Search

We investigate the effects of magnetic fields on neutrino production in microquasars. We calculate the steady particle distributions for the pions and muons generated in p-gamma and p-p interactions in the jet taking the effects of all energy losses into account. The obtained neutrino emission is significantly modified due to the synchrotron losses suffered by secondary pions and muons. The estimates made for neutrino fluxes arriving on the Earth imply that detection of high-energy neutrinos from the vicinity of the compact object can be difficult. However, in the case of windy microquasars, the interaction of energetic protons in the jet with matter of dense clumps of the wind could produce detectable neutrinos. This is because the pions and muons at larger distances from the compact object will not be affected by synchrotron losses., Comment: 12 pages, 11 figures, accepted for publication in A&A

Star-forming regions have been tentatively associated with gamma-ray sources since the early days of the COS B satellite. After the Compton Gamma-Ray Observatory, the statistical evidence for such an association has became overwhelming. Recent results from Cherenkov telescopes indicate that some high-energy sources are produced in regions of active star formation like Cygnus OB2 and Westerlund 2. In this paper I will briefly review what kind of stellar objects can produce gamma-ray emission in star-forming regions and I will suggest that the formation process of massive stars could in principle result in the production of observable gamma rays., Comment: 7 pages, 8 figures (in low resolution version). Invited talk at the "Fourth Heidelberg International Symposium on High Energy Gamma-Ray Astronomy 2008"

We present a model for high-energy emission in microquasars where the energy content of the jets is dominated by relativistic protons. We also include a primary leptonic component. Particles are accelerated up to relativistic energies in a compact region located near the base of the jet, where most of the emission is produced. We calculate the production spectrum due to proton and electron synchrotron radiation and photohadronic interactions. The target field for proton-photon collisions is provided by the synchrotron radiation in the acceleration region. In models with a significant leptonic component, strong internal photon-photon absorption can attenuate the emission spectrum at high energies. Depending on the values of the parameters, our model predicts luminosities in the range 10^34-10^37 erg s^-1 up to GeV energies, with a high-energy tail that can extend up to 10^16 eV. In some cases, however, absorption effects can completely suppress the emission above 10 GeV, giving rise to different spectral shapes. These results can be tested in the near future by observations with instruments like GLAST-Fermi, HESS II and MAGIC II., Comment: 4 pages, 3 figures, contribution to the "Fourth Heidelberg International Symposium on High-Energy Gamma-Ray Astronomy 2008"

If the jets of microquasars carry a significant power in the form of relativistic hadrons, then gamma rays and neutrinos can be produced by interactions with matter and photon fields either external or internal to the jet. In this paper I present some recent results of calculations of the interaction of hadronic jets with 1) matter of the jet itself, 2) photon fields generated by synchrotron radiation of both protons and electrons, and 3) matter external to the jet (e.g. a clumped wind). I briefly discuss neutrino production in these scenarios and the prospects of detection with new gamma-ray instruments. Finally, I make a few comments on the controversy about the nature of LS I +61 303., Comment: 10 pages and 5 figures. Contribution to the 7th. Microquasar Workshop, Foca 2008

We model the high-energy emission that results from the interaction of relativistic particles with photons and matter in the cluster of galaxies Abell 3376. The presence of relativistic particles is inferred from the recently found radio relics in this cluster, being one of the most prominent examples of double opposite, giant ringlike radio structures. Assuming that diffusive shock acceleration takes place in the cluster regions where radio relics are observed, we calculate the spectral energy distribution resulting from the most relevant non-thermal processes, which are synchrotron radiation, inverse Compton scattering, relativistic Bremsstrahlung, and inelastic proton-proton collisions. In the context of our model, the major radiative component at high energies is inverse Compton scattering, which could reach luminosities L ~ 9x10^{41} erg/s in the energy range between ~ 1 MeV and 10 TeV. Hadronic interactions would yield a minor contribution to the overall non-thermal emission, but would dominate at ultra-high energies. The cluster Abell 3376 might be detectable at gamma-rays by HESS, GLAST satellite and future planned Cherenkov arrays., Comment: 9 pages and 5 figures. Accepted for publication in MNRAS

Thermal radio and X-ray emission has been traditionally associated with the formation of stars. However, in recent years, non-thermal radiation from massive star forming regions has been detected. Synchrotron radio emission and non-thermal X-rays from the outflows of massive young stellar objects (YSOs) provide evidence of the presence of relativistic particles in these sources. In YSOs, the acceleration of particles is likely produced where the thermal jet impacts on the surrounding medium and a shock wave is formed. Thus, particles might be accelerated up to relativistic energies through a Fermi-I type mechanism. Relativistic electrons and protons can interact with thermal particles and photons, producing then gamma-rays. These energetic photons could be detected by the new generation of instruments, making massive YSOs a new population of gamma-ray surces. In the present contribution we briefly describe some massive star forming regions from which non-thermal radio emission has been detected. In addition, we present a general model for high-energy radiation from the massive YSOs embedded in these regions. We take into account both leptonic and hadronic interactions of particles accelerated at the termination points of the collimated outflows ejected from the protostar., Comment: 4 pages, 3 figures. Student contribution to the "First La Plata International School: Compact Objects and their Emission", held in La Plata, Argentina, March 10-14, 2008

We study the incidence of the underlying host galaxy light on the measured optical linear polarization of blazars. Our methodology consists of the implementation of simulated observations obtained under different atmospheric conditions, which are characterised by the Gaussian $\sigma$ of the seeing function. The simulated host plus active nucleus systems span broad ranges in luminosity, structural properties, redshift, and polarization; this allows us to test the response of the results against each of these parameters. Our simulations show that, as expected, the measured polarization is always lower than the intrinsic value, due to the contamination by non-polarized star light from the host. This effect is more significant when the host is brighter than the active nucleus, and/or a large photometric aperture is used. On the other hand, if seeing changes along the observing time under certain particular conditions, spurious microvariability could be obtained, especially when using a small photometric aperture. We thus give some recommendations in order to minimise both unwanted effects, as well as basic guidelines to estimate a lower limit of the true (nuclear) polarization. As an example, we apply the results of our simulations to real polarimetric observations, with high temporal resolution, of the blazar PKS 0521-365., Comment: 9 pages, 9 figures, 4 tables. Accepted for publication in MNRAS

Black holes are perhaps the most strange and fascinating objects known to exist in the universe. Our understanding of space and time is pushed to its limits by the extreme conditions found in these objects. They can be used as natural laboratories to test the behavior of matter in very strong gravitational fields. Black holes seem to play a key role in the universe, powering a wide variety of phenomena, from X-ray binaries to active galactic nuclei. In this article we will review the basics of black hole physics., Comment: 25 pages, 6 figures, Lecture Notes from the First La Plata International School on Astronomy and Geophysics

Unlike high-mass gamma-ray binaries, low-mass microquasars lack external sources of radiation and matter that could produce high-energy emission through interactions with relativistic particles. In this work we consider the synchrotron emission of protons and leptons that populate the jet of a low-mass microquasar. In our model photohadronic and inverse Compton (IC) interactions with synchrotron photons produced by both protons and leptons result in a high-energy tail of the spectrum. We also estimate the contribution from secondary pairs injected through photopair production. The high-energy emission is dominated by radiation of hadronic origin, so we can call these objects proton microquasars., Comment: 4 pages, 2 figures, accepted for publication in the International Journal of Modern Physics D, proceedings of HEPRO meeting, held in Dublin, in September 2007

A population of unidentified gamma-ray sources is forming a structure resembling a halo around the Galactic center. These sources are highly variable, and hence they should be associated with compact objects. Microquasars are objects undergoing accretion with relativistic jets; if such an object has a low-mass, evolved, donor star, it might be found in the Galactic halo. If these low-mass microquasars can generate detectable gamma-ray emission, then they are natural candidates to account for the halo high-energy sources. We aim to construct models for high-energy emission of low-mass microquasars, which could produce a significant luminosity in the gamma-ray domain. We consider that a significant fraction of the relativistic particles in the jets of low-mass microquasars are protons and then we study the production of high-energy emission through proton synchrotron radiation and photopion production. Photopair production and leptonic processes are considered as well. We compute a number of specific models with different parameters to explore the possibilities of this scenario.} We find that important luminosities, in the range of $10^{34}-10^{37}$ erg s$^{-1}$, can be achieved by proton synchrotron radiation in the Gamma-Ray Large Area Space Telescope (GLAST) energy range, and lower, but still significant luminosities at higher energies for some models. We conclude that the "proton microquasar" model offers a very interesting alternative to account for the halo gamma-ray sources and presents a variety of predictions that might be tested in the near future by instruments like GLAST, the High-Energy Stereoscopic System II (HESS II), the Major Atmospheric Gamma-ray Imaging Cherenkov telescope II (MAGIC II), and neutrino telescopes like IceCube., Comment: 11 pages, 7 figures, final version, accepted for publication in A&A

In this work we study gravitational lensing of neutrinos by Schwarzschild black holes. In particular, we analyze the case of a neutrino transient source associated with a gamma-ray burst lensed by a supermassive black hole located at the center of an interposed galaxy. We show that the primary and secondary images have an angular separation beyond the resolution of forthcoming km-scale detectors, but the signals from each image have time delays between them that in most cases are longer than the typical duration of the intrinsic events. In this way, the signal from different images can be detected as separate events coming from the very same location in the sky. This would render an event that otherwise might have had a low signal-to-noise ratio a clear detection, since the probability of a repetition of a signal from the same direction is negligible. The relativistic images are so faint and proximate that are beyond the sensitivity and resolution of the next-generation instruments., Comment: 9 pages; v2: improved version, new references added. Accepted for publication in Physics Letters B

We study the spectral energy distribution of gamma rays and neutrinos in the precessing microquasar SS433 as a result of pp interactions within its dark jets. Gamma-ray absorption due to interactions with matter of the extended disk and of the star is found to be important, as well as absorption caused by the UV and mid-IR radiation from the equatorial envelopment. We analyze the range of precessional phases for which this attenuation is at a minimum and the chances for detection of a gamma-ray signal are enhanced. The power of relativistic protons in the jets, a free parameter of the model, is constrained by HEGRA data. This imposes limits on the gamma-ray fluxes to be detected with instruments such as GLAST, VERITAS and MAGIC II. A future detection of high energy neutrinos with cubic kilometer telescopes such as IceCube would also yield important information about acceleration mechanisms that may take place in the dark jets. Overall, the determination of the ratio of gamma-ray to neutrino flux will result in a key observational tool to clarify the physics of heavy jets., Comment: 10 pages, 13 figures. Accepted for publication in MNRAS

We present optical photo-polarimetric observations with high temporal resolution of the blazar AO 0235+164. Our data, the first to test the photo-polarimetric behaviour of this object at very short time-scales, show significant micro-variability in total flux, colour index, linear polarization degree, and position angle. Strong inter-night variations are also detected for these parameters. Although no correlation between colour index and total flux was found, our data seem to support the general bluer-when-brighter trend already known for this object. The polarization degree, in turn, shows no correlation with total flux, but a clear trend in the sense that colour index is redder (the spectrum is softer) when the measured polarization is higher., Comment: Accepted by MNRAS (Letters), 5 pages

We discuss the gamma-ray absorption in the inner region of the microquasar SS433. Our investigation includes several contributions to the opacity of this system. They result from the ambient fields generated by the primary star, possibly an A-type supergiant, and a very extended disk around the black hole. Besides the sharp and dramatic absorption effect that occurs every time the star crosses the emission zone, we find in the UV photon field from the extended disk an important source of absorption for very high energy gamma-rays. This results in periodic gamma-ray observational signatures., Comment: 8 pages, 9 figures, to appear in Astropart.Phys

Massive X-ray binaries are formed by a compact object that accretes matter from the stellar wind of an early-type donor star. In some of these systems, called microquasars, relativistic jets are launched from the surroundings of the compact object. Such jets interact with the photon field of the companion star, the stellar wind, and, at large distances, with the interstellar medium. In this paper I will review the main results of such interactions with particular emphasis on the production of high-energy photons and neutrinos. The case of some specific systems, like LS I +61 303, will be discussed in some detail. Prospects for future observations at different wavelengths of this type of objects will be presented., Comment: 5 pages, 5 figures. Invited talk presented at the meeting: "Massive Stars: fundamental parameters and circumstellar interactions", Carilo, Dec 2006

The COMPTEL unidentified source GRO J 1411-64 was observed by INTEGRAL and XMM-Newton in 2005. The Circinus Galaxy is the only source detected within the 4$\sigma$ location error of GRO J1411-64, but in here excluded as the possible counterpart. At soft X-rays, 22 reliable and statistically significant sources (likelihood $> 10$) were extracted and analyzed from XMM-Newton data. Only one of these sources, XMMU J141255.6 -635932, is spectrally compatible with GRO J1411-64 although the fact the soft X-ray observations do not cover the full extent of the COMPTEL source position uncertainty make an association hard to quantify and thus risky. At the best location of the source, detections at hard X-rays show only upper limits, which, together with MeV results obtained by COMPTEL suggest the existence of a peak in power output located somewhere between 300-700 keV for the so-called low state. Such a spectrum resembles those in blazars or microquasars, and might suggest at work by a similar scenario. However, an analysis using a microquasar model consisting on a magnetized conical jet filled with relativistic electrons, shows that it is hard to comply with all observational constrains. This fact and the non-detection at hard X-rays introduce an a-posteriori question mark upon the physical reality of this source, what is discussed here., Comment: Accepted for publication in Astrophysics and Space Science, as proceedings of "The Multi-Messenger Approach to High-Energy Gamma-Ray Sources", Barcelona, July 4-7, 2006, J. M. Paredes, O. Reimer, and D. F. Torres, editors

We report the first XMM detection of the SNR candidate G337.2+0.1 (=AX J1635.9-4719). The object shows centrally filled and diffuse X-ray emission. The emission peaks in the hard 3.0-10.0 keV band. A spatially resolved spectral study confirms that the column density of the central part of the SNR is about N_{H}~5.9 +/- 1.5*10^{22} cm^{-2} and its X-ray spectrum is well represented by a single power-law with a photon index Gamma=0.96 +/- 0.56. The non-detection of line emission in the central spectrum is consistent with synchrotron radiation from a population of relativistic electrons. Detailed spectral analysis indicates that the outer region is highly absorbed and quite softer than the inner region, with N_{H}~16.2(+/-5.2)*10^{22} cm^{-2} and kT=4.4(+/-2.8) keV. Such characteristics are already observed in other X-ray plerions. Based on the morphological and spectral X-ray information, we confirm the SNR nature of G337.2+0.1, and suggest that the central region of the source is a pulsar wind nebula (PWN), originated by an energetic though yet undetected pulsar, that is currently losing energy at a rate of ~ 10^{36} erg s^{-1}., Comment: 13 pages, 5 figures, Accepted for publication in Astrophysical Journal Letters

Variability amplitudes larger than 1 magnitude over time-scales of a few tens of minutes have recently been reported in the optical light-curves of several blazars. In order to independently verify the real occurrence of such extremely violent events, we undertook an observational study of a selected sample of three blazars: PKS 0048-097, PKS 0754+100, and PKS 1510-089. Possible systematic error sources during data acquisition and reduction were carefully evaluated. We indeed found flux variability at intra-night time-scales in all three sources, although no extremely violent behaviour, as reported by other authors, was detected. We show that an incorrect choice of the stars used for differential photometry will, under fairly normal conditions, lead to spurious variability with large amplitudes on short time-scales. Wrong results of this kind can be avoided with the use of simple error-control techniques., Comment: 8 pages, accepted for publication in MNRAS

The COMPTEL unidentified source GRO J1411-64 was observed by INTEGRAL, and its central part, also by XMM-Newton. The data analysis shows no hint for new detections at hard X-rays. The upper limits in flux herein presented constrain the energy spectrum of whatever was producing GRO J1411-64, imposing, in the framework of earlier COMPTEL observations, the existence of a peak in power output located somewhere between 300-700 keV for the so-called low state. The Circinus Galaxy is the only source detected within the 4$\sigma$ location error of GRO J1411-64, but can be safely excluded as the possible counterpart: the extrapolation of the energy spectrum is well below the one for GRO J1411-64 at MeV energies. 22 significant sources (likelihood $> 10$) were extracted and analyzed from XMM-Newton data. Only one of these sources, XMMU J141255.6-635932, is spectrally compatible with GRO J1411-64 although the fact the soft X-ray observations do not cover the full extent of the COMPTEL source position uncertainty make an association hard to quantify and thus risky. The unique peak of the power output at high energies (hard X-rays and gamma-rays) resembles that found in the SED seen in blazars or microquasars. However, an analysis using a microquasar model consisting on a magnetized conical jet filled with relativistic electrons which radiate through synchrotron and inverse Compton scattering with star, disk, corona and synchrotron photons shows that it is hard to comply with all observational constrains. This and the non-detection at hard X-rays introduce an a-posteriori question mark upon the physical reality of this source, which is discussed in some detail.

We study the precession of accretion disks in the context of gamma-ray burst inner engines. With an accretion disk model that allows for neutrino cooling, we evaluate the possible periods of disk precession and nutation due to the Lense-Thirring effect. Assuming jet ejection perpendicular to the disk midplane and a typical intrinsic time dependence for the burst, we find possible gamma-ray light curves with temporal microstructure similar to what is observed in some subsamples. We conclude that the precession and nutation of a neutrino-cooled accretion disk in the burst engine might be responsible for some events, especially those with a slow rise and a fast decay., Comment: 7 pages, 8 figures, accepted for publication in A&A

We show that high-energy neutrinos can be efficiently produced in X-ray binaries with relativistic jets and high-mass primary stars. We consider a system where the star presents a dense equatorial wind and the jet has a small content of relativistic protons. In this scenario, neutrinos and correlated gamma-rays result from pp interactions and the subsequent pion decays. As a particular example we consider the microquasar LS I +61 303. Above 1 TeV, we obtain a mean-orbital neutrino muon induced luminosity of 5 10^34 erg/s which can be related to an event rate of 4-5 muon-type neutrinos per kilometer-squared per year after considering the signal attenuation due to maximal neutrino oscillations. The maximal neutrino energies here considered will range between 20 and 85 TeV along the orbit. The local infrared photon-field is responsible for opacity effects on the associated gamma radiation at high energies, but below 50 GeV the source could be detected by MAGIC telescope. GLAST observations at photon energies E>100 MeV should also reveal a strong source., Comment: Version improved. To appear in Phys Rev D. 5 figures

A 50 ksec Chandra observation of the unidentified TeV source in Cygnus reported by the HEGRA collaboration reveals no obvious diffuse X-ray counterpart. However, 240 Pointlike X-ray sources are detected within or nearby the extended TeV J2032+4130 source region, of which at least 36 are massive stars and 2 may be radio emitters. That the HEGRA source is a composite, having as counterpart the multiple point-like X-ray sources we observe, cannot be ruled out. Indeed, the distribution of point-like X-ray sources appears non-uniform and concentrated broadly within the extent of the TeV source region. We offer a hypothesis for the origin of the very high energy gamma-ray emission in Cyg OB2 based on the local acceleration of TeV range cosmic rays and the differential distribution of OB vs. less massive stars in this association., Comment: Substantially revised version; incorporates referee suggestions & expanded discussion

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., Comment: 18 pages,5 figures. Accepted for publication in Astrophysical Journal

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., Comment: 9 pages, 5 figures, accepted (Astronomy & Astrophysics)

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., Comment: 8 pages, 2 figures, accepted for publication in A&A

We show that changes in the orientation of the inner accretion disk of Cygnus X-1 affect the shape of the broad Fe K$\alpha$ 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., Comment: To appear in The Astrophysics Journal

Population studies of EGRET gamma-ray sources indicate that there is a distinctive population of bright sources at low galactic latitudes. The sources have a distribution consistent with that of young galactic objects, with a concentration toward the inner spiral arms. There is a subgroup that displays strong variability with timescales from days to months. Following an earlier suggestion by Kaufman Bernado et al. (2002), we explore the possibility that these sources could be high-mass microquasars. Detailed models for the gamma-ray emission that include inverse Compton interactions of electrons in the relativistic jets and photons from all local fields (stellar UV photons, synchrotron photons, soft X-ray photons from the accretion disk, and hard X-ray photons from a corona) are presented. We conclude that microquasars are excellent candidates for the parent population of the subgroup of variable low-latitude EGRET sources., Comment: (1)Universitat de Barcelona (2)Instituto Argentino de Radioastronomia (3)Universidad Nacional de La Plata 12 pages, 5 figures. Presented as talk to the "Multiwavelength Approach to Unidentified Gamma-Ray Sources", Eds. K.S. Cheng & G.E. Romero, Accepted for publication in Astrophysics and Space Science journal (title and some references corrected)

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., Comment: Talk presented at the V Microquasar Workshop, Beijing, China, June 5-13, 2004. Proceedings to appear in the Chinese Journal of Astronomy and Astrophysics

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., Comment: Invited talk presented at the V Microquasar Workshop, Beijing, June 2004. 12 pages, 7 figures

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., Comment: 14 pages, to appear in: "The multiwavelength approach to unidentified gamma-ray sources", Eds. K. S. Cheng & G.E. Romero, Kluwer Academic Publisher (Astrophysics and Space Sciences Journal). The present version has two additional figures respect to the version to be published in the journal

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 $\gamma$-ray flux., Comment: Presented for the proceedings of the II Workshop on Unidentified Gamma-Ray Sources, Hong Kong, June 1-4, 2004. To appear in Astrophysics and Space Science. Some changes to address referee's and readers' remarks. References added. Results unchanged

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., Comment: Presented at 5th INTEGRAL Workshop: The INTEGRAL Universe, Munich, Germany, 16-20 Feb 2004. To be published in the proceedings. Version #2, with minimal changes after Referee's comments

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., Comment: 10 pages, 1 figure. Accepted for publication in Astrophysical Journal Letters

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., Comment: Accepted for publication in Astronomy and Astrophysics as a Letter to the Editor

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., Comment: Astrophysical Journal Letters, in press. 2 figures, 1 table

The origins of all extra-solar cosmic rays -- both the ultra high-energy and lower energy Galactic variety -- remain unclear. It has been argued that the large scale shocks and turbulence induced by the multiple interacting supersonic winds from the many hot stars in young stellar associations may play a role in accelerating Galactic cosmic rays -- with or without the associated multiple supernova remnants (eg. Cesarsky & Montmerle, 1983; Bykov 2001). In this context, the report by the HEGRA Cherenkov telescope group of a steady and extended unidentified TeV gamma-ray source lying at the outskirts of Cygnus OB2 is particularly significant. This is the most massive stellar association known in the Galaxy, estimated to contain ~2600 OB type members alone indeed, Cyg OB2 also coincides with the non-variable MeV-GeV range unidentified EGRET source, 3EG 2033+4118. We summarise here the near-simultaneous follow-up observations of the extended TeV source region with the CHANDRA X-ray Observatory and the Very Large Array (VLA) radio telescope. Applying a numerical simulation which accurately tracks the radio to gamma-ray emission from primary hadrons as well as primary and secondary e+/-, we find that the broadband spectrum of the TeV source region favors a predominantly nucleonic rather than electronic origin of the high-energy flux, though deeper X-ray and radio observations will help confirm this. A very reasonable, ~0.1%, conversion efficiency of Cyg OB2's extreme stellar wind mechanical luminosity to nucleonic acceleration to ~PeV (10^15 eV) energies is sufficient to explain the multifrequency emissions., Comment: To appear in the Proceedings of the 2nd VERITAS Symposium on the TeV Astrophysics of Extragalactic Sources, Chicago, Il, April, 2003. (New Astronomy Reviews)

The HEGRA Cherenkov telescope array group recently reported a steady and extended unidentified TeV gamma-ray source lying at the outskirts of Cygnus OB2. This is the most massive stellar association known in the Galaxy, estimated to contain ~2600 OB type members alone. It has been previously argued that the large scale shocks and turbulence induced by the multiple interacting supersonic winds from the many young stars in such associations may play a role in accelerating Galactic cosmic rays. Indeed, Cyg OB2 also coincides with the non-variable MeV-GeV range unidentified EGRET source, 3EG 2033+4118. We report on the near-simultaneous follow-up observations of the extended TeV source region with the CHANDRA X-ray Observatory and the Very Large Array (VLA) radio telescope obtained in order to explore this possibility. Analysis of the CO, HI, and IRAS 100 micron emissions shows that the TeV source region coincides with an outlying sub-group of powerful OB stars which have evacuated or destroyed much of the ambient atomic, molecular and dust material, and which may be related to the very high-energy emissions. An interesting SNR-like structure is also revealed near the TeV source region in the CO, HI and radio emission maps. Applying a numerical simulation which accurately tracks the radio to gamma-ray emission from primary hadrons as well as primary and secondary e+/-, we find that the broadband spectrum of the TeV source region favors a predominantly nucleonic - rather than electronic - origin of the high-energy flux, though deeper X-ray and radio observations are needed to confirm this. A very reasonable, ~0.1%, conversion efficiency of Cyg OB2's extreme stellar wind mechanical luminosity to nucleonic acceleration to ~PeV (10^15 eV) energies is sufficient to explain the multifrequency emissions., Comment: ApJ accepted

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 $\gamma$-ray generation. Close starbursts are also found to be potential sources for km-scale neutrino telescopes, like ICECUBE, within reasonable integration times., Comment: To appear in Astrophysical Journal Letters. One reference added

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., Comment: 4 pages; to be submitted to the proceedings of the international workshop on pulsars, AXPs and SGRs observed with Beppo-SAX and other observatories

It might be thought that supernova remnants (SNRs) more distant than a few kiloparsec from Earth could not have been detected by the EGRET experiment. This work analyzes the observational status of this statement in the light of new CO studies of SNRs., Comment: Accepted for publication in Advances in Space Research, in High Energy Studies of Supernova Remnants and Neutron Stars, eds. W. Becker and W. Hermsen (2003)

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 $\nu$-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 $\nu$-spectrum with extensive disc shower simulations. We show that the $\nu$-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 $\nu$-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 $\nu$-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., Comment: 7 pages, 5 figures. Updates to match accepted version in Astrophys. J

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., Comment: Final version of a review article, to appear in the Physics Reports (82 pages, 31 figures). Figures requiring high quality are just too large and too many to be included here. Please download them from http://www.angelfire.com/id/dtorres/down3.html

It is thought that Galactic cosmic ray (CR) nuclei are gradually accelerated to high energies (up to ~300 TeV/nucleon, where 1TeV=10^12eV) in the expanding shock-waves connected with the remnants of powerful supernova explosions. However, this conjecture has eluded direct observational confirmation^1,2 since it was first proposed in 1953 (ref. 3). Enomoto et al.^4 claim to have finally found definitive evidence that corroborates this model, proposing that the very-high-energy, TeV-range, gamma-rays from the supernova remnant (SNR) RX J1713.7-3946 are due to the interactions of energetic nuclei in this region. Here we argue that their claim is not supported by the existing multiwavelength spectrum of this source. The search for the origin(s) of Galactic cosmic ray nuclei may be closing in on the long-suspected supernova-remnant sources, but it is not yet over., Comment: 4 pages, 1 Figure

The unidentified EGRET source 3EG J1828+0142, located at (l,b) ~ (31.9,5.8) (Hartman et al. 1999), is particularly enigmatic since it displays very high variability over timescales of months (Torres et al. 2000, Tompkins 1999) yet no blazar has been found within the 95% confidence contour of the gamma-ray location error-box (Punsly et al 2000). It also has an unusually steep gamma-ray spectral index of Gamma=2.76 +/- 0.39. It's relatively low latitude may be indicative of a galactic origin of the variable GeV flux. A multifrequency study of this source has been initiated and may reveal the presence of a novel type of compact object: a galactic 'microblazar'. X-ray data, especially those of high spatial resolution from CHANDRA, would be particularly useful in establishing such a connection., Comment: To be Submitted to Proceedings of the 4th Microquasar Workshop. Updated Figure

The gamma-ray & neutrino visibility of cosmic ray (CR) accelerators will be dramatically increased by the presence of molecular material abutting such sites due to the increased probability of pion production -- and, in the case of neutral pions, subsequent gamma-decay. This was recognized by Pinkau, Montmerle, and Black & Fazio, and others in the 1970's. In an effort to examine the long-standing -- but unproven -- conjecture that galactic supernova remnants (SNRs) are indeed the sites of nucleonic CR acceleration to <~300TeV/n we have carried out a 3-way coincidence search between the SNRs in Green's (2001) catalog, the GeV range unidentified sources from the third EGRET catalog (Hartman et al.,1999), and molecular clouds, at low Galactic latitudes (|b|<~5 deg). In order to be quantitative regarding the distribution and amount of the ambient molecular masses we have extracted the CO(J=1->0) mm wavelength data from the compilation of Dame, Hartmann & Thaddeus (2001), at the best estimates of the various SNR distances. We outline the overall correlative study and examine the interesting case of SNR IC443, a likely accelerator of CR nuclei., Comment: To be submitted to the Proceedings of the 2002 Moriond Gamma-Ray Universe Conference. Ammended to include clarification from A. Bykov regarding fits in Bykov et al. 2000

We present a detailed study of the effects of gravitational microlensing on compact and distant $\gamma$-ray blazars. These objects have $\gamma$-ray emitting regions which are small enough as to be affected by microlensing effects produced by stars lying in intermediate galaxies. We analyze the temporal evolution of the gamma-ray magnification for sources moving in a caustic pattern field, where the combined effects of thousands of stars are taken into account using a numerical technique. We propose that some of the unidentified $\gamma$-ray sources (particularly some of those lying at high galactic latitude whose gamma-ray statistical properties are very similar to detected $\gamma$-ray blazars) are indeed the result of gravitational lensing magnification of background undetected Active Galactic Nuclei (AGNs)., Comment: 30 pages, 27 figures. Four figures are being submitted only as .gif files, and should be printed separately. The abstract below has been shortened from the actual version appearing in the paper

We propose that some of the high-latitude unidentified EGRET gamma-ray sources could be the result of gravitational lensing amplification of the innermost regions of distant, faint, active galactic nuclei. These objects have gamma-ray emitting regions small enough as to be affected by microlensing of stars in interposed galaxies. We compute the gravitational amplification taking into account effects of the host galaxy of the lens and prove that, whereas the innermost gamma-ray regions can be magnified up to thousand times, there is no amplification at radio frequencies, leading to the observed absence of strong counterparts. Some new effects in the spectral evolution of a gravitational microlensed gamma-ray AGN are predicted. Within a reasonable range of lensing parameters, and/or types of sources, both variable and non-variable EGRET detections at high latitudes can be explained by microlensing. The same phenomenon could also have an important incidence among the future GLAST detections at high-latitudes., Comment: Accepted for publication in The Astrophysical Journal. Scheduled to appear in April, 2002. 2 figures

The non-thermal supernova remnant RX J1713.7-3946 (G347.3-0.5) has recently been shown to be a site of cosmic ray (CR) electron acceleration to TeV energies (Muraishi et al. 2000). Here we present evidence that this remnant is also accelerating CR nuclei. Such nuclei can interact with ambient interstellar gas to produce high energy gamma-rays via the decay of neutral pions. We associate the unidentified EGRET GeV gamma- ray source 3EG J1714-3857 with a massive (~3*10 5 Mo) and dense (~500 nucleons cm -3) molecular cloud interacting with RX J1713.7-3946. Direct evidence for such interaction is provided by observations of the lowest two rotational transitions of CO in the cloud; as in other clear cases of interaction, the CO(J=2-1)/CO(J=1-0) ratio is significantly enhanced. Since the cloud is of low radio and X-ray brightness, CR electrons cannot be responsible for the bulk of its GeV emission there. A picture thus emerges where both electrons and nuclei are being accelerated by the SNR: whereas the CR electrons dominate the local non-thermal radio, X-ray and TeV emission, the shock accelerated CR protons and ions (hadrons) are revealed through their interactions in the adjacent massive cloud. Such a scenario had been anticipated by Aharonian, Drury and Volk (1994)., Comment: 15 pages, 2 low-resolution figures, submitted to ApJL Aug 10, 2001; this version includes referee's suggestions

We study the colour changes induced by blending in a wormhole-like microlensing scenario with extended sources. The results are compared with those obtained for limb darkening. We assess the possibility of an actual detection of the colour curve using the difference image analysis method., Comment: Accepted for publication in Modern Physics Letters A. 13 report pages, 7 figures

We present a study of the unidentified gamma-ray source 3EG J0542+2610. This source is spatially superposed to the supernova remnant G180.0-1.7, but its time variability makes unlikely a physical link. We have searched into the EGRET location error box for compact radio sources that could be the low energy counterpart of the gamma-ray source. Although 29 point-like radio sources were detected and measured, none of them is strong enough as to be considered the counterpart of a background gamma-ray emitting AGN. We suggest that the only object within the 95 % error box capable of producing the required gamma-ray flux is the X-ray transient A0535+26. We show that this Be/accreting pulsar can produce variable hadronic gamma-ray emission through the mechanism originally proposed by Cheng & Ruderman (1989), where a proton beam accelerated in a magnetospheric electrostatic gap impacts the transient accretion disk., Comment: 6 pages, 3 figures, Astronomy & Astrophysics in press