arXiv subjects

E. Gallo

Publications and source records attributed to E. Gallo.

At least 19 recordsLinked to original sources

arXiv2025

Determination of Parton Densities for QCD partons and Electroweak Bosons

Parton densities are obtained from a solution of the extended DGLAP-type evolution equation that includes both QCD and electroweak contributions. The equations are solved using the Parton-Branching (PB) approach, and the evolution is performed at next-to-leading order for QCD partons and leading order for electroweak bosons. The initial QCD parton distributions are fitted to HERA deep inelastic scattering data, while photon and weak-boson densities are generated perturbatively and validated against $d\sigma/dQ^2$. The resulting collinear and transverse-momentum dependent (TMD) densities are provided in LHApdf and TMDlib formats for direct phenomenological use.

hep-ph↗

arXiv2025

The peculiar hard state behaviour of the black hole X-ray binary Swift J1727.8$-$1613

Tracking the correlation between radio and X-ray luminosities during black hole X-ray binary outbursts is a key diagnostic of the coupling between accretion inflows (traced by X-rays) and relativistic jet outflows (traced by radio). We present the radio--X-ray correlation of the black hole low-mass X-ray binary Swift~J1727.8$-$1613 during its 2023--2024 outburst. Our observations span a broad dynamic range, covering $\sim$4 orders of magnitude in radio luminosity and $\sim$6.5 in X-ray luminosity. This source follows an unusually radio-quiet track, exhibiting significantly lower radio luminosities at a given X-ray luminosity than both the standard (radio-loud) track and most previously known radio-quiet systems. Across most of the considered distance range ($D {\sim} 1.5-4.3$ kpc), Swift~J1727.8$-$1613 appears to be the most radio-quiet black hole binary identified to date. For distances ${\geq} 4$ kpc, while Swift~J1727.8$-$1613 becomes comparable to one other extremely radio-quiet system, its peak X-ray luminosity (${\gtrsim} 5{\times}10^{38}$ erg/s) exceeds that of any previously reported hard-state black hole low-mass X-ray binary, emphasising the extremity of this outburst. Additionally, for the first time in a radio-quiet system, we identify the onset of X-ray spectral softening to coincide with a change in trajectory through the radio--X-ray plane. We assess several proposed explanations for radio-quiet behaviour in black hole systems in light of this dataset. As with other such sources, however, no single mechanism fully accounts for the observed properties, highlighting the importance of regular monitoring and the value of comprehensive (quasi-)simultaneous datasets.

astro-ph.HE↗

arXiv2024

The azimuthal correlation between the leading jet and the scattered lepton in deep inelastic scattering at HERA

The azimuthal correlation angle, $\Delta\phi$, between the scattered lepton and the leading jet in deep inelastic $e^{\pm}p$ scattering at HERA has been studied using data collected with the ZEUS detector at a centre-of-mass energy of $\sqrt{s} = 318 \;\mathrm{GeV}$, corresponding to an integrated luminosity of $326 \;\mathrm{pb}^{-1}$. A measurement of jet cross sections in the laboratory frame was made in a fiducial region corresponding to photon virtuality $10 \;\mathrm{GeV}^2 < Q^2 < 350 \;\mathrm{GeV}^2$, inelasticity $0.04 < y < 0.7$, outgoing lepton energy $E_e > 10 \;\mathrm{GeV}$, lepton polar angle $140^\circ < \theta_e < 180^\circ$, jet transverse momentum $2.5 \;\mathrm{GeV} < p_\mathrm{T,jet} < 30 \;\mathrm{GeV}$, and jet pseudorapidity $-1.5 < \eta_\mathrm{jet} < 1.8$. Jets were reconstructed using the $k_\mathrm{T}$ algorithm with the radius parameter $R = 1$. The leading jet in an event is defined as the jet that carries the highest $p_\mathrm{T,jet}$. Differential cross sections, $d\sigma/d\Delta\phi$, were measured as a function of the azimuthal correlation angle in various ranges of leading-jet transverse momentum, photon virtuality and jet multiplicity. Perturbative calculations at $\mathcal{O}(\alpha_{s}^2)$ accuracy successfully describe the data within the fiducial region, although a lower level of agreement is observed near $\Delta\phi \rightarrow \pi$ for events with high jet multiplicity, due to limitations of the perturbative approach in describing soft phenomena in QCD. The data are equally well described by Monte Carlo predictions that supplement leading-order matrix elements with parton showering.

hep-ex↗

arXiv2023

The 50 Mpc Galaxy Catalog (50MGC): Consistent and Homogeneous Masses, Distances, Colors, and Morphologies

We assemble a catalog of 15424 nearby galaxies within 50 Mpc with consistent and homogenized mass, distance, and morphological type measurements. Our catalog combines galaxies from HyperLeda, the NASA-Sloan Atlas, and the Catalog of Local Volume Galaxies. Distances for the galaxies combine best-estimates for flow-corrected redshift-based distances with redshift independent distances. We also compile magnitude and color information for 11740 galaxies. We use the galaxy colors to estimate masses by creating self-consistent color -- mass-to-light ratio relations in four bands; we also provide color transformations of all colors into Sloan (g-i) by using galaxies with overlapping color information. We compile morphology information for 13744 galaxies, and use galaxy color information to separate early and late-type galaxies. This catalog is widely applicable for studies of nearby galaxies, and placing these studies in the context of more distant galaxies. We present one application here; a preliminary analysis of the nuclear X-ray activity of galaxies. Out of 1506 galaxies within the sample that have available Chandra X-ray observations, we find 291 have detected nuclear sources. Of the 291 existing Chandra detections, 249 have log(L$_{X}$)$>$38.3 and available stellar mass estimates. We find that the X-ray active fractions in early-type galaxies are higher than in late-type galaxies, especially for galaxy stellar masses between 10$^9$ and 10$^{10.5}$ M$_\odot$. We show that these differences may be due at least in part to the increased astrometric uncertainties in late-type galaxies relative to early-types.

astro-ph.GA↗

arXiv2023

Measurement of jet production in deep inelastic scattering and NNLO determination of the strong coupling at ZEUS

A new measurement of inclusive-jet cross sections in the Breit frame in neutral current deep inelastic scattering using the ZEUS detector at the HERA collider is presented. The data were taken in the years 2004 to 2007 at a centre-of-mass energy of $318\,\text{GeV}$ and correspond to an integrated luminosity of $347\,\text{pb}^{-1}$. Massless jets, reconstructed using the $k_t$-algorithm in the Breit reference frame, have been measured as a function of the squared momentum transfer, $Q^2$, and the transverse momentum of the jets in the Breit frame, $p_{\perp,\text{Breit}}$. The measured jet cross sections are compared to previous measurements and to perturbative QCD predictions. The measurement has been used in a next-to-next-to-leading-order QCD analysis to perform a simultaneous determination of parton distribution functions of the proton and the strong coupling, resulting in a value of $\alpha_s(M_Z^2) = 0.1142 \pm 0.0017~\text{(experimental/fit)}$ ${}^{+0.0006}_{-0.0007}~\text{(model/parameterisation)}$ ${}^{+0.0006}_{-0.0004}~\text{(scale)}$, whose accuracy is improved compared to similar measurements. In addition, the running of the strong coupling is demonstrated using data obtained at different scales.

hep-ex↗

arXiv2023

ReveaLLAGN 0: First Look at JWST MIRI data of Sombrero and NGC 1052

We present the first results from the Revealing Low-Luminosity Active Galactic Nuclei (ReveaLLAGN) survey, a JWST survey of seven nearby LLAGN. We focus on two observations with the Mid-Infrared Instrument's (MIRI) Medium Resolution Spectrograph (MRS) of the nuclei of NGC 1052 and Sombrero (NGC 4594 / M104). We also compare these data to public JWST data of a higher-luminosity AGN, NGC 7319 and NGC 7469. JWST clearly separates the AGN spectrum from the galaxy light even in Sombrero, the faintest target in our survey; the AGN components have very red spectra. We find that the emission-line widths in both NGC 1052 and Sombrero increase with increasing ionization potential, with FWHM > 1000 km/s for lines with ionization potential > 50 eV. These lines are also significantly blue-shifted in both LLAGN. The high ionization potential lines in NGC 7319 show neither broad widths or significant blue shifts. Many of the lower ionization potential emission lines in Sombrero show significant blue wings extending > 1000 km/s. These features and the emission-line maps in both galaxies are consistent with outflows along the jet direction. Sombrero has the lowest luminosity high-ionization potential lines ([Ne V] and [O IV]) ever measured in the mid-IR, but the relative strengths of these lines are consistent with higher luminosity AGN. On the other hand, the [Ne V] emission is much weaker relative to the [Ne III] and [Ne II] lines of higher-luminosity AGN. These initial results show the great promise that JWST holds for identifying and studying the physical nature of LLAGN.

astro-ph.GA↗

arXiv2022

Measurement of the cross-section ratio $σ_{ψ(2S )}/σ_{J/ψ(1S )}$ in exclusive photoproduction at HERA

The exclusive photoproduction reactions $γp \to J/ψ(1S) p$ and $γp \to ψ(2S) p$ have been measured at an $ep$ centre-of-mass energy of 318 GeV with the ZEUS detector at HERA using an integrated luminosity of 373 pb$^{-1}$. The measurement was made in the kinematic range $30 < W < 180$ GeV, $Q^2 < 1$ GeV$^2$ and $|t| < 1$ GeV$^2$, where $W$ is the photon--proton centre-of-mass energy, $Q^2$ is the photon virtuality and $t$ is the squared four-momentum transfer at the proton vertex. The decay channels used were $J/ψ(1S) \to μ^+ μ^-$, $ψ(2S) \to μ^+ μ^-$ and $ψ(2S) \to J/ψ(1S) π^+ π^-$ with subsequent decay $J/ψ(1S) \to μ^+ μ^-$. The ratio of the production cross sections, $R = σ_{ψ(2S)} / σ_{J/ψ(1S)}$, has been measured as a function of $W$ and $|t|$ and compared to previous data in photoproduction and deep inelastic scattering and with predictions of QCD-inspired models of exclusive vector-meson production, which are in reasonable agreement with the data.

hep-ex↗

arXiv2022

Search for effective Lorentz and CPT violation using ZEUS data

Lorentz and CPT symmetry in the quark sector of the Standard Model are studied in the context of an effective field theory using ZEUS $e^{\pm} p$ data. Symmetry-violating effects can lead to time-dependent oscillations of otherwise time-independent observables, including scattering cross sections. An analysis using five years of inclusive neutral-current deep inelastic scattering events corresponding to an integrated HERA luminosity of $372\; \text{pb}^{-1}$ at $\sqrt{s} = 318$ Gev has been performed. No evidence for oscillations in sidereal time has been observed within statistical and systematic uncertainties. Constraints, most for the first time, are placed on 42 coefficients parameterising dominant CPT-even dimension-four and CPT-odd dimension-five spin-independent modifications to the propagation and interaction of light quarks.

hep-ex↗

arXiv2022

Planetary parameters, XUV environments and mass-loss rates for nearby gaseous planets with X-ray detected host-stars

We leverage Gaia DR2 parallactic distances to deliver new or revised estimates of planetary parameters and X-ray irradiation for a distance-limited ($\lesssim 100$ pc) sample of 27 gaseous planets (from super-Earths to hot Jupiters) with publicly available Chandra and/or XMM observations, for which we carry out a homogeneous data reduction. For 20 planets with X-ray detected host stars we make use of the photoionization hydrodynamics code ATES to derive updated atmospheric mass outflow rates. The newly derived masses/radii are not consistent with the exoplanet.eu values for five systems; HD 149026b and WASP-38, for mass; and Au Mic b, HAT-P-20 and HAT-P-2 for radii. Notably, the lower mass implies a (Saturn-like) density of $0.86\pm 0.09$ g cm$^{-3}$ for HD 149026 b. This independent estimate is consistent with the lowest values reported in the literature. Separately, we report on the X-ray detection of GJ 9827, HD 219134 and LHS 1140 for the first time. The inferred stellar X-ray luminosity of LHS 1140 ($1.34^{+0.19}_{-0.21} \times 10^{26}$ erg sec$^{-1}$) implies that LHS 1140 b is the least irradiated transiting super-Earth known to orbit within the habitable zone of a nearby M-dwarf.

astro-ph.EP↗

arXiv2022

Subarcsecond view on the high-redshift blazar GB 1508+5714 by the International LOFAR Telescope

Studies of the most distant AGNs allow us to test our current understanding of the physics present in radio-jetted AGNs across a range of environments. The decrease in apparent luminosity with distance is the primary difficulty to overcome in the study of these distant AGNs, which requires highly sensitive instruments. Our goal is to employ new long wavelength radio data to better parametrise the broad-band SED of GB 1508+5714, a high-redshift (z=4.30) AGN. Its high redshift, high intrinsic luminosity, and classification as a blazar allow us to test emission models that consider the efficient cooling of jet electrons via inverse Compton losses in interactions with the dense CMB photon field at high redshifts. A significant detection of this effect in GB 1508+5714 may partly explain the apparent sparsity of high-redshift radio galaxies in wide-field surveys; detections of this kind are only becoming possible with the current generation of SKA precursors. We used international LOFAR telescope to image the long wavelength radio emission around the high-redshift blazar GB 1508+5714 on arcsecond scales at frequencies between 128 MHz and 160 MHz. This allowed us to compare the spatially resolved structure with higher frequency observations, and to construct spectral index maps. The LOFAR image shows a compact unresolved core and two resolved emission regions around 2 arcsec to the east and to the west of the radio core. We find structure consistent with previous VLA observations, as well as a previously unreported emission region to the east. We interpret the arcsecond-scale radio structure of GB 1508+5714 as a FR II-like radio galaxy at a small viewing angle. Our SED modelling shows that a scenario featuring significant quenching effects caused by interaction with the CMB provides a good description of the data, and notably explains the suppressed radio emission.

astro-ph.HE↗

arXiv2022

Azimuthal correlations in photoproduction and deep inelastic $\boldsymbol{ep}$ scattering at HERA

Collective behaviour of final-state hadrons, and multiparton interactions are studied in high-multiplicity $ep$ scattering at a centre-of-mass energy $\sqrt{s}=318$ GeV with the ZEUS detector at HERA. Two- and four-particle azimuthal correlations, as well as multiplicity, transverse momentum, and pseudorapidity distributions for charged-particle multiplicities $N_{\textrm ch} \geq 20$ are measured. The dependence of two-particle correlations on the virtuality of the exchanged photon shows a clear transition from photoproduction to neutral current deep inelastic scattering. For the multiplicities studied, neither the measurements in photoproduction processes nor those in neutral current deep inelastic scattering indicate significant collective behaviour of the kind observed in high-multiplicity hadronic collisions at RHIC and the LHC. Comparisons of PYTHIA predictions with the measurements in photoproduction strongly indicate the presence of multiparton interactions from hadronic fluctuations of the exchanged photon.

hep-ex↗

arXiv2021

Impact of jet-production data on the next-to-next-to-leading-order determination of HERAPDF2.0 parton distributions

The HERAPDF2.0 ensemble of parton distribution functions (PDFs) was introduced in 2015. The final stage is presented, a next-to-next-to-leading-order (NNLO) analysis of the HERA data on inclusive deep inelastic $ep$ scattering together with jet data as published by the H1 and ZEUS collaborations. A perturbative QCD fit, simultaneously of $α_s(M_Z^2)$ and and the PDFs, was performed with the result $α_s(M_Z^2) = 0.1156 \pm 0.0011~{\rm (exp)}~ ^{+0.0001}_{-0.0002}~ {\rm (model}$ ${\rm +~parameterisation)}~ \pm 0.0029~{\rm (scale)}$. The PDF sets of HERAPDF2.0Jets NNLO were determined with separate fits using two fixed values of $α_s(M_Z^2)$, $α_s(M_Z^2)=0.1155$ and $0.118$, since the latter value was already chosen for the published HERAPDF2.0 NNLO analysis based on HERA inclusive DIS data only. The different sets of PDFs are presented, evaluated and compared. The consistency of the PDFs determined with and without the jet data demonstrates the consistency of HERA inclusive and jet-production cross-section data. The inclusion of the jet data reduced the uncertainty on the gluon PDF. Predictions based on the PDFs of HERAPDF2.0Jets NNLO give an excellent description of the jet-production data used as input.

hep-ex↗

arXiv2021

A new radio census of neutron star X-ray binaries

We report new radio observations of a sample of thirty-six neutron star (NS) X-ray binaries, more than doubling the sample in the literature observed at current-day sensitivities. These sources include thirteen weakly-magnetised ($B<10^{10}$ G) and twenty-three strongly-magnetised ($B\geq10^{10}$ G) NSs. Sixteen of the latter category reside in high-mass X-ray binaries, of which only two systems were radio-detected previously. We detect four weakly and nine strongly-magnetised NSs; the latter are systematically radio fainter than the former and do not exceed $L_R \approx 3\times10^{28}$ erg/s. In turn, we confirm the earlier finding that the weakly-magnetized NSs are typically radio fainter than accreting stellar-mass black holes. While an unambiguous identification of the origin of radio emission in high-mass X-ray binaries is challenging, we find that in all but two detected sources (Vela X-1 and 4U 1700-37) the radio emission appears more likely attributable to a jet than the donor star wind. The strongly-magnetised NS sample does not reveal a global correlation between X-ray and radio luminosity, which may be a result of sensitivity limits. Furthermore, we discuss the effect of NS spin and magnetic field on radio luminosity and jet power in our sample. No current model can account for all observed properties, necessitating the development and refinement of NS jet models to include magnetic field strengths up to $10^{13}$ G. Finally, we discuss jet quenching in soft states of NS low-mass X-ray binaries, the radio non-detections of all observed very-faint X-ray binaries in our sample, and future radio campaigns of accreting NSs.

astro-ph.HE↗

arXiv2020

Observations of the Disk/Jet Coupling of MAXI J1820+070 During its Descent to Quiescence

Black hole X-ray binaries in the quiescent state (Eddington ratios typically $\lesssim$10$^{-5}$) display softer X-ray spectra (photon indices $Γ\sim2$) compared to higher-luminosity black hole X-ray binaries in the hard state ($Γ\sim1.7$). However, the cause of this softening, and its implications for the underlying accretion flow, are still uncertain. Here, we present quasi-simultaneous X-ray and radio spectral monitoring of the black hole X-ray binary MAXI J1820$+$070 during the decay of its 2018 outburst and of a subsequent re-flare in 2019, providing an opportunity to monitor a black hole X-ray binary as it actively transitions into quiescence. We probe 1-10 keV X-ray luminosities as low as $L_{\rm X}\sim4\times10^{32}$ erg s$^{-1}$, equivalent to Eddington fractions of $\sim4\times10^{-7}$. During its decay towards quiescence, the X-ray spectrum of MAXI J1820$+$070 softens from $Γ\sim1.7$ to $Γ\sim2$, with the softening taking $\sim30$d, and completing at $L_{\rm X}\approx10^{34}$ erg s$^{-1}$ ($\approx10^{-5} L_{\rm Edd}$). While the X-ray spectrum softens, the radio spectrum generally remains flat/inverted throughout the decay. We also find that MAXI J1820$+$070 follows a radio ($L_{\rm R}$) -- X-ray luminosity correlation of the form $L_{\rm R} \propto L_{\rm X}^{0.52\pm0.07}$, making it the fourth black hole system to follow the so-called `standard track' unbroken over several (in this case, four) decades in $L_{\rm X}$. Comparing the radio/X-ray spectral evolution(s) with the $L_{\rm R}$ -- $L_{\rm X}$ plane, we find that the X-ray softening is consistent with X-rays produced by Comptonization processes in a radiatively inefficient accretion flow. We generally disfavor X-ray emission originating solely from within the jet, with the possible exception of X-rays produced via synchrotron self-Compton processes.

astro-ph.HE↗

arXiv2020

An Obscured, Seyfert-2-like State of the Stellar-mass Black Hole GRS 1915+105 Caused by Failed Disk Winds

We report on Chandra gratings spectra of the stellar-mass black hole GRS 1915+105 obtained during a novel, highly obscured state. As the source entered this state, a dense, massive accretion disk wind was detected through strong absorption lines. Photionization modeling indicates that it must originate close to the central engine, orders of magnitude from the outer accretion disk. Strong, nearly sinusoidal flux variability in this phase yielded a key insight: the wind is blue-shifted when its column density is relatively low, but red-shifted as it approaches the Compton-thick threshold. At no point does the wind appear to achieve the local escape velocity; in this sense, it is a "failed wind." Later observations suggest that the disk ultimately fails to keep even the central engine clear of gas, leading to heavily obscured and Compton-thick states characterized by very strong Fe K emission lines. Indeed, these later spectra are successfully described using models developed for obscured AGN. We discuss our results in terms the remarkable similarity of GRS 1915+105 deep in its "obscured state" to Seyfert-2 and Compton-thick AGN, and explore how our understanding of accretion and obscuration in massive black holes is impacted by our observations.

astro-ph.HE↗

arXiv2020

An extremely powerful long-lived superluminal ejection from the black hole MAXI J1820+070

Black holes in binary systems execute patterns of outburst activity where two characteristic X-ray states are associated with different behaviours observed at radio wavelengths. The hard state is associated with radio emission indicative of a continuously replenished, collimated, relativistic jet, whereas the soft state is rarely associated with radio emission, and never continuously, implying the absence of a quasi-steady jet. Here we report radio observations of the black hole transient MAXI J1820$+$070 during its 2018 outburst. As the black hole transitioned from the hard to soft state we observed an isolated radio flare, which, using high angular resolution radio observations, we connect with the launch of bi-polar relativistic ejecta. This flare occurs as the radio emission of the core jet is suppressed by a factor of over 800. We monitor the evolution of the ejecta over 200 days and to a maximum separation of 10$''$, during which period it remains detectable due to in-situ particle acceleration. Using simultaneous radio observations sensitive to different angular scales we calculate an accurate estimate of energy content of the approaching ejection. This energy estimate is far larger than that derived from state transition radio flare, suggesting a systematic underestimate of jet energetics.

astro-ph.HE↗

arXiv2020

Accretion-Driven Sources in Spatially Resolved Ly$α$ Emitters

Ly$α$ emission is a standard tracer of starburst galaxies at high redshift. However, a number of local Ly$α$ emitters (LAEs) are X-ray sources, suggesting a possible origin of Ly$α$ photons other than young, hot stars, and which may be active at much later ages relative to the parent starburst. Resolved, nearby LAEs offer the opportunity to discriminate between diffuse X-ray emission arising from supernova-heated gas, high-mass X-ray binaries (HMXBs), or low-luminosity active galactic nuclei (LLAGN). We examine archival X-ray imaging from Chandra and XMM-Newton for 11 galaxies with spatially resolved Ly$α$ imaging to determine the luminosity, morphology, and spectral hardness of the X-ray sources. The data are consistent with 9 of the 12, bright Ly$α$ sources being driven by luminous, $10^{40}$ erg s$^{-1}$ X-ray sources. Half of the 8 Chandra sources are unresolved. The data suggest that nuclear activity, whether from LLAGN or nuclear starbursts, may play an important role in Ly$α$ emission. Our results also suggest a significant link between Ly$α$ emission and HMXBs, ULXs, and/or LLAGN, which would imply that Ly$α$ may be generated over timescales 1 - 2 orders of magnitude longer than produced by photoionization from OB stars. This highlights a critical need to quantify the relative contributions of different sources across cosmic time, to interpret Ly$α$ observations and the resulting properties of distant galaxies.

astro-ph.GA↗

arXiv2019

GS 2000+25: The Least Luminous Black Hole X-ray Binary

Little is known about the properties of the accretion flows and jets of the lowest-luminosity quiescent black holes. We report new, strictly simultaneous radio and X-ray observations of the nearby stellar-mass black hole X-ray binary GS 2000+25 in its quiescent state. In deep Chandra observations we detect the system at a faint X-ray luminosity of $L_X = 1.1^{+1.0}_{-0.7} \times 10^{30}\,(d/2 {\rm \,\, kpc})^2$ erg s$^{-1}$ (1-10 keV). This is the lowest X-ray luminosity yet observed for a quiescent black hole X-ray binary, corresponding to an Eddington ratio $L_X/L_{\rm Edd} \sim 10^{-9}$. In 15 hours of observations with the Karl G. Jansky Very Large Array, no radio continuum emission is detected to a $3σ$ limit of $< 2.8\ μ$Jy at 6 GHz. Including GS 2000+25, four quiescent stellar-mass black holes with $L_X < 10^{32}$ erg s$^{-1}$ have deep simultaneous radio and X-ray observations and known distances. These sources all have radio to X-ray luminosity ratios generally consistent with, but slightly lower than, the low state radio/X-ray correlation for stellar-mass black holes with $L_X > 10^{32}$ erg s$^{-1}$. Observations of these sources tax the limits of our current X-ray and radio facilities, and new routes to black hole discovery are needed to study the lowest-luminosity black holes.

astro-ph.HE↗