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Massimo Persic

Publications and source records attributed to Massimo Persic.

35 records · Page 2Linked to original sources
arXiv2011

High-energy emission from star-forming galaxies

Adopting the convection-diffusion model for energetic electron and proton propagation, and accounting for all the relevant hadronic and leptonic processes, the steady-state energy distributions of these particles in the starburst galaxies M82 and NGC253 can be determined with a detailed numerical treatment. The electron distribution is directly normalized by the measured synchrotron radio emission from the central starburst region; a commonly expected theoretical relation is then used to normalize the proton spectrum in this region, and a radial profile is assumed for the magnetic field. The resulting radiative yields of electrons and protons are calculated: the predicted >100MeV and >100GeV fluxes are in agreement with the corresponding quantities measured with the orbiting Fermi telescope and the ground-based VERITAS and HESS Cherenkov telescopes. The cosmic-ray energy densities in central regions of starburst galaxies, as inferred from the radio and gamma-ray measurements of (respectively) non-thermal synchrotron and neutral-pion-decay emission, are U=O(100) eV/cm3, i.e. at least an order of magnitude larger than near the Galactic center and in other non-very-actively star-forming galaxies. These very different energy density levels reflect a similar disparity in the respective supernova rates in the two environments. A L(gamma) ~ SFR^(1.4) relationship is then predicted, in agreement with preliminary observational evidence.

astro-ph.HE↗
arXiv2010

High-frequency-peaked BL Lacertae objects as spectral candles to measure the Extragalactic Background Light in the Fermi and air Cherenkov telescopes era

The Extragalactic Background Light (EBL) is the integrated light from all the stars that have ever formed, and spans the IR-UV range. The interaction of very-high-energy (VHE: E>100 GeV) gamma-rays, emitted by sources located at cosmological distances, with the intervening EBL results in electron-positron pair production that leads to energy-dependent attenuation of the observed VHE flux. This introduces a fundamental ambiguity in the interpretation of measured VHE gamma-ray spectra: neither the intrinsic spectrum, nor the EBL, are separately known -- only their combination is. In this paper we propose a method to measure the EBL photon number density. It relies on using simultaneous observations of BL Lac objects in the optical, X-ray, high-energy (HE: E>100 MeV) gamma-ray (from the Fermi telescope), and VHE gamma-ray (from Cherenkov telescopes) bands. For each source, the method involves best-fitting the spectral energy distribution (SED) from optical through HE gamma-rays (the latter being largely unaffected by EBL attenuation as long as z<1) with a Synchrotron Self-Compton (SSC) model. We extrapolate such best-fitting models into the VHE regime, and assume they represent the BL Lacs' intrinsic emission. Contrasting measured versus intrinsic emission leads to a determination of the photon-photon opacity to VHE photons. Using, for each given source, different states of emission will only improve the accuracy of the proposed method. We demonstrate this method using recent simultaneous multi-frequency observations of the high-frequency-peaked BL Lac object PKS 2155-304 and discuss how similar observations can more accurately probe the EBL.

astro-ph.CO↗
arXiv2010

Cosmic rays in galaxies: a probe of star formation

Cosmic-ray energy densities in central regions of starburst galaxies, as inferred from radio and gamma-ray measurements of, respectively, non-thermal synchrotron and neutral pion decay emission, are typically U_p = O(100)eV/cm3, i.e. typically at least an order of magnitude larger than near the Galactic center and in other non-very-actively star-forming galaxies. We first show that these very different energy-density levels reflect a similar disparity in the respective supernova rates in the two environments, which is not unexpected given the supernova origin of (Galactic) energetic particles. As a consequence of this correspondence, we then demonstrate that there is partial quantitative evidence that the stellar initial mass function (IMF) in starburst nuclei has a low-mass truncation at ~2M_sun, as predicted by theoretical models of turbulent media, in contrast with the much smaller value of 0.1M_sun that characterizes the low-mass cutoff of the stellar IMF in `normal' galactic environments.

astro-ph.HE↗
arXiv2010

Blazars as beamlights to probe the Extragalactic Background Light in the Fermi and Cherenkov telescopes era

The Extragalactic Background Light (EBL) is the integrated light from all the stars that have ever formed, and spans the IR-UV range. The interaction of very-high-energy (VHE: E>100 GeV) gamma-rays, emitted by sources located at cosmological distances, with the intervening EBL results in electron-positron pair production that leads to energy-dependent attenuation of the observed VHE flux. This introduces a fundamental ambiguity in the interpretation of the measured VHE blazar spectra: neither the intrinsic spectra, nor the EBL, are separately known - only their combination is. In this paper we propose a method to measure the EBL photon number density. It relies on using simultaneous observations of blazars in the optical, X-ray, high-energy (HE: E>100 MeV) gamma-ray (from the Fermi telescope), and VHE gamma-ray (from Cherenkov telescopes) bands. For each source, the method involves best-fitting the spectral energy distribution (SED) from optical through HE gamma-rays (the latter being largely unaffected by EBL attenuation as long as z<1) with a Synchrotron Self-Compton (SSC) model. We extrapolate such best-fitting models into the VHE regime, and assume they represent the blazars' intrinsic emission. Contrasting measured versus intrinsic emission leads to a determination of the gamma-gamma opacity to VHE photons - hence, upon assuming a specific cosmology, we derive the EBL photon number density. Using, for each given source, different states of emission will only improve the accuracy of the proposed method. We demonstrate this method using recent simultaneous multi-frequency observations of the blazar PKS2155-304 and discuss how similar observations can more accurately probe the EBL.

astro-ph.CO↗
arXiv2009

Simultaneous multi-frequency observations of PG 1553+113

We report simultaneous multi-frequency observations of the blazar PG 1553+113, that were carried out in March-April 2008. Optical, X-ray, high-energy (HE; greater than 100 MeV) gamma-ray, and very-high- energy (VHE; greater than 100 GeV) gamma-ray data were obtained with the KVA, REM, RossiXTE/ASM, AGILE and MAGIC telescopes. This is the first simultaneous broad-band (i.e., HE+VHE) gamma-ray observation of a blazar. The source spectral energy distribution derived combining these data shows the usual double-peak shape, and is interpreted in the framework of a synchrotron-self-Compton model.

astro-ph.HE↗
arXiv2008

Very-High Energy Gamma Astrophysics

High-energy photons are a powerful probe for astrophysics and for fundamental physics under extreme conditions. During the recent years, our knowledge of the most violent phenomena in the Universe has impressively progressed thanks to the advent of new detectors for high-energy gamma-rays. Observation of gamma-rays gives an exciting view of the high-energy universe thanks to satellite-based telescopes (AGILE, GLAST) and to ground-based detectors like the Cherenkov telescopes (H.E.S.S. and MAGIC in particular), which recently discovered more than 60 new very-high-energy sources. The progress achieved with the last generation of Cherenkov telescopes is comparable to the one drawn by EGRET with respect to the previous gamma-ray satellite detectors. This article reviews the present status of high-energy gamma astrophysics, with emphasis on the recent results and on the experimental developments.

astro-ph↗
arXiv2008

Intergalactic absorption and blazar gamma-ray spectra

The distribution of TeV spectral slopes versus redshift for currently known TeV blazars (16 sources with z<0.21, and one with z>0.25) is essentially a scatter plot with hardly any hint of a global trend. We suggest that this is the outcome of two combined effects of intergalactic gamma-gamma absorption, plus an inherent feature of the SSC (synchro-self-Compton) process of blazar emission. First, flux dimming introduces a bias that favors detection of progressively more flaring sources at higher redshifts. According to mainstream SSC models, more flaring source states imply sources with flatter TeV slopes. This results in a structured relation between intrinsic TeV slope and redshift. The second effect, spectral steepening by intergalactic absorption, affects sources progressively with distance and effectively wipes out the intrinsic slope-redshift correlation.

astro-ph↗
arXiv2008

Constraints on Large-Scale Magnetic Fields from the Auger Results

A recent article from the Pierre Auger Collaboration links the direction of charged cosmic rays to possible extragalactic sites of emission. The correlation of the direction of such particles with the direction of the emitter allows constraining the value of large-scale magnetic fields B. Assuming for B a coherence length in the range between 1 Mpc and 10 Mpc, we find values of B between 0.3 and 0.9 nG.

astro-ph↗
arXiv2005

Discovery of Rapid X-ray Oscillations in the Tail of the SGR 1806-20 Hyperflare

We have discovered rapid Quasi Periodic Oscillations (QPOs) in RXTE/PCA measurements of the pulsating tail of the 27th December 2004 giant flare of SGR 1806-20. QPOs at about 92.5Hz are detected in a 50s interval starting 170s after the onset of the giant flare. These QPOs appear to be associated with increased emission by a relatively hard unpulsed component and are seen only over phases of the 7.56s spin period pulsations away from the main peak. QPOs at about 18 and 30Hz are also detected, 200-300s after the onset of the giant flare. This is the first time that QPOs are unambiguously detected in the flux of a Soft Gamma-ray Repeater, or any other magnetar candidate. We interpret the highest QPOs in terms of the coupling of toroidal seismic modes with Alfven waves propagating along magnetospheric field lines. The lowest frequency QPO might instead provide indirect evidence on the strength of the internal magnetic field of the neutron star.

astro-ph↗
arXiv2004

2-100 keV Spectrum of an Actively Star Forming Galaxy

We compute the synthetic 2-100 keV spectrum of an actively star forming galaxy. To this aim we use a luminosity function of point sources appropriate for starbursts (based on Chandra data), as well as the types of stellar sources and their corresponding spectra. Our estimates indicate that a Compton spectral component - resulting from scattering of SN-accelerated electrons by ambient FIR and CMB photons - could possibly be detected, in deep INTEGRAL observations of nearby starburst galaxies, only if there is a break in the spectra of the brightest (L > 2x10^{38} erg/s) point sources.

astro-ph↗
arXiv1997

Dark Matter Halos around Galaxies

We present evidence that all galaxies, of any Hubble type and luminosity, bear the kinematical signature of a mass component distributed differently from the luminous matter. We review and/or derive the DM halo properties of galaxies of different luminosities and morphologies: spirals, LSBs, ellipticals, dwarf irregulars and dwarf spheroidals.

astro-ph↗
arXiv1996

Dark Baryons in the Universe: the Quest Goes On

We show that the high local baryonic fraction, M(bar) ~ 1/3 M(tot), found in groups and clusters of galaxies does not reconcile the observed cosmological baryon density with the standard Big-Bang prediction. Taking into account recent measurements on the hot-gas content and temperature functions of clusters and groups, we get Omega(gas) = 0.0023 h_{50}^{-1.5} ~ 4% Omega(BBN). Including the contributions of galaxies and of (local) Ly-alpha clouds we estimate Omega(bar) ~ 0.004-0.006 < 10% Omega(BBN) as the amount of detected baryons. Most of the synthesised atoms are still to be discovered. We propose to relate the impressive presence of the hot gas component in clusters with the very low, mass-dependent efficiency of the process of galaxy formation in making stars from the primordial gas.

astro-ph↗
arXiv1995

ROTATION CURVES OF 967 SPIRAL GALAXIES: IMPLICATIONS FOR DARK MATTER.

We present the rotation curves of 967 spiral galaxies, obtained by deprojecting and folding the raw data published by Mathewson et al. (1992). Of these, 80 meet objective excellence criteria and are suitable for individual detailed mass modelling, while 820 are suitable for statistical studies. A preliminary analysis of theire properties confirms that rotation curves are a universal function of luminosity and that the dark matter fraction in spirals increase with decreasing luminosity. Quantitative considerations on the virial radius of dark halos allow us to make hypotheses about their structure and nature. The deprojected folded curves, the smoothed curves, and various related quantities are available via anonymous ftp at ftp://galileo.sissa.it/users/ftp/pub/psrot

astro-ph↗
arXiv1995

ROTATION CURVES OF 967 SPIRAL GALAXIES

We present the rotation curves (RCs) of 967 spiral galaxies, obtained by folding and deprojecting the raw optical data by Mathewson et al. (1992). Out of these, we have identified 80 excellent RCs and 820 fair RCs. The 80 excellent RCs are smooth, symmetric, have negligible rms internal error, extend to at least the optical radius, and have high and homogeneus radial data coverage; they rival with the best optical one-slit RCs available in the literature and are ideally suited for detailed mass structure modelling. The 820 fair RCs do not meet at least one of the above criteria and hence are not suitable for direct mass modelling; however, they constitute a large database for methods aiming to recover statistical properties of DM. For these 900 RCs we also present, in figures and tables, the rotation curves smoothed on scales corresponding to 5%-20% of the optical size. By size, homogeneity, intrinsic quality, and spanned range of luminosities and asymptotic velocities, this sample constitutes by far the best sample of RCs available to date. As such, it will offer a unique opportunity for investigating in considerable depth the properties of DM in galaxies. The the deprojected folded curves, the smoothed curves, and various related quantities are available via anonymous ftp at galileo.sissa.it in the directory /users/ftp/pub/psrot.

astro-ph↗
arXiv1994

The contribution of galaxies to OMEGA_0

We estimate the contribution of galaxies to the cosmological density Omega_0 including also their extended dark halos. We find Omega_gal< 0.1, so implying that the matter in galaxies cannot by itself close the Universe but marginally could be baryonic. (latex file)two figures available upon request).

astro-ph↗
arXiv1993

Decaying Neutrinos in Galaxy Clusters

Davidsen et al. (1991) have argued that the failure to detect uv photons from the dark matter DM) in cluster A665 excludes the decaying neutrino hypothesis. Sciama et al. (1993) argued that because of high central concentration the DM in that cluster must be baryonic. We study the DM profile in clusters of galaxies simulated using the Harrison--Zel'dovich spectrum of density fluctuations, and an amplitude previously derived from numerical simulations (Melott 1984b; Anninos et al. 1991) and in agreement with microwave background fluctuations (Smoot et al. 1992). We find that with this amplitude normalization cluster neutrino DM densities are comparable to observed cluster DM values. We conclude that given this normalization, the cluster DM should be at least largely composed of neutrinos. The constraint of Davidsen et al. can be somewhat weakened by the presence of baryonic DM; but it cannot be eliminated given our assumptions.

astro-ph↗