arXiv subjects

Sergio Colafrancesco

Publications and source records attributed to Sergio Colafrancesco.

At least 19 recordsLinked to original sources

arXiv2021

Making light of gravitational-waves

Mixing between photons and low-mass bosons is well considered in the literature. The particular case of interest here is with hypothetical gravitons, as we are concerned with the direct conversion of gravitons into photons in the presence of an external magnetic field. We examine whether such a process could produce direct low-frequency radio counterparts to gravitational-wave events. Our work differs from previous work in the literature in that we use the results of numerical simulations to demonstrate that, although a single such event may be undetectable without at least 100000 dipoles, an unresolved gravitational wave background from neutron star mergers could be potentially detectable with a lunar telescope composed of 1000 elements. This is provided the gravitational wave spectrum only experiences exponential damping above 80 kHz, a full order of magnitude above the limit achieved by present simulation results. In addition, the extrapolation cannot have a power-law slope < -2 (for 100 hours of observation time) and background and foregrounds must be effectively subtracted to obtain the signal. This does not make detection impossible, but suggests it may be unlikely. Furthermore, assuming a potentially detectable spectral scenario we show that, for the case when no detection is made by a lunar array, a lower bound, competitive with those from Lorentz-invariance violation, may be placed on the energy-scale of quantum gravitational effects. The SKA is shown to have very limited prospects for the detection of either a single merger or a background.

astro-ph.HE↗

arXiv2020

Simulated differential observations of the Sunyaev-Zel'dovich Effect: Probing the Dark Ages and Epoch of Reionization

This work presents an analytical approach for studying the cosmological 21cm background signal from the Dark Ages (DA) and subsequent Epoch of Reionization (EoR). We simulate differential observations of a galaxy cluster to demonstrate how these epochs can be studied with a specific form of the Sunyaev-Zel'dovich Effect called the SZE-21cm. This work produces simulated maps of the SZE-21cm and shows that the SZE-21cm can be extracted from future observations with low-frequency radio interferometers such as the Hydrogen Epoch of Reionization Array (HERA) and the Square Kilometre Array (SKA). In order to simulate near realistic scenarios, we look into cosmic variance noise, incorporate and take into account the effects of foregrounds, thermal noise, and angular resolution for our simulated observations. We further extend this exploration by averaging over a sample of galaxy clusters to mitigate the effects of cosmic variance and instrumental noise. The impact of point source contamination is also studied. Lastly, we apply this technique to the results of the EDGES collaboration, which in 2018 reported an absorption feature of the global 21cm background signal centred at 78 MHz. The challenges to be addressed in order to achieve the objectives of this work include errors that arise due to cosmic variation, instrumental noise and point source contamination. Our approach demonstrates the potential of the SZE-21cm as an indirect probe for the DA and EoR, and we conclude that the spectral features of the SZE-21cm from our simulated observations yield results that are close to prior theoretical predictions and that the SZE-21cm can be used to test the validity of the EDGES detection.

astro-ph.CO↗

arXiv2019

Dark matter gets DAMPE

The DArk Matter Particle Explorer (DAMPE) recently reported an excess of electrons/positrons above expected background fluxes even when a double power-law background spectrum is assumed. Several dark matter models that involve TeV-scale leptophilic WIMPs have been suggested in the literature to account for this excess. All of these models are associated with the presence of a nearby dark matter clump/over-density. In this work we set out to explore how current constraints from observational data impact the suggested parameter space for a dark matter explanation of the DAMPE excess, as well as make projections of the capacity of LOFAR and the up-coming SKA to observe indirect radio emissions from the nearby dark matter over-density. We show that LOFAR is incapable of probing the parameter space for DAMPE excess models, unless the dark matter clump is in the form of an ultra-compact mini halo. Fermi-LAT limits on dark matter annihilation are unable to probe these models in all cases. Limits derived from diffuse Coma cluster radio emission can probe a substantial portion of the parameter space and muon neutrino limits inferred from galactic centre gamma-ray fluxes heavily restrict muon coupling for the proposed WIMPs. The SKA is shown to able to fully probe the parameter space of all the studied models using indirect emissions from the local dark matter over-density.

astro-ph.HE↗

arXiv2019

Differential Observation Techniques for the SZE-21cm and radio sources

The SZE-21cm has been proposed as an alternative probe for the Dark Ages (DA) and the Epoch of Reionization (EoR). The effect is produced when photons of the 21cm background are inverse Compton (up-)scattered by electrons residing in hot plasma of cosmic structures and can be studied through differential observations of the large-scale structure, towards and away a region of the radio sky containing the cosmic structure of interest. This work makes use of the 21cmFAST code to simulate low frequency radio observations of a galaxy cluster from which we then extract the SZE-21cm signal. We further explore uses of the differential technique to radio observations of active radio galaxies using radio observations from the MWA GLEAM survey. The differential techniques this work studies enable us to extract the SZE-21cm signal from simulated galaxy cluster observations, which can then be used to obtain the global features of the 21cm signal during the DA and EoR. The technique shows further benefits in source extraction and characterisation, and feature enhancement in radio observations particularly for low-surface brightness and extended radio sources.

astro-ph.CO↗

arXiv2019

Probing quantum gravity using high-energy astrophysics

The events observed by LIGO indicate the existence of a large population of intermediate mass black holes. This unexpected result lead to a resurgence in the interest in theories of the formation of primordial black holes with several studies showing that broad mass distributions can evade or satisfy the stringent constraints on monochromatic populations. If such large populations exist they provide the perfect test-bed for theories beyond the standard model of physics that modify black hole evolution. The case we studied is that of "Planck stars", a hypothetical modification of the black hole evolution where it explodes via quantum loop gravity motivated tunnelling. We determine what the high-frequency background signal of such objects exploding over the whole of cosmic history would look like for various black hole populations to place actual empirical constraints on quantum loop gravity via comparison to observed isotropic background signals at the same frequencies. We find that stringent constraints heavily restrict the amount of energy released via the high-energy channel, thereby casting doubt on whether or not the high-energy signal could result in gamma-ray bursts as speculated in the literature.

astro-ph.CO↗

arXiv2019

Dark matter gets DAMPE at high energies

The DArk Matter Particle Explorer (DAMPE) mission revealed a break in the spectrum of cosmic-ray electons and positrons. This is associated with an excess above the expected backgrounds at energies around 1 TeV. Several authors have argued that such an excess can be explained in terms of dark matter models that feature heavy leptophilic WIMPs. These models, however, require some form annihilation enchancement above that expected from the Milky-Way galactic centre. This can take the form of either a local over-density near to our solar system or some form of Sommerfeld enhancement of the annihilation rate. In this work we will explore the detectability of local over-densities using gamma-ray and neutrino observatories. We conclude that KM3NET may be the only up-coming high-energy instrument capable of ruling out the presence of such objects. However, in the case where the local over-density is an Ultra-Compact Mini Halo, CTA can also explore the parameter space of these proposed dark matter models.

astro-ph.CO↗

arXiv2019

Probing quantum gravity at low energies

Planck stars form when a collapsing shell of matter within a black hole reaches the Planck density, roughly equivalent to the mass being compressed into a volumetric size near that of the proton, and rebounds outwards. These planck stars have been considered as accounting for both fast radio bursts and short gamma ray bursts, whilst offering a comparatively low energy perspective onto quantum gravity. The observation of such an event would require black hole masses much smaller than a solar mass, which could be provided by primordial black hole dark matter models. We discuss the low energy isotropic background emissions produced by decaying primordial black holes at all epochs and derive constraints from the spectrum of the extragalactic background light. We find that, in order to avoid exceeding known extragalactic background light emissions, we must restrict the total energy emitted at low frequencies by a planck star exploding in the present epoch to be less than $10^{13}$ erg or restrict the primordial black hole population far below any existing limits. This casts doubt on whether exploding planck stars could actually account for fast radio bursts, as they are speculated to in the literature.

astro-ph.CO↗

arXiv2019

Through a Mini Halo, Darkly

In this Letter we explore the effects of the scattering of photons incident on a dark matter halo through their interaction with either electrons or photons produced by dark matter annihilation. Particularly, we examine the effects of this scattering upon the observed spectrum of a distant AGN or of the Cosmic Microwave Background. Our results indicate that ultra-compact mini halos and other dense dark matter clumps can induce an observable Comptonisation of AGN spectra as well as a Sunyaev-Zel'dovich effect (SZE) with an optical depth similar to that attained by thermal electrons in the Coma cluster. The rate of encounters between a distant AGN and these dense mini-halos is also estimated using micro-lensing limits existing on the population of dark compact bodies.

astro-ph.CO↗

arXiv2018

Gamma-rays and LHC-inspired dark matter

The Madala hypothesis was formulated to explain anomalies in LHC data from run-1. Subsequently, these anomalies have endured into run-2 and been strengthened. This makes the analysis of the proposal highly pertinent, and, since some of the properties of the model are beyond the reach of current collider data it is also important to determine alternative means of analysis. Here, we examine the consequences of WIMPs linked to the Madala hypothesis providing a dark matter candidate and annihilating in the galactic centre of the Milky-Way and the Andromeda galaxy. These targets have been observed to have similar gamma-ray spectra by Fermi-LAT and their emissions have been widely discussed in terms of dark matter annihilation in the literature. We show that, when the decay branchings of the hidden-sector mediator in the Madala hypothesis are assumed Higgs-like, the emissions of Andromeda and the Milky-Way are not compatible with both being produced by dark matter annihilation, apart from when a steeply contracted NFW profile is assumed for the halos or the WIMP has a mass above 1 TeV. Additionally, similar results are displayed for a wide variety of model independent cases

hep-ph↗

arXiv2018

Analysis of differential observations of the cosmological radio background : studying the SZE-21cm

In pursuit of understanding the early Universe and early processes occurring particularly in the Dark Ages (DA) and the Epoch of Reionization (EoR), it is vital that a suitable probe is identified. Probing these epochs will be useful in studies of the origin of first galaxies and most importantly formation of early black-holes. Over the past decade numerous probes have been proposed, with one of the most promising being the SZE-21cm, a specific form of the Sunyaev-Zeldovich effect (SZE) produced when photons of the 21cm background are inverse Compton up-scattered by electrons residing in hot plasma of cosmic structures such as galaxy clusters and active radio galaxies. The SZE-21cm is calculated in a full relativistic approach of the scattering processes of the CMB photons modified by the cosmological redshifted 21cm background in the hot intra-cluster medium of galaxy clusters capturing effects induced by relativistic corrections to this scattering and by multiple scattering effects. We apply image differencing techniques to simulated radio observations of galaxy clusters using the redshifted 21cm background, we conduct this making use of the public semi-numeric code 21cmFAST. We are able to achieve subtraction of contaminating foregrounds through pixel by pixel operations on the data retrieved from our simulated data cubes. We demonstrate that SZE-21cm can be recovered through differential observations of the 21cm background.

astro-ph.CO↗

arXiv2018

Using Gravitational Wave Observations to Probe Quantum Gravity

Electromagnetic radiation is known to be associated with certain gravitational waves events, i.e. the collision of binary neutron stars. Establishing this connection is non-trivial. However, if electromagnetic counterparts could be produced by directly converting gravitons into photons, then a simple smoking gun test exists linking the two events. This model uses the general and conversion mechanism discussed by Raffelt and Stodolsky. Furthermore, because this mechanism is generic to the symmetries of general relativity and the standard model and because it assumes a quantised gravitational field, we may probe both the existence of the graviton and the scale at which quantum gravity effects (and thus very high-energy phenomena) become relevant using simple low-energy experiments.

astro-ph.HE↗

arXiv2018

`Rings' of diffuse radio emission surrounding the Bullet cluster

We present the discovery of ringlike diffuse radio emission structures in the peripheral regions of the Bullet cluster 1E 0657$-$55.8. Ring formations are spanning between 1--3 Mpc away from the center of the cluster, significantly further away from the two already reported relics. Integrated fluxes of four of the sub-regions in the inner `ring' from 4.5 to 10 GHz have also been reported. To understand the possible origin of these structures, here we present a maiden attempt of numerical modelling of a 3D and realistic `bullet' like event in a full cosmological ($Λ$CDM) environment with N-body plus hydrodynamics code. We report a simulated `bullet' found inside a (128 Mpc)$^3$ volume simulation with a speed of 2700 km s$^{-1}$, creating a high supersonic bow shock of Mach $M=3.5$ and a clear evidence of temporal separation of dark matter and baryons, assuring no challenge to $Λ$CDM cosmology from the bullet event as of now. We are also able to unveil the physics behind the formation of these observed multiple shock structures. Modelled radio emissions in our simulation support a complex combination of merger-associated processes that accelerates and re-accelerates fossil and cosmic-ray electrons. With a time evolution study and the computed radio emissions, we have shown that the ring like formation around the bullet is originated due to the interaction of the strong merger shocks with the accretion shocks at the periphery. The multiple shock structures observed are possibly originated from multiple mergers that have taken place at different times and much before the bullet event.

astro-ph.HE↗

arXiv2017

Hunting Dark Matter in ultra-compact structures within the Milky-Way

The local environment is ideal for searching out compact dark structures via the microlensing and multi-frequency emissions as these objects are expected to be faint and microlensing experiments have already hinted at their possibility. In the case that these objects are composed of Dark Matter (DM) then there are both few limits on their abundance but many consequences of their existence or non-existence on both local and cosmic scales. In this work we examine the possibility of Ultra-Compact Mini-Halos (UCMHs) formed in the early universe. These objects can persist to the present epoch due to their large central density inuring them to the worst effects of later tidal stripping. As such, these objects could constitute probes of many details of early universe physics, such as primordial phase-transitions, inflation, small scale exploration of the primordial density perturbation field and non-Gaussianity of these density perturbations. The fact that they are also highly dark matter-dominated objects means that they are attractive objects of study in the continuing hunt for the nature of Dark Matter (DM) through indirect detection. Another reason to study such objects in the local environment is found in the conjecture that encounters with UCMHs could induce catastrophic events on planets within our solar system, e.g. mass-extinction events on Earth. We will outline a strategy for multi-frequency UCMH searches within the region of the Milky-Way in which Gaia can accurately reconstruct microlens masses. This methodology ensures that any candidate UCMH DM emission should correlate to some unidentified microlensing object with determinable mass and demonstrate that large-scale volcanic extinctions on Earth due to UCMHs are unlikely but that it is possible for loss of the Martian geodynamo to be driven encounters with such compact objects.

astro-ph.CO↗

arXiv2017

A Multi-frequency analysis of possible Dark Matter Contributions to M31 Gamma-Ray Emissions

We examine the possibility of a dark matter (DM) contribution to the recently observed gamma-ray spectrum seen in the M31 galaxy. In particular, we apply limits on Weakly Interacting Massive Particle DM annihilation cross-sections derived from the Coma galaxy cluster and the Reticulum II dwarf galaxy to determine the maximal flux contribution by DM annihilation to both the M31 gamma-ray spectrum and that of the Milky-Way galactic centre. We limit the energy range between 1 and 12 GeV in M31 and galactic centre spectra due to the limited range of former's data, as well as to encompass the high-energy gamma-ray excess observed in the latter target. In so doing, we will make use of Fermi-LAT data for all mentioned targets, as well as diffuse radio data for the Coma cluster. The multi-target strategy using both Coma and Reticulum II to derive cross-section limits, as well as multi-frequency data, ensures that our results are robust against the various uncertainties inherent in modelling of indirect DM emissions. Our results indicate that, when a Navarro-Frenk-White (or shallower) radial density profile is assumed, severe constraints can be imposed upon the fraction of the M31 and galactic centre spectra that can be accounted for by DM, with the best limits arising from cross-section constraints from Coma radio data and Reticulum II gamma-ray limits. These particular limits force all the studied annihilation channels to contribute 1% or less to the total integrated gamma-ray flux within both M31 and galactic centre targets. In contrast, considerably more, 10-100%, of the flux can be attributed to DM when a contracted Navarro-Frenk-White profile is assumed. This demonstrates how sensitive DM contributions to gamma-ray emissions are to the possibility of cored profiles in galaxies.

astro-ph.CO↗

arXiv2017

MeerKLASS: MeerKAT Large Area Synoptic Survey

We discuss the ground-breaking science that will be possible with a wide area survey, using the MeerKAT telescope, known as MeerKLASS (MeerKAT Large Area Synoptic Survey). The current specifications of MeerKAT make it a great fit for science applications that require large survey speeds but not necessarily high angular resolutions. In particular, for cosmology, a large survey over $\sim 4,000 \, {\rm deg}^2$ for $\sim 4,000$ hours will potentially provide the first ever measurements of the baryon acoustic oscillations using the 21cm intensity mapping technique, with enough accuracy to impose constraints on the nature of dark energy. The combination with multi-wavelength data will give unique additional information, such as exquisite constraints on primordial non-Gaussianity using the multi-tracer technique, as well as a better handle on foregrounds and systematics. Such a wide survey with MeerKAT is also a great match for HI galaxy studies, providing unrivalled statistics in the pre-SKA era for galaxies resolved in the HI emission line beyond local structures at z > 0.01. It will also produce a large continuum galaxy sample down to a depth of about 5\,$μ$Jy in L-band, which is quite unique over such large areas and will allow studies of the large-scale structure of the Universe out to high redshifts, complementing the galaxy HI survey to form a transformational multi-wavelength approach to study galaxy dynamics and evolution. Finally, the same survey will supply unique information for a range of other science applications, including a large statistical investigation of galaxy clusters as well as produce a rotation measure map across a huge swathe of the sky. The MeerKLASS survey will be a crucial step on the road to using SKA1-MID for cosmological applications and other commensal surveys, as described in the top priority SKA key science projects (abridged).

astro-ph.CO↗

arXiv2017

Exploring the Potential of the Dark Matter Candidate from the Madala Hypothesis with Multi-frequency Indirect Detection

The Madala hypothesis was proposed by members of the Wits-ATLAS group to account for several anomalies in both ATLAS and CMS data at the LHC. This hypothesis extends the standard model through the addition of two scalar bosons and a hidden sector that can provide a dark matter candidate. This hidden sector interacts with the standard model only through the mediation of one of these scalars S. The couplings of S are not amenable to investigation in current collider data and so are assumed to be Higgs-like to reduce the parameter space of the model. Our previous work (Beck & Colafrancesco 2016) has shown that these couplings can be limited via indirect dark matter detection experiments in gamma-rays (for resonant annihilations into S). Here we will treat the dark matter and S masses independently, and we generalise our previous work and examine what fraction of the cosmological dark matter abundance can be accounted for by particles in the hidden sector of the Madala hypothesis dark matter when these annihilate to standard model products via a Higgs-like S. We will also extend our gamma-ray analysis of Madala hypothesis dark matter to include the constraints of diffuse radio data from the Coma galaxy cluster in addition to the Fermi-LAT gamma-ray data from both this target and the Reticulum II dwarf galaxy. Our analysis indicates that either the Madala hypothesis cannot provide the bulk of cosmological dark matter, or the S boson cannot be simply Higgs-like. These apply unless the candidate particle exceeds a mass of 200 GeV. Both these scenarios may reduce the attractiveness of the hypothesis as the second case will imply that many free parameters must be added to describe S, greatly weakening fits for the model. To investigate the full consequences of this further work will necessitate using larger astrophysical data sets to strongly constrain details about S.

astro-ph.CO↗

arXiv2017

Dark matter in the Reticulum II dSph: a radio search

We present a deep radio search in the Reticulum II dwarf spheroidal (dSph) galaxy performed with the Australia Telescope Compact Array. Observations were conducted at 16 cm wavelength, with an rms sensitivity of 0.01 mJy/beam, and with the goal of searching for synchrotron emission induced by annihilation or decay of weakly interacting massive particles (WIMPs). Data were complemented with observations on large angular scales taken with the KAT-7 telescope. We find no evidence for a diffuse emission from the dSph and we derive competitive bounds on the WIMP properties. In addition, we detect more than 200 new background radio sources. Among them, we show there are two compelling candidates for being the radio counterpart of the possible gamma-ray emission reported by other groups using Fermi-LAT data.

astro-ph.HE↗

arXiv2017

Relativistic inverse Compton scattering of photons from the early universe

Electrons at relativistic speeds, diffusing in magnetic fields, cause copious emission at radio frequencies in both clusters of galaxies and radio galaxies, through the non-thermal radiation emission called synchrotron. However, the total power radiated through this mechanism is ill constrained, as the lower limit of the electron energy distribution, or low-energy cutoffs, for radio emission in galaxy clusters and radio galaxies have not yet been determined. This lower limit, parametrized by the lower limit of the electron momentum - pmin - is critical for estimating the energetics of non-thermal electrons produced by cluster mergers or injected by radio galaxy jets, which impacts the formation of large-scale structure in the universe, as well as the evolution of local structures inside galaxy clusters. The total pressure due to the relativistic, non-thermal population of electrons is critically dependent on pmin, making the measurement of this non-thermal pressure a promising technique to estimate the electron low-energy cutoff. We present here the first unambiguous detection of this pressure for a non-thermal population of electrons in a radio galaxy jet/lobe, located at a significant distance away from the center of the Bullet cluster of galaxies.

astro-ph.CO↗