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Einstein Probe Discovery of EP J005245.1−722843: A Rare Be–White Dwarf Binary in the Small Magellanic Cloud?
On 2024 May 27, the Wide-field X-ray Telescope on board the Space Sciences, University of Chinese Academy of Einstein Probe (EP) mission detected enhanced X-ray emission from a new transient source in the Small Magellanic Cloud during its commissioning phase. Prompt follow-up with the EP Follow-up X-ray Telescope, the Swift X-ray Telescope. and NICER have revealed a very soft, thermally emitting source (kT ~ 0.1 keV at the outburst peak) with an X-ray luminosity of L ~ 4 × 1038 erg s−1, labeled EP J005245.1−722843. This supersoft outburst faded very quickly in a week's time. Several emission lines and absorption edges were present in the X-ray spectrum, including deep nitrogen (0.67 keV) and oxygen (0.87 keV) absorption edges. The X-ray emission resembles the supersoft source phase of typical nova outbursts from an accreting white dwarf (WD) in a binary system, despite the X-ray source being historically associated with an O9-B0e massive star exhibiting a 17.55 day periodicity in the optical band. The discovery of this supersoft outburst suggests that EP J005245.1−722843 is a BeWD X-ray binary: an elusive evolutionary stage where two main-sequence massive stars have undergone a common envelope phase and experienced at least two episodes of mass transfer. In addition, the very short duration of the outburst and the presence of Ne features hint at a rather massive, i.e., close to the Chandrasekhar limit, Ne–O WD in the system.
Journal Article
Hunting for High-mass X-Ray Binaries in the Galactic Center with NuSTAR
The central 2 × 0.8 deg2 region of our Galaxy contains ∼10,000 X-ray point sources that were detected by a series of Chandra observations over the last two decades. However, the limited bandpass of Chandra below 8 keV hampered their spectroscopic classification. In 2016, the initial NuSTAR Galactic center (GC) survey detected 77 X-ray sources above 10 keV. The hard X-ray detections indicate magnetic cataclysmic variables, low-mass X-ray binaries, high-mass X-ray binaries (HMXBs), or even pulsars. The possibility of HMXB detections is particularly interesting given the dearth of identified HMXBs in the GC. We conducted a search for bright (Ks ≲ 16 mag) near-infrared (NIR) counterparts to the hard X-ray sources—utilizing their Chandra positions—in order to identify HMXB candidates. We identified seven NuSTAR sources with NIR counterpart candidates whose magnitudes are consistent with HMXBs at the GC. We assessed the likelihood of random association for these seven sources, and determined that two have a nonrandom association with a probability exceeding 99.98%, making them strong HMXB candidates. We analyzed broadband NuSTAR, Chandra, and XMM-Newton spectral data for these two candidates, one of which was previously identified as a red supergiant. We find that the X-ray spectra are consistent with HMXBs. If confirmed through follow-up NIR spectroscopic studies, our findings will open a new window into our understanding of the intrinsic luminosity distribution of HMXBs in our Galaxy in general and the GC HMXB population in particular.
Journal Article
The Long Stare at Hercules X-1. I. Emission Lines from the Outer Disk, the Magnetosphere Boundary, and the Accretion Curtain
Hercules X-1 is a nearly edge-on accreting X-ray pulsar with a warped accretion disk, precessing with a period of about 35 days. The disk precession allows for unique and changing sightlines toward the X-ray source. To investigate the accretion flow at a variety of sightlines, we obtained a large observational campaign on Her X-1 with XMM-Newton (380 ks exposure) and Chandra (50 ks exposure) for a significant fraction of a single disk precession cycle, resulting in one of the best data sets taken to date on a neutron star X-ray binary. Here we present the spectral analysis of the high state high-resolution grating and CCD data sets, including the extensive archival data available for this famous system. The observations reveal a complex Fe K region structure, with three emission line components of different velocity widths. Similarly, the high-resolution soft X-ray spectra reveal a number of emission lines of various widths. We correct for the uncertain gain of the European Photon Imaging Camera pn Timing mode spectra, and track the evolution of these spectral components with Her X-1 precession phase and observed luminosity. We find evidence for three groups of emission lines, the first of which originates in the outer accretion disk (105 R G from the neutron star). The second line group plausibly originates at the boundary between the inner disk and the pulsar magnetosphere (103 R G). The last group is too broad to arise in the magnetically truncated disk and instead must originate very close to the neutron star surface, likely from X-ray reflection from the accretion curtain (∼102 R G).
Journal Article
X-Ray Spectra of Black Hole X-Ray Binaries with Returning Radiation
In the disk–corona model, the X-ray spectrum of a stellar-mass black hole in an X-ray binary is characterized by three components: a thermal component from a thin and cold accretion disk, a Comptonized component from a hot corona, and a reflection component produced by illumination of the cold disk by the hot corona. In this paper, we assume a lamppost corona, and we improve previous calculations of the X-ray spectrum of black hole X-ray binaries. The reflection spectrum is produced by the direct radiation from the corona as well as by the returning radiation of the thermal and reflection components and is calculated considering the actual spectrum illuminating the disk. If we turn the corona off, the reflection spectrum is completely generated by the returning radiation of the thermal component, as it may happen for some sources in soft spectral states. After choosing the radial density profile of the accretion disk, the ionization parameter is calculated self-consistently at any radial coordinate of the disk from the illuminating X-ray flux and the local electron density. We show the predictions of our model in different regimes, and we discuss its current limitations as well as the next steps to improve it.
Journal Article
X-Ray Coronal Properties of Swift/BAT-selected Seyfert 1 Active Galactic Nuclei
The corona is an integral component of active galactic nuclei (AGNs) which produces the bulk of the X-ray emission above 1–2 keV. However, many of its physical properties and the mechanisms powering this emission remain a mystery. In particular, the temperature of the coronal plasma has been difficult to constrain for large samples of AGNs, as constraints require high-quality broadband X-ray spectral coverage extending above 10 keV in order to measure the high-energy cutoff, which provides constraints on the combination of coronal optical depth and temperature. We present constraints on the coronal temperature for a large sample of Seyfert 1 AGNs selected from the Swift/BAT survey using high-quality hard X-ray data from the NuSTAR observatory combined with simultaneous soft X-ray data from Swift/XRT or XMM-Newton. When applying a physically motivated, nonrelativistic disk-reflection model to the X-ray spectra, we find a mean coronal temperature kT e = 84 ± 9 keV. We find no significant correlation between the coronal cutoff energy and accretion parameters such as the Eddington ratio and black hole mass. We also do not find a statistically significant correlation between the X-ray photon index, Γ, and Eddington ratio. This calls into question the use of such relations to infer properties of supermassive black hole systems.
Journal Article
Constraining the White-dwarf Mass and Magnetic Field Strength of a New Intermediate Polar through X-Ray Observations
We report timing and broadband spectral analysis of a Galactic X-ray source, CXOGBS J174517.0−321356 (J1745), with a 614 s periodicity. Chandra discovered the source in the direction of the Galactic Bulge. Gong proposed that J1745 was either an intermediate polar (IP) with a mass of ∼1 M ⊙, or an ultracompact X-ray binary (UCXB). To confirm J1745's nature, we jointly fit XMM-Newton and NuSTAR spectra, ruling out a UCXB origin. We have developed a physically realistic model that considers a finite magnetosphere radius, X-ray absorption from the preshock region, and reflection from the white-dwarf (WD) surface to properly determine the IP properties, especially its WD mass. To assess systematic errors on the WD mass measurement, we consider a broad range of specific accretion rates ( ṁ=0.6–44 g cm−2 s−1) based on the uncertain source distance (d = 3–8 kpc) and fractional accretion area (f = 0.001–0.025). Our model properly implements the fitted accretion column height in the X-ray reflection model and accounts for the underestimated mass accretion rate due to the (unobserved) soft X-ray blackbody and cyclotron cooling emissions. We found that the lowest accretion rate of ṁ = 0.6 g cm−2 s−1, which corresponds to the nearest source distance and maximum f value, yields a WD mass of (0.92 ± 0.08)M ⊙. On the other hand, as long as the accretion rate is ṁ≳3 g cm−2 s−1, the WD mass is robustly measured to be (0.81 ± 0.06)M ⊙, nearly independent of ṁ . The derived WD mass range is consistent with the mean WD mass of nearby IPs. Assuming spin equilibrium between the WD and accretion disk, we constrained the WD magnetic field to B ≳ 7 MG, indicating that it could be a highly magnetized IP. Our analysis presents the most comprehensive methodology for constraining the WD mass and B field of an IP by consolidating the effects of cyclotron cooling, finite magnetospheric radius, and accretion column height.
Journal Article