arXiv subjects

A. Del Popolo

Publications and source records attributed to A. Del Popolo.

At least 73 records · Page 4Linked to original sources

arXiv2022

Gamma-ray observations of MAXI J1820+070 during the 2018 outburst

MAXI J1820+070 is a low-mass X-ray binary with a black hole as a compact object. This binary underwent an exceptionally bright X-ray outburst from March to October 2018, showing evidence of a non-thermal particle population through its radio emission during this whole period. The combined results of 59.5 hours of observations of the MAXI J1820+070 outburst with the H.E.S.S., MAGIC and VERITAS experiments at energies above 200 GeV are presented, together with Fermi-LAT data between 0.1 and 500 GeV, and multiwavelength observations from radio to X-rays. Gamma-ray emission is not detected from MAXI J1820+070, but the obtained upper limits and the multiwavelength data allow us to put meaningful constraints on the source properties under reasonable assumptions regarding the non-thermal particle population and the jet synchrotron spectrum. In particular, it is possible to show that, if a high-energy gamma-ray emitting region is present during the hard state of the source, its predicted flux should be at most a factor of 20 below the obtained Fermi-LAT upper limits, and closer to them for magnetic fields significantly below equipartition. During the state transitions, under the plausible assumption that electrons are accelerated up to ~ 500 GeV, the multiwavelength data and the gamma-ray upper limits lead consistently to the conclusion that a potential high-energy and very-high-energy gamma-ray emitting region should be located at a distance from the black hole ranging between 10^11 and 10^13 cm. Similar outbursts from low-mass X-ray binaries might be detectable in the near future with upcoming instruments such as CTA.

astro-ph.HE↗

arXiv2022

Arrival Directions of Cosmic Rays above 32 EeV from Phase One of the Pierre Auger Observatory

A promising energy range to look for angular correlation between cosmic rays of extragalactic origin and their sources is at the highest energies, above few tens of EeV ($1\:{\rm EeV}\equiv 10^{18}\:$eV). Despite the flux of these particles being extremely low, the area of ${\sim}\:3{,}000 \: \text{km}^2$ covered at the Pierre Auger Observatory, and the 17-year data-taking period of the Phase 1 of its operations, have enabled us to measure the arrival directions of more than 2,600 ultra-high energy cosmic rays above $32\:\text{EeV}$. We publish this data set, the largest available at such energies from an integrated exposure of $122{,}000 \: \text{km}^2\:\text{sr}\:\text{yr}$, and search it for anisotropies over the $3.4π$ steradians covered with the Observatory. Evidence for a deviation in excess of isotropy at intermediate angular scale, with ${\sim}\:15^\circ$ Gaussian spread or ${\sim}\:25^\circ$ top-hat radius, is obtained at the $4\:σ$ significance level for cosmic-ray energies above ${\sim}\:40\:\text{EeV}$.

astro-ph.HE↗

arXiv2022

Investigating Hadronic Interactions at Ultra-High Energies with the Pierre Auger Observatory

The development of an extensive air shower depends not only on the nature of the primary ultra-high-energy cosmic ray but also on the properties of the hadronic interactions. For energies above those achievable in human-made accelerators, hadronic interactions are only accessible through the studies of extensive air showers, which can be measured at the Pierre Auger Observatory. With its hybrid detector design, the Pierre Auger Observatory measures both the longitudinal development of showers in the atmosphere and the lateral distribution of particles that arrive at the ground. This way, observables that are sensitive to hadronic interactions at ultra-high energies can be obtained. While the hadronic interaction cross-section can be assessed from the longitudinal profiles, the number of muons and their fluctuations measured with the ground detectors are linked to other physical properties. In addition to these direct studies, we discuss here how measurements of the atmospheric depth of the maximum of air-shower profiles and the characteristics of the muon signal at the ground can be used to test the self-consistency of the post-LHC hadronic models.

astro-ph.HE↗

arXiv2022

A search for photons with energies above $2{\times}10^{17}$ eV using hybrid data from the low-energy extensions of the Pierre Auger Observatory

Ultra-high-energy photons with energies exceeding $10^{17}$ eV offer a wealth of connections to different aspects of cosmic-ray astrophysics as well as to gamma-ray and neutrino astronomy. The recent observations of photons with energies in the $10^{15}$ eV range further motivate searches for even higher-energy photons. In this paper, we present a search for photons with energies exceeding $2{\times}10^{17}$ eV using about 5.5 years of hybrid data from the low-energy extensions of the Pierre Auger Observatory. The upper limits on the integral photon flux derived here are the most stringent ones to date in the energy region between $10^{17}$ and $10^{18}$ eV.

astro-ph.HE↗

arXiv2022

Investigating the blazar TXS 0506+056 through sharp multi-wavelength eyes during 2017-2019

The blazar TXS 0506+056 got into the spotlight of the astrophysical community in September 2017, when a high-energy neutrino detected by IceCube (IceCube-170922A) was associated at the 3 $σ$ level to a $γ$-ray flare from this source. This multi-messenger photon-neutrino association remains, as per today, the most significant one ever observed. TXS 0506+056 was a poorly studied object before the IceCube-170922A event. To better characterize its broad-band emission, we organized a multi-wavelength campaign lasting 16 months (November 2017 to February 2019), covering the radio-band (Metsähovi, OVRO), the optical/UV (ASAS-SN, KVA, REM, Swift/UVOT), the X-rays (Swift/XRT, NuSTAR), the high-energy $γ$ rays (Fermi/LAT) and the very-high-energy (VHE) $γ$ rays (MAGIC). In $γ$ rays, the behaviour of the source was significantly different from the 2017 one: MAGIC observations show the presence of flaring activity during December 2018, while the source only shows an excess at the 4$σ$ level during the rest of the campaign (74 hours of accumulated exposure); Fermi/LAT observations show several short (days-to-week timescale) flares, different from the long-term brightening of 2017. No significant flares are detected at lower energies. The radio light curve shows an increasing flux trend, not seen in other wavelengths. We model the multi-wavelength spectral energy distributions in a lepto-hadronic scenario, in which the hadronic emission emerges as Bethe-Heitler and pion-decay cascade in the X-rays and VHE $γ$ rays. According to the model presented here, the December 2018 $γ$-ray flare was connected to a neutrino emission that was too brief and not bright enough to be detected by current neutrino instruments.

astro-ph.HE↗

arXiv2022

Testing effects of Lorentz invariance violation in the propagation of astroparticles with the Pierre Auger Observatory

Lorentz invariance violation (LIV) is often described by dispersion relations of the form $E_i^2=m_i^2+p_i^2+δ_{i,n} E^{2+n}$ with delta different based on particle type $i$, with energy $E$, momentum $p$ and rest mass $m$. Kinematics and energy thresholds of interactions are modified once the LIV terms become comparable to the squared masses of the particles involved. Thus, the strongest constraints on the LIV coefficients $δ_{i,n}$ tend to come from the highest energies. At sufficiently high energies, photons produced by cosmic ray interactions as they propagate through the Universe could be subluminal and unattenuated over cosmological distances. Cosmic ray interactions can also be modified and lead to detectable fingerprints in the energy spectrum and mass composition observed on Earth. The data collected at the Pierre Auger Observatory are therefore possibly sensitive to both the electromagnetic and hadronic sectors of LIV. In this article, we explore these two sectors by comparing the energy spectrum and the composition of cosmic rays and the upper limits on the photon flux from the Pierre Auger Observatory with simulations including LIV. Constraints on LIV parameters depend strongly on the mass composition of cosmic rays at the highest energies. For the electromagnetic sector, while no constraints can be obtained in the absence of protons beyond $10^{19}$ eV, we obtain $δ_{γ,0} > -10^{-21}$, $δ_{γ,1} > -10^{-40}$ eV$^{-1}$ and $δ_{γ,2} > -10^{-58}$ eV$^{-2}$ in the case of a subdominant proton component up to $10^{20}$ eV. For the hadronic sector, we study the best description of the data as a function of LIV coefficients and we derive constraints in the hadronic sector such as $δ_{\mathrm{had},0} < 10^{-19}$, $δ_{\mathrm{had},1} < 10^{-38}$ eV$^{-1}$ and $δ_{\mathrm{had},2}< 10^{-57}$ eV$^{-2}$ at 5$σ$ CL.

astro-ph.HE↗

arXiv2020

On the absence of a universal surface density, and a maximum Newtonian acceleration in dark matter haloes: consequences for MOND

We study the dark matter (DM) surface density using the SPARC sample and {compare} it to Donato et al. (2009) result. By means of MCMC method, we infer the best-fitting parameters for each galaxy. We reobtain the scaling relation between the surface density and luminosity, and several other scaling laws relating the dark matter halo properties to that of the galactic disc properties. We conclude, in contrast with Donato et al. \cite{Donato}, that the dark matter surface density is not a universal (constant) quantity but correlates with the luminosity as well as with other galactic disc properties. A derived posterior probability distribution of $ρ_0 r_0$ shows that the null hypothesis of constancy is rejected at a very high confidence level. These results leave little room for the claimed universality of dark matter surface density. Since MOND has strong prediction on the surface density \cite{Milgrom2009}, we compared our result with those predictions, finding that MOND predictions are violated by data. To strengthen the previous result, we compared our results to another prediction of MOND (Milgrom 2005), the existence of a maximum Newtonian dark matter acceleration in the halo. Also in this case, MOND predictions are in contradiction with data. The dark matter Newtonian acceleration correlates with all the previously presented galactic disc properties, and data are distributed outside the bound predicted by Milgrom $\&$ Sanders (Milgrom 2005). We also find that the null hypothesis (constancy of DM Newtonian acceleration) is rejected at a very high confidence level.

astro-ph.GA↗

arXiv2019

Thermodynamic Equilibrium in General Relativity

The thermodynamic equilibrium condition for a static self-gravitating fluid in the Einstein theory is defined by the Tolman-Ehrenfest temperature law, $T{\sqrt {g_{00}(x^{i})}} = constant$, according to which the proper temperature depends explicitly on the position within the medium through the metric coefficient $g_{00}(x^{i})$. By assuming the validity of Tolman-Ehrenfest "pocket temperature", Klein also proved a similar relation for the chemical potential, namely, $μ{\sqrt {g_{00}(x^{i})}} = constant$. In this letter we prove that a more general relation uniting both quantities holds regardless of the equation of state satisfied by the medium, and that the original Tolman-Ehrenfest law form is valid only if the chemical potential vanishes identically. In the general case of equilibrium, the temperature and the chemical potential are intertwined in such a way that only a definite (position dependent) relation uniting both quantities is obeyed. As an illustration of these results, the temperature expressions for an isothermal gas (finite spherical distribution) and a neutron star are also determined.

gr-qc↗

arXiv2016

On the dark matter haloes inner structure and galaxy morphology

In the present paper, we extend the study of Del Popolo (2010) to determine the slope of the inner density profile of galaxy haloes with different morphologies. We study how galaxy morphology changes the relation between the inner slope of the galaxy halo density profile, $α$, and the stellar mass, $M_{*}$, or rotation velocity $V_{\rm rot}$. For this, we use the model of Del Popolo (2009) in combination with observed data from the Romanowsky \& Fall (2012) sample of elliptical and spiral galaxies, the Local Group sample compiled by McConnachie (2012), and the simulation results by Cloet-Osselaer et al. (2014). We find that the slope $α$ flattens monotonically, from $α\simeq -1 $ at $V_{\rm rot} \simeq 250$ km/s, to $α\simeq 0 $. After $V_{\rm rot}\simeq 25$ km/s the slope starts to steepen. The steepening happens in the mass range dominated by non-rotationally supported galaxies (e.g., dSphs) and depends on the level of offset in the angular momentum of rotationally and non-rotationally dominated galaxies. The steepening is a consequence of the decrease in baryons content, and angular momentum in spheroidal dwarf galaxies. We finally compare our result to the SPH simulations of Di Cintio. Our result is in qualitatively agreement with their simulations, with the main difference that the inner slope $α$ at small stellar masses ($M_* \lesssim10^{8} M_{\odot}$) is flatter than that in their simulations. As a result, the claim that finding a core in dwarf galaxies with masses slightly smaller than $\simeq 10^6 M_{\odot}$, (as in the Di Cintio, or Governato, supernovae feedback mechanism) would be a problem for the $Λ$CDM model must be probably revised.

astro-ph.GA↗

arXiv2016

The Cusp/Core problem: supernovae feedback versus the baryonic clumps and dynamical friction model

In the present paper, we compare the predictions of two well known mechanisms considered able to solve the cusp/core problem (a. supernova feedback; b. baryonic clumps-DM interaction) by comparing their theoretical predictions to recent observations of the inner slopes of galaxies with masses ranging from dSphs to normal spirals. We compare the $α$-$V_{\rm rot}$ and the $α$-$M_{\ast}$ relationships, predicted by the two models with high resolution data coming from \citep{Adams2014,Simon2005}, LITTLE THINGS \citep{Oh2015}, THINGS dwarves \citep{Oh2011a,Oh2011b}, THINGS spirals \citep{Oh2015}, Sculptor, Fornax and the Milky Way. The comparison of the theoretical predictions with the complete set of data shows that the two models perform similarly, while when we restrict the analysis to a smaller subsample of higher quality, we show that the method presented in this paper (baryonic clumps-DM interaction) performs better than the one based on supernova feedback. We also show that, contrarily to the first model prediction, dSphs of small mass could have cored profiles. This means that observations of cored inner profiles in dSphs having a stellar mass $<10^6 M_{\odot}$ not necessarily imply problems for the $Λ$CDM model.

astro-ph.GA↗

arXiv2016

Chandrasekhar's Dynamical Friction and non-extensive statistics

The motion of a point like object of mass $M$ passing through the background potential of massive collisionless particles ($m << M$) suffers a steady deceleration named dynamical friction. In his classical work, Chandrasekhar assumed a Maxwellian velocity distribution in the halo and neglected the self gravity of the wake induced by the gravitational focusing of the mass $M$. In this paper, by relaxing the validity of the Maxwellian distribution due to the presence of long range forces, we derive an analytical formula for the dynamical friction in the context of the $q$-nonextensive kinetic theory. In the extensive limiting case ($q = 1$), the classical Gaussian Chandrasekhar result is recovered. As an application, the dynamical friction timescale for Globular Clusters spiraling to the galactic center is explicitly obtained. Our results suggest that the problem concerning the large timescale as derived by numerical $N$-body simulations or semi-analytical models can be understood as a departure from the standard extensive Maxwellian regime as measured by the Tsallis nonextensive $q$-parameter.

astro-ph.GA↗

arXiv2016

Gravitational microlensing as a probe for dark matter clumps

Extended dark matter (DM) substructures may play the role of microlenses in the Milky Way and in extragalactic gravitational lens systems (GLSs). We compare microlensing effects caused by point masses (Schwarzschild lenses) and extended clumps of matter using a simple model for the lens mapping. A superposition of the point mass and the extended clump is also considered. For special choices of the parameters, this model may represent a cusped clump of cold DM, a cored clump of self-interacting dark matter (SIDM) or an ultra compact minihalo of DM surrounding a massive point-like object. We built the resulting micro-amplification curves for various parameters of one clump moving with respect to the source in order to estimate differences between the light curves caused by clumps and by point lenses. The results show that it may be difficult to distinguish between these models. However, some region of the clump parameters can be restricted by considering the high amplification events at the present level of photometric accuracy. Then we estimate the statistical properties of the amplification curves in extragalactic GLSs. For this purpose, an ensemble of amplification curves is generated yielding the autocorrelation functions (ACFs) of the curves for different choices of the system parameters. We find that there can be a significant difference between these ACFs if the clump size is comparable with typical Einstein radii; as a rule, the contribution of clumps makes the ACFs less steep.

astro-ph.GA↗

arXiv2014

On the surface density of dark matter haloes

In this article, we test the conclusion of Donato et al. (2009) concerning the universality of the DM halo surface density $μ_{0D}=ρ_0r_0$. According to our study, the dispersion of values of $μ_{0D}$ is twice higher than that found by Donato et al. (2009). We conclude, in contrast with Donato et al. (2009), that the DM surface density and its Newtonian acceleration are not constant but correlate with the luminosity, morphological type, $(B-V)_0$ colour index, and the content of neutral hydrogen. These DM parameters are higher for more luminous systems of early types with red colour and low gas content. We also found that the correlation of DM parameters with colour index appears to be the manifestation of a stronger relation between DM halo mass and the colour of a galaxy. This finding is in agreement with cosmological simulations (Guo et al, 2011). These results leave little room for the recently claimed universality of DM column density. We also found that isolated galaxies in our sample (contained in the Analysis of the interstellar Medium of Isolated GAlaxies (AMIGA) catalogue) do not differ significantly in their value of $μ_{0D}$ from the entire sample. Thus, since the AMIGA catalogue gives a sample of galaxies that have not interacted with a significant mass neighbour in the past 3 Gyr, the difference between the systems with low and high values of $μ_{0D}$ is not related to the merging events during this period of time.

astro-ph.GA↗

arXiv2014

Effects of shear and rotation on the spherical collapse model for clustering dark energy

In the framework of the spherical collapse model we study the influence of shear and rotation terms for dark matter fluid in clustering dark energy models. We evaluate, for different equations of state, the effects of these terms on the linear overdensity threshold parameter, $δ_{\rm c}$, and on the virial overdensity, $Δ_{\rm V}$. The evaluation of their effects on $δ_{\rm c}$ allows us to infer the modifications occurring on the mass function. Due to ambiguities in the definition of the halo mass in the case of clustering dark energy, we consider two different situations: the first is the classical one where the mass is of the dark matter halo only, while the second one is given by the sum of the mass of dark matter and dark energy. As previously found, the spherical collapse model becomes mass dependant and the two additional terms oppose to the collapse of the perturbations, especially on galactic scales, with respect to the spherical non-rotating model, while on clusters scales the effects of shear and rotation become negligible. The values for $δ_{\rm c}$ and $Δ_{\rm V}$ are higher than the standard spherical model. Regarding the effects of the additional non-linear terms on the mass function, we evaluate the number density of halos. As expected, major differences appear at high masses and redshifts. In particular, quintessence (phantom) models predict more (less) objects with respect to the $Λ$CDM model and the mass correction due to the contribution of the dark energy component has negligible effects on the overall number of structures.

astro-ph.CO↗

arXiv2014

The flat density profiles of massive, and relaxed galaxy clusters

In the present paper, we studied by means of the SIM introduced in Del Popolo (2009), the total and DM density profiles, and the correlations among different quantities, observed by Newman et al. (2012a,b), in seven massive and relaxed clusters, namely MS2137, A963, A383, A611, A2537, A2667, A2390. Similarly to Newman et al. (2012a,b), the total density profile, in the radius range 0.003 - 0.03$r_{200}$, has a mean total density profile in agreement with dissipationless simulations. The slope of the DM profiles of all clusters is flatter than -1. The slope, $α$, has a maximum value (including errors) of $α=-0.88$ in the case of A2390, and minimum value $α=-0.14$ for A2537. The baryonic component dominates the mass distribution at radii $< 5-10$ kpc, while the outer distribution is dark matter dominated. We found an anti-correlation among the slope $α$, the effective radius, $R_e$, and the BCG mass, and a correlation among the core radius $r_{core}$, and $R_e$. Moreover, the mass in 100 kpc (mainly dark matter) is correlated with the mass inside 5 kpc (mainly baryons). The behavior of the total mass density profile, the DM density profile, and the quoted correlations can be understood in a double phase scenario. In the first dissipative phase the proto-BCG forms, and in the second dissipationless phase, dynamical friction between baryonic clumps (collapsing to the center) and the DM halo flattens the inner slope of the density profile. In simple terms, the large scatter in the inner slope from cluster to cluster, and the anti-correlation among the slope, $α$ and $R_e$ is due to the fact that in order to have a total mass density profile which is NFW-like, clusters having more massive BCGs at their centers must contain less DM in their center. Consequently the inner profile has a flatter slope.

astro-ph.GA↗

arXiv2014

A unified solution to the small scale problems of the $Λ$CDM model

We study, by means of the model proposed in Del Popolo (2009), the effect of baryon physics on the small scale problems of the CDM model. We show that, using this model, the cusp/core problem, the missing satellite problem (MSP), the Too Big to Fail (TBTF) problem, and the angular momentum catastrophe can be reconciled with observations. Concerning the cusp/core problem, the interaction among dark matter (DM) and baryonic clumps of 1% the mass of the halo, through dynamical friction (DF), is able to flatten the inner cusp of the density profiles. We moreover assume that haloes form primarily through quiescent accretion, in agreement with the spherical collapse model (SCM)-secondary infall model (SIM) prescriptions. The results of this paper follow from the two assumptions above. Concerning the MSP and TBTF problem, applying to the Via Lactea II (VL2) subhaloes a series of corrections similar to those of Brooks et al. (2013), namely applying a Zolotov et al. (2012)-like correction obtained with our model, and further correcting for the UV heating and tidal stripping, we obtain that the number of massive, luminous satellites is in agreement with the number observed in the MW. The model also produces an angular momentum distribution in agreement with observations, that is with the distribution of the angular spin parameter and angular momentum of the dwarfs studied by van den Bosch, Burkert, & Swaters (2001). In conclusion, the small scale problems of the CDM model can all be solved by introducing baryon physics.

astro-ph.CO↗

arXiv2013

Non-Baryonic Dark Matter in Cosmology

This paper is a broad-band review of the current status of non-baryonic dark matter research. I start with a historical overview of the evidences of dark matter existence, then I discuss how dark matter is distributed from small scale to large scale, and I then verge the attention to dark matter nature: dark matter candidates and their detection. I finally discuss some of the limits of the $Λ$CDM model, with particular emphasis on the small scale problems of the paradigm.

astro-ph.CO↗

arXiv2013

Shear and rotation in Chaplygin cosmology

We study the effect of shear and rotation on results previously obtained dealing with the application of the spherical collapse model (SCM) to generalized Chaplygin gas (gCg) dominated universes. The system is composed of baryons and gCg and the collapse is studied for different values of the parameter $α$ of the gCg. We show that the joint effect of shear and rotation is that of slowing down the collapse with respect to the simple SCM. This result is of utmost importance for the so-called unified dark matter models, since the described slow down in the growth of density perturbation can solve one of the main problems of the quoted models, namely the instability described in previous papers [e.g., H. B. Sandvik {\it et al.}, Phys. Rev. D {\bf 69}, 123524 (2004)] at the linear perturbation level.

astro-ph.CO↗