arXiv subjects

Gustavo E. Romero

Publications and source records attributed to Gustavo E. Romero.

At least 163 records · Page 9Linked to original sources

arXiv2005

Hadronic high-energy gamma-ray emission from the microquasar LS I $+61 303$

We present a hadronic model for gamma-ray production in the microquasar LS I $+61 303$. The system is formed by a neutron star that accretes matter from the dense and slow equatorial wind of the Be primary star. We calculate the gamma-ray emission originated in $pp$ interactions between relativistic protons in the jet and cold protons from the wind. After taking into account opacity effects on the gamma-rays introduced by the different photons fields, we present high-energy spectral predictions that can be tested with the new generation Cherenkov telescope MAGIC.

astro-ph↗

arXiv2005

Multiwavelength studies of WR 21a and its surroundings

We present results of high-resolution radio continuum observations towards the binary star WR 21a (Wack 2134) obtained with the Australia Telescope Compact Array (ATCA) at 4.8 and 8.64 GHz. We detected the system at 4.8 GHz (6 cm) with a flux density of 0.25+-0.06 mJy and set an upper limit of 0.3 mJy at 8.64 GHz (3 cm). The derived spectral index of alpha < 0.3 [S(nu)=k.nu**(alpha)] suggests the presence of non-thermal emission, probably originating in a colliding-wind region. A second, unrelated radio source was detected ~10" north of WR 21a at (RA, Dec)(J2000)=(10h 25m 56.49, -57deg 48' 34.4", with flux densities of 0.36 and 0.55 mJy at 4.8 and 8.64 GHz, respectively, resulting in alpha = 0.72. HI observations in the area are dominated by absorption against the prominent HII region RCW 49. Analysis of a complete set of archived X-ray observations of WR 21a confirms its strong variability but throws into doubt previous suggestions by Reig (1999) of a period of years for the system. Finally, we comment on the association with the nearby EGRET source 3EG J1027-5817.

astro-ph↗

arXiv2005

Gamma-ray and neutrino emission from misaligned microquasars

Microquasars are accreting X-ray binary systems with non-thermal radio jets. In some of these systems the jet is expected to be strongly misaligned with the perpendicular to the orbital plane. If the donor star is an early-type star, the jet could collide with the stellar wind producing a standing shock between the compact object and the stellar surface. Relativistic particles injected by the jet can be re-accelerated and isotropized at the colliding region. If the jet has hadronic content, TeV protons will diffuse into the inner, dense wind leading to gamma-ray and neutrino production from interactions with the matter of the wind. In the case of very powerful jets, the wind pressure can be overbalanced and the jet might impact directly onto the stellar surface. We present estimates of the gamma-ray and neutrino luminosities for different sets of parameters in these scenarios and we briefly discuss the effects of this radiation on the donor star and its detectability with current instruments.

astro-ph↗

arXiv2005

Probing the precession of the inner accretion disk in Cygnus X-1

We show that changes in the orientation of the inner accretion disk of Cygnus X-1 affect the shape of the broad Fe K$α$ emission line emitted from this object, in such a way that eV-level spectral resolution observations (such as those that will be carried out by the {\it ASTRO-E2} satellite) can be used to analyze the dynamics of the disk. We here present a potential diagnostic tool, supported by numerical simulations, by which a few observations of Cygnus X-1, separated in time, can determine whether its accretion disk actually precesses, and if so, determine its period and precession angle. This approach could also be used for similar studies in other microquasar systems.

astro-ph↗

arXiv2004

Status of the connection between unidentified EGRET sources and supernova remnants: The case of CTA 1

In this paper we briefly comment on the observational status of the possible physical association between unidentified EGRET sources and supernova remnants (SNRs) in our Galaxy. We draw upon recent results presented in the review by Torres et al. (Physics Reports, 2003), concerning molecular gas in the vicinity of all 19 SNRs found to be positionally coincident with EGRET sources at low Galactic latitudes. In addition, we present new results regarding the supernova remnant CTA~1. Our findings disfavor the possibility of a physical connection with the nearby (in projection) EGRET source. There remains possible, however, that the compact object produced in the supernova explosion be related with the observed $γ$-ray flux.

astro-ph↗

arXiv2004

Neutrinos from microquasars

The jets of microquasars with high-mass stellar companions are exposed to the dense matter field of the stellar wind as well as to the photon densities found in the surrounding medium. Photopion and proton-proton interactions could then lead to copious production of neutrinos. In this work, we analyze the hadronic microquasar model, particularly in what concerns to the neutrino production. Limits to this kind of models using data from AMANDA-II are established. New constraints are also imposed upon specific microquasar models based on photopion processes. These are very restrictive particularly for the case of SS433, a microquasar for which the presence of accelerated hadrons has been already inferred from iron X-ray line observations.

astro-ph↗

arXiv2004

Microquasars and gamma-ray sources

Microquasars are X-ray binary systems with non-thermal radio emission originated in jet-like features. They are attractive sites for gamma-ray production, since relativistic particles in the jet should traverse locally strong both photon and matter fields. In this review we discuss whether some of the unidentified gamma-ray sources detected by the EGRET instrument of the Compton Gamma-Ray Observatory might be associated with microquasars. Relevant models for gamma-ray emission in such binaries are described and prospects for the detection of microquasars with instruments of new generation are briefly evaluated.

astro-ph↗

arXiv2004

Gamma-Ray Emission From Be/X-Ray Binaries

Be/X-ray binaries are systems formed by a massive Be star and a magnetized neutron star, usually in an eccentric orbit. The Be star has strong equatorial winds occasionally forming a circumstellar disk. When the neutron star intersects the disk the accretion rate dramatically increases and a transient accretion disk can be formed around the compact object. This disk can last longer than a single orbit in the case of major outbursts. If the disk rotates faster than the neutron star, the Cheng-Ruderman mechanism can produce a current of relativistic protons that would impact onto the disk surface, producing gamma-rays from neutral pion decays and initiating electromagnetic cascades inside the disk. In this paper we present calculations of the evolution of the disk parameters during both major and minor X-ray events, and we discuss the generation of gamma-ray emission at different energies within a variety of models that include both screened and unscreened disks.

astro-ph↗

arXiv2004

On the nature of the unidentified MeV gamma-ray source GRO 1411-64

A recent analysis of COMPTEL data has revealed the existence of an unidentified and variable gamma-ray source, designated GRO J1411-64, at MeV energies in the galactic plane (Zhang et al. 2002). We discuss the possibility that such a source can be produced in the accreting neutron star system 2S 1417-624 through the bombardment of the accretion disk by relativistic protons accelerated in the magnetosphere by the Cheng-Ruderman mechanism.

astro-ph↗

arXiv2003

High-energy gamma-rays from stellar associations

It is proposed that TeV gamma-rays and neutrinos can be produced by cosmic rays (CRs) through hadronic interactions in the innermost parts of the winds of massive O and B stars. Convection prevents low-energy particles from penetrating into the wind, leading to an absence of MeV-GeV counterparts. It is argued that groups of stars located close to the CR acceleration sites in OB stellar associations may be detectable by ground-based Cherenkov telescopes.

astro-ph↗

arXiv2003

Linearized stability of charged thin-shell wormholes

The linearized stability of charged thin shell wormholes under spherically symmetric perturbations is analized. It is shown that the presence of a large value of charge provides stabilization to the system, in the sense that the constrains onto the equation of state are less severe than for non-charged wormholes.

gr-qc↗

arXiv2003

Hadronic gamma-ray emission from windy microquasars

The jets of microquasars with high-mass stellar companions are exposed to the dense matter field of the stellar wind. We present estimates of the gamma-ray emission expected from the jet-wind hadronic interaction and we discuss the detectability of the phenomenon at high energies. The proposed mechanism could explain some of the unidentified gamma-ray sources detected by EGRET instrument on the galactic plane.

astro-ph↗

arXiv2003

Testing the binary black hole paradigm through the Fe Kalpha line profile: application to 3C 273

We propose the study of long-term variations in the Fe Kalpha line profile as a discriminator for binary black hole (BH) systems. The existence of a binary BH in the center of an active galaxy will produce a particular signature in the evolution of the line profile, as a result of disk precession. This signature is a periodic variation of the position of the blue edge of the profile, accompanied by periodic change of its intensity. We show that detection of the former is clearly within the observational capabilities of planned X-ray missions. Detecting a periodic variation of line profiles would provide the first direct evidence for precessing discs in active galactic nuclei, as opposed to the existing evidence supporting only the precession of jets. We apply these ideas to 3C 273.

astro-ph↗

arXiv2003

Supernova remnants and gamma-ray sources

A review of the possible relationship between gamma-ray sources and supernova remnants (SNRs) is presented. Particular emphasis is given to the analysis of the observational status of the problem of cosmic ray acceleration at SNR shock fronts. All positional coincidences between SNRs and unidentified gamma-ray sources listed in the Third EGRET Catalog at low Galactic latitudes are discussed on a case by case basis. For several coincidences of particular interest, new CO(J=1-0) and radio continuum maps are shown, and the mass content of the SNR surroundings is determined. The contribution to the gamma-ray flux observed that might come from cosmic ray particles (particularly nuclei) locally accelerated at the SNR shock fronts is evaluated. We discuss the prospects for future research in this field and remark on the possibilities for observations with forthcoming gamma-ray instruments.

astro-ph↗

arXiv2003

Reissner-Nordstrom black hole lensing

In this paper we study the strong gravitational lensing scenario where the lens is a Reissner-Nordstrom black hole. We obtain the basic equations and show that, as in the case of Schwarzschild black hole, besides the primary and secondary images, two infinite sets of relativistic images are formed. We find analytical expressions for the positions and amplifications of the relativistic images. The formalism is applied to the case of a low-mass black hole placed at the galactic halo.

gr-qc↗

arXiv2003

Signatures of hadronic cosmic rays in starbursts? High-energy photons and neutrinos from NGC253

We show that it appears possible for starburst galaxies, like the nearby NGC 253, recently identified as a TeV source by the CANGAROO collaboration, to emit a significant amount of high-energy gamma-rays and neutrinos through hadronic processes in their cores. We suggest that proton illumination of the inner winds of massive stars can be a viable mechanism for producing TeV gamma-rays and neutrinos without a strong MeV-GeV counterpart. The rich stellar content of the starbursts, with millions of early-type stars concentrated in the central regions, where collective effects of the stellar winds and supernovae can produce a significant enhancement of the cosmic ray density, provides an adequate scenario for TeV $γ$-ray generation. Close starbursts are also found to be potential sources for km-scale neutrino telescopes, like ICECUBE, within reasonable integration times.

astro-ph↗

arXiv2003

ULXs, Microblazars, and the Unidentified EGRET sources

We suggest that ultraluminous X-ray sources (ULXs) and some of the variable low latitude EGRET gamma-ray sources may be two different manifestations of the same underlying phenomena: high-mass microquasars with relativistic jets forming a small angle with the line of sight (i.e. microblazars). Microblazars with jets formed by relatively cool plasma (Lorentz factors for the leptons up to a few hundreds) naturally lead to ULXs. If the jet contains very energetic particles (high-energy cutoff above Lorentz factors of several thousands) the result is a relatively strong gamma-ray source. As pointed out by Kaufman Bernado, Romero & Mirabel (2002), a gamma-ray microblazar will always have an X-ray counterpart (although it might be relatively weak), whereas X-ray microblazars might have no gamma-ray counterparts.

astro-ph↗

arXiv2003

Neutrinos from Accreting Neutron Stars

The magnetospheres of accreting neutron stars develop electrostatic gaps with huge potential drops. Protons and ions, accelerated in these gaps along the dipolar magnetic field lines to energies greater than 100 TeV, can impact onto a surrounding accretion disc. A proton-induced cascade so develops, and $ν$-emission is produced from charged pion decays. Using GEANT4, a computer code that tracks particles produced in high energy collisions, we have calculated the resulting $ν$-spectrum with extensive disc shower simulations. We show that the $ν$-spectrum produced out of the proton beam is a power law. We use this result to propose accretion-powered X-ray binaries (with highly magnetized neutron stars) as a new population of point-like $ν$-sources for km-scale detectors, such as ICECUBE. As a particular example we discuss the case of A0535+26. We show that ICECUBE should find A0535+26 to be a periodic $ν$-source: one for which the formation and loss of its accretion disc can be fully detected. Finally, we briefly comment on the possibility that smaller telescopes, like AMANDA, could also detect A0535+26 by folding observations with the orbital period.

hep-ph↗