arXiv subjects

Diego F. Torres

Publications and source records attributed to Diego F. Torres.

At least 109 records · Page 6Linked to original sources

arXiv2010

Gamma-ray binaries as non-accreting pulsar systems

The gamma-ray binaries LS 5039 and LS I +61 303 have been detected by Cerenkov telescopes at TeV energies, exhibiting periodic behavior correlated with the orbital period. These gamma-ray binary systems have also been recently detected by the Fermi Gamma-ray Telescope at GeV energies, and combination of GeV and TeV observations are providing both, expected and surprising results. We summarize these results, also considering the multi-frequency scenario, from the perspective of pulsar systems. We discuss similarities and differences of models in which pulsar wind/star wind shocks, or pulsar wind zone processes lead to particles accelerated enough to emit TeV photons. We discuss in detail the caveats of the current observations for detecting either accretion lines or pulsations from these objects. We also comment on the possibility for understanding the GeV to TeV emission from these binaries with a 2-components contribution to their spectrum. We show that it would be possible to accommodate both, normal pulsar emission and GeV / TeV fluxes that vary with orbital phase. We point out several aspects of this idea that are subject to test with data being currently taken.

astro-ph.HE↗

arXiv2010

Variability in the orbital profiles of the X-ray emission of the gamma-ray binary LS I +61 303

We report on the analysis of Rossi X-ray Timing Explorer (RXTE) Proportional Counter Array (PCA) monitoring observations of the $γ$-ray binary system LS I +61 303, covering 35 full cycles of its orbital motion. This constitutes the largest continuous X-ray monitoring dataset analyzed to date for this source. Such an extended analysis allows us to report: a) the discovery of variability in the orbital profiles of the X- ray emission, b) the existence of a few (recent) short flares on top of the overall behavior typical of the source, which, given the PCA field-of-view, may or may not be associated with LS I +61 303, and c) the determination of the orbital periodicity using soft X-ray data alone.

astro-ph.HE↗

arXiv2010

The GeV to TeV connection in the environment of SNR IC 443

We have recently interpreted the source MAGIC J0616+225 as a result of delayed TeV emission of cosmic-rays diffusing from IC 443 and interacting with a cloud in the foreground of the remnant. This model was used to make predictions for future observations, especially those to be made with the Fermi satellite. Just recently, AGILE, Fermi, and VERITAS have released new results of their observations of IC 443. In this work, we compare them with the predictions of our model, exploring the GeV to TeV connection in this region of space. We use Fermi data to consider the possibility of constraining the cosmic-ray diffusion features of the environment. We analyze the cosmic-ray distributions, their interactions, and a possible detection of the SNR environment in the neutrino channel.

astro-ph.HE↗

arXiv2010

Long-term monitoring of LS I+61303 with INTEGRAL

LS I+61303 is one of the few high-mass X-ray binaries that have been recently observed at TeV and GeV energies. Here we investigate the hard X-ray spectral and timing properties of this source using the IBIS/ISGRI instrument on-board the INTEGRAL satellite. We carry out a systematic analysis based on all available INTEGRAL data since December 28, 2002 up to April 30, 2008. The total exposure time analyzed amounts to 2.1 Ms, hence more than doubling the previous reported sample. The source is best detected in the 18-60 keV band, with a significance level of 12.0 sigma. The hard X-ray data are best fit with a simple power law with a photon index of ~ 1.7+-0.2. We detect a periodical signal at 27 +- 4 days, matching the orbital period of 26.496 days previously reported at other wavelengths. The hard X-rays orbital lightcurve is obtained and compared with those derived at other frequencies.

astro-ph.HE↗

arXiv2010

INTEGRAL and Swift/XRT observations of the source PKS 0208-512

The active galaxy PKS 0208-512, detected at lower energies by COMPTEL, has been claimed to be a MeV blazar from EGRET. We report on the most recent INTEGRAL observations of the blazar PKS 0208-512, which are supplemented by Swift ToO observations. The high energy X-ray and gamma-ray emission of PKS 0208-512 during August - December 2008 has been studied using 682 ks of INTEGRAL guest observer time and ~ 56 ks of Swift/XRT observations. These data were collected during the decay of a gamma-ray flare observed by Fermi/LAT. At X-ray energies (0.2 - 10 keV) PKS 0208-512 is significantly detected by Swift/XRT, showing a power-law spectrum with a photon index of ~ 1.64. Its X-ray luminosity varied by roughly 30% during one month. At hard X-/soft gamma-ray energies PKS 0208-512 shows a marginally significant (~ 3.2 sigma) emission in the 0.5-1 MeV band when combining all INTEGRAL/SPI data. Non-detections at energies below and above this band by INTEGRAL/SPI may indicate intrinsic excess emission. If this possible excess is produced by the blazar, one possible explanation could be that its jet consists of an abundant electron-positron plasma, which may lead to the emission of an annihilation radiation feature. Assuming this scenario, we estimate physical parameters of the jet of PKS 0208-512.

astro-ph.CO↗

arXiv2010

Type-I bursts within outbursts of IGR J17473-2721

Two outbursts were observed by RXTE in the history of the atoll source IGR J17473-2721. During the most recent outburst in 2008, the source showed a complete series of spectral states/transitions. The neutron star system was prolific in type-I X-ray bursts, and we investigate them in the context of complete outbursts evolution. A total exposure of ~ 309 ks was collected by RXTE during the two outbursts of IGR J17473-2721. We carried out a systematic search for type-I bursts in this data set. For each burst found, we investigated the burst profile, the peak flux, and their dependence on the accretion rate along the evolution of the outbursts. Eighteen type-I X-ray bursts were found from IGR J17473-2721: two from the outburst in 2005 and the other 16 from the recent outburst in 2008. Among them, 3 bursts show photospheric radius expansion (PRE). The distance to the source is estimated as 6.4 kpc with a 15% uncertainty based on the three bursts that show PRE. In the recent outburst, there are 6 bursts showing up in the low/hard state prior to the state transition to a high/soft state, 3 bursts at the end phase of the high/soft state, and 7 in the following low/hard state. The blackbody radius of these bursts presents a variety of interesting features. We find that at the end of the recent outburst, the profile of the blackbody radius is anti-correlated with the blackbody temperature and the burst flux. The durations of the type-I burst are found to correlate with the Eddington ratio and to have two parallel evolution groups. Along the decreasing Eddington ratio, the burst duration decreases and ends in each group the PRE bursts occurred. This provides new clues to the type-I bursts in the context of outbursts for atoll XRBs.

astro-ph.HE↗

arXiv2009

Present and Future Gamma-Ray Probes of the Cygnus OB2 Environment

The MAGIC Collaboration has provided new observational data pertaining to the TeV J2032+4130 gamma-ray source (within the Cygnus OB2 region), for energies E_gamma >400 GeV. It is then appropriate to update the impact of these data on gamma-ray production mechanisms in stellar associations. We consider two mechanisms of gamma-ray emission, pion production and decay (PION) and photo-excitation of high-energy nuclei followed by prompt photo-emission from the daughter nuclei (A*). We find that while the data can be accommodated with either scenario, the A* features a spectral bump, corresponding to the threshold for exciting the Giant Dipole Resonance, which can serve to discriminate between them. We comment on neutrino emission and detection from the region if the PION and/or A* processes are operative. We also touch on the implications for this analysis of future Fermi and Cerenkov Telescope Array data.

astro-ph.HE↗

arXiv2009

Gamma-ray astronomy in the summer of 2009

This paper presents a snapshot of the field of $γ$-ray astrophysics in the early summer of 2009, as it was discussed in about 200 presentations at the International Cosmic Ray Conference (ICRC) held in Łódź, Poland. This is the written wrap up of a Rapporteur, one-hour talk and as such it is thus an atypical review, a still picture in a moment of great advancement in an observationally driven field, led by the full operation of ground-based arrays and the launch and operations of Fermi and Agile.

astro-ph.HE↗

arXiv2009

INTEGRAL and Swift/XRT observations of IGR J19405-3016

IGR J19405-3016 is reported in the 3rd IBIS catalog as being one of its lowest significance sources (~4.6 sigma under an exposure of about 371 ks). This leads to a caveat in multi-wavelength study although the source was identified in optical as a Seyfert 1. The currently increased INTEGRAL data stimulate us to investigate the reality of this source. We analyze all available observations carried out by INTEGRAL and Swift on IGR J19405-3016. We find that IGR J19405-3016 has a detection significance of ~ 9.4 sigma in the 20-60 keV band during the observational period between March 2003 and March 2008. Thus confirms a real source detection reported previously. The source position and error location are therefore updated. The source is found to be constant over years at the hard X-rays. Over the three XRT observations, the source flux varied by up to 39% from the average, and the spectrum is generally soft. The combined XRT/ISGRI spectrum is well fitted with a simple power law model (photon index 2.11+-0.03). Such a photon index is well consistent with the mean value 1.98 (dispersion 0.27) as obtained from Swift/BAT AGN samples at 14-195 keV. The spectral slope of IGR J19405-3016 is larger than the average spectral slope found by Molina et al. (2009). A similar discrepancy is found with the results of Beckmann et al. (2009) regarding Seyfert 1 AGNs. A possible explanation of this simple spectral description may be that the low level of the column density allows for the `true' spectrum to appear at soft X-rays as well.

astro-ph.HE↗

arXiv2009

Outbursts from IGR J17473-2721

We have investigated the outbursts of IGR J17473-2721. We analyzed all available observations carried out by RXTE on IGR J17473-2721 during its later outburst and as well all the available SWIFT/BAT data. The flux of the latter outburst rose in ~ one month and then kept roughly constant for the following ~ two months. During this time period, the source was in a low/hard state. The source moved to a high/soft state within the following three days, accompanied by the occurrence of an additional outburst at soft X-rays and the end of the preceding outburst in hard X-rays. During the decay of this soft outburst, the source went back to a low/hard state within 6 days, with a luminosity 4 times lower than the first transition. This shows a full cycle of the hysteresis in transition between the hard and the soft states. The fact that the flux remained roughly constant for ~ two months at times prior to the spectral transition to a high/soft state might be regarded as the result of balancing the evaporation of the inner disk and the inward accretion flow, in a model in which the state transition is determined by the mass flow rate. Such a balance might be broken via an additional mass flow accreting onto the inner disk, which lightens the extra soft outburst and causes the state transition. However, the possibility of an origin of the emission from the jet during this time period cannot be excluded. The spectral analysis suggests an inclined XRB system for IGR J17473-2721. Such a long-lived preceding low/hard state makes IGR J17473-2721 resemble the behavior of outbursts seen in black hole X-ray binaries like GX 339-4.

astro-ph.HE↗

arXiv2009

First bounds on the VHE gamma-ray emission from isolated Wolf-Rayet binary systems

Different theoretical models predict VHE gamma-ray emission to arise in tight binary star systems (high mass-loss and high wind speeds), which has not been confirmed experimentally so far. Here we present the first bounds on the VHE emission from two isolated Wolf-Rayet star binaries, WR147 and WR146, obtained with the MAGIC telescope.

astro-ph.HE↗

arXiv2009

Multi-messenger model for the starburst galaxy M82

In this paper, a consistent model of the multifrequency emission of the starburst galaxy M82, from radio to gamma-rays is presented and discussed. Predictions for observations with Fermi, MAGIC II/VERITAS and CTA telescopes are made. The model is also used to self-consistenty compute the (all flavors) emission of neutrinos resulting from this starburst galaxy, what can be used in considerations of the diffuse contributions of such objects.

astro-ph.GA↗

arXiv2009

gamma-ray emission from LS I +61 303: The impact of basic system uncertainties

LS I +61 303 has been recently detected as a periodic gamma-ray source by the Major Atmospheric Imaging Cerenkov (MAGIC) telescope. A distinctive orbital correlation of the $γ$-ray emission was found. This work shows that the range of uncertainties yet at hand in the orbital elements of the binary system LSI as well as in the possible assumptions on the stellar wind of the optical companion play a non-negligible role in the computation of opacities to high energy processes leading to $γ$-ray predictions. The geometry influence on the propagation and escape of $γ$-ray photons is explored. With this study at hand, we analyse the results of a pulsar wind zone model for the production of gamma-rays and compare it with recent MAGIC observations.

astro-ph↗

arXiv2009

The GeV to TeV view of SNR IC443: predictions for Fermi

We present a theoretical model that explains the high energy phenomenology of the neighborhood of SNR IC 443, as observed with the Major Atmospheric Gamma Imaging Cherenkov (MAGIC) telescope and the Energetic Gamma-Ray Experiment Telescope (EGRET). We also discuss how the model can be tested with observations by the Fermi Gamma-ray Large Area Space Telescope. We interpret MAGIC J0616+225 as delayed TeV emission of cosmic-rays diffusing from IC 443 and interacting with a known cloud located at a distance of about 20 pc in the foreground of the remnant. This scenario naturally explains the displacement between EGRET and MAGIC sources, their fluxes, and their spectra. Finally, we predict how this context can be observed by Fermi.

astro-ph.HE↗

arXiv2008

Future science issues for Galactic very-high-energy gamma-ray astronomy

This work intends to provide a brief summary of some of the Galactic science issues for the next generation of very high energy (VHE) instruments. The latter is here generically understood, as an instrument or set of instruments providing about one order of magnitude more sensitivity at its central energy (at about 1 TeV), but extending the observational window to have a real broadband capability (from a few tens of GeV up to tens of TeV); exceeding at low energies the current VHE threshold for observations set by MAGIC as well as the few-tens-of-GeV sensitivity set by Fermi. Science topics regarding populations of emitters, pulsars and their nebula, binaries, supernova remnants, stars, and their associations, are discussed.

astro-ph↗

arXiv2008

The nature of LS 5039 under the scrutiny of gamma-rays

Several gamma-ray binaries have been recently detected by the High-Energy Stereoscopy Array (H.E.S.S.) and the Major Atmospheric Imaging Cerenkov (MAGIC) telescope. In two cases, their nature is unknown, since a final observational feature for a black hole or a pulsar compact object companion is still missing. One such system is LS 5039. Here we present results from a model (it includes a detailed account of the system geometry, the angular dependence of processes such as Klein-Nishina inverse Compton and gamma-gamma absorption, and a Monte Carlo simulation of cascading) of the high energy phenomenology of LS 5039 in which it is assumed that the companion object is a pulsar rotating around an O6.5V star in the 3.9 days orbit. We show that the H.E.S.S. phenomenology at all scales (spectra along the orbit in both broad and short phase-bins and lightcurve) is described within this model. We focus on presenting predictions for photons with lower energies (for E>1 GeV), subject to test in the forthcoming months with the GLAST satellite, and we also present predictions for future observations with high-energy arrays, such as H.E.S.S. II. Both set of predictions go beyond the description of the current data, and could provide a high-energy determination of the system's composition.

astro-ph↗

arXiv2008

Pulsar wind zone processes in LS 5039

Several $γ$-ray binaries have been recently detected by the High-Energy Stereoscopy Array (H.E.S.S.) and the Major Atmospheric Imaging Cerenkov (MAGIC) telescope. In at least two cases, their nature is unknown. In this paper we aim to provide the details of a theoretical model of close $γ$-ray binaries containing a young energetic pulsar as compact object, earlier presented in recent Letters. This model includes a detailed account of the system geometry, the angular dependence of processes such as Klein-Nishina inverse Compton and $γγ$ absorption in the anisotropic radiation field of the massive star, and a Monte Carlo simulation of leptonic cascading. We present and derive the used formulae and give all details about their numerical implementation, particularly, on the computation of cascades. In this model, emphasis is put in the processes occurring in the pulsar wind zone of the binary, since, as we show, opacities in this region can be already important for close systems. We provide a detailed study on all relevant opacities and geometrical dependencies along the orbit of binaries, exemplifying with the case of LS 5039. This is used to understand the formation of the very high-energy lightcurve and phase dependent spectrum. For the particular case of LS 5039, we uncover an interesting behavior of the magnitude representing the shock position in the direction to the observer along the orbit, and analyze its impact in the predictions. We show that in the case of LS 5039, the H.E.S.S. phenomenology is matched by the presented model, and explore the reasons why this happens while discussing future ways of testing the model.

astro-ph↗

arXiv2008

Diffusion of cosmic-rays and the Gamma-ray Large Area Telescope: Phenomenology at the 1-100 GeV regime

This paper analyzes astrophysical scenarios that may be detected at the upper end of the energy range of the Gamma Ray Large Area Space Telescope (GLAST), as a result of cosmic-ray (CR) diffusion in the interstellar medium (ISM). Hadronic processes are considered as the source of $γ$-ray photons from localized molecular enhancements nearby accelerators. Two particular cases are presented: a) the possibility of detecting spectral energy distributions (SEDs) with maxima above 1 GeV, which may be constrained by detection or non-detection at very-high energies (VHE) with observations by ground-based Cerenkov telescopes, and b) the possibility of detecting V-shaped, inverted spectra, due to confusion of a nearby (to the line of sight) arrangement of accelerator/target scenarios with different characteristic properties. We show that the finding of these signatures (in particular, a peak at the 1--100 GeV energy region) is indicative for an identification of the underlying mechanism producing the $γ$-rays that is realized by nature: which accelerator (age and relative position to the target cloud) and under which diffusion properties CR propagate.

astro-ph↗