Gamma Rays from Compton Scattering in the Jets of Microquasars: Application to LS 5039

Recent HESS observations show that microquasars in high-mass systems are sources of VHE gamma-rays. A leptonic jet model for microquasar gamma-ray emission is developed. Using the head-on approximation for the Compton cross section and taking into account angular effects from the star's orbital motion, we derive expressions to calculate the spectrum of gamma rays when nonthermal jet electrons Compton-scatter photons of the stellar radiation field. Calculations are presented for power-law distributions of nonthermal electrons that are assumed to be isotropically distributed in the comoving jet frame, and applied to $\gamma$-ray observations of LS 5039. We conclude that (1) the TeV emission measured with HESS cannot result only from Compton-scattered stellar radiation (CSSR), but could be synchrotron self-Compton (SSC) emission or a combination of CSSR and SSC; (2) fitting both the HESS data and the EGRET data associated with LS 5039 requires a very improbable leptonic model with a very hard electron spectrum. Because the gamma rays would be variable in a leptonic jet model, the data sets are unlikely to be representative of a simultaneously measured gamma-ray spectrum. We therefore attribute EGRET gamma rays primarily to CSSR emission, and HESS gamma rays to SSC emission. Detection of periodic modulation of the TeV emission from LS 5039 would favor a leptonic SSC or cascade hadron origin of the emission in the inner jet, whereas stochastic variability alone would support a more extended leptonic model. The puzzle of the EGRET gamma rays from LS 5039 will be quickly solved with GLAST. (Abridged)Comment: 17 pages, 11 figures, ApJ, in press, June 1, 2006, corrected eq.

On the Redshift Distribution of Gamma Ray Bursts in the Swift Era

A simple physical model for long-duration gamma ray bursts (GRBs) is used to fit the redshift (z) and the jet opening-angle distributions measured with earlier GRB missions and with Swift. The effect of different sensitivities for GRB triggering is sufficient to explain the difference in the z distributions of the pre-Swift and Swift samples, with mean redshifts of ~1.5 and ~2.7, respectively. Assuming that the emission properties of GRBs do not change with time, we find that the data can only be fitted if the comoving rate-density of GRB sources exhibits positive evolution to z >~ 3-5. The mean intrinsic beaming factor of GRBs is found to range from ~34-42, with the Swift average opening half-angle ~10 degree, compared to the pre-Swift average of ~7 degree. Within the uniform jet model, the GRB luminosity function is proportional to L^{-3.25}_*, as inferred from our best fit to the opening angle distribution. Because of the unlikely detection of several GRBs with z <~ 0.25, our analysis indicates that low redshift GRBs represent a different population of GRBs than those detected at higher redshifts. Neglecting possible metallicity effects on GRB host galaxies, we find that ~1 GRB occurs every 600,000 yrs in a local L_* spiral galaxy like the Milky Way. The fraction of high-redshift GRBs is estimated at 8-12% and 2.5-6% at z >= 5 and z >= 7, respectively, assuming continued positive evolution of the GRB rate density to high redshifts.Comment: Accepted for publication in ApJ. The paper contains 29 pages and 24 figure

On the equipartition of thermal and non-thermal energy in clusters of galaxies

Clusters of galaxies are revealing themselves as powerful sources of non thermal radiation in a wide range of wavelengths. In order to account for these multifrequency observations equipartition of cosmic rays (CRs) with the thermal gas in clusters of galaxies is often invoked. This condition might suggest a dynamical role played by cosmic rays in the virialization of these large scale structures and is now testable through gamma ray observations. We show here, in the specific case of the Coma and Virgo clusters, for which upper limits on the gamma ray emission exist, that equipartition implies gamma ray fluxes that are close or even in excess of the EGRET limit, depending on the adopted model of CR injection. We use this bound to limit the validity of the equipartition condition. We also show that, contrary to what claimed in previous calculations, the equipartition assumption implies gamma ray fluxes in the TeV range which can be detectable even by currently operating gamma ray observatories if the injection cosmic ray spectrum is flatter than $E^{-2.4}$.Comment: 20 pages + 2 figures. To appear in the Astrophysical Journa

On the numerical analysis of triplet pair production cross-sections and the mean energy of produced particles for modelling electron-photon cascade in a soft photon field

The double and single differential cross-sections with respect to positron and electron energies as well as the total cross-section of triplet production in the laboratory frame are calculated numerically in order to develop a Monte Carlo code for modelling electron-photon cascades in a soft photon field. To avoid numerical integration irregularities of the integrands, which are inherent to problems of this type, we have used suitable substitutions in combination with a modern powerful program code Mathematica allowing one to achieve reliable higher-precission results. The results obtained for the total cross-section closely agree with others estimated analytically or by a different numerical approach. The results for the double and single differential cross-sections turn out to be somewhat different from some reported recently. The mean energy of the produced particles, as a function of the characteristic collisional parameter (the electron rest frame photon energy), is calculated and approximated by an analytical expression that revises other known approximations over a wide range of values of the argument. The primary-electron energy loss rate due to triplet pair production is shown to prevail over the inverse Compton scattering loss rate at several ($\sim$2) orders of magnitude higher interaction energy than that predicted formerly.Comment: 18 pages, 8 figures, 2 tables, LaTex2e, Iopart.cls, Iopart12.clo, Iopams.st

The obscured gamma-ray and UHECR universe

Auger results on clustering of > 60 EeV ultra-high energy cosmic ray (UHECR) ions and the interpretation of the gamma-ray spectra of TeV blazars are connected by effects from the extragalactic background light (EBL). The EBL acts as an obscuring medium for gamma rays and a reprocessing medium for UHECR ions and protons, causing the GZK cutoff. The study of the physics underlying the coincidence between the GZK energy and the clustering energy of UHECR ions favors a composition of > 60 EeV UHECRs in CNO group nucleons. This has interesting implications for the sources of UHECRs. We also comment on the Auger analysis.Comment: 11 pages, 10 figures, in the International Conference on Topics in Astroparticle and Underground Physics (TAUP) 2007, Sendai, Japan, September 11-15, 200

Can Galactic Cosmic Rays Account for Solar 6Li Without Overproducing Gamma Rays?

Cosmic-ray interactions with interstellar gas produces both 6Li, which accumulates in the interstellar medium (ISM), and $\pi^0$ mesons, which decay to gamma-rays which propagate throughout the cosmos. Local 6Li abundances and extragalactic gamma-rays thus have a common origin which tightly links them. We exploit this connection to use gamma-ray observations to infer the contribution to 6Li nucleosynthesis by standard Galactic cosmic-ray (GCR) interactions with the ISM. Our calculation uses a carefully propagated cosmic-ray spectrum and accounts for 6Li production from both fusion reactions ($\alpha \alpha \to ^6Li$) as well as from spallation channels ({p,\alpha+CNO \to ^6Li). We find that although extreme assumptions yield a consistent picture, more realistic ones indicate that solar 6Li cannot be produced by standard GCRs alone without overproducing the hadronic gamma rays. Implications for the primordial 6Li production by decaying dark matter and cosmic rays from cosmological structure formation are discussed. Upcoming gamma-ray observations by GLAST will be crucial for determining the resolution of this problem.Comment: 4 pages, 1 figure To be published in ApJ

On Spectral and Temporal Variability in Blazars and Gamma Ray Bursts

A simple model for variability in relativistic plasma outflows is studied, in which nonthermal electrons are continuously and uniformly injected in the comoving frame over a time interval dt. The evolution of the electron distribution is assumed to be dominated by synchrotron losses, and the energy- and time-dependence of the synchrotron and synchrotron self-Compton (SSC) fluxes are calculated for a power-law electron injection function with index s = 2. The mean time of a flare or pulse measured at photon energy E with respect to the onset of the injection event varies as E^{-1/2} and E^{-1/4} for synchrotron and SSC processes, respectively, until the time approaches the limiting intrinsic mean time (1+z)dt/(2 D), where z is the redshift and D is the Doppler factor. This dependence is in accord with recent analyses of blazar and GRB emissions, and suggests a method to discriminate between external Compton and SSC models of high-energy gamma radiation from blazars and GRBs. The qualititative behavior of the X-ray spectral index/flux relation observed from BL Lac objects can be explained with this model. This demonstrates that synchrotron losses are primarily responsible for the X-ray variability behavior and strengthens a new test for beaming from correlated hard X-ray/TeV observations.Comment: 10 pages, 2 figures, accepted for publication in Astrophysical Journal Letters; uses aaspp4.sty, epsf.st