Search results for "LSA"
showing 10 items of 832 documents
Gamma ray sources observation with the ARGO-YBJ detector
2011
In this paper we report on the observations of TeV gamma ray sources performed by the air shower de- tector ARGO-YBJ. The objects studied in this work are the blazar Markarian 421 and the extended galactic source MGROJ1908+06, monitored during 2 years of operation. Mrk421 has been detected by ARGO-YBJ with a statisti- cal significance of 11 standard deviations. The observed TeV emission was highly variable, showing large enhance- ments of the flux during active periods. The study of the spectral behaviour during flares revealed a positive correla- tion of the hardness with the flux, as already reported in the past by the Whipple telescope, suggesting that this is a long term property of the…
The optical counterpart of SAX J1808.4-3658 in quiescence: evidence of an active radio pulsar?
2004
Abstract The optical counterpart of the binary millisecond X-ray pulsar SAX J1808.4–3658 during quiescence was detected at V = 21.5 mag by Homer et al. [MNRAS 325 (2001) 1471]. It was proposed that the bulk of the optical emission arises from viscous dissipation in the innermost zones of a remnant disk. The serious difficulty in this scenario lies in the estimate of the irradiating luminosity required to match the observational data, that is a factor 10–50 higher than the observed quiescent X-ray luminosity of this source. To overcome this problem, we propose an alternative scenario, in which the irradiation is due to the release of rotational energy by the fast spinning neutron star, switc…
Electron neutrino opacity in magnetised media
1997
We study the effects of strong magnetic fields ($B> 10^{13}$~G) in the cross section for $��_e n\to p e$ scattering in the presence of a degenerate electron background. This can be relevant for the $��_e$ propagation in the proto-neutron star stage after supernovae collapse. We find that for field strengths $B> 10^{16}$~G$(E_��/10$ MeV$)^2$ the $��_e$ opacity is sizeably affected by the magnetic field and can lead to a shift in the location of the electron neutrino sphere towards lower densities. We discuss the implications that this may have for scenarios proposed to explain the observed pulsar velocities.
Conservative upper limits on WIMP annihilation cross section from Fermi-LAT γ rays
2012
The spectrum of an isotropic extragalactic {\gamma}-ray background (EGB) has been measured by the Fermi-LAT telescope at high latitudes. Two new models for the EGB are derived from the subraction of unresolved point sources and extragalactic diffuse processes, which could explain from 30% to 70% of the Fermi-LAT EGB. Within the hypothesis that the two residual EGBs are entirely due to the annihilation of dark matter (DM) particles in the Galactic halo, we obtain stringent upper limits on their annihilation cross section. Severe bounds on a possible Sommerfeld enhancement of the annihilation cross section are set as well. Finally, we consider models for DM annihilation depending on the inver…
Pulsar Velocities without Neutrino Mass
1998
We show that pulsar velocities may arise from anisotropic neutrino emission induced by resonant conversions of massless neutrinos in the presence of a strong magnetic field. The main ingredient is a small violation of weak universality and neither neutrino masses nor magnetic moments are required.
Neutrino propagation in magnetized media
1999
Abstract After a short presentation of the general techniques used to determine neutrino potentials in a magnetized medium I will discuss some applications to MSW resonant oscillations. I will also consider the relevance of our results for the pulsar velocity problem.
Direct URCA process in neutron stars with strong magnetic fields
1997
We calculate the emissivity for the direct URCA process in strongly magnetized, degenerate matter in neutron stars, under $\beta $-equilibrium. We show that, if the magnetic field is large enough for protons and electrons to be confined to the ground Landau levels, the field-free threshold condition on proton concentration no longer holds, and direct URCA reactions are open for an arbitrary proton concentration. Direct URCA process leads to an early phase of fast neutron star cooling. This circumstance allows us to constrain the initial magnetic field inside observed pulsars.
The discovery of the serendipitous X-ray pulsar SAX J1802.7–2017 from a BeppoSAX observation of GX 9+1
2004
Abstract We have discovered a new X-ray source, SAX J1802.7–2017, ∼22′ away from the bright X-ray source GX 9+1, during a BeppoSAX observation of this latter one on 2001 September 16-20. The count rate of SAX J1802.7–2017 is comparable to the background (∼0.04 c/s) during the first 50 ks of the observation, then the count rate shows a large variability increasing up to ∼0.28 c/s. We have performed a temporal analysis finding that SAX J1802.7–2017 has a pulse period of ∼139.61 s, a projected orbital separation a x sin i of 48±5 lt-s and an orbital period of 3.7 +0.4 −0.2 days, allowing to classify the new object as an X-ray pulsar.
BeppoSAX observation of the transient X-ray pulsar GS 1843+009
1999
Abstract The transient X-ray pulsar GS 1843+009 was observed by BeppoSAX satellite on 1997 April 4, while it was at flux level of 50 mCrab in the 20–200 keV energy band. Using the MECS and LECS concentrators, the source position was measured with unprecedented accuracy of 30. In this poster we present results on both spectral and temporal analysis.
Hard X-ray emission from neutron star X-ray binaries
2004
Abstract In this paper we review our current knowledge of the hard X–ray emission properties of accreting X–ray Binary Pulsars and old accreting neutron stars in Low Mass X–ray Binaries in light of 7 years of BeppoSAX and RXTE observations. The paper is divided in two parts. In the first part we review the more recent findings on the phase-dependent broad band continua and cyclotron resonance scattering features observed in many systems of the X-ray Binary Pulsar class. In the second part we review the hard X-ray emission of LMXRB focussing on the hard X-ray components extending up to energies of a few hundred keV that have been clearly detected in sources of both the atoll and Z classes. T…