arXiv subjects

F. De Paolis

Publications and source records attributed to F. De Paolis.

At least 91 records · Page 5Linked to original sources

arXiv2000

High-energy $γ$-ray emission from GRBs

GRBs are nowadays a rather well understood phenomenon in the soft (KeV-MeV) $γ$-ray energy band, while only a few GRBs have been observed at high photon energies ($E_γ \ut > 1$ GeV). It is also widely recognized that GRBs accelerate protons to relativistic energies and that dense media are often present nearby the sources. Within this framework we compute in detail the high-energy $γ$-ray flux from the decay of neutral pions produced through the interaction of accelerate protons with nucleons in the surrounding medium. We also take into account the local and intergalactic $γ$-ray absorption. The presence of magnetic fields around the GRB sources causes the deflection of the accelerated protons and so a temporal spread of the produced high-energy $γ$-rays with respect to the signal in the soft $γ$-ray band. Moreover, we analyze the possibility to detect the $γ$-ray signal in the GeV-TeV energy range by the ARGO detector under construction in Tibet.

astro-ph↗

arXiv2000

Gamma ray emission from a baryonic dark halo

A recent re-analysis of EGRET data by Dixon et al. has led to the discovery of a statistically significant diffuse $γ$-ray emission from the galactic halo. We show that this emission can naturally be accounted for within a previously-proposed model for baryonic dark matter, according to which dark clusters of brown dwarfs and cold self-gravitating $H_2$ clouds populate the outer galactic halo and can show up in microlensing observations. Basically, cosmic-ray protons in the galactic halo scatter on the clouds clumped into dark clusters, giving rise to the observed $γ$-ray flux. We derive maps for the corresponding intensity distribution, which turn out to be in remarkably good agreement with those obtained by Dixon et al. We also address future prospects to test our predictions.

astro-ph↗

arXiv1999

Slott-Agape Project

SLOTT-AGAPE (Systematic Lensing Observation at Toppo Telescope - Andromeda Gravitational Amplification Pixel Lensing Experiment) is a new collaboration project among international partners from England, France, Germany, Italy and Switzerland that intends to perform microlensing observation by using M31 as target. The MACHOs search is made thanks to the pixel lensing technique.

astro-ph↗

arXiv1999

Gamma ray astronomy and baryonic dark matter

Recently, Dixon et al. have re-analyzed the EGRET data, finding a statistically significant diffuse $γ$-ray emission from the galactic halo. We show that this emission can naturally be explained within a previously-proposed model for baryonic dark matter, in which $γ$-rays are produced through the interaction of high-energy cosmic-ray protons with cold $H_2$ clouds clumped into dark clusters - these dark clusters supposedly populate the outer galactic halo and can show up in microlensing observations. Our estimate for the halo $γ$-ray flux turns out to be in remarkably good agreement with the discovery by Dixon et al. We also address future prospects to test our predictions.

astro-ph↗

arXiv1998

MACHOs as brown dwarfs

Recent observations of microlensing events in the Large Magellanic Cloud suggest that a sizable fraction of the galactic halo is in the form of Massive Astrophysical Compact Halo Objects (MACHOs). Although the average MACHO mass is presently poorly known, the value $\sim 0.1 M_{\odot}$ looks as a realistic estimate, thereby implying that brown dwarfs are a viable and natural candidate for MACHOs. We describe a scenario in which dark clusters of MACHOs and cold molecular clouds (mainly of $H_2$) naturally form in the halo at galactocentric distances larger than 10-20 kpc. Moreover, we discuss various experimental tests of this picture.

astro-ph↗

arXiv1998

Binary brown dwarfs in the galactic halo?

Microlensing events towards the Large Magellanic Cloud entail that a sizable fraction of dark matter is in the form of MACHOs (Massive Astrophysical Compact Halo Objects), presumably located in the halo of the Galaxy. Within the present uncertainties, brown dwarfs are a viable candidate for MACHOs. Various reasons strongly suggest that a large amount of MACHOs should actually consist of binary brown dwarfs. Yet, this circumstance looks in flat contradiction with the fact that MACHOs have been detected as unresolved objects so far. We show that such an apparent paradox does not exist within a model in which MACHOs are clumped into dark clusters along with cold molecular clouds, since dynamical friction on these clouds makes binary brown dwarfs very close. Moreover, we argue that future microlensing experiments with a more accurate photometric observation can resolve binary brown dwarfs.

astro-ph↗

arXiv1998

Halo dark clusters of brown dwarfs and molecular clouds

The discovery of Massive Astrophysical Compact Halo Objects (MACHOs) in microlensing experiments makes it compelling to understand their physical nature, as well as their formation mechanism. Within the present uncertainties, brown dwarfs are a viable candidate for MACHOs, and the present paper deals with this option. According to a recently proposed scenario, brown dwarfs are clumped along with cold molecular clouds into dark clusters -- in several respects similar to globular clusters -- which form in the outer part of the galactic halo. Here, we analyze the dynamics of these dark clusters and we address the possibility that a sizable fraction of MACHOs can be binary brown dwarfs. Moreover, we point out that Ly-$α$ absorption systems naturally fit within the present picture.

astro-ph↗

arXiv1997

Microlensing results and the galactic models

Microlensing results towards the LMC strongly depend on the properties of both the luminous and the dark matter distribution in the Galaxy. The two main sources of uncertainty come from the poor knowledge of the rotation curve at large galactocentric distance and from the possibility that MACHOs follow radial orbits in the outer Galaxy. Given these uncertainties MACHO mass values $ \sim 0.1~ M_{\odot}$ are still consistent with observations and thus brown dwarfs are to date viable candidates for MACHOs.

astro-ph↗

arXiv1997

X-ray emission from dark clusters of MACHOs

MACHOs (Massive Astrophysical Compact Halo Objects) - as discovered by microlensing experiments towards the LMC - provide a natural explanation for the galactic halo dark matter. A realistic possibility is that MACHOs are brown dwarfs of mass $\sim 0.1 M_{\odot}$. Various arguments suggest that brown dwarfs should have a coronal X-ray emission of $\sim 10^{27}$ erg s$^{-1}$. As MACHOs are presumably clumped into dark clusters (DCs), each DC is expected to have a total X-ray luminosity of $\sim 10^{29}-10^{32}$ erg s$^{-1}$. We discuss the possibility that dark clusters contribute to the diffuse X-ray background (XRB) or show up as discrete sources in very deep field X-ray satellite observations. Moreover, from the observed diffuse XRB we infer that the amount of virialized diffuse gas present in the galactic halo can at most make up 5% of the halo dark matter.

astro-ph↗

arXiv1997

Microlensing implications for halo dark matter

The observations of microlensing events in the Large Magellanic Cloud suggest that a sizable fraction ($\sim$ 50%) of the galactic halo is in the form of MACHOs (Massive Astrophysical Compact Halo Objects) with an average mass $\sim 0.27 M_{\odot}$, assuming a standard spherical halo model. We describe a scenario in which dark clusters of MACHOs and cold molecular clouds (mainly of $H_2$) naturally form in the halo at galactocentric distances larger than 10--20 kpc.

astro-ph↗

arXiv1997

Baryonic Dark Matter

Reasons supporting the idea that most of the dark matter in galaxies and clusters of galaxies is baryonic are discussed. Moreover, it is argued that most of the dark matter in galactic halos should be in the form of MACHOs and cold molecular clouds.

astro-ph↗

arXiv1997

Halo dark clusters of brown dwarfs and molecular clouds

The observations of microlensing events in the Large Magellanic Cloud suggest that a sizable fraction (\sim 50%) of the galactic halo is in the form of MACHOs (Massive Astrophysical Compact Halo Objects) with an average mass $\sim 0.27 M_{\odot}$, assuming a standard spherical halo model. We describe a scenario in which dark clusters of MACHOs and cold molecular clouds (mainly of H_2) naturally form in the halo at galactocentric distances larger than 10-20 kpc.

astro-ph↗

arXiv1996

Pulsars Tracing Black Holes in Globular Clusters

Results of precise measurements of the periods of pulsars discovered in the central regions of globular clusters are shown to be approaching the capabilities of testing the existence of a central black hole. For example, in the case of M 15 the available data on two pulsars PSR 2129+1210A and PSR 2129+1209D seem to exclude the existence of a black hole, the presence of which was instead supported by recent Hubble Space Telescope data on surface brightness profile (Yanny et al. 1994). The fluctuations of the gravitational field caused by the stars of the system are enough to explain the acceleration observed for both pulsars. In the case of the Galactic center, additional data are needed for similar definite conclusions.

astro-ph↗

arXiv1996

Baryonic dark matter

Reasons supporting the idea that most of the dark matter in galaxies and clusters of galaxies is baryonic is discussed. Moreover it is argued that most of the dark matter in galactic halo should be in the form of MACHOs and cold molecular clouds.

astro-ph↗

arXiv1995

Microlensing rates from self-consistent galactic models

We study different models of dark matter distribution for the halo of our galaxy. In particular, we consider Eddington and King-Michie models, which [C[C, which include anisotropy in the velocity space, and compute in a self-consistent way the amount of dark matter present in the halo. Assuming that the dark matter is in form of MACHOs, we find for each model the expected number of microlensing events and their average time duration for an experiment monitoring stars in the LMC. The main effect of including anisotropy is to reduce the microlensing rate by about 30% and to increase, but only slightly, the mean event duration as compared to the standard halo model. Consideration of different luminous models for the visible part of the galaxy also induce variations in the microlensing results by roughly the same amount as mentioned above. The main uncertainty, in order to be able to discriminate between different dark matter distributions and to estimate the fraction of it in form of MACHOs is due to the poor knowledge of the rotation velocity at large galactocentric distances.

astro-ph↗

arXiv1995

MACHOs and molecular clouds in galactic halos

Recent observations of microlensing events in the Large Magellanic Cloud by the MACHO and EROS collaborations suggest that an important fraction of the galactic halo is in form of Massive Astrophysical Compact Halo Objects (MACHOs) with mass ~ 0.1 M_{\odot}. We outline a scenario in which dark clusters of MACHOs and molecular clouds form in the halo at galactocentric distances larger than ~ 10-20 kpc, provide baryons are a major constituent of the halo. Possible signatures of the presence of molecular clouds in our galaxy are discussed. We also discuss how molecular clouds as well as MACHOs can be observed directly in the nearby M31 galaxy.

hep-ph↗

arXiv1995

Is the galactic halo baryonic?

Recent observations of microlensing events in the Large Magellanic Cloud suggest that a sizeable fraction of the galactic halo is in form of MACHOs with mass less than abou 0.1 M_{\odot}. Here we argue that molecular clouds (mainly H_2) located in the galactic halo can contribute substantially to its total mass. We outline a scenario in which dark clusters of MACHOs and molecular clouds naturally form in the halo at large galactocentric distances. Possible ways of detecting MACHOs via infrared emission and molecular clouds via the induced gamma-ray flux are discussed. Molecular clouds located in the M31 dark halo could be discovered through cosmic background radiation (CBR) anisotropies or emission lines in the microwave band.

astro-ph↗

arXiv1995

Observing molecular hydrogen clouds and dark massive objects in galactic halos

Molecular hydrogen clouds can contribute substantially to the galactic halo< dark matter and may lead to the birth of massive halo objects (MHOs) observed indirectly by microlensing. We present a method to detect these molecular clouds in the halo of M31 using the Doppler shift effect. We also consider the possibility to directly observe MHOs in the halo of M31 via their infrared emission.

astro-ph↗