arXiv subjects

V. Bosch-Ramon

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

At least 55 records · Page 3Linked to original sources

arXiv2012

Fermi~I particle acceleration in converging flows mediated by magnetic reconnection

Context. Converging flows with strong magnetic fields of different polarity can accelerate particles through magnetic reconnection. If the particle mean free path is longer than the reconnection layer is thick, but much shorter than the entire reconnection structure, the particle will mostly interact with the incoming flows potentially with a very low escape probability. Aims. We explore, in general and also in some specific scenarios, the possibility of particles to be accelerated in a magnetic reconnection layer by interacting only with the incoming flows. Methods. We characterize converging flows that undergo magnetic reconnection, and derive analytical estimates for the particle energy distribution, acceleration rate, and maximum energies achievable in these flows. We also discuss a scenario, based on jets dominated by magnetic fields of changing polarity, in which this mechanism may operate. Results. The proposed acceleration mechanism operates if the reconnection layer is much thinner than its transversal characteristic size, and the magnetic field has a disordered component. Synchrotron losses may prevent electrons from entering in this acceleration regime. The acceleration rate should be faster, and the energy distribution of particles harder than in standard diffusive shock acceleration. The interaction of obstacles with the innermost region of jets in active galactic nuclei and microquasars may be suitable sites for particle acceleration in converging flows.

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arXiv2012

Detection of VHE γ-rays from HESS J0632+057 during the 2011 February X-ray outburst with the MAGIC Telescopes

The very high energy (VHE) γ-ray source HESS J0632+057 has recently been confirmed to be a γ-ray binary. The optical counterpart is the Be star MWC 148, and a compact object of unknown nature orbits it every ~321 d with a high eccentricity of ~0.8. We monitored HESS J0632+057 with the stereoscopic MAGIC telescopes from 2010 October to 2011 March and detected significant VHE γ-ray emission during 2011 February, when the system exhibited an X-ray outburst. We find no γ-ray signal in the other observation periods when the system did not show increased X-ray flux. Thus HESS J0632+057 exhibits γ-ray variability on timescales of the order of one to two months possibly linked to the X-ray outburst that takes place about 100 days after the periastron passage. Furthermore our measurements provide for the first time the γ-ray spectrum down to about 140 GeV and indicate no turnover of the spectrum at low energies. We compare the properties of HESS J0632+057 with the similar γ-ray binary LS I +61 303, and discuss on the possible origin of the multi-wavelength emission of the source

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arXiv2011

Detection of the gamma-ray binary LS I +61 303 in a low flux state at Very High Energy gamma-rays with the MAGIC Telescopes in 2009

We present very high energy (VHE, E > 100 GeV) γ-ray observations of the γ-ray binary system LS I+61 303 obtained with the MAGIC stereo system between 2009 October and 2010 January. We detect a 6.3σ γ-ray signal above 400 GeV in the combined data set. The integral flux above an energy of 300 GeV is F(E>300 GeV)=(1.4 +- 0.3stat +- 0.4syst) * 10^{-12} cm^{-2} s^{-1}, which corresponds to about 1.3% of the Crab Nebula flux in the same energy range. The orbit-averaged flux of LS I +61 303 in the orbital phase interval 0.6--0.7, where a maximum of the TeV flux is expected, is lower by almost an order of magnitude compared to our previous measurements between 2005 September and 2008 January. This provides evidence for a new low emission state in LS I +61 303. We find that the change to the low flux state cannot be solely explained by an increase of photon-photon absorption around the compact star.

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arXiv2011

Thermal X-ray Emission from the Shocked Stellar Wind of Pulsar Gamma-ray Binaries

Gamma-ray loud X-ray binaries are binary systems that show non-thermal broadband emission from radio to gamma rays. If the system comprises a massive star and a young non-accreting pulsar, their winds will collide producing broadband non-thermal emission, most likely originated in the shocked pulsar wind. Thermal X-ray emission is expected from the shocked stellar wind, but until now it has neither been detected nor studied in the context of gamma-ray binaries. We present a semi-analytic model of the thermal X-ray emission from the shocked stellar wind in pulsar gamma-ray binaries, and find that the thermal X-ray emission increases monotonically with the pulsar spin-down luminosity, reaching luminosities of the order of 10^33 erg/s. The lack of thermal features in the X-ray spectrum of gamma-ray binaries can then be used to constrain the properties of the pulsar and stellar winds. By fitting the observed X-ray spectra of gamma-ray binaries with a source model composed of an absorbed non-thermal power law and the computed thermal X-ray emission, we are able to derive upper limits on the spin-down luminosity of the putative pulsar. We applied this method to LS 5039, the only gamma-ray binary with a radial, powerful wind, and obtain an upper limit on the pulsar spin-down luminosity of ~6x10^36 erg/s. Given the energetic constraints from its high-energy gamma-ray emission, a non-thermal to spin-down luminosity ratio very close to unity may be required.

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arXiv2011

Large-scale flow dynamics and radiation in pulsar gamma-ray binaries

Several gamma-ray binaries show extended X-ray emission that may be associated to interactions of an outflow with the medium. Some of these systems are, or may be, high-mass binaries harboring young nonaccreting pulsars, in which the stellar and the pulsar winds collide, generating a powerful outflow that should terminate at some point in the ambient medium. This work studies the evolution and termination, as well as the related radiation, of the shocked-wind flow generated in high-mass binaries hosting powerful pulsars. A characterization, based on previous numerical work, is given for the stellar/pulsar wind interaction. Then, an analytical study of the further evolution of the shocked flow and its dynamical impact on the surrounding medium is carried out. Finally, the expected nonthermal emission from the flow termination shock, likely the dominant emitting region, is calculated. The shocked wind structure, initially strongly asymmetric, becomes a quasi-spherical, supersonically expanding bubble, with its energy coming from the pulsar and mass from the stellar wind. This bubble eventually interacts with the environment on ~pc scales, producing a reverse and, sometimes, a forward shock. Nonthermal leptonic radiation can be efficient in the reverse shock. Radio emission is expected to be faint, whereas X-rays can easily reach detectable fluxes. Under very low magnetic fields and large nonthermal luminosities, gamma rays may also be significant. We conclude that the complexity of the stellar/pulsar wind interaction is likely to be smoothed out outside the binary system, where the wind-mixed flow accelerates and eventually terminates in a strong reverse shock. This shock may be behind the extended X-rays observed in some binary systems. For very powerful pulsars, part of the unshocked pulsar wind may directly interact with the large-scale environment.

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arXiv2011

Exploring Particle Acceleration in Gamma-Ray Binaries

Binary systems can be powerful sources of non-thermal emission from radio to gamma rays. When the latter are detected, then these objects are known as gamma-ray binaries. In this work, we explore, in the context of gamma-ray binaries, different acceleration processes to estimate their efficiency: Fermi I, Fermi II, shear acceleration, the converter mechanism, and magnetic reconnection. We find that Fermi I acceleration in a mildly relativistic shock can provide, although marginally, the multi-10 TeV particles required to explain observations. Shear acceleration may be a complementary mechanism, giving particles the final boost to reach such a high energies. Fermi II acceleration may be too slow to account for the observed very high energy photons, but may be suitable to explain extended low-energy emission. The converter mechanism seems to require rather high Lorentz factors but cannot be discarded a priori. Standard relativistic shock acceleration requires a highly turbulent, weakly magnetized downstream medium; magnetic reconnection, by itself possibly insufficient to reach very high energies, could perhaps facilitate such a conditions. Further theoretical developments, and a better source characterization, are needed to pinpoint the dominant acceleration mechanism, which need not be one and the same in all sources.

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arXiv2011

Gamma rays from extragalactic astrophysical sources

Presently there are several classes of detected gamma-ray extragalatic sources. They are mostly associated to active galactic nuclei (AGN) and (at soft gamma rays) to gamma-ray bursts (GRB), but not only. Active galactic nuclei consist of accreting supermassive black holes hosted by a galaxy that present in some cases powerful relativistic jet activity. These sources, which have been studied in gamma rays for several decades, are probably the most energetic astrophysical objects, and their appearance depends much on whether their jets point to us. Gamma-ray bursts, thought to be associated to collapsing or merging stellar-mass objects at cosmological distances, are also accreting highly relativistic jet sources that shine strongly at high energies. These are very short-duration events, but they are also the most luminous. Recently, star formation galaxies have turned out to be also gamma-ray emitters. On the other hand, clusters of galaxies have not been detected beyond X-rays yet. These are the largest known structures in the Universe; in their formation through accretion and merging, shocks and turbulence are generated, which may lead to gamma-ray production. In this work, the gamma-ray physics of AGNs is briefly presented, as well as that of starburst galaxies, GRBs and clusters of galaxies. Afterwards, we consider some particular cases of gamma-ray production in non-blazar AGN jets interacting with their medium at different scales.

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arXiv2011

Non-Thermal Emission from Galactic Jets

Jets are ubiquitous in the Universe. They are collimated outflows whose origin is associated to an accretion disc and a central object, and can be very powerful non-thermal emitters. Jets form in active galactic nuclei, gamma-ray bursts, microquasars, and young stellar objects. Galactic jets emitting non-thermal emission are typically associated to microquasars, although the jets of massive young stellar objects are also non-thermal sources. The production of non-thermal radiation, in particular radio synchrotron emission, is a clear indication that particle acceleration is taking place in the source, which hints to the generation of photons even at high energies. In this work, we will discuss the emitting sites in, or related to, microquasar jets, and briefly comment on the possibility of high-energy emission in jets from young stellar objects.

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arXiv2011

Jet/medium interactions at large scales

High energy emission can be produced in the interaction sites of both galactic and extragalactic jets with the surrounding medium. We have developed a radiative model that accounts for the continuous injection of relativistic electrons in the forward, reverse and recollimation shocks developed in the shell, cocoon and reconfinement interaction regions, respectively. We also performed hydrodynamical simulations to establish the physical properties in both galactic and extragalactic systems. The resulting non-thermal emission is studied assuming different values for the jet power, the external mass density and the source age for both FR I galaxies and galactic microquasars. The obtained fluxes are compared to current instrument sensitivities at radio, X-ray and gamma-ray bands.

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arXiv2011

Multifrequency Behavior of Microquasars in the GeV--TeV era: A review

Microquasars are X-ray binaries that present non-thermal radio jets. Efficient particle acceleration can take place in different regions of the jets of microquasars. The accelerated particles can emit gamma-rays via leptonic or hadronic processes, with a complex spectral and temporal behavior. The jet termination region can be also an efficient non-thermal emitter, as well as, in high-mass microquasars, the region of the binary system outside the jet. In this work, I briefly describe the physics behind the non-thermal emission observed in microquasars at different scales, focusing in the GeV and TeV bands.

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arXiv2011

A search for Very High Energy gamma-ray emission from Scorpius X-1 with the MAGIC telescopes

The acceleration of particles up to GeV or higher energies in microquasars has been the subject of considerable theoretical and observational efforts in the past few years. Sco X-1 is a microquasar from which evidence of highly energetic particles in the jet has been found when it is in the so-called Horizontal Branch (HB), a state when the radio and hard X-ray fluxes are higher and a powerful relativistic jet is present. Here we present the first very high energy gamma-ray observations of Sco X-1 obtained with the MAGIC telescopes. An analysis of the whole dataset does not yield a significant signal, with 95% CL flux upper limits above 300 GeV at the level of 2.4x10^{-12} ph/cm^2/s. Simultaneous RXTE observations were conducted to search for TeV emission during particular X-ray states of the source. A selection of the gamma-ray data obtained during the HB based on the X-ray colors did not yield a signal either, with an upper limit of 3.4x10^{-12} ph/cm^2/s. These upper limits place a constraint on the maximum TeV luminosity to non-thermal X-ray luminosity of L_{VHE}/L_{ntX}<0.02, that can be related to a maximum TeV luminosity to jet power ratio of L_{VHE}/L_{j}<10^{-3}. Our upper limits indicate that the underlying high-energy emission physics in Sco X-1 must be inherently different from that of the hitherto detected gamma-ray binaries.

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arXiv2011

MAGIC Upper Limits for two Milagro-detected, Bright Fermi Sources in the Region of SNR G65.1+0.6

We report on the observation of the region around supernova remnant G65.1+0.6 with the stand-alone MAGIC-I telescope. This region hosts the two bright GeV gamma-ray sources 1FGL J1954.3+2836 and 1FGL J1958.6+2845. They are identified as GeV pulsars and both have a possible counterpart detected at about 35 TeV by the Milagro observatory. MAGIC collected 25.5 hours of good quality data, and found no significant emission in the range around 1 TeV. We therefore report differential flux upper limits, assuming the emission to be point-like (<0.1 deg) or within a radius of 0.3 deg. In the point-like scenario, the flux limits around 1 TeV are at the level of 3 % and 2 % of the Crab Nebula flux, for the two sources respectively. This implies that the Milagro emission is either extended over a much larger area than our point spread function, or it must be peaked at energies beyond 1 TeV, resulting in a photon index harder than 2.2 in the TeV band.

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arXiv2011

Radio emission from high-mass binaries with non-accreting pulsars

Binary systems that harbor a non-accreting pulsar are efficient non-thermal emitters, from radio to gamma rays. This broadband emission is thought to come from the region where the companion star and pulsar winds collide. A paradigmatic example of this source type is PSR B1259$-$63. Whereas the high-energy radiation probes the shock structure at the binary scale, the radio emission is produced well outside the system, in regions where the shocked stellar and pulsar winds are likely mixing due to hydrodynamical instabilities. Understanding the evolution of the shocked flow depends strongly on a proper characterization of the low-energy radiation. We have performed numerical calculations of the radio emission produced in a high-mass binary hosting a young pulsar. Adopting a prescription for the shocked flow in the system and the non-thermal particle injection, we have generated synthetic radio emission maps that can be compared with observations. Preliminary results suggest that the emitting bulk flow should be rather slow, with a pulsar wind magnetization at the shock higher than in isolated pulsars.

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arXiv2011

The termination region of high-mass microquasar jets

The environment of high-mass X-ray binaries can be characterized either by the SNR that forms these systems, or by the wind from the companion massive star. These regions should be tenuous but very hot, and surrounded by a dense and cold shocked ISM shell. The interaction between the jet and such a complex medium, also affected by the system proper motion, can lead to very different jet termination structures. The evolution of the jet termination regions during the life of a high-mass microquasar is simulated to improve the present understanding of these structures. Also, the evolving emission characteristics are modeled to inform potential observational campaigns for this class of object. We have performed 2D numerical simulations of jets propagating in different scenarios, corresponding to different epochs after the formation of the high-mass X-ray binary, using the code Ratpenat. We have also made simple estimates of the non-thermal emission that could be produced in the jet termination regions. We find that, in the way through the hot and tenuous medium of the shocked wind/SNR ejecta, the jet suffers recollimation shocks in which it loses part of its thrust and ends in a strong shock inflating a hot cocoon. The jet head propagates with a speed similar to the medium sound speed, until it eventually reaches the denser and colder shocked ISM and the unperturbed ISM later on. In these last stages of evolution, the jet is significantly slowed down and can be disrupted. For relatively old sources, the microquasar peculiar velocity becomes important, leading to complete jet destruction. Extended non-thermal radiation can be generated in the jet termination regions, being hard X-rays and TeV photons the best suited wavelengths to observe these structures.

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arXiv2011

On the origin of correlated X-ray/VHE emission from LS I +61 303

The MAGIC collaboration has recently reported correlated X-ray and very high-energy gamma-ray emission from the gamma-ray binary LS I +61 303 during ~60% of one orbit. These observations suggest that the emission in these two bands has its origin in a single particle population. We aim at improving our understanding of the source behaviour by explaining the simultaneous X-ray and VHE data through a radiation model. We use a model based on a one zone population of relativistic leptonic particles assuming dominant adiabatic losses located at the position of the compact object. The adiabatic cooling timescale is inferred from the X-ray fluxes. The model can reproduce the spectra and lightcurves in the X-ray and VHE bands. Adiabatic losses could be the key ingredient to explain the X-ray and partially the VHE lightcurves. From the best fit result, we obtain a magnetic field of B=0.2 G, a minimum luminosity budget of ~2x10^35 erg/s and a relatively high acceleration efficiency. In addition, our results seem to confirm that the GeV emission detected by Fermi does not come from the same parent particle population as the X-ray and VHE emission and the Fermi spectrum poses a constraint on the hardness of the particle spectrum at lower energies. In the context of our scenario, more sensitive observations would allow to constrain the inclination angle, which could determine the nature of the compact object.

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arXiv2011

Gamma-ray excess from a stacked sample of high- and intermediate-frequency peaked blazars observed with the MAGIC telescope

Between 2004 and 2009 a sample of 28 X-ray selected high- and intermediate-frequency peaked blazars with a X-ray flux larger than 2 uJy at 1 keV in the redshift range from 0.018 to 0.361 was observed with the MAGIC telescope at energies above 100 GeV. Seven among them were detected and the results of these observations are discussed elsewhere. Here we concentrate on the remaining 21 blazars which were not detected during this observation campaign and present the 3 sigma (99.7 %) confidence upper limits on their flux. The individual flux upper limits lie between 1.6 % and 13.6 % of the integral flux from the Crab Nebula. Applying a stacking method to the sample of non-detections with a total of 394.1 hours exposure time, we find evidence for an excess with a cumulative significance of 4.9 standard deviations. It is not dominated by individual objects or flares, but increases linearly with the observation time as for a constant source with an integral flux level of ~1.5 % of that observed from the Crab Nebula above 150 GeV.

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arXiv2010

Observations of the Blazar 3C 66A with the MAGIC Telescopes in Stereoscopic Mode

We report new observations of the intermediate-frequency peaked BL Lacertae object 3C 66A with the MAGIC telescopes. The data sample we use were taken in 2009 December and 2010 January, and comprises 2.3 hr of good quality data in stereoscopic mode. In this period, we find a significant signal from the direction of the blazar 3C 66A. The new MAGIC stereoscopic system is shown to play an essential role for the separation between 3C 66A and the nearby radio galaxy 3C 66B, which is at a distance of only $6^\prime$. The derived integral flux above $100\eh{GeV}$ is 8.3% of Crab Nebula flux and the energy spectrum is reproduced by a power law of photon index $3.64 \pm 0.39_{\rm stat} \pm 0.25_{\rm sys}$. Within errors, this is compatible with the one derived by VERITAS in 2009. From the spectra corrected for absorption by the extragalactic background light, we only find small differences between the four models that we applied, and constrain the redshift of the blazar to $z < 0.68$.

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arXiv2010

Spectral Energy Distribution of Markarian 501: Quiescent State vs. Extreme Outburst

The very high energy (VHE; E > 100 GeV) blazar Markarian 501 has a well-studied history of extreme spectral variability and is an excellent laboratory for studying the physical processes within the jets of active galactic nuclei. However, there are few detailed multiwavelength studies of Markarian 501 during its quiescent state, due to its low luminosity. A short-term multiwavelength study of Markarian 501 was coordinated in March 2009, focusing around a multi-day observation with the Suzaku X-ray satellite and including γ-ray data from VERITAS, MAGIC, and the Fermi Gamma-ray Space Telescope with the goal of providing a well-sampled multiwavelength baseline measurement of Markarian 501 in the quiescent state. The results of these quiescent-state observations are compared to the historically extreme outburst of April 16, 1997, with the goal of examining variability of the spectral energy distribution between the two states. The derived broadband spectral energy distribution shows the characteristic double-peaked profile. We find that the X-ray peak shifts by over two orders of magnitude in photon energy between the two flux states while the VHE peak varies little. The limited shift in the VHE peak can be explained by the transition to the Klein-Nishina regime. Synchrotron self-Compton models are matched to the data and the implied Klein-Nishina effects are explored.

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