 second. Fourteen periods are newly discovered in the X-ray band. We classify these periodic sources into four quiescent low-mass X-ray binaries, one milli-second pulsar, two coronally-active binaries and eleven cataclysmic variabl…
We investigate low mass X-ray binaries (LMXBs) in the M31 globular cluster (GC) system using data from the 2XMMi catalogue. These X-ray data are based on all publicly available XMM-Newton observations of the galaxy. This new survey provides the most complete and homogeneous X-ray survey of M31's GCs to date, covering >80% of the confirmed old clusters in the galaxy. We associate 41 X-ray sources with confirmed old clusters in the M31 cluster catalogue of Peacock et al. (2010). Comparing these data with previous surveys of M31, it is found that three of these clusters are newly identified, incl…
A new Chandra look at the globular cluster NGC 6540 and its peculiar X-ray flaring source
INAF–Istituto di Astrofisica Spaziale e Fisica Cosmica di Milano（意大利国家天体物理研究所空间天体物理和宇宙物理米兰研究所）；Department of Physics and Astronomy, University of Alabama（阿拉巴马大学物理与天文系）
We present an analysis of the spin and orbital properties of the newly discovered accreting pulsar IGR J17480-2446, located in the globular cluster Terzan 5. Considering the pulses detected by the Rossi X-ray Timing Explorer at a period of 90.539645(2) ms, we derive a solution for the 21.27454(8) hr binary system. The binary mass function is estimated to be 0.021275(5) Msun, indicating a companion star with a mass larger than 0.4 Msun. The X-ray pulsar spins up while accreting at a rate of between 1.2 and 1.7E-12 Hz/s, in agreement with the accretion of disc matter angular momentum given the o…
Ultraluminous X-ray sources in the first eROSITA survey I. Candidate catalogs
Dr. Karl Remeis-Sternwarte and Erlangen Centre for Astroparticle Physics, Friedrich-Alexander Universität Erlangen-Nürnberg（弗里德里希-亚历山大埃尔朗根-纽伦堡大学卡尔·雷梅斯天文台和埃尔朗根高能天体物理中心）；Durham University（杜伦大学）；Institute of Astrophysics, Foundation for Research and Technology-Hellas（希腊研究基金会天体物理研究所）；Physics Department, & Institute of Theoretical and Computational Physics, University of Crete（克里特大学物理系及理论与计算物理研究所）；Max-Planck-Institut für Extraterrestrische Physik（马克斯·普朗克地外物理研究所）
We report on the discovery of coherent pulsations at a period of 2.9 ms from the X-ray transient MAXI J0911-655 in the globular cluster NGC 2808. We observed X-ray pulsations at a frequency of $\sim339.97$ Hz in three different observations of the source performed with XMM-Newton and NuSTAR during the source outburst. This newly discovered accreting millisecond pulsar is part of an ultra-compact binary system characterised by an orbital period of $44.3$ minutes and a projected semi-major axis of $\sim17.6$ lt-ms. Based on the mass function we estimate a minimum companion mass of 0.024 M$_{\odo…
We report the discovery of the second accreting millisecond X-ray pulsar (AMXP) in the globular cluster NGC 6440. Pulsations with a frequency of 205.89 Hz were detected with the Rossi X-Ray Timing Explorer on August 30th, October 1st and October 28th, 2009, during the decays of ~4 day outbursts of a newly X-ray transient source in NGC 6440. By studying the Doppler shift of the pulsation frequency, we find that the system is an ultra-compact binary with an orbital period of 57.3 minutes and a projected semi-major axis of 6.22 light-milliseconds. Based on the mass function, we estimate a lower l…
Globular clusters contain a unique pulsar population, with many exotic systems that can form only in their dense stellar environments. The leap in sensitivity of the upgraded Giant Metrewave Radio Telescope (uGMRT) in India, especially at low radio frequencies ($<$ 1 GHz) has motivated a new search for radio pulsars in a group of eight Southern globular clusters. We discovered PSR J1835$-$3259B, a 1.83-ms pulsar in NGC 6652; this is in a near-circular wide orbit of 28.7 hr with a low-mass ($ \sim 0.2 \, M_{\rm \odot}$) companion, likely a Helium white dwarf. We derived a 10-year timing solutio…
Globular clusters are dense stellar systems that have typical ages of ~13 billion years, implying that they formed at redshifts of z>~6. Massive stars in newly formed or primordial globular clusters could have played an important role during the epoch of cosmological reionisation (z>~6) as sources of energetic, neutral hydrogen ionising UV photons. We investigate whether or not these stars could have been as important in death as sources of energetic X-ray photons as they were during their main sequence lives. Most massive stars are expected to form in binaries, and an appreciable fraction of …
