arXiv subjects

Riley M. T. Connors

Publications and source records attributed to Riley M. T. Connors.

At least 19 recordsLinked to original sources

arXiv2026

Systematic assessment of disk truncation in the black hole X-ray binary Swift J1727.8-1613 using NICER

The 2023/24 NICER monitoring campaign of the 7 Crab bright black hole X-ray binary Swift J1727.8-1613 covered the outburst in almost all accretion states. High-quality data are available in the high-Eddington-fraction hard-intermediate state, hard-to-soft transition, the soft state, and the poorly studied back-transition to the dim hard state, making it an ideal dataset to compare the accretion flow at vastly different accretion rates. We apply disk continuum fitting techniques to investigate the evolution of the inner disk radius throughout the outburst. Taking a temperature-dependent color-correction factor into account, we see evolution of the disk inner radius by a factor of a few comparing the hard states to the thermal/soft state. We tentatively detect an onset of disk truncation in the soft-to-hard transition, right after the source leaves the soft state. After accounting for model systematics, we find the disk to be more truncated in the high-luminosity bright hard state compared to the low-luminosity dim hard state.

astro-ph.HE↗

arXiv2025

The 2025 Failed Outburst of IGR J17091-3624: Spectral Evolution and the Role of Ionized Absorbers

IGR J17091-3624 is the only black hole X-ray binary candidate, aside from the well-studied black hole system GRS 1915+105, observed to exhibit a wide range of structured variability patterns in its light curves. In 2025, the source underwent a ``failed'' outburst: it brightened in the hard state but did not transition to the soft state before returning to quiescence within a few weeks. During this period, IGR J17091-3624 was observed by multiple ground- and space-based facilities. Here, we present results from six pointed NuSTAR observations obtained during the outburst. None of the NuSTAR light curves showed the exotic variability classes typical of the soft state in this source; however, we detected, for the first time, strong dips in the count rate during one epoch, with a total duration of $\sim4\,\mathrm{ks}$ as seen by NuSTAR. Through spectral and timing analysis of all six epochs, we investigate the hard-state spectral evolution and the nature of the dips. A clear evolution of the coronal properties with luminosity is observed over all six epochs, with clear signatures of relativistic disk reflection which remain largely unchanged across the first five epochs. The first five epochs also show a strong and stable quasi-periodic oscillation (QPO) feature in the power spectra. The dips observed in Epoch 5 are consistent with partial obscuration by ionized material with a column density $N_{\mathrm{H}} \approx 2.0 \times 10^{23}\,\mathrm{cm^{-2}}$. We discuss possible origins for this material and place constraints on the orbital parameters and distance of the system.

astro-ph.HE↗

arXiv2025

Reprocessing from highly ionized gas in the soft spectral state of V4641 Sgr with NuSTAR

V4641 Sgr is a low-mass black hole X-ray binary system with somewhat puzzling spectral characteristics during its soft state. Recent high-resolution spectroscopic studies of V4641 Sgr have revealed strong ionized emission line features in both the optical and X-ray bands, including P-Cygni signatures, and an unusually low soft state luminosity, indicating that the central engine is obscured. Here we present an analysis of five NuSTAR observations of V4641 Sgr taken during its recent outburst in 2021, when the source was in the soft state. We identify highly ionized Fe K emission lines, consistent with a combination of the near-neutral $6.4$~keV Fe K$α$ line, and the H-like and He-like Fe K$α$ and Fe K$β$ transitions found at $6.7\mbox{--}7$~keV and $\sim8$~keV, and find no evidence for strong relativistic broadening. The line fluxes correlate linearly with the observed disk continuum flux, implying a direct connection between the central engine and the reprocessing region. Most interestingly, all five spectra also show a persistent highly ionized Fe K continuum edge feature at $\sim9$~keV with a stable optical depth, which is likely smeared, implying a localized reprocessing zone. We find tentative supporting evidence for obscuration of the inner accretion disk based on its unusually low intrinsic luminosity, however, the NuSTAR spectra do not require obscuration from cold, optically thick gas.

astro-ph.HE↗

arXiv2024

Self-Consistent Disk-Reflection Analysis of the Black-Hole Candidate X-ray Binary MAXI J1813-095 with NICER, Swift, Chandra, and NuSTAR

We present our analysis of MAXI J1813-095 during its hard state ``stalled'' outburst in 2018. This self-consistent analysis has been carried out using \NICER, \Swift, \Chandra, and {\NuSTAR} throughout seven observations of MAXI J1813-095. We find a relativistic iron line at $\sim$6.5 keV from the inner region of the accretion disk. Our results are consistent with a slightly truncated disk or non-truncated disk for an inner radius of $\sim$2$R_\mathrm{g}$ and minimum spin of $>$0.7 with a best value of $\sim0.9$, assuming $R_\mathrm{in}$ reaches the innermost stable circular orbit at $\it{L_\mathrm{x}}$ $\sim$ 1\% $\it{L_\mathrm{Edd}}$. We analyzed MAXI J1813-095 over its outburst employing a spectral model which self-consistently couples the seed disk photons to the Comptonization and reflection components, also inclusive of reflection Comptonization. The unique aspect of this work is a reflection fraction of order unity, which is significantly higher than previous studies of this source, and is a consequence of applying the self-consistent disk-Comptonization-reflection spectral model. Other key parameters such as inclination and inner radius are found to be consistent with other works. works.

astro-ph.HE↗

arXiv2023

Reflection and timing study of the transient black hole X-ray binary MAXI J1803-298 with NuSTAR

The transient black hole X-ray binary MAXI J1803-298 was discovered on 2021 May 1, as it went into outburst from a quiescent state. As the source rose in flux it showed periodic absorption dips and fit the timing and spectral characteristics of a hard state accreting black hole. We report on the results of a Target-of-Opportunity observation with NuSTAR obtained near the peak outburst flux beginning on 2021 May 13, after the source had transitioned into an intermediate state. MAXI J1803-298 is variable across the observation, which we investigate by extracting spectral and timing products separately for different levels of flux throughout the observation. Our timing analysis reveals two distinct potential QPOs which are not harmonically related at 5.4+/-0.2 Hz and 9.4+/-0.3 Hz, present only during periods of lower flux. With clear relativistic reflection signatures detected in the source spectrum, we applied several different reflection models to the spectra of MAXI J1803-298. Here we report our results, utilizing high density reflection models to constrain the disk geometry, and assess changes in the spectrum dependent on the source flux. With a standard broken power-law emissivity, we find a near-maximal spin for the black hole, and we are able to constrain the inclination of the accretion disk at 75+/-2 degrees, which is expected for a source that has shown periodic absorption dips. We also significantly detect a narrow absorption feature at 6.91+/-0.06 keV with an equivalent width between 4 and 9 eV, which we interpret as the signature of a disk wind.

astro-ph.HE↗

arXiv2022

The long-stable hard state of XTE J1752-223 and the disk truncation dilemma

The degree to which the thin accretion disks of black hole X-ray binaries are truncated during hard spectral states remains a contentious open question in black hole astrophysics. During its singular observed outburst in $2009\mbox{--}2010$, the black hole X-ray binary XTE J1752-223 spent $\sim1$~month in a long-stable hard spectral state at a luminosity of $\sim0.02\mbox{--}0.1~L_{\rm Edd}$. It was observed with 56 RXTE pointings during this period, with simultaneous Swift-XRT daily coverage during the first 10 days of the RXTE observations. Whilst reflection modeling has been extensively explored in the analysis of these data, there is a disagreement surrounding the geometry of the accretion disk and corona implied by the reflection features. We re-examine the combined, high signal-to-noise, simultaneous Swift and RXTE observations, and perform extensive reflection modeling with the latest relxill suite of reflection models, including newer high disk density models. We show that reflection modeling requires that the disk be within $\sim5~R_{\rm ISCO}$ during the hard spectral state, whilst weaker constraints from the thermal disk emission imply higher truncation ($R_{\rm in}=6\mbox{--}80~R_{\rm ISCO}$). We also explore more complex coronal continuum models, allowing for two Comptonization components instead of one, and show that the reflection features still require only a mildly truncated disk. Finally we present a full comparison of our results to previous constraints found from analyses of the same dataset.

astro-ph.HE↗

arXiv2022

MAXI and NuSTAR observations of the faint X-ray transient MAXI J1848-015 in the GLIMPSE-C01 Cluster

We present the results of MAXI monitoring and two NuSTAR observations of the recently discovered faint X-ray transient MAXI J1848-015. Analysis of the MAXI light-curve shows that the source underwent a rapid flux increase beginning on 2020 December 20, followed by a rapid decrease in flux after only $\sim5$ days. NuSTAR observations reveal that the source transitioned from a bright soft state with unabsorbed, bolometric ($0.1$-$100$ keV) flux $F=6.9 \pm 0.1 \times 10^{-10}\,\mathrm{erg\,cm^{-2}\,s^{-1}}$, to a low hard state with flux $F=2.85 \pm 0.04 \times 10^{-10}\,\mathrm{erg\,cm^{-2}\,s^{-1}}$. Given a distance of $3.3$ kpc, inferred via association of the source with the GLIMPSE-C01 cluster, these fluxes correspond to an Eddington fraction of order $10^{-3}$ for an accreting neutron star of mass $M=1.4M_\odot$, or even lower for a more massive accretor. However, the source spectra exhibit strong relativistic reflection features, indicating the presence of an accretion disk which extends close to the accretor, for which we measure a high spin, $a=0.967\pm0.013$. In addition to a change in flux and spectral shape, we find evidence for other changes between the soft and hard states, including moderate disk truncation with the inner disk radius increasing from $R_\mathrm{in}\approx3\,R_\mathrm{g}$ to $R_\mathrm{in}\approx8\,R_\mathrm{g}$, narrow Fe emission whose centroid decreases from $6.8\pm0.1$ keV to $6.3 \pm 0.1$ keV, and an increase in low-frequency ($10^{-3}$-$10^{-1}$ Hz) variability. Due to the high spin we conclude that the source is likely to be a black hole rather than a neutron star, and we discuss physical interpretations of the low apparent luminosity as well as the narrow Fe emission.

astro-ph.HE↗

arXiv2021

Evidence for an expanding corona based on spectral-timing modelling of multiple black hole X-ray binaries

Galactic black hole X-ray binaries (BHXBs) provide excellent laboratories to study accretion, as their relatively quick evolution allows us to monitor large changes in the in-flowing and/or out-flowing material over human timescales. However, the details of how the inflow-outflow coupling evolves during a BHXB outburst remain an area of active debate. In this work we attempt to probe the physical changes underlying the system evolution, by performing a systematic analysis of the multi-wavelength data of three BHXB sources: XTE J1752-223, MAXI J1659-152, and XTE J1650-500, during hard and hard-intermediate states. Using the power spectral hue which characterises the X-ray variability properties, we identify several clusters of BHXB epochs and perform the joint multi-wavelength spectral modelling to test their commonality with a physical jet model. Under the assumption that the corona is related to the base of the jet, we find that the power spectral hue traces the variation of the coronal radius(from ~10Rg - ~40Rg) in multiple BHXBs at hard and hard-intermediate states, and that the data are consistent with moderately truncated accretion discs (<25Rg) during hard-intermediate states. We also find that all epochs of low disc reflection have high hue located near the hard-intermediate to soft-intermediate state transition, indicating that in these states the vertical extent of the corona and/or its bulk speed are increasing. Our results link the geometrical similarity in the corona among multiple BHXB sources to their timing characteristics, and probe the corona responding to the disc-jet interactions at hard and intermediate states during outbursts.

astro-ph.HE↗

arXiv2021

Spectral and Timing Analysis of NuSTAR and Swift/XRT Observations of the X-Ray Transient MAXI J0637-430

We present results for the first observed outburst from the transient X-ray binary source MAXI J0637-430. This study is based on eight observations from the Nuclear Spectroscopic Telescope Array (NuSTAR) and six observations from the Neil Gehrels Swift Observatory X-Ray Telescope (Swift/XRT) collected from 2019 November 19 to 2020 April 26 as the 3-79 keV source flux declined from 8.2e-10 to 1.4e-12 erg/cm^2/s. We see the source transition from a soft state with a strong disk-blackbody component to a hard state dominated by a power-law or thermal Comptonization component. NuSTAR provides the first reported coverage of MAXI J0637-430 above 10 keV, and these broadband spectra show that a two-component model does not provide an adequate description of the soft state spectrum. As such, we test whether blackbody emission from the plunging region could explain the excess emission. As an alternative, we test a reflection model that includes a physical Comptonization continuum. Finally, we also test a spectral component based on reflection of a blackbody illumination spectrum, which can be interpreted as a simple approximation to the reflection produced by returning disk radiation due to the bending of light by the strong gravity of the black hole. We discuss the physical implications of each scenario and demonstrate the value of constraining the source distance.

astro-ph.HE↗

arXiv2021

The Nature of Soft Excess in ESO 362-G18 Revealed by XMM-Newton and NuSTAR Spectroscopy

We present a detailed spectral analysis of the joint XMM-Newton and NuSTAR observations of the active galactic nuclei (AGN) in the Seyfert 1.5 Galaxy ESO 362-G18. The broadband ($0.3\mbox{--}79$ keV) spectrum shows the presence of a power-law continuum with a soft excess below $2$ keV, iron K$α$ emission ($\sim 6.4$ keV), and a Compton hump (peaking at $\sim 20$ keV). We find that the soft excess can be modeled by two different possible scenarios: a warm ($kT_\mathrm{e}\sim0.2$ keV) and optically thick ($τ\sim34$) Comptonizing corona; or with relativistically-blurred reflection off a high-density ($\log{[n_\mathrm{e}/\mathrm{cm}^{-3}]}>18.3$) inner disk. These two models cannot be easily distinguished solely from their fit statistics. However, the low temperature ($kT_\mathrm{e}\sim20$ keV) and the thick optical depth ($τ\sim5$) of the hot corona required by the warm corona scenario are uncommon for AGNs. We also fit a 'hybrid' model, which includes both disk reflection and a warm corona. Unsurprisingly, as this is the most complex of the models considered, this provides the best fit, and more reasonable coronal parameters. In this case, the majority of the soft excess flux arises in the warm corona component. However, based on recent simulations of warm coronae, it is not clear whether such a structure can really exist at the low accretion rates relevant for ESO 362-G18 ($\dot{m}\sim0.015$). This may therefore argue in favor of a scenario in which the soft excess is instead dominated by the relativistic reflection. Based on this model, we find that the data would require a compact hot corona ($h\sim3\,R_\mathrm{Horizon}$) around a highly spinning ($a_\star>0.927$) black hole.

astro-ph.HE↗

arXiv2020

Evolution of the accretion disk-corona during bright hard-to-soft state transition: A reflection spectroscopic study with GX 339-4

We present the analysis of several observations of the black hole binary GX 339--4 during its bright intermediate states from two different outbursts (2002 and 2004), as observed by RXTE/PCA. We perform a consistent study of its reflection spectrum by employing the relxill family of relativistic reflection models to probe the evolutionary properties of the accretion disk including the inner disk radius ($R_{\rm in}$), ionization parameter ($ξ$), temperatures of the inner disk ($T_{\rm in}$), corona ($kT_{\rm e}$), and its optical depth ($τ$). Our analysis indicates that the disk inner edge approaches the inner-most stable circular orbit (ISCO) during the early onset of bright hard state, and that the truncation radius of the disk remains low ($\lesssim 14 R_{\rm g}$) throughout the transition from hard to soft state. This suggests that the changes observed in the accretion disk properties during the state transition are driven by variation in accretion rate, and not necessarily due to changes in the inner disk's radius. We compare the aforementioned disk properties in two different outbursts, with state transitions occurring at dissimilar luminosities, and find identical evolutionary trends in the disk properties, with differences only seen in corona's $kT_{\rm e}$ and $τ$. We also perform an analysis by employing a self-consistent Comptonized accretion disk model accounting for the scatter of disk photons by the corona, and measure low inner disk truncation radius across the bright intermediate states, using the temperature dependent values of spectral hardening factor, thereby independently confirming our results from the reflection spectrum analysis.

astro-ph.HE↗

arXiv2020

Evidence for Returning Disk Radiation in the Black Hole X-ray Binary XTE J1550-564

We explore the accretion properties of the black hole X-ray binary \j1550\ during its outbursts in 1998/99 and 2000. We model the disk, corona, and reflection components of X-ray spectra taken with the {\it Rossi X-ray Timing Explorer} (\rxte), using the {\tt relxill} suite of reflection models. The key result of our modeling is that the reflection spectrum in the very soft state is best explained by disk self-irradiation, i.e., photons from the inner disk are bent by the strong gravity of the black hole, and reflected off the disk surface. This is the first known detection of thermal disk radiation reflecting off the inner disk. There is also an apparent absorption line at $\sim6.9$ keV which may be evidence of an ionized disk wind. The coronal electron temperature ($kT_{\rm e}$) is, as expected, lower in the brighter outburst of 1998/99, explained qualitatively by more efficient coronal cooling due to irradiating disk photons. The disk inner radius is consistent with being within a few times the innermost stable circular orbit (ISCO) throughout the bright-hard-to-soft states (10s of $r_{\rm g}$ in gravitational units). The disk inclination is low during the hard state, disagreeing with the binary inclination value, and very close to $90^{\circ}$ in the soft state, recovering to a lower value when adopting a blackbody spectrum as the irradiating continuum.

astro-ph.HE↗

arXiv2019

The 2017 Failed Outburst of GX 339-4: Relativistic X-ray Reflection near the Black Hole Revealed by NuSTAR and Swift Spectroscopy

We report on the spectroscopic analysis of the black hole binary GX 339-4 during its recent 2017-2018 outburst, observed simultaneously by the Swift and NuSTAR observatories. Although during this particular outburst the source failed to make state transitions, and despite Sun constraints during the peak luminosity, we were able to trigger four different observations sampling the evolution of the source in the hard state. We show that even for the lowest luminosity observations the NuSTAR spectra show clear signatures of X-ray reprocessing (reflection) in an accretion disk. Detailed analysis of the highest signal-to-noise spectra with our family of relativistic reflection models RELXILL indicates the presence of both broad and narrow reflection components. We find that a dual-lamppost model provides a superior fit when compared to the standard single lamppost plus distant neutral reflection. In the dual lamppost model two sources at different heights are placed on the rotational axis of the black hole, suggesting that the narrow component of the Fe K emission is likely to originate in regions far away in the disk, but still significantly affected by its rotational motions. Regardless of the geometry assumed, we find that the inner edge of the accretion disk reaches a few gravitational radii in all our fits, consistent with previous determinations at similar luminosity levels. This confirms a very low degree of disk truncation for this source at luminosities above ~1% Eddington. Our estimates of Rin reinforces the suggested behavior for an inner disk that approaches the inner-most regions as the luminosity increases in the hard state.

astro-ph.HE↗

arXiv2019

Multi-wavelength astronomical searches for primordial black holes

If primordial black holes of $\mathcal{O}(1-100) M_{\odot}$ constitute a significant portion of the dark matter in the Universe, they should be very abundant in our Galaxy. We present here a detailed analysis of the radio and X-ray emission that these objects are expected to produce due to the accretion of gas from the interstellar medium. With respect to previous studies, we relax the assumption of a monochromatic mass function, and introduce an improved treatment of the physics of gas accretion onto isolated, moving compact objects, based on a set of state-of-the-art numerical simulations. By comparing our predictions with known radio and X-ray sources in the Galactic center region, we show that the maximum relic density of primordial black holes in the mass range of interest is $\sim 10^{-3}$ smaller than that of dark matter. The new upper bound is two orders of magnitude stronger with respect to previous results, based on a conservative phenomenological treatment of the accretion physics. We also provide a comprehensive critical discussion on the reliability of this bound, and on possible future developments in the field. We argue in particular that future multi-wavelength searches will soon start to probe the galactic population of astrophysical black holes.

astro-ph.HE↗

arXiv2019

Combining timing characteristics with physical broadband spectral modelling of black hole X-ray binary GX~339$-$4

GX~339$-$4 is a black hole X-ray binary that is a key focus of accretion studies since it goes into outburst roughly every two-to-three years. Tracking of its radio, IR and X-ray flux during multiple outbursts reveals tight broadband correlations. The radio emission originates in a compact, self-absorbed jet, however the origin of the X-ray emission is still debated: jet base or corona? We fit 20 quasi-simultaneous radio, IR, optical and X-ray observations of GX~339$-$4 covering three separate outbursts in 2005, 2007, 2010--2011, with a composite corona + jet model, where inverse Compton emission from both regions contributes to the X-ray emission. Using a recently-proposed identifier of the X-ray variability properties known as power-spectral hue, we attempt to explain both the spectral and evolving timing characteristics, with the model. We find the X-ray spectra are best fit by inverse Compton scattering in a dominant hot corona ($kT_{\rm e}\sim$ hundreds of keV). However, radio and IR-optical constraints imply a non-negligible contribution from inverse Compton scattering off hotter electrons ($kT_{\rm e} \ge 511$~keV) in the base of the jets, ranging from a few up to $\sim50$\% of the integrated 3--100~keV flux. We also find that the physical properties of the jet show interesting correlations with the shape of the broadband X-ray variability of the source, posing intriguing suggestions for the connection between the jet and corona.

astro-ph.HE↗

arXiv2018

The Soft-Excess in Mrk 509: Warm Corona or Relativistic Reflection?

We present the analysis of the first NuSTAR observations ($\sim 220$ ks), simultaneous with the last SUZAKU observations ($\sim 50$ ks), of the active galactic nucleus of the bright Seyfert 1 galaxy Mrk 509. The time-averaged spectrum in the $1-79$ keV X-ray band is dominated by a power-law continuum ($Γ\sim 1.8-1.9$), a strong soft excess around 1 keV, and signatures of X-ray reflection in the form of Fe K emission ($\sim 6.4$ keV), an Fe K absorption edge ($\sim 7.1$ keV), and a Compton hump due to electron scattering ($\sim 20-30$ keV). We show that these data can be described by two very different prescriptions for the soft excess: a warm ($kT\sim 0.5-1$ keV) and optically thick ($τ\sim10-20$) Comptonizing corona, or a relativistically blurred ionized reflection spectrum from the inner regions of the accretion disk. While these two scenarios cannot be distinguished based on their fit statistics, we argue that the parameters required by the warm corona model are physically incompatible with the conditions of standard coronae. Detailed photoionization calculations show that even in the most favorable conditions, the warm corona should produce strong absorption in the observed spectrum. On the other hand, while the relativistic reflection model provides a satisfactory description of the data, it also requires extreme parameters, such as maximum black hole spin, a very low and compact hot corona, and a very high density for the inner accretion disk. Deeper observations of this source are thus necessary to confirm the presence of relativistic reflection, and to further understand the nature of its soft excess.

astro-ph.HE↗

arXiv2018

Reflection Spectroscopy of the Black Hole Binary XTE J1752-223 in its Long-Stable Hard State

We present a detailed spectral analysis of the Black Hole Binary XTE J1752-223 in the hard state of its 2009 outburst. Regular monitoring of this source by RXTE provided high signal-to-noise spectra along the outburst rise and decay. During one full month this source stalled at $\sim$30\% of its peak count rate at a constant hardness and intensity. By combining all the data in this exceptionally-stable hard state, we obtained an aggregate PCA spectrum (3-45 keV) with 100 million counts, and a corresponding HEXTE spectrum (20-140 keV) with 5.8 million counts. Implementing a version of our reflection code with a physical model for Comptonization, we obtain tight constraints on important physical parameters for this system. In particular, the inner accretion disk is measured very close in, at $R_\mathrm{in}=1.7\pm0.4$ $R_g$. Assuming $R_\mathrm{in}=R_\mathrm{ISCO}$, we find a relatively high black hole spin ($a_*=0.92\pm0.06$). Imposing a lamppost geometry, we obtain a low inclination ($i=35\pm4$ deg), which agrees with the upper limit found in the radio ($i<49$ deg). However, we note that this model cannot be statistically distinguished from a non-lamppost model with free emissivity index, for which the inclination is markedly higher. Additionally, we find a relatively cool corona ($57-70$ keV), and large iron abundance ($3.3-3.7$ solar). We further find that properly accounting for Comptonization of the reflection emission improves the fit significantly and causes an otherwise low reflection fraction ($\sim 0.2-0.3$) to increase by an order of magnitude, in line with geometrical expectations for a lamppost corona. We compare these results with similar investigations reported for GX 339-4 in its bright hard state.

astro-ph.HE↗

arXiv2017

Searching for Primordial Black Holes in the radio and X-ray sky

We model the accretion of gas onto a population of massive primordial black holes in the Milky Way, and compare the predicted radio and X-ray emission with observational data. We show that under conservative assumptions on the accretion process, the possibility that ${\cal O}(10) \, M_\odot$ primordial black holes can account for all of the dark matter in the Milky Way is excluded at $5σ$ by a comparison with a VLA radio catalog at $1.4$ GHz, and at $\simeq 40σ$ by a comparison with a Chandra X-ray catalog ($0.5 - 8$ keV). We argue that this method can be used to identify such a population of primordial black holes with more sensitive future radio and X-ray surveys.

astro-ph.HE↗