arXiv subjects

Sergio Colafrancesco

Publications and source records attributed to Sergio Colafrancesco.

At least 37 records · Page 2Linked to original sources

arXiv2017

Multi-frequency search for Dark Matter: the role of HESS, CTA, and SKA

Dark Matter (DM) remains a vital, but elusive, component in our current understanding of the universe. Accordingly, many experimental searches are devoted to uncovering its nature. However, both the existing direct detection methods, and the prominent $γ$-ray search with the Fermi Large Area Telescope (Fermi-LAT), are most sensitive to DM particles with masses below 1 TeV, and are significantly less sensitive to the hard spectra produced in annihilation via heavy leptons. The High Energy Stereoscopic System (HESS) has had some success in improving on the Fermi-LAT search for higher mass DM particles, particularly annihilating via heavy lepton states. However, the recent discovery of high J-factor dwarf spheroidal galaxies by the Dark Energy Survey (DES) opens up the possibility of investing more HESS observation time in the search for DM $γ$-ray signatures in dwarf galaxies. This work explores the potential of HESS to extend its current limits using these new targets, as well as the future constraints derivable with the up-coming Cherenkov Telescope Array (CTA). These limits are further compared with those we derived at low radio frequencies for the Square Kilometre Array (SKA). Finally, we explore the impact of HESS, CTA, and Fermi-LAT on the phenomenology of the "Madala" boson hypothesized based on anomalies in the data from the Large Hadron Collider (LHC) run 1. The power of these limits from differing frequency bands is suggestive of a highly effective multi-frequency DM hunt strategy making use of both existing and up-coming Southern African telescopes.

astro-ph.CO↗

arXiv2017

What Can Gamma-rays from Space tell us About the Madala Hypothesis?

The recent Madala hypothesis, a conjecture that seeks to explain anomalies within Large Hadron Collider (LHC) data (particularly in the transverse momentum of the Higgs boson), is interesting for more than just a statistical hint at unknown and unpredicted physics. This is because the model itself contains additional new particles that may serve as Dark Matter (DM) candidates. These particles interact with the Standard Model via a scalar mediator boson $S$. More interesting still, the conjectured mass range for the DM candidate ($65$ - $100$ GeV) lies within the region of models viable to try explain the recent Galactic Centre (GC) gamma-ray excess seen by Fermi Large Area Telescope (Fermi-LAT) and the High Energy Stereoscopic System (HESS). Therefore, assuming $S$ decays promptly, it should be possible to check what constraints are imposed upon the effective DM annihilation cross-section in the Madala scenario by hunting signatures of $S$ decay that follows DM annihilation within dense astrophysical structures. In order to make use of existing data, we use the Reticulum II dwarf galaxy and the galactic centre gamma-ray excess data sets from Fermi-LAT, and compare these to the consequences of various decay paths for $S$ in the aforementioned environments. We find that, based on this existing data, we can limit $τ$ lepton, quark, direct gamma-ray, and weak boson channels to levels below the canonical relic cross-section. This allows us to set new limits on the branching ratios of $S$ decay, which can rule out a Higgs-like decay branching for $S$, in the case where the Madala DM candidate is assumed to comprise all DM.

astro-ph.CO↗

arXiv2015

Using SKA Rotation Measures to Reveal the Mysteries of the Magnetised Universe

We know that magnetic fields are pervasive across all scales in the Universe and over all of cosmic time and yet our understanding of many of the properties of magnetic fields is still limited. We do not yet know when, where or how the first magnetic fields in the Universe were formed, nor do we fully understand their role in fundamental processes such as galaxy formation or cosmic ray acceleration or how they influence the evolution of astrophysical objects. The greatest challenge to addressing these issues has been a lack of deep, broad bandwidth polarimetric data over large areas of the sky. The Square Kilometre Array will radically improve this situation via an all-sky polarisation survey that delivers both high quality polarisation imaging in combination with observations of 7-14 million extragalactic rotation measures. Here we summarise how this survey will improve our understanding of a range of astrophysical phenomena on scales from individual Galactic objects to the cosmic web.

astro-ph.CO↗

arXiv2015

The impact of non-Planckian effects on cosmological radio backgrounds

Non-Planckian (NP) spectral modifications of the CMB radiation spectrum can be produced due to the existence of a non-zero value of the plasma frequency at the recombination epoch. We present here an analysis of NP effects on the radio cosmological background and we derive, for the first time, predictions of their amplitude on three different observables: the CMB spectrum, the Sunyaev-Zel'dovich (SZ) effect in cosmic structures, and the 21-cm background temperature brightness change. We find that NP effect can manifest in the CMB spectrum at $ν\simlt 400$ MHz as a drastic cut-off in the CMB intensity. Using the available CMB data in the relevant $ν$ range (i.e., mainly at $\simlt 1$ GHz and in the COBE-FIRAS data frequency range), we derive upper limits on the plasma frequency $ν_p$ = 206, 346 and 418 MHz at 1, 2 and 3 $σ$ confidence level, respectively. We find that the difference between the pure Planck spectrum and the one modified by NP effects is of the order of mJy/arcmin$^2$ at $ν\simlt 0.5$ GHz and it becomes smaller at higher frequencies where it is $\sim 0.1$ mJy/arcmin$^2$ at $ν\simgt 150$ GHz, thus indicating that the experimental route to probe NP effects in the early universe is to observe the radio cosmological background at very low frequencies.(abridged)

astro-ph.CO↗

arXiv2015

Structure, dynamical impact and origin of magnetic fields in nearby galaxies in the SKA era

Magnetic fields are an important ingredient of the interstellar medium (ISM). Besides their importance for star formation, they govern the transport of cosmic rays, relevant to the launch and regulation of galactic outflows and winds, which in turn are pivotal in shaping the structure of halo magnetic fields. Mapping the small-scale structure of interstellar magnetic fields in many nearby galaxies is crucial to understand the interaction between gas and magnetic fields, in particular how gas flows are affected. Elucidation of the magnetic role in, e.g., triggering star formation, forming and stabilising spiral arms, driving outflows, gas heating by reconnection and magnetising the intergalactic medium has the potential to revolutionise our physical picture of the ISM and galaxy evolution in general. Radio polarisation observations in the very nearest galaxies at high frequencies (>= 3 GHz) and with high spatial resolution (<= 5") hold the key here. The galaxy survey with SKA1 that we propose will also be a major step to understand the galactic dynamo, which is important for models of galaxy evolution and for astrophysical magnetohydrodynamics in general. Field amplification by turbulent gas motions, which is crucial for efficient dynamo action, has been investigated so far only in simulations, while compelling evidence of turbulent fields from observations is still lacking.

astro-ph.GA↗

arXiv2015

Probing the nature of Dark Matter with the SKA

Dark Matter (DM) is a fundamental ingredient of our Universe and of structure formation, and yet its nature is elusive to astrophysical probes. Information on the nature and physical properties of the WIMP (neutralino) DM (the leading candidate for a cosmologically relevant DM) can be obtained by studying the astrophysical signals of their annihilation/decay. Among the various e.m. signals, secondary electrons produced by neutralino annihilation generate synchrotron emission in the magnetized atmosphere of galaxy clusters and galaxies which could be observed as a diffuse radio emission (halo or haze) centered on the DM halo. A deep search for DM radio emission with SKA in local dwarf galaxies, galaxy regions with low star formation and galaxy clusters (with offset DM-baryonic distribution, like e.g. the Bullet cluster) can be very effective in constraining the neutralino mass, composition and annihilation cross-section. For the case of a dwarf galaxy, like e.g. Draco, the constraints on the DM annihilation cross-section obtainable with SKA1-MID will be at least a factor $\sim 10^3$ more stringent than the limits obtained by Fermi-LAT in the $γ$-rays. These limits scale with the value of the B field, and the SKA will have the capability to determine simultaneously both the magnetic field in the DM-dominated structures and the DM particle properties. The optimal frequency band for detecting the DM-induced radio emission is around $\sim 1$ GHz, with the SKA1-MID Band 1 and 4 important to probe the synchrotron spectral curvature at low-$ν$ (sensitive to DM composition) and at high-$ν$ (sensitive to DM mass).

astro-ph.HE↗

arXiv2014

Non-thermal emission from galaxy clusters: feasibility study with SKA1

Galaxy clusters are known to host a variety of extended radio sources: tailed radio galaxies whose shape is modelled by the interaction with the intra-cluster medium (ICM); radio bubbles filling cavities in the ICM distribution and rising buoyantly through the thermal gas; diffuse giant radio sources ("halos" and "relics") revealing the presence of relativistic electrons and magnetic fields in the intra-cluster volume. It is currently the subject of an active debate how the non-thermal components that we observe at radio wavelengths affect the physical properties of the ICM and depend on the dynamical state of galaxy clusters. In this work we start our SKA1 feasibility study of the "radio cluster zoo" through simulations of a typical radio-loud cluster, hosting several bright tailed radio galaxies and a diffuse radio halo. Realistic simulations of SKA1 observations are obtained through the MeqTrees software. A new deconvolution algorithm, based on sparse representations and optimised for the detection of faint diffuse astronomical sources, is tested and compared to the classical CLEAN method.

astro-ph.CO↗

arXiv2014

Galaxy Cluster Science with the Sunyaev-Zel'dovich Effect

Studying galaxy clusters through their Sunyaev-Zel'dovich (SZ) imprint on the Cosmic Microwave Background has many important advantages. The total SZ signal is an accurate and precise tracer of the total pressure in the intra-cluster medium and of cluster mass, the key observable for using clusters as cosmological probes. Band 5 observations with SKA-MID towards cluster surveys from the next generation of X-ray telescopes such as e-ROSITA and from Euclid will provide the robust mass estimates required to exploit these samples. This will be especially important for high redshift systems, arising from the SZ's unique independence to redshift. In addition, galaxy clusters are very interesting astrophysical systems in their own right, and the SKA's excellent surface brightness sensitivity down to small angular scales will allow us to explore the detailed gas physics of the intra-cluster medium.

astro-ph.CO↗

arXiv2014

PRISM (Polarized Radiation Imaging and Spectroscopy Mission): An Extended White Paper

PRISM (Polarized Radiation Imaging and Spectroscopy Mission) was proposed to ESA in May 2013 as a large-class mission for investigating within the framework of the ESA Cosmic Vision program a set of important scientific questions that require high resolution, high sensitivity, full-sky observations of the sky emission at wavelengths ranging from millimeter-wave to the far-infrared. PRISM's main objective is to explore the distant universe, probing cosmic history from very early times until now as well as the structures, distribution of matter, and velocity flows throughout our Hubble volume. PRISM will survey the full sky in a large number of frequency bands in both intensity and polarization and will measure the absolute spectrum of sky emission more than three orders of magnitude better than COBE FIRAS. The aim of this Extended White Paper is to provide a more detailed overview of the highlights of the new science that will be made possible by PRISM

astro-ph.CO↗

arXiv2013

Design, Environmental and Sustainability Constraints of new African Observatories: The example of the Mozambique Radio Astronomy Observatory

The Mozambique Radio Astronomy Observatory (MRAO) will be a first milestone towards development of radioastronomy in Mozambique. Development of MRAO will constitute a preparation step towards participation in the upcoming Africa VLBI Network and the Square Kilometer Array project. The MRAO first antenna is planned to serve as a capacitation and training facility and will be installed after the conversion of a 7-meter telecom dish in South Africa. Therefore, this first radiotelescope design has to comply with local spectral and environmental constraints. Furthermore, power availability and long term sustainability with potential inclusion of solar power and control of Radio Frequency Interference are analyzed. Here we outline some of the design, environmental and power sustainability constraints.

astro-ph.IM↗

arXiv2013

PRISM (Polarized Radiation Imaging and Spectroscopy Mission): A White Paper on the Ultimate Polarimetric Spectro-Imaging of the Microwave and Far-Infrared Sky

PRISM (Polarized Radiation Imaging and Spectroscopy Mission) was proposed to ESA in response to the Call for White Papers for the definition of the L2 and L3 Missions in the ESA Science Programme. PRISM would have two instruments: (1) an imager with a 3.5m mirror (cooled to 4K for high performance in the far-infrared---that is, in the Wien part of the CMB blackbody spectrum), and (2) an Fourier Transform Spectrometer (FTS) somewhat like the COBE FIRAS instrument but over three orders of magnitude more sensitive. Highlights of the new science (beyond the obvious target of B-modes from gravity waves generated during inflation) made possible by these two instruments working in tandem include: (1) the ultimate galaxy cluster survey gathering 10e6 clusters extending to large redshift and measuring their peculiar velocities and temperatures (through the kSZ effect and relativistic corrections to the classic y-distortion spectrum, respectively) (2) a detailed investigation into the nature of the cosmic infrared background (CIB) consisting of at present unresolved dusty high-z galaxies, where most of the star formation in the universe took place, (3) searching for distortions from the perfect CMB blackbody spectrum, which will probe a large number of otherwise inaccessible effects (e.g., energy release through decaying dark matter, the primordial power spectrum on very small scales where measurements today are impossible due to erasure from Silk damping and contamination from non-linear cascading of power from larger length scales). These are but a few of the highlights of the new science that will be made possible with PRISM.

astro-ph.CO↗

arXiv2011

Low-resolution spectroscopy of the Sunyaev-Zel'dovich effect and estimates of cluster parameters

The Sunyaev-Zel'dovich (SZ) effect is a powerful tool for studying clusters of galaxies and cosmology. Large mm-wave telescopes are now routinely detecting and mapping the SZ effect in a number of clusters, measure their comptonisation parameter and use them as probes of the large-scale structure and evolution of the universe. We show that estimates of the physical parameters of clusters (optical depth, plasma temperature, peculiar velocity, non-thermal components etc.) obtained from ground-based multi-band SZ photometry can be significantly biased, owing to the reduced frequency coverage, to the degeneracy between the parameters and to the presence of a number of independent components larger than the number of frequencies measured. We demonstrate that low-resolution spectroscopic measurements of the SZ effect that also cover frequencies $> 270$ GHz are effective in removing the degeneracy. We used accurate simulations of observations with lines-of-sight through clusters of galaxies with different experimental configurations (4-band photometers, 6-band photometer, multi-range differential spectrometer, full coverage spectrometers) and different intracluster plasma stratifications. We find that measurements carried out with ground-based few-band photometers are biased towards high electron temperatures and low optical depths, and require coverage of high frequency and/or independent complementary observations to produce unbiased information; a differential spectrometer that covers 4 bands with a resolution of $\sim 6 \ GHz$ eliminates most if not all bias; full-range differential spectrometers are the ultimate resource that allows a full recovery of all parameters.

astro-ph.CO↗

arXiv2010

The Herschel-SPIRE Legacy Survey (HSLS): the scientific goals of a shallow and wide submillimeter imaging survey with SPIRE

A large sub-mm survey with Herschel will enable many exciting science opportunities, especially in an era of wide-field optical and radio surveys and high resolution cosmic microwave background experiments. The Herschel-SPIRE Legacy Survey (HSLS), will lead to imaging data over 4000 sq. degrees at 250, 350, and 500 micron. Major Goals of HSLS are: (a) produce a catalog of 2.5 to 3 million galaxies down to 26, 27 and 33 mJy (50% completeness; 5 sigma confusion noise) at 250, 350 and 500 micron, respectively, in the southern hemisphere (3000 sq. degrees) and in an equatorial strip (1000 sq. degrees), areas which have extensive multi-wavelength coverage and are easily accessible from ALMA. Two thirds of the of the sources are expected to be at z > 1, one third at z > 2 and about a 1000 at z > 5. (b) Remove point source confusion in secondary anisotropy studies with Planck and ground-based CMB data. (c) Find at least 1200 strongly lensed bright sub-mm sources leading to a 2% test of general relativity. (d) Identify 200 proto-cluster regions at z of 2 and perform an unbiased study of the environmental dependence of star formation. (e) Perform an unbiased survey for star formation and dust at high Galactic latitude and make a census of debris disks and dust around AGB stars and white dwarfs.

astro-ph.CO↗

arXiv2007

Blazar Duty-Cycle at gamma-ray Frequecies: Constraints from Extragalactic Background Radiation and Prospects for AGILE and GLAST

We take into account the constraints from the observed extragalactic gamma-ray background to estimate the maximum duty cycle allowed for a selected sample of WMAP Blazars, in order to be detectable by AGILE and GLAST gamma-ray experiments. For the nominal sensitivity values of both instruments, we identify a subset of sources which can in principle be detectable also in a steady state without over-predicting the extragalactic background. This work is based on the results of a recently derived Blazar radio LogN-LogS obtained by combining several multi-frequency surveys.

astro-ph↗

arXiv2003

Cooling flows or warming rays?

The radiative cooling time of the X-ray-emitting plasma near the center in many clusters of galaxies is shorter than the age of the cluster, but neither the expected large drop in central temperature --nor the expected mass flow towards the pressure-depleted cluster centers-- are observed. We propose a solution to these ``cooling flow'' problems wherein energy is supplied to the plasma by hadronic cosmic rays (CRs). The solution requires an energy deposition more intense and more distributed than in conventional CR models, but this alternative model is also successful in describing the origin of CRs, as well as the properties of gamma ray bursts and of the diffuse gamma background radiation. The X-ray energy emitted by clusters is supplied, in a quasi-steady state, by the hadronic CRs, which act as ``warming rays''. The temperature distribution in the intracluster space is successfully predicted from the measured plasma-density distribution. Four other puzzling features of clusters can also be explained in simple terms: the discrepancy between their ``virial'' and ``lensing'' masses, their large magnetic fields, the correlation between their optical and X-ray luminosities, and the non-thermal tail of their X-ray spectrum.

astro-ph↗

arXiv2002

Low-energy X-ray emission from the Abell 2199 cluster of galaxies

In a recent Letter, Berghoefer & Bowyer rediscussed the analysis of BeppoSAX LECS data of the cluster of galaxies Abell 2199 as presented by Kaastra et al., in particular the detection of a soft X-ray excess. Berghoefer & Bowyer stated that their analysis method is better suited and does not show evidence for a soft X-ray excess. Here we find it necessary to publish a rebuttal, because it can be demonstrated that the method used by Berghoefer & Bowyer is oversimplified, leading to an erroneous result. As a consequence, their statement that our initial analysis is incorrect is invalid and the detection of a soft X-ray excess in A 2199 is still confirmed.

astro-ph↗

arXiv2002

First gamma-rays from galaxy clusters. Preliminary evidence of the association of galaxy clusters with EGRET unidentified gamma-ray sources

The vast majority of the celestial gamma-ray sources detected so far have not yet been identified with secure counterparts at other wavelenghts. Here we report the preliminary evidence of a probable association between galaxy clusters and unidentified gamma-ray sources of high galactic latitude (|b|>20 deg) in the Third EGRET catalog. All of the clusters which are most probably associated with EGRET sources show evidence of strong radio emission either because they host radio galaxies/sources in their environments or because they have a radio halo or relic inhabiting their intracluster medium. The cluster radio emission suggests that the relativistic particles (electrons, protons), which are diffusing in the intracluster medium, might be also responsible for their gamma-ray emission. Beyond the spatial associations of clusters with unidentified EGRET sources, we found a correlation between the radio flux at 1.4 GHz of the cluster's brightest source and the gamma-ray flux, F(>100 MeV), of the associated EGRET source. For the most probable EGRET-cluster associations we also found a further correlation between the X-ray luminosity of galaxy clusters and the gamma-ray luminosity of the associated gamma-ray source under the hypothesis that the EGRET sources have the same cluster redshifts. Such correlations are consistent with the theoretical expectations and strengthen the probability of a true, physical association between galaxy clusters and gamma-ray sources.

astro-ph↗

arXiv2000

The Sunyaev-Zeldovich MITO Project

Compton scattering of the cosmic microwave background radiation by electrons in the hot gas in clusters of galaxies - the Sunyaev-Zeldovich effect - has long been recognized as a uniquely important feature, rich in cosmological and astrophysical information. We briefly describe the effect, and emphasize the need for detailed S-Z and X-ray measurements of nearby clusters in order to use the effect as a precise cosmological probe. This is the goal of the MITO project, whose first stage consisted of observations of the S-Z effect in the Coma cluster. We report the results of these observations.

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