Abstract
Ultraluminous X-ray sources (ULXs) are thought to be powerful X-ray binaries (XRBs) and may contribute significantly to the redshift-dependent X-ray emission from star-forming galaxies. We ...have assembled a uniform sample of 259 ULXs over the redshift range
z
= 0.002–0.51 to constrain their physical nature and their contribution to the cosmic X-ray background. The sample is constructed by crossmatching galaxies from the Sloan Digital Sky Survey with the Chandra Source Catalog and selecting off-nuclear X-ray sources after applying astrometric corrections. The fraction of contaminants is ∼30% and shows no evolution with redshift. The host-galaxy star formation rates (SFRs) are systematically elevated relative to the parent sample when matched in host stellar mass. The specific SFRs suggest a slight preference for high-mass XRBs, and the X-ray luminosity scaling relations with host-galaxy stellar mass and SFR indicate that the highest-redshift sources represent relatively luminous XRB populations that dominate their host-galaxy X-ray emission. The fraction of galaxies hosting at least one ULX of a given luminosity increases with redshift over the full range of our sample, as expected if ULXs are preferentially found in galaxies with high SFRs and low metallicities. At
z
∼ 0.5, the ULX X-ray flux is consistent with the X-ray emission from star-forming galaxies. Moreover, ULXs may account for up to ∼40% of the integrated flux from XRBs in the normal galaxy population out to
z
∼ 0.5, suggesting they may contribute significantly to the overall ionizing radiation from galaxies.
Abstract
M51 ULX7 is among a small group of known ultraluminous X-ray pulsars (ULXPs). The neutron star powering the source has a spin period of 2.8 s, orbits its companion star with a period of 2 ...days, and a superorbital period of 38 days is evident in its X-ray lightcurve. Here we present NuSTAR and XMM-Newton data on the source from 2019 obtained when the source was near its peak brightness. We detect the pulsations, having spun up at a rate of 3 ± 0.5 × 10
−10
s s
−1
since they were previously detected in 2018. The data also provide the first high-quality broadband spectrum of the source. We find it to be very similar to that of other ULXPs, with two disk-like components, and a high-energy tail. When combined with XMM-Newton data obtained in 2018, we explore the evolution of the spectral components with superorbital phase, finding that the luminosity of the hotter component drives the superorbital flux modulation. The inclination the disk components appear to change with phase, which may support the idea that these superorbital periods are caused by disk precession. We also reexamine the superorbital period with 3 yr of Swift/XRT monitoring, finding that the period is variable, increasing from 38.2 ± 0.5 days in 2018–2019 to 44.2 ± 0.9 days in 2020–2021, which rules out alternative explanations for the superorbital period.
We present a mid-infrared (IR) sample study of nearby ultraluminous X-ray sources (ULXs) using multiepoch observations with the Infrared Array Camera (IRAC) on the Spitzer Space Telescope. ...Spitzer/IRAC observations taken after 2014 were obtained as part of the Spitzer Infrared Intensive Transients Survey. Our sample includes 96 ULXs located within 10 Mpc. Of the 96 ULXs, 12 have candidate counterparts consistent with absolute mid-IR magnitudes of supergiants, and 16 counterparts exceeded the mid-IR brightness of single supergiants and are thus more consistent with star clusters or non-ULX background active galactic nuclei. The supergiant candidate counterparts exhibit a bimodal color distribution in a Spitzer/IRAC color-magnitude diagram, where "red" and "'blue" ULXs fall in IRAC colors 3.6 - 4.5 ∼ 0.7 and 3.6 - 4.5 ∼ 0.0, respectively. The mid-IR colors and absolute magnitudes of four "red" and five "blue" ULXs are consistent with those of supergiant Be (sgBe) and red supergiant (RSG) stars, respectively. Although "blue," RSG-like mid-IR ULX counterparts likely host RSG mass donors; we propose that "red" counterparts are ULXs exhibiting the "Be phenomenon" rather than hosts of sgBe mass donors. We show that the mid-IR excess from the "red" ULXs is likely due to thermal emission from circumstellar or circumbinary dust. Using dust as a probe for total mass, we estimate mass-loss rates of M ˙ ∼ 1 × 10 − 4 M yr−1 in dust-forming outflows of red ULXs. Based on the transient mid-IR behavior and its relatively flat spectral index, = −0.19 0.1, we suggest that the mid-IR emission from Holmberg IX X-1 originates from a variable jet.
We report the discovery of a third ULX in NGC 925 (ULX-3), detected in 2017 November by Chandra at a luminosity of LX = (7.8 0.8) × 1039 erg s−1. Examination of archival data for NGC 925 reveals that ...ULX-3 was detected by Swift at a similarly high luminosity in 2011, as well as by XMM-Newton in 2017 January at a much lower luminosity of LX = (3.8 0.5) × 1038 erg s−1. With an additional Chandra nondetection in 2005, this object demonstrates a high dynamic range of flux of factor 26. In its high-luminosity detections, ULX-3 exhibits a hard power-law spectrum with Γ = 1.6 0.1, whereas the XMM-Newton detection is slightly softer, with , and is also well-fitted with a broadened disk model. The long-term light curve is sparsely covered and could be consistent either with the propeller effect or with a large-amplitude superorbital period, both of which are seen in ULXs, in particular those with neutron star accretors. Further systematic monitoring of ULX-3 will allow us to determine the mechanism by which ULX-3 undergoes its extreme variability and to better understand the accretion processes of ULXs.
The Variability Behavior of NGC 925 ULX-3 Earnshaw, Hannah P.; Brightman, Murray; Harrison, Fiona A. ...
Astrophysical journal/The Astrophysical journal,
07/2022, Letnik:
934, Številka:
1
Journal Article
Recenzirano
Odprti dostop
Abstract
We report the results of a 2019–2021 monitoring campaign with Swift and associated target-of-opportunity observations with XMM-Newton and NuSTAR, examining the spectral and timing behavior ...of the highly variable ultraluminous X-ray source (ULX) NGC 925 ULX-3. We find that the source exhibits a 127–128-day periodicity, with fluxes typically ranging from 1 × 10
−13
to 8 × 10
−13
erg s
−1
cm
−2
. We do not find strong evidence for a change in period over the time that NGC 925 ULX-3 has been observed, although the source may have been in a much lower flux state when first observed with Chandra in 2005. We do not detect pulsations, and we place an upper limit on the pulsed fraction of ∼40% in the XMM-Newton band, consistent with some previous pulsation detections at low energies in other ULXs. The source exhibits a typical ULX spectrum that turns over in the NuSTAR band and can be fitted using two thermal components. These components have a high temperature ratio that may indicate the lack of extreme inner disk truncation by a magnetar-level magnetic field. We examine the implications for a number of different models for superorbital periods in ULXs, finding that a neutron star with a magnetic field of ∼10
12
G may be plausible for this source. The future detection of pulsations from this source would allow for the further testing and constraining of such models.
A Broadband Look at the Old and New ULXs of NGC 6946 Earnshaw, Hannah P.; Grefenstette, Brian W.; Brightman, Murray ...
Astrophysical journal/The Astrophysical journal,
08/2019, Letnik:
881, Številka:
1
Journal Article
Recenzirano
Odprti dostop
Two recent observations of the nearby galaxy NGC 6946 with NuSTAR, one simultaneous with an XMM-Newton observation, provide an opportunity to examine its population of bright accreting sources from a ...broadband perspective. We study the three known ultraluminous X-ray sources (ULXs) in the galaxy, and find that ULX-1 and ULX-2 have very steep power-law spectra with in both cases. Their properties are consistent with being super-Eddington accreting sources with the majority of their hard emission obscured and down-scattered. ULX-3 (NGC 6946 X-1) is significantly detected by both XMM-Newton and NuSTAR at LX = (6.5 0.1) × 1039 erg s−1, and has a power-law spectrum with Γ = 2.51 0.05. We are unable to identify a high-energy break in its spectrum like that found in other ULXs, but the soft spectrum likely hinders our ability to detect one. We also characterize the new source, ULX-4, which is only detected in the joint XMM-Newton and NuSTAR observation, at LX = (2.27 0.07) × 1039 erg s−1, and is absent in a Chandra observation 10 days later. It has a very hard cutoff power-law spectrum with Γ = 0.7 0.1 and keV. We do not detect pulsations from ULX-4, but its transient nature can be explained either as a neutron star ULX briefly leaving the propeller regime or as a micro-tidal disruption event induced by a stellar-mass compact object.
Abstract
Time series of spectroscopic, speckle-interferometric, and optical long-baseline-interferometric observations confirm that
ν
Gem is a hierarchical triple system. It consists of an inner ...binary composed of two B-type stars and an outer classical Be star. Several photospheric spectral lines of the inner components were disentangled, revealing two stars with very different rotational broadening (∼260 and ∼140 km s
−1
, respectively), while the photospheric lines of the Be star remain undetected. From the combined spectroscopic and astrometric orbital solution it is not possible to unambiguously cross identify the inner astrometric components with the spectroscopic components. In the preferred solution based on modeling of the disentangled line profiles, the inner binary is composed of two stars with nearly identical masses of 3.3
M
⊙
and the more rapidly rotating star is the fainter one. These two stars are in a marginally elliptical orbit (
e
= 0.06) about each other with a period of 53.8 days. The third star also has a mass of 3.3
M
⊙
and follows a more eccentric (
e
= 0.24) orbit with a period of 19.1 yr. The two orbits are codirectional, and at inclinations of 79° and 76° of the inner and the outer orbit, respectively, about coplanar. No astrometric or spectroscopic evidence could be found that the Be star itself is double. The system appears dynamically stable and not subject to eccentric Lidov–Kozai oscillations. After disentangling, the spectra of the components of the inner binary do not exhibit peculiarities that would be indicative of past interactions. Motivations for a wide range of follow-up studies are suggested.
Ultraluminous X-ray sources (ULXs) are thought to be powerful X-ray binaries (XRBs) and may contribute significantly to the redshift-dependent X-ray emission from star forming galaxies. We have ...assembled a uniform sample of 259 ULXs over the redshift range z=0.002-0.51 to constrain their physical nature and their contribution to the Cosmic X-Ray Background (CXB). The sample is constructed by crossmatching galaxies from the Sloan Digital Sky Survey with the Chandra Source Catalog and selecting off-nuclear X-ray sources after applying astrometric corrections. The fraction of contaminants is ~30% and shows no evolution with redshift. The host galaxy star formation rates (SFRs) are systematically elevated relative to the parent sample when matched in host stellar mass. The specific SFRs suggest a slight preference for high-mass XRBs, and the X-ray luminosity scaling relations with host galaxy stellar mass and SFR indicate that the highest redshift sources represent relatively luminous XRB populations that dominate their host galaxy X-ray emission. The fraction of galaxies hosting at least one ULX of a given luminosity increases with redshift over the full range of our sample, as expected if ULXs are preferentially found in galaxies with high SFRs and low metallicities. At z~0.5, the ULX X-ray flux is consistent with the X-ray emission from star-forming galaxies. Moreover, ULXs may account for up to ~40% of the integrated flux from XRBs in the normal galaxy population out to z~0.5, suggesting they may contribute significantly to the overall ionizing radiation from galaxies.
In 2010 May, an intermediate luminosity optical transient was discovered in the nearby galaxy NGC 300 by a South African amateur astronomer. In the decade since its discovery, multi-wavelength ...observations of the misnamed ``SN 2010da'' have continually re-shaped our understanding of this high mass X-ray binary system. In this review, we present an overview of the multi-wavelength observations and attempts to understand the 2010 transient event and, later, the re-classification of this system as NGC~300 ULX-1: a red supergiant + neutron star ultraluminous X-ray source.
PDF