Hugo R. Christiansen

HADRONIC HIGH-ENERGY GAMMA-RAY EMISSION FROM THE MICROQUASAR LS I +61 303

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 originating from 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 photon fields, we present high-energy spectral predictions that can be tested with the new-generation Cerenkov telescope MAGIC.

High-energy neutrino emission from x-ray binaries

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 {nu}{sub {mu}}-luminosity of {approx}5 10{sup 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 E{sub {gamma}}>100 MeV should also reveal a strong source.

Gravitational radiation from precessing accretion disks in gamma-ray bursts

We study the precession of accretion disks in the context of gamma-ray burst inner engines. Our aim is to quantitatively estimate the characteristics of gravitational waves produced by the precession of the transient accretion disk in gamma-ray bursts. We evaluate the possible periods of disk precession caused by the Lense-Thirring effect using an accretion disk model that allows for neutrino cooling. Assuming jet ejection perpendicular to the disk plane and a typical intrinsic time-dependence for the burst, we find gamma-ray light curves that have a temporal microstructure similar to that observed in some reported events. The parameters obtained for the precession are then used to evaluate the production of gravitational waves. We find that the precession of accretion disks of outer radius smaller than

Production of gamma rays and neutrinos in the dark jets of the microquasar SS433

10^8

Exact solutions for a Maxwell-Kalb-Ramond action with dilaton: Localization of massless and massive modes in a sine-Gordon brane-world

cm and accretion rates above 1 solar mass per second could be detected by Advanced LIGO if they occur at distances of less than 100 Mpc. We conclude that the precession of a neutrino-cooled accretion disk in long gamma-ray bursts can be probed by gravitational wave astronomy. Precession of the disks in short gamma-ray events is undetectable with the current technology.

Gamma-ray absorption in the microquasar SS433

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 disc and of the star is found to be important, as well as absorption caused by the ultraviolet and mid-infrared radiation from the equatorial envelopment. We analyse 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 kilometre 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.

High-energy gamma-ray production in microquasars

We analytically find the exact propagation modes of the electromagnetic and the Kalb-Ramond fields together in a five-dimensional curved space-time. The existence and localization of gauge particles into our four-dimensional world (4D) is studied in detail on a brane-world scenario in which two gauge fields interact with a dilaton and a gravitational background. The coupling to the dilaton is different in each case causing the splitting between gauge spectra. The gauge-field zero-modes and an infinite tower of Kaluza-Klein massive states are analytically obtained. Relevant conditions on the dilaton coupling constant are found in order to identify with precision every finite tensor and vector eigenstate in the theory. An exact quantization condition on the whole mass spectrum, depending on the dilaton coupling constant and the bulk Planck mass, is inherited from the extra-dimension. This allows finding an exact rule to prevent tachyons in the theory and, by the same token, predicting a possible tensor zero-mode in 4D world. We also show that KK massive-modes contributions onto 4D physics are strongly suppressed.

Gravitational waves from precessing GRBs

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.

Gravitational waves from precessing engines in GRBs

A hadronic model for gamma-ray production in microquasars is presented. Microquasars are galactic binary systems with jets, which have, presumably, hadronic components. We consider a microquasar formed by a neutron star that accretes matter from the equatorial wind of a Be primary star. The collision between the jet, emitted by the compact object, and the dense equatorial disk of the companion massive star is responsible for the gamma-ray production. Gamma-rays result from the decay of neutral pions produced in relativistic pp interactions arising from this collision all along the orbit. Assuming a simple, positional independent set of parameters, our calculations are consistent with a peak of gamma-ray flux at the periastron passage with a secondary maximum near apastron. Under this assumption, gamma-ray signals would be in contrast with the radio/X-ray outbursts which peak clearly after periastron. We finally calculate the opacity of the ambient photon field to the propagation of the gamma-rays. The spectral energy distribution appears strongly attenuated in a wide band (50 GeV - 50 TeV) due to local absorption. These spectral features should be detectable by an instrument like MAGIC through exposures integrated along several periastron passages.