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Gustavo E. Romero

Publications and source records attributed to Gustavo E. Romero.

At least 19 recordsLinked to original sources
arXiv2026

The unidentified PeV source LHAASO J2108+5157: A microquasar remnant?

Context. LHAASO~J2108+5157 is one of the most intriguing Galactic ultra-high-energy $γ$-ray sources. It is detected up to hundreds of TeV but lacks a clear counterpart at lower frequencies. Fermi-LAT observations have revealed a compact GeV source spatially coincident with the PeV emission, while VERITAS and LST-1 impose stringent upper limits in the intermediate TeV band, producing a pronounced spectral suppression between the GeV and TeV domains. Aims. We investigate a time-dependent hadronic scenario in which a past episode of microquasar jet activity injected relativistic protons that are now observed in a relic phase. Methods. Energy-dependent diffusion naturally segregates the particle population: Lower-energy protons remain confined near dense molecular gas and account for the GeV emission, whereas higher-energy particles reach more distant material, producing the hard TeV--PeV component. Results. For representative interstellar conditions, the model is simultaneously consistent with the broadband spectral energy distribution, the spatial compact--extended dichotomy, and the lack of detectable radio or X-ray counterparts. Conclusions. The observed bimodal $γ$-ray phenomenology thus emerges as a transport effect of a single injected proton population, supporting the interpretation of LHAASO~J2108+5157 as a fossil microquasar remnant observed at an intermediate evolutionary stage and highlighting relic jet systems as a viable class of Galactic PeVatrons.

astro-ph.HE↗
arXiv2026

A Galactic microblazar as a potential accelerator of ultra-high-energy particles

Context. Persistent jets from X-ray binaries which are aligned very close to the line of sight could be considered to be Galactic equivalents of blazars, or 'microblazars'. They are also expected to power gamma-ray sources. Aims. We intend to assess a serious candidate apparently fulfilling many of the requirements to be considered a genuine member of this class: IRAS 18293-0941. Methods. An intense multi-wavelength observational and theoretical study has been carried out on our proposed candidate source. Results. With photometric and spectroscopic properties typical of a binary star, this system exhibits clear collimated and one-sided radio emission matching the effects of relativistic motion along a reduced ejection angle. Only fast variability is not observed possibly smoothed by a dense circumstellar envelope. A physical scenario is consistently modeled that also gives credibility to its likely connection with LHAASO J1831-1007u*, an ultra-high-energy source in its immediate vicinity. Conclusions. Our reported identification not only helps to fill a gap in Galactic taxonomy, but also potentially strengthens the role of the microblazar and microquasar families in our understanding of the most energetic Milky Way phenomena.

astro-ph.HE↗
arXiv2026

Broadband emission of microquasar remnants

Microquasar remnants (MQRs), the long-lived cocoons inflated by extinct microquasar jets, have recently been proposed as hidden Galactic PeVatrons capable of producing ultra-high-energy gamma rays without an active central engine. While hadronic interactions can account for bright gamma-ray emission from nearby clouds, the direct detection of MQRs remains challenging because their intrinsic emission is expected to be extended and of low surface brightness. In this work, we explore the broadband emission of MQRs by focusing on the leptonic component confined within the cocoon and on particle interactions in the shocked shell surrounding it. We model the injection and time-dependent transport of relativistic particles, including stochastic re-acceleration driven by internal turbulence, treated as a second-order Fermi process. We consider sub-Eddington and super-Eddington microquasar systems and compute the resulting non-thermal emission from radio to gamma-ray energies, together with the thermal soft X-ray emission produced in the shocked shell. In the super-Eddington case, the intrinsic emission reaches peak values of $νL_ν\sim 10^{35}-10^{36}\,{\rm erg\,s^{-1}}$, whereas sub-Eddington remnants are typically several orders of magnitude fainter. At 1.3 GHz, the modeled cocoon surface brightness is of order $Σ_ν\sim 10^{-19}\,{\rm W\,m^{-2}\,Hz^{-1}\,sr^{-1}}$ for young powerful remnants and decreases rapidly as the remnant evolves. We find that the direct detectability of MQRs is therefore controlled mainly by surface brightness rather than by integrated luminosity. Powerful remnants may be detectable as extended synchrotron radio cocoons and shell-dominated soft X-ray structures, whereas sub-Eddington remnants are expected to be much harder to identify directly. Our results suggest that MQRs may constitute a hidden population of extended Galactic non-thermal sources.

astro-ph.HE↗
arXiv2026

First fast radio burst search campaign at the Argentine Institute of Radio Astronomy

Fast radio bursts (FRBs) are intense millisecond-duration radio transients of extragalactic origin whose physical nature remains under active investigation, and which also serve as probes of the intergalactic medium. We report on the first FRB search campaign carried out at the Argentine Institute of Radio Astronomy (IAR) between December 2024 and March 2026, targeting nearby galaxy superclusters in the southern sky. We observed fields in the Ophiuchus, Shapley, and Sculptor/Phoenix supercluster regions with one of the two 30~m antennas of the IAR, using a ROACH-based backend with 400~MHz of bandwidth centred at 1400~MHz and a time resolution of 41--82~$μ$s, for a total net observing time of 212~h. The data were searched for dispersed single pulses with \texttt{PRESTO} in the dispersion measure range $100 \leq \mathrm{DM} \leq 500$, and candidates were classified with the FETCH machine learning classifier. The pipeline was validated on archival Parkes data containing known FRBs and on synthetic bursts injected into IAR observations. One FRB candidate, FRB~20251018, was identified on 18 October 2025 in an observation pointed towards the galaxy cluster A2870, in the Phoenix supercluster, with a dispersion measure of $243$, a signal-to-noise ratio of 8.2, and a FETCH probability of $p=0.99$. To the best of our knowledge, this would be the first FRB detected from South America. A set of more marginal candidates is also presented. These results demonstrate the capability of the IAR antennas to detect FRBs and support the continuation and extension of the monitoring campaign, including coincident dual-antenna observations and cross-matches with gravitational-wave events and electromagnetic transients.

astro-ph.HE↗
arXiv2026

Neutrinos from super-Eddington Seyfert galaxies

Multimessenger observations suggest that Seyfert galaxies are promising sources of high-energy neutrinos, but their dense inner environments can strongly suppress the emerging very-high-energy gamma-ray emission. Active galactic nuclei (AGN) undergoing intense accretion episodes can enter a super-Eddington state, in which the accretion flow becomes geometrically and optically thick within a critical radius and develops strong magnetic fields in its innermost region. At the same time, large amounts of matter are expelled from the disk surface in the form of powerful, radiation-driven winds. In this work, we explore a scenario in which the cores of super-Eddington AGN provide suitable conditions for the acceleration of relativistic particles, including hadrons, via magnetic reconnection in a magnetically confined region close to the supermassive black hole. The accelerated hadronic component interacts with the intense photon field of the disk, leading to a copious neutrino flux peaking at 10-100 TeV that may be detectable with current observatories such as IceCube and KM3NeT, while the surrounding outflow efficiently absorbs the accompanying gamma-ray emission from the inner core. We also apply the model to the nearby super-Eddington Seyfert 1 NGC 7469 as a representative case with two reported neutrino events. In this framework, super-Eddington AGN, in particular Seyfert galaxies undergoing transient intense accretion episodes, emerge as plausible hidden neutrino sources, offering a natural explanation for the coexistence of efficient neutrino production and a strongly attenuated gamma-ray counterpart.

astro-ph.HE↗
arXiv2026

Does a wormhole survive a cosmological bounce?

We investigate whether a dynamical wormhole could survive in a universe that undergoes a cosmological bounce. First, the conditions under which a wormhole could persist from a contracting to an expanding phase of the cosmos are presented. Then, the only two known cosmological solutions of Einstein's equations representing wormholes are analyzed, and it is shown that both dynamical wormholes exist for all cosmic times on both sides of a bouncing universe and at the bounce itself. We also provide a detailed analysis of the causal structure of such spacetimes and the matter content of the wormhole. Finally, some possible astrophysical manifestations of surviving wormholes in a bouncing universe are mentioned. Our results show that, at least for the Kim and Pérez-Raia Neto solutions, there is no topology change in the chosen cosmological model with a bounce.

gr-qc↗
arXiv2026

Microquasar remnants as reservoirs of PeV cosmic rays

The Large High Altitude Air Shower Observatory (LHAASO) has revealed a population of Galactic gamma-ray sources radiating beyond 100 TeV, but the nature of several of them is still uncertain. In this contribution, we explore the idea that some of these ultrahigh-energy emitters are not powered by currently active accelerators, but by the fossil remains of microquasars (MQs). We consider systems in which mass transfer onto the stellar-mass black hole has already stopped, so that the central engine and its jets are permanently quenched. During the active phase, powerful transrelativistic jets inflate a hot cocoon whose interior is filled with cosmic rays (CRs) accelerated at the jet termination shocks. Once the jets switch off, the cocoon enters a long afterlife stage in which it behaves as a large reservoir of PeV CRs. If the remnant lies in or near a star-forming region, these relic CRs can still interact with dense clumps and molecular clouds, inside the cocoon or in the surrounding interstellar medium, leading to delayed gamma-ray emission via inelastic pp collisions and the subsequent decay of neutral pions. We present a time-dependent model for the jet-cocoon system, follow the evolution of the CR population during and after the MQ phase, and discuss the conditions under which the resulting microquasar remnants can account for some of the unidentified LHAASO sources.

astro-ph.HE↗
arXiv2026

Spacetime singularities and incompleteness: epistemic and ontological remarks

I argue that spacetime singularities entail no ontological commitment to material entities. First, I show that Penrose's singularity theorem is best understood as a theorem of incompleteness, it demonstrates the failure of specific spacetime models within General Relativity (or any theory incorporating the Raychaudhuri equation) under certain general conditions. Although this has been done before, I adopt a novel approach based on differentiating between physical and purely formal assumptions in the axiomatic foundation of general relativity. Next, I compare Penrose's result with Gödel's incompleteness theorem, highlighting key similarities and differences. Finally, I draw philosophical conclusions regarding the limits and prospects of our epistemic reconstructions of the physical world.

physics.hist-ph↗
arXiv2026

Probing Cosmic Curvature with Fast Radio Bursts and DESI DR2

The spatial curvature of the Universe remains a central question in modern cosmology. In this work, we explore the potential of localized Fast Radio Bursts (FRBs) as a novel tool to constrain the cosmic curvature parameter $Ω_k$ in a cosmological model-independent way. Using a sample of 120 FRBs with known redshifts and dispersion measures, we reconstruct the Hubble parameter $H(z)$ via artificial neural networks, and use it to obtain angular-diameter distances $D_A(z)$ through two complementary approaches. First, we derive the comoving distance $D_C(z)$ and $D_A(z)$ directly from FRBs without assuming a fiducial cosmology. Then, we combine the FRB-based $H(z)$ with Baryon Acoustic Oscillation (BAO) DESI DR2 measurements to infer $D_A(z)$. By comparing the FRB-derived and BAO+FRB-derived $D_A(z)$, we constrain spatial curvature. Our covariance-based likelihood (accounting for correlated uncertainties) yields $Ω_k = -0.31\pm0.57$, while a diagonal (Gaussian) treatment gives $Ω_k = -0.13\pm0.46$. Both estimations are consistent with spatial flatness at the $1σ$ level, albeit with a mild preference for negative curvature. Explicitly accounting for the full covariance broadens the intervals and avoids underestimation of uncertainties. These results highlight the growing relevance of FRBs in precision cosmology and their synergy with BAO as a powerful, cosmological model-independent probe of the large-scale geometry of the Universe.

astro-ph.CO↗
arXiv2026

Neutrinos from hidden ultraluminous X-ray sources in the Galaxy

Ultraluminous X-ray sources (ULXs) are point-like sources that exhibit apparent X-ray luminosities exceeding the Eddington limit for stellar-mass compact objects. A widely accepted interpretation is that these systems are X-ray binaries accreting matter possibly at super-Eddington rates. In this regime, photon trapping inflates the accretion disk, making it geometrically and optically thick. Radiation-driven winds launched from the supercritical disk form funnel-shaped walls along the symmetry axis. While the apparent X-ray luminosity can exceed the Eddington limit due to geometrical beaming within this funnel, a misalignment with the observer's line of sight strongly suppresses the X-ray emission, rendering the ULX electromagnetically obscured. This work explores the potential for high-energy neutrino production in black hole-hosting ULXs. We model proton acceleration via magnetic reconnection in the region above the super-accreting black hole. Although electromagnetic emission is efficiently absorbed by the dense wind and radiation fields, neutrinos generated from photomeson interactions can escape. Our model self-consistently accounts for energy losses of pions and muons in this environment. The results indicate that misaligned, electromagnetically obscured Galactic ULXs could produce a neutrino flux detectable by instruments like KM3NeT and IceCube within several years of observation.

astro-ph.HE↗
arXiv2025

Microquasar remnants as hidden PeVatrons

The Large High Altitude Air Shower Observatory (LHAASO) has revealed numerous ultrahigh-energy gamma-ray sources of unknown origin. We propose that a fraction of them can be explained by microquasar remnants, i.e., binary systems where mass transfer has ceased and the central engine is quenched. Cosmic rays injected during the active phase of a microquasar may remain confined within its cocoon and subsequently interact with nearby molecular clouds, producing bright gamma-ray emission through $pp$ collisions. Remnants of former super-Eddington systems can act as dark PeVatrons, releasing particles up to $\sim$10 PeV that illuminate surrounding clouds producing gamma rays reaching hundreds of TeV. This scenario provides a natural explanation for several unidentified Galactic LHAASO sources.

astro-ph.HE↗
arXiv2025

Super-Accreting Active Galactic Nuclei as Neutrino Sources

Active galactic nuclei (AGNs) often exhibit broad-line regions (BLRs), populated by high-velocity clouds in Keplerian orbits around the central supermassive black hole (SMBH) at subparsec scales. During episodes of intense super-Eddington accretion, the disk can launch a powerful radiation-driven wind that overtakes the BLR clouds, forming bowshocks. Two shocks arise: one into the wind and another into the cloud. If adiabatic, electrons and protons are efficiently accelerated via Fermi processes to relativistic energies. In dense winds, the resulting high-energy photons are absorbed and reprocessed within the photosphere, while neutrinos from inelastic $pp$ collisions escape. We explore the potential of super-accreting AGNs as neutrino sources and propose a new class of emitter: an AGN without jets or gamma-ray counterparts, but with a strong opaque disk wind. As a case study, we consider a SMBH with $M_{\rm BH}=10^6,M_{\odot}$ and accretion rates consistent with tidal disruption events (TDEs). We compute the main cooling processes for relativistic particles and show that super-Eddington SMBHs can produce detectable neutrino fluxes with only weak electromagnetic signatures. Such fluxes may be observable by IceCube-Gen2 in nearby galaxies with a high BLR cloud filling factor. For more massive black holes, detection remains possible with moderate filling factors if the source is close, or at larger distances if the filling factor is high. Our model thus provides a plausible scenario for extragalactic neutrino sources, where both flux and timescale are determined by the number of orbiting clouds and the duration of the super-accreting phase.

astro-ph.HE↗
arXiv2025

Gravitational entropy in black hole transformations

There are several reasons to support the idea that entropy might be associated to gravity itself. In the absence of a quantum theory of gravity, classical estimators for the gravitational entropy have been proposed. Any viable description of the gravitational entropy should reproduce the Hawking-Bekenstein entropy at the event horizon of black holes. Furthermore, in any black hole transformation, these estimators must satisfy the second law of black hole thermodynamics. In this work, we analyze whether two entropy estimators, one based on the Weyl tensor and the other on the Bel-Robinson tensor, satisfy the second law in the transformation process from a Schwarzschild to a Reissner-Nordström black hole by the absorption of a charge test particle. We also address the inverse process. We show that depending whether the process is reversible or not, both estimators fulfill the second law.

gr-qc↗
arXiv2024

The remarkable microquasar S26: a super-Eddington PeVatron?

Context. S26 is an extragalactic microquasar with the most powerful jets ever discovered. They have a kinetic luminosity of $L_{\rm j}\sim5\times 10^{40}\,{\rm erg\,s^{-1}}$. This implies that the accretion power to the black hole should be super-Eddington, of the order of $L_{\rm acc}\sim L_{\rm j}$. However, the observed X-ray flux of this system indicates an apparent very sub-Eddington accretion luminosity of $L_{\rm X}\approx 10^{37}\,{\rm erg\,s^{-1}}$. Aims. We aim to characterize the nature of S26, explain the system emission, and study the feasibility of super-Eddington microquasars as potential PeVatron sources. Methods. We first analyze X-ray observations of S26 obtained with XMM-Newton and model the super-Eddington disk and its wind. We then develop a jet model and study the particle acceleration and radiative processes that occur in shocks generated near the base of the jet and in its terminal region. Results. We find that the discrepancy between the jet and the apparent disk luminosities in S26 is caused by the complete absorption of the disk radiation by the wind ejected from the super-Eddington disk. The nonthermal X-rays are produced near the base of the jet, and the thermal X-rays are emitted in the terminal regions. The radio emission observed with the Australia Telescope Compact Array can be explained as synchrotron radiation produced at the reverse shock in the lobes. We also find that S26 can accelerate protons to PeV energies in both the inner jet and the lobes. The ultra-high energy protons accelerated in the lobes are injected into the ISM with a total power of $\sim 10^{36}\,{\rm erg\,s^{-1}}$. Conclusions. We conclude that S26 is a super-Eddington microquasar with a dense disk-driven wind that obscures the X-ray emission from the inner disk, and that the supercritical nature of the system allows the acceleration of cosmic rays to PeV energies.

astro-ph.HE↗
arXiv2024

Electromagnetic signatures of black hole clusters in the center of super-Eddington galaxies

Supermassive black holes (SMBHs) at the centers of active galaxies are fed by accretion disks that radiate from the infrared or optical to the X-ray bands. Several types of objects can orbit SMBHs, including massive stars, neutron stars, clouds from the broad- and narrow-line regions, and X-ray binaries. Isolated black holes with a stellar origin (BHs of $\sim10\,M_{\odot}$) should also be present in large numbers within the central parsec of the galaxies. These BHs are expected to form a cluster around the SMBH as a result of the enhanced star formation rate in the inner galactic region and the BH migration caused by gravitational dynamical friction. However, except for occasional microlensing effects on background stars or gravitational waves from binary BH mergers, the presence of a BH population is hard to verify. In this paper, we explore the possibility of detecting electromagnetic signatures of a central cluster of BHs when the accretion rate onto the central SMBH is greater than the Eddington rate. In these supercritical systems, the accretion disk launches powerful winds that interact with the objects orbiting the SMBH. Isolated BHs can capture matter from this dense wind, leading to the formation of small accretion disks around them. If jets are produced in these "single" microquasars, they could be sites of particle acceleration to relativistic energies. These particles in turn are expected to cool by various radiative processes. Therefore, the wind of the SMBH might illuminate the BHs through the production of both thermal and nonthermal radiation. We conclude that, under these circumstances, a cluster of isolated BHs could be detected at X-rays (with Chandra and XMM-Newton) and radio wavelengths (e.g., with the Very Large Array and the Square Kilometer Array) in the center of nearby super-Eddington galaxies.

astro-ph.HE↗
arXiv2024

General Relativistic magneto-hydrodynamical simulations of accretion flows through traversable wormholes

We present the first dynamical model of plasma accretion onto traversable wormholes by performing General Relativistic magneto-hydrodynamical (GRMHD) simulations of the flow on both sides of the wormhole. We evolve the ideal MHD equations on a wormhole spacetime described by the spherically symmetric Simpson--Visser metric. The disk is initialized on one side of the wormhole and accretes onto the throat driven by the magneto-rotational instability (MRI). We show that the inflowing plasma quickly settles in the throat and forms a hot, rotating cloud. The wormhole cloud acts as an engine in which gas coming from one side accumulates at the center, dissipates energy, and powers a mildly relativistic thermal wind toward the other side. Our novel predictions show that accreting wormholes behave very differently from black holes (BHs) in astrophysical environments. In particular, one mouth presents outflows without accretion signatures, contradicting the jet-disk symbiotic relation that holds for black holes.

astro-ph.HE↗
arXiv2024

Simultaneous NICER and NuSTAR observations of the Ultraluminous source NGC 4190 ULX-1

We present an X-ray analysis of three different XMM-Newton observations together with simultaneous NICER and NuSTAR observations of the ultraluminous X-ray source NGC 4190 ULX-1. Our goal is to constrain the structure of the accretion disk and the geometrical properties of the source. We performed a temporal and spectral analyses in the 0.4--30 keV energy range where the source is significantly detected in dedicated XMM-Newton, NICER and NuSTAR observations. The temporal analysis shows no flaring activity in the light curves. No pulsation is detected throughout. The source exhibits a typical ULX spectrum, which can be fitted with two thermal blackbody components plus a Comptonization tail at high energies. The luminosity-temperature relation of each thermal spectral component is consistent with the $L \propto T^{2}$ relation expected from an advection-dominated supercritical disk. We interpret these results as a super-Eddington accreting black hole seen almost face-on. A dense wind ejected from the disk obscures the central source, and a hot electron plasma is evacuated through the funnel formed above the hole. Geometric beaming is responsible for the ULX soft emission, whereas the hard tail is the result of Comptonization of soft photons by the electrons ejected through the funnel.

astro-ph.HE↗