arXiv subjects

Francesca Scarcella

Publications and source records attributed to Francesca Scarcella.

8 recordsLinked to original sources

arXiv2026

The swallowed spike: the formation of light primordial black hole structures around heavy seeds

Spikes are steep enhancements in the dark matter (DM) distribution around a heavy compact object. If the compact object is primordial, and the bulk of the DM is also composed of (lighter) primordial compact objects, for instance asteroid-mass primordial black holes (PBHs), the phenomenology of spike formation is highly non-trivial. In fact, lighter PBHs have negligible angular momentum at formation with respect to the massive central object and would therefore be captured unless enough torque is exerted from either small-scale or large-scale matter fluctuations. In this paper, we present the first comprehensive assessment of this scenario. We define the mechanisms and the initial conditions that allow light PBHs to avoid capture. We then quantify the different types of torque and follow the corresponding angular momentum evolution with a combination of analytical prescriptions and numerical simulations. We find that in the innermost region no mechanism studied here is capable of providing enough torque; the resulting inner core is expected to be significantly less dense than in particle scenarios, potentially leading to interesting phenomenology.

astro-ph.CO↗

arXiv2026

On the survival of dark matter spikes: Stellar and compact-object perturbations

Establishing realistic expectations for the dark matter (DM) distribution near a supermassive black hole (BH) is essential for assessing environmental imprints on gravitational wave (GW) signals. Using the Galactic Center as an observationally constrained case study, we investigate the evolution of DM density spikes under gravitational perturbations from the nuclear stellar and black hole populations surrounding the central BH. We find that scattering of DM particles by the nuclear star cluster depletes the DM distribution at radii $r \sim 10^{-1}~\mathrm{pc}$, far outside the region where the relevant GW signals are produced. At smaller radii, at $r \sim 10^{-3}~\mathrm{pc}$, the closest known stellar perturbers, S2 and S38, induce only negligible changes to the density profile. At still smaller radii, where no stellar perturbers are currently known, we assess the cumulative impact of past EMRIs by modelling successive mergers with stellar-mass BHs of mass $\sim10~\mathrm{M}_\odot$, in numbers consistent with the expected rate over $10~\mathrm{Gyr}$. We find that these events do not erase the central overdensity, reducing the density only to approximately $82 \%$ of its initial value at $r \lesssim 10^{-5}~\mathrm{pc}$. Our results indicate that, at least at the Galactic Center, DM overdensities around the central BH are expected to remain largely intact under stellar perturbations and plausible stellar-BH merger histories.

astro-ph.GA↗

arXiv2025

Hints of Dark Matter Spikes in Low-mass X-ray Binaries: a critical assessment

Three black-hole low-mass X-ray binaries (LMXBs) in the Milky Way show rates of period decay which cannot be easily explained by standard mechanisms. Recently, it has been claimed that the anomalous period decays in two of these systems may be explained by dynamical friction due to very high dark matter (DM) densities around the black holes. We critically assess these claims by performing $N$-body simulations of binaries embedded in dense DM ``spikes". We simulate the previously-studied systems XTE J1118+480 and A0620--00, as well as studying the third binary Nova Muscae 1991 for the first time in this context. These simulations show that feedback on the DM distribution plays a crucial role and we rule out previously-claimed shallow DM spikes. We set lower limits on the steepness $\gamma$ of DM density profiles required to explain the period decay in these LMXBs, requiring $\gamma \gtrsim 2.15-2.20$ in XTE J1118+480 and A0620--00 and $\gamma \gtrsim 2.3$ in Nova Muscae 1991. Improved modeling of the long-term evolution of binaries embedded in DM spikes may allow us to exclude even larger densities in future.

hep-ph↗

arXiv2024

CMB bounds on primordial black holes with dark matter mini-halos: the role of radiative feedback

Observations of the cosmic microwave background constrain the abundance of primordial black holes, as these would accrete gas and inject energy into the cosmological medium. We have revisited these constraints, taking into account the local heating and ionisation of the gas around the black holes. While constraints for \textit{naked} black holes are not significantly affected, bounds including dark matter mini-halos are drastically relaxed. This result suggests that previous analysis may have significantly overestimated the role of dark matter mini-halos in boosting the accretion rates.

astro-ph.CO↗

arXiv2024

Feedback in the dark: a critical examination of CMB bounds on primordial black holes

If present in the early universe, primordial black holes (PBHs) will accrete matter and emit high-energy photons, altering the statistical properties of the {Cosmic Microwave Background (CMB)}. This mechanism has been used to constrain the fraction of dark matter that is in the form of PBHs to be much smaller than unity for PBH masses well above one solar mass. Moreover, the presence of dense dark matter mini-halos around the PBHs has been used to set even more stringent constraints, as these would boost the accretion rates. In this work, we critically revisit CMB constraints on PBHs taking into account the role of the local ionization of the gas around them. We discuss how the local increase in temperature around PBHs can prevent the dark matter mini-halos from strongly enhancing the accretion process, in some cases significantly weakening previously derived CMB constraints. We explore in detail the key ingredients of the CMB bound and derive a conservative limit on the cosmological abundance of massive PBHs.

hep-ph↗

arXiv2023

Black Hole Phenomenology and Dark Matter Searches

Motivated by recent detections of black hole binary systems through gravitational waves, in this thesis we discuss two complementary channels for the observation of Primordial Black Holes (PBHs) with masses between a few and a hundred solar masses. First, we consider the possibility of detecting black holes in the Milky Way through the electromagnetic radiation emitted in the process of gas accretion, separately examining the astrophysical black hole population and an hypothetical primordial one. We employ a state-of-the-art accretion model, able to account for radiative feedback. Our findings suggest that the detection of astrophysical isolated black holes in the vicinity of the galactic center is around the corner. We perform a complete parametric study of the uncertainty associated with this prediction. We then turn to constraining PBH abundance through the same channel, finding that existing bounds can be significantly relaxed when modelling uncertainties are taken into account. The PBH mass function motivated by the Universe's thermal history is considered. In the second part, we turn to gravitational wave observations. The possibility of disentangling the astrophysical background form a possible primordial signal in present data is hampered by large theoretical uncertainties on the properties of both populations. However, third-generation gravitational wave detectors will be able to detect mergers up to the dark ages, where the astrophysical background is expected to be absent. Through mock data generation and analysis, we assess the ability of the Einstein Telescope (ET) to identify a subdominant population of PBHs, disentangling it from the astrophysical one based exclusively on event distance measurements. We find that the ET should be able to detect and constrain the PBH abundance if these constitute at least approximately one part in $10^5$ of the dark matter.

astro-ph.CO↗

arXiv2022

Dancing in the dark: detecting a population of distant primordial black holes

Primordial black holes (PBHs) are compact objects proposed to have formed in the early Universe from the collapse of small-scale over-densities. Their existence may be detected from the observation of gravitational waves (GWs) emitted by PBH mergers, if the signals can be distinguished from those produced by the merging of astrophysical black holes. In this work, we forecast the capability of the Einstein Telescope, a proposed third-generation GW observatory, to identify and measure the abundance of a subdominant population of distant PBHs, using the difference in the redshift evolution of the merger rate of the two populations as our discriminant. We carefully model the merger rates and generate realistic mock catalogues of the luminosity distances and errors that would be obtained from GW signals observed by the Einstein Telescope. We use two independent statistical methods to analyse the mock data, finding that, with our more powerful, likelihood-based method, PBH abundances as small as $f_\mathrm{PBH} \approx 7 \times 10^{-6}$ ($f_\mathrm{PBH} \approx 2\times10^{-6}$) would be distinguishable from $f_\mathrm{PBH} = 0$ at the level of $3\sigma$ with a one year (ten year) observing run of the Einstein Telescope. Our mock data generation code, darksirens, is fast, easily extendable and publicly available on GitLab.

astro-ph.CO↗

arXiv2020

Multi-wavelength detectability of isolated black holes in the Milky Way

Isolated black holes in our Galaxy have eluded detection so far. We present here a comprehensive study on the detectability of isolated stellar-mass astrophysical black holes that accrete interstellar gas from molecular clouds in both the local region and the Central Molecular Zone. We adopt a state-of-the-art model for the accretion physics backed up by numerical simulations, and study the number of observable sources in both the radio and X-ray band, as a function of a variety of parameters. We discuss in particular the impact of the astrophysical uncertainties on our prediction for the number of bright X-ray sources in the central region of the Galaxy. We finally consider future developments in the radio domain, and assess the potential of SKA to detect a population of astrophysical black holes accreting gas in our Galaxy.

astro-ph.HE↗