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V. Bosch-Ramon

Publications and source records attributed to V. Bosch-Ramon.

At least 109 records · Page 6Linked to original sources
arXiv2007

Gamma-ray emission from massive young stellar objects

Massive stars form in dense and massive molecular cores. The exact formation mechanism is unclear, but it is possible that some massive stars are formed by processes similar to those that produce the low-mass stars, with accretion/ejection phenomena occurring at some point of the evolution of the protostar. This picture seems to be supported by the detection of a collimated stellar wind emanating from the massive protostar IRAS 16547-4247. A triple radio source is associated with the protostar: a compact core and two radio lobes. The emission of the southern lobe is clearly non-thermal. Such emission is interpreted as synchrotron radiation produced by relativistic electrons locally accelerated at the termination point of a thermal jet. Since the ambient medium is determined by the properties of the molecular cloud in which the whole system is embedded, we can expect high densities of particles and infrared photons. Because of the confirmed presence of relativistic electrons, inverse Compton and relativistic Bremsstrahlung interactions are unavoidable. Proton-proton collision should also occur, producing an injection of neutral pions. In this paper we aim at making quantitative predictions of the spectral energy distribution of the non-thermal spots generated by massive young stellar objects, with emphasis on the particular case of IRAS 16547-4247. We present spectral energy distributions for the southern lobe of this source, for a variety of conditions. We show that high-energy emission might be detectable from this object in the gamma-ray domain (MeV to TeV). The source may also be detectable at X-rays through long exposures with current X-ray instruments.

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arXiv2007

Exploring the connection between the stellar wind and the non-thermal emission in LS 5039

LS 5039 has been observed with several X-ray instruments so far. The source presents X-ray variability at orbital timescales in flux and photon index. The system harbors an O-type main sequence star with moderate mass loss. At present, the link between the X-rays and the stellar wind is unclear. New XMM-Newton observations have been performed around periastron and apastron passages in September 2005, when the stellar wind activity was apparently higher. April 2005 Chandra observations on LS 5039 are also revisited. Moreover, a compilation of Halpha EW data obtained since 1992, from which the stellar mass loss evolution can be approximately inferred, is carried out. XMM-Newton observations show higher and harder emission around apastron than around periastron. No signatures of thermal emission or a reflection iron line indicating the presence of an accretion disk are found in the spectrum, and the hydrogen column density (NH) is compatible with being the same in both observations and consistent with the interstellar value. 2005 Chandra observations show a hard X-ray spectrum, and possibly high fluxes. The Halpha EW shows yearly variations of a ~ 10%, and does not seem to be correlated with X-ray fluxes obtained at similar phases, unlike it would be expected in the wind accretion scenario. 2005 XMM-Newton and Chandra observations are consistent with 2003 RXTE/PCA results. The constancy of the NH seems to imply that either the X-ray emitter is located at >~ 1012 cm from the compact object, or the real N_H is 3-27 times smaller than the one predicted by a spherical symmetric wind model. We suggest that the multiwavelength non-thermal emission of LS 5039 is related to the radio jets and unlikely produced inside the binary system.

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arXiv2007

MAGIC upper limits on the very high energy emission from GRBs

The fast repositioning system of the MAGIC Telescope has allowed during its first data cycle, between 2005 and the beginning of year 2006, observing nine different GRBs as possible sources of very high energy gammas. These observations were triggered by alerts from Swift, HETE-II, and Integral; they started as fast as possible after the alerts and lasted for several minutes, with an energy threshold varying between 80 and 200 GeV, depending upon the zenith angle of the burst. No evidence for gamma signals was found, and upper limits for the flux were derived for all events, using the standard analysis chain of MAGIC. For the bursts with measured redshift, the upper limits are compatible with a power law extrapolation, when the intrinsic fluxes are evaluated taking into account the attenuation due to the scattering in the Metagalactic Radiation Field (MRF).

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arXiv2007

Chandra Observations of the Gamma-ray Binary LSI+61303: Extended X-ray Structure?

We present a 50 ks observation of the gamma-ray binary LSI+61303 carried out with the ACIS-I array aboard the Chandra X-ray Observatory. This is the highest resolution X-ray observation of the source conducted so far. Possible evidence of an extended structure at a distance between 5 and 12 arcsec towards the North of LSI+61303 have been found at a significance level of 3.2 sigma. The asymmetry of the extended emission excludes an interpretation in the context of a dust-scattered halo, suggesting an intrinsic nature. On the other hand, while the obtained source flux, of F_{0.3-10 keV}=7.1^{+1.8}_{-1.4} x 10^{-12} ergs/cm^2/s, and hydrogen column density, N_{H}=0.70+/-0.06 x 10^{22} cm^{-2}, are compatible with previous results, the photon index Gamma=1.25+/-0.09 is the hardest ever found. In light of these new results, we briefly discuss the physics behind the X-ray emission, the location of the emitter, and the possible origin of the extended emission ~0.1 pc away from LSI+61303.

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arXiv2006

Theoretical overview on high-energy emission in microquasars

Microquasar (MQ) jets are sites of particle acceleration and synchrotron emission. Such synchrotron radiation has been detected coming from jet regions of different spatial scales, which for the instruments at work nowadays appear as compact radio cores, slightly resolved radio jets, or (very) extended structures. Because of the presence of relativistic particles and dense photon, magnetic and matter fields, these outflows are also the best candidates to generate the very high-energy (VHE) gamma-rays detected coming from two of these objects, LS 5039 and LS I +61 303, and may be contributing significantly to the X-rays emitted from the MQ core. In addition, beside electromagnetic radiation, jets at different scales are producing some amount of leptonic and hadronic cosmic rays (CR), and evidences of neutrino production in these objects may be eventually found. In this work, we review on the different physical processes that may be at work in or related to MQ jets. The jet regions capable to produce significant amounts of emission at different wavelengths have been reduced to the jet base, the jet at scales of the order of the size of the system orbital semi-major axis, the jet middle scales (the resolved radio jets), and the jet termination point. The surroundings of the jet could be sites of multiwavelegnth emission as well, deserving also an insight. We focus on those scenarios, either hadronic or leptonic, in which it seems more plausible to generate both photons from radio to VHE and high-energy neutrinos. We briefly comment as well on the relevance of MQ as possible contributors to the galactic CR in the GeV-PeV range.

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arXiv2006

INTEGRAL serendipitous detection of the gamma-ray microquasar LS 5039

LS 5039 is the only X-ray binary persistently detected at TeV energies by the Cherenkov HESS telescope. It is moreover a gamma-ray emitter in the GeV and possibly MeV energy ranges. To understand important aspects of jet physics, like the magnetic field content or particle acceleration, and emission processes, such as synchrotron and inverse Compton (IC), a complete modeling of the multiwavelength data is necessary. LS 5039 has been detected along almost all the electromagnetic spectrum thanks to several radio, infrared, optical and soft X-ray detections. However, hard X-ray detections above 20 keV have been so far elusive and/or doubtful, partly due to source confusion for the poor spatial resolution of hard X-ray instruments. We report here on deep (300 ksec) serendipitous INTEGRAL hard X-ray observations of LS 5039, coupled with simultaneous VLA radio observations. We obtain a 20-40 keV flux of 1.1 +/- 0.3 mCrab (5.9 (+/-1.6) X 10^{-12} erg cm^{-2} s^{-1}), a 40-100 keV upper limit of 1.5 mCrab (9.5 x 10^{-12} erg cm^{-2}s^{-1}), and typical radio flux densities of about 25 mJy at 5GHz. These hard X-ray fluxes are significantly lower than previous estimates obtained with BATSE in the same energy range but, in the lower interval, agree with extrapolation of previous RXTE measurements. The INTEGRAL observations also hint to a break in the spectral behavior at hard X-rays. A more sensitive characterization of the hard X-ray spectrum of LS 5039 from 20 to 100 keV could therefore constrain key aspects of the jet physics, like the relativistic particle spectrum and the magnetic field strength. Future multiwavelength observations would allow to establish whether such hard X-ray synchrotron emission is produced by the same population of relativistic electrons as those presumably producing TeV emission through IC.

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arXiv2006

On the multiwavelength spectrum of the microquasar 1E 1740.7-2942

Context. The microquasar 1E 1740.7-2942 is a source located in the direction of the Galactic Center. It has been detected at X-rays, soft gamma-rays, and in the radio band, showing an extended radio component in the form of a double-sided jet. Although no optical counterpart has been found so far for 1E 1740.7-2942, its X-ray activity strongly points to a galactic nature. Aims. We aim to improve our understanding of the hard X-ray and gamma-ray production in the system, exploring whether the jet can emit significantly at high energies under the light of the present knowledge. Methods. We have modeled the source emission, from radio to gamma-rays, with a cold-matter dominated jet model. INTEGRAL data combined with radio and RXTE data, as well as EGRET and HESS upper-limits, are used to compare the computed and the observed spectra. Results. From our modeling, we find out that jet emission cannot explain the high fluxes observed at hard X-rays without violating at the same time the constraints from the radio data, favoring the corona origin of the hard X-rays. Also, 1E 1740.7-2942 might be detected by GLAST or AGILE at GeV energies, and by HESS and HESS-II beyond 100 GeV, with the spectral shape likely affected by photon-photon absorption in the disk and corona photon fields.

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arXiv2006

Spectral and variability properties of LS 5039 from radio to very high-energy gamma-rays

Microquasars are X-ray binaries with relativistic jets. The microquasar LS 5039 turned out to be the first high-energy gamma-ray microquasar candidate due to its likely association with the EGRET source 3EG J1824-1514. Further theoretical studies supported this association, which could be extended to other EGRET sources. Very recently, Aharonian et al. (2005) have communicated the detection of the microquasar LS 5039 at TeV energies. This fact confirms the EGRET source association and leaves no doubt about the gamma-ray emitting nature of this object. The aim of the present work is to show that, applying a cold-matter dominated jet model to LS 5039, we can reproduce many of the spectral and variability features observed in this source. Jet physics is explored, and some physical quantities are estimated as a by-product of the performed modeling. Although at the moment only LS 5039 has been detected on the entire electromagnetic spectrum, it does not seem unlikely that other microquasars will show similar spectral properties. Therefore, an in-depth study of the first gamma-ray microquasar, on theoretical grounds supported by observations, can render a useful knowledge applicable elsewhere.

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arXiv2006

Radio to TeV radiation initiated by termination of hadronic jets from microquasars in the ISM

Microquasars (MQs) are potential candidates to produce a non-negligible fraction of the observed galactic cosmic rays. The protons accelerated at the jet termination shock interact with the interstellar medium and may produce extended emission detectable at different energy bands through several processes: neutral pion-decay produce high-energy and very high-energy gamma-rays, secondary electrons produced by charged pion-decay generate synchrotron and bremsstrahlung emission. In addition, the jets of MQs themselves are likely sources of gamma-rays. We discuss about the association between the intrinsic and the indirect emission coming from these objects.

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arXiv2006

Microquasar hadronic jets at very high-energy gamma-rays

Microquasars (MQs) present emission over the whole spectrum, from radio wavelengths to gamma-rays. The microquasar spectral energy distribution is very complex, being a signature of the different physical processes that generate the radiation emitted by these objects. In this work, we estimate the amount of broad-band emission produced by relativistic protons, released from the jet of a MQ, interacting with high density regions of the ISM. We show that a two components source, the microquasar itself and the region of interaction between the jets and the ISM, could be unveiled by the new instruments at high-energy and very high-energy gamma-rays.

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arXiv2006

A microquasar model applied to unidentified gamma-ray sources

Among unidentified gamma-ray sources in the galactic plane, there are some that present significant variability and have been proposed to be high-mass microquasars. To deepen the study of the possible association between variable low galactic latitude gamma-ray sources and microquasars, we have applied a leptonic jet model based on the microquasar scenario that reproduces the gamma-ray spectrum of three unidentified gamma-ray sources, 3EG J1735-1500, 3EG J1828+0142 and GRO J1411-64, and is consistent with the observational constraints at lower energies. We conclude that if these sources were generated by microquasars, the particle acceleration processes could not be as efficient as in other objects of this type that present harder gamma-ray spectra. Moreover, the dominant mechanism of high-energy emission should be synchrotron self-Compton (SSC) scattering, and the radio jets may only be observed at low frequencies. For each particular case, further predictions of jet physical conditions and variability generation mechanisms have been made in the context of the model. Although there might be other candidates able to explain the emission coming from these sources, microquasars cannot be excluded as counterparts. Observations performed by the next generation of gamma-ray instruments, like GLAST, are required to test the proposed model.

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arXiv2006

Spectral energy distribution of the gamma-ray microquasar LS 5039

The microquasar LS 5039 has recently been detected as a source of very high energy (VHE) $γ$-rays. This detection, that confirms the previously proposed association of LS 5039 with the EGRET source 3EG~J1824$-$1514, makes of LS 5039 a special system with observational data covering nearly all the electromagnetic spectrum. In order to reproduce the observed spectrum of LS 5039, from radio to VHE $γ$-rays, we have applied a cold matter dominated jet model that takes into account accretion variability, the jet magnetic field, particle acceleration, adiabatic and radiative losses, microscopic energy conservation in the jet, and pair creation and absorption due to the external photon fields, as well as the emission from the first generation of secondaries. The radiative processes taken into account are synchrotron, relativistic Bremsstrahlung and inverse Compton (IC). The model is based on a scenario that has been characterized with recent observational results, concerning the orbital parameters, the orbital variability at X-rays and the nature of the compact object. The computed spectral energy distribution (SED) shows a good agreement with the available observational data.

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arXiv2005

A broadband leptonic model for gamma-ray emitting microquasars

Observational and theoretical studies point to microquasars (MQs) as possible counterparts of a significant fraction of the unidentified gamma-ray sources detected so far. At present, a proper scenario to explain the emission beyond soft X-rays from these objects is not known, nor what the precise connection is between the radio and the high-energy radiation. We develop a new model where the MQ jet is dynamically dominated by cold protons and radiatively dominated by relativistic leptons. The matter content and power of the jet are both related with the accretion process. The magnetic field is assumed to be close to equipartition, although it is attached to and dominated by the jet matter. For the relativistic particles in the jet, their maximum energy depends on both the acceleration efficiency and the energy losses. The model takes into account the interaction of the relativistic jet particles with the magnetic field and all the photon and matter fields. Such interaction produces significant amounts of radiation from radio to very high energies through synchrotron, relativistic Bremsstrahlung, and inverse Compton (IC) processes. Variability of the emission produced by changes in the accretion process (e.g. via orbital eccentricity) is also expected. The effects of the gamma-ray absorption by the external photon fields on the gamma-ray spectrum have been taken into account, revealing clear spectral features that might be observed. This model is consistent to the accretion scenario, energy conservation laws, and current observational knowledge, and can provide deeper physical information of the source when tested against multiwavelength data.

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arXiv2005

Broad-band electromagnetic radiation from microquasars interacting with ISM

Microquasars (MQs) are galactic objects with relativistic jets that constitute a source population which can be responsible for production of a non-negligible fraction of the observed galactic cosmic rays. These relativistic protons, associated with the termination of the jet, interact with the interstellar medium and, at certain surrounding conditions, may lead to production of detectable fluxes of high-energy and very high-energy gamma-rays. This radiation is accompanied by the broad-band emission of secondary electrons from decays of $π^\pm$-mesons produced through synchrotron, bremstrahlung and inverse Compton process. The features of broad-band emission initiated by proton-proton (pp) interactions in such a scenario is discussed in the context of the strategy of search for counterparts of high-energy and very high-energy gamma-ray sources in the galactic plane.

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arXiv2005

Orbital X-Ray Variability of the Microquasar LS 5039

The properties of the orbit and the donor star in the high mass X-ray binary microquasar LS 5039 indicate that accretion processes should mainly occur via a radiatively driven wind. In such a scenario, significant X-ray variability would be expected due to the eccentricity of the orbit. The source has been observed at X-rays by several missions, although with a poor coverage that prevents to reach any conclusion about orbital variability. Therefore, we conducted RossiXTE observations of the microquasar system LS 5039 covering a full orbital period of 4 days. Individual observations are well fitted with an absorbed power-law plus a Gaussian at 6.7 keV, to account for iron line emission that is probably a diffuse background feature. In addition, we have taken into account that the continuum is also affected by significant diffuse background contamination. Our results show moderate power-law flux variations on timescales of days, as well as the presence of miniflares on shorter timescales. The new orbital ephemeris of the system recently obtained by Casares et al. have allowed us to show, for the first time, that an increase of emission is seen close to the periastron passage, as expected in an accretion scenario. Moreover, the detected orbital variability is a factor of ~4 smaller than the one expected by using a simple wind accretion model, and we suggest that an accretion disk around the compact object could be responsible for this discrepancy. On the other hand, significant changes in the photon index are also observed clearly anti-correlated with the flux variations. We interpret the overall X-ray spectral characteristics of LS 5039 in the context of X-ray radiation produced by inverse Compton and/or synchrotron processes in the jet of this microquasar.

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arXiv2004

Electromagnetic radiation initiated by hadronic jets from microquasars in the ISM

Microquasars are potential candidates to produce a non-negligible fraction of the observed galactic cosmic rays. The protons accelerated at the jet termination shock interact with the interstellar medium and may produce detectable fluxes of extended emission at different energy bands: high-energy and very high-energy gamma-rays produced by neutral pion-decay, synchrotron and bremsstrahlung emission in a wide energy range generated by the secondary electrons produced by charged pion-decay. We discuss the association between this scenario and some of the unidentified EGRET sources in the galactic plane.

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arXiv2004

High-mass microquasars and low-latitude gamma-ray sources

Population studies of unidentified EGRET sources suggest that there exist at least three different populations of galactic gamma-ray sources. One of these populations is formed by young objects distributed along the galactic plane with a strong concentration toward the inner spiral arms of the Galaxy. Variability, spectral and correlation analysis indicate that this population is not homogeneous. In particular, there is a subgroup of sources that display clear variability in their gamma-ray fluxes on timescales from days to months. Following the proposal by Kaufman Bernadó et al. (2002), we suggest that this group of sources might be high-mass microquasars, i.e. accreting black holes or neutron stars with relativistic jets and early-type stellar companions. We present detailed inhomogeneous models for the gamma-ray emission of these systems that include both external and synchrotron self-Compton interactions. We have included effects of interactions between the jet and all external photon fields to which it is exposed: companion star, accretion disk, and hot corona. We make broadband calculations to predict the spectral energy distribution of these objects from radio up to GeV energies. The results and predictions can be tested by present and future gamma-ray instruments like INTEGRAL, AGILE, and GLAST.

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