arXiv subjects

G. Ingrosso

Publications and source records attributed to G. Ingrosso.

At least 73 records · Page 4Linked to original sources

arXiv2002

A note on gravitational wave lensing

In a recent paper [Ruffa] it was proposed that the massive black hole at the Galactic center may act as a gravitational lens focusing gravitational wave energy to the Earth. Considering the gravitational wave signal emitted by galactic spinning pulsars, an enhancement in the gravitational wave intensity by a factor of a few thousand is found. For galactic and extra-galactic sources the intensity enhancement can be as high as 4,000 and 17,000, respectively. In this note we consider the probability of significant signal enhancement from galactic and extra-galactic pulsars by the proposed mechanism and find that it is actually negligible. The lensing effect due to a possible companion object (a star or the galactic center black hole) of the gravitational wave source is also investigated in the framework of the classical microlensing theory.

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arXiv2002

Astrophysical implications of binary black holes in BL Lacertae objects

Some BL Lacertae objects show a periodic behaviour in their light curves that is often attributed to the orbital motion of a central binary black hole system. On this basis, and assuming a circular orbit, Rieger and Mannheim have recently proposed a method to determine the orbital parameters of the binary system from the observed quantities, {\it i.e.} the signal periodicity, the flux ratio between maximum and minimum signal and the power law spectral index of the photon flux. However, since these binary black holes are expected to originate from galactic mergers, they could well be on eccentric orbits, which might not circularize for a substantial time. We therefore generalize the treatment proposed by Rieger and Mannheim by taking into account the effect of the orbital eccentricity of the binary system. We apply the model to three well-observed Markarian objects: MKN 501, MKN 421 and MKN 766 that most likely host a binary system in their centers. Some astrophysical implications of this model are also investigated with particular emphasis to the gravitational radiation emission from the binary black holes. Under particular conditions (e.g. for some values of the orbital separation, eccentricity and Lorentz factor) one can obtain signals above the sensitivity threshold of the LISA detector.

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arXiv2001

Microlensing search towards M31

We present the first results of the analysis of data collected during the 1998-99 observational campaign at the 1.3 meter McGraw-Hill Telescope, towards the Andromeda galaxy (M31), aimed to the detection of gravitational microlensing effects as a probe of the presence of dark matter in our and in M31 halo. The analysis is performed using the pixel lensing technique, which consists in the study of flux variations of unresolved sources and has been proposed and implemented by the AGAPE collaboration. We carry out a shape analysis by demanding that the detected flux variations be achromatic and compatible with a Paczynski light curve. We apply the Durbin-Watson hypothesis test to the residuals. Furthermore, we consider the background of variables sources. Finally five candidate microlensing events emerge from our selection. Comparing with the predictions of a Monte Carlo simulation, assuming a standard spherical model for the M31 and Galactic haloes, and typical values for the MACHO mass, we find that our events are only marginally consistent with the distribution of observable parameters predicted by the simulation.

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arXiv2001

High-energy neutrino emission from Gamma-Ray Bursts

Gamma-Ray Bursts (GRBs) are expected to efficiently accelerate protons up to relativistic energies. High-energy photons can originate from decay of neutral pions produced by the interaction of these protons with the medium surrounding the source. In the same hadronic chain, high energy neutrinos are expected to be produced from decay of charged pions, as well as from decay of generated muons. Neutrinos can travel cosmological distances being much less absorbed than photons, thus providing a powerful tool for the investigation of cosmic sources. In the frame of a recently proposed hadronic model for the emission of high-energy gamma-rays, we estimate the neutrino emission from GRBs and calculate the neutrino fluxes at Earth as a function of the model parameters. The detectability of the expected signal by current and future experiments is also extensively discussed.

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arXiv2001

Astrophysical constraints on a possible neutrino ball at the Galactic Center

The nature of the massive object at the Galactic Center (Sgr A$^{*}$) is still unclear even if various observational campaigns led many authors to believe that our Galaxy hosts a super-massive black hole with mass $M\simeq 2.6\times 10^6$ M$_{\odot}$. However, the black hole hypothesis, which theoretically implies a luminosity $\simeq 10^{41}$ erg s$^{-1}$, runs into problems if one takes into account that the observed luminosity, from radio to $γ$-ray wavelengths, is below $10^{37}$ erg s$^{-1}$. In order to solve this blackness problem, alternative models have been recently proposed. In particular, it has been suggested that the Galactic Center hosts a ball made up by non-baryonic matter (e.g. massive neutrinos and anti-neutrinos) in which the degeneracy pressure of fermions balances their self-gravity. Requiring to be consistent with all the available observations towards the Galactic Center allows us to put severe astrophysical constraints on the neutrino ball parameters. The presence of such an object in the Galactic Center may be excluded if the constituent neutrino mass $m_ν$ is $\ut> 24$ keV, while if $m_ν\ut< 24$ keV observations can not give a definite answer.

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arXiv2000

Astrophysical implications of gravitational microlensing of gravitational waves

Astrophysical implications of gravitational microlensing of gravitational waves emitted by rotating neutron stars (NSs) are investigated. In particular, attention is focused on the following situations: i) NSs in the galactic bulge lensed by a central black hole of $2.6\times 10^6 M_{\odot}$ or by stars and MACHOs distributed in the galactic bulge, disk and halo between Earth and the sources; ii) NSs in globular clusters lensed by a central black hole of $\sim 10^3 M_{\odot}$ or by stars and MACHOs distributed throughout the Galaxy. The detection of such kind of microlensing events will give a unique opportunity for the unambiguous mapping of the central region of the Galaxy and of globular clusters. In addition, the detection of such events will provide a new test of the General Theory of Relativity. Gravitational microlensing will, moreover, increase the challenge of detecting gravitational waves from NSs.

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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.

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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.

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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.

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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.

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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.

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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.

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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.

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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.

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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.

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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.

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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.

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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.

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