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G. Giovannini

Publications and source records attributed to G. Giovannini.

At least 37 records · Page 2Linked to original sources
arXiv2019

A joint XMM-NuSTAR observation of the galaxy cluster Abell 523: constraints on Inverse Compton emission

We present the results of a joint XMM-Newton and NuSTAR observation (200 ks) of the galaxy cluster Abell 523 at $z=0.104$. The peculiar morphology of the cluster radio halo and its outlier position in the radio power P(1.4 GHz) - X-ray luminosity plane make it an ideal candidate for the study of radio-X-ray correlations and for the search of inverse Compton (IC) emission. We constructed thermodynamic maps derived from the XMM observations to describe in detail the physical and dynamical state of the ICM. We performed a point-to-point comparison in terms of surface brightness between the X-ray and radio emissions, to quantify their morphological discrepancies. Making use of NuSTAR's hard X-ray focusing capability, we looked for IC emission both globally and locally, after modeling the purely thermal component with a multi-temperature description. The thermodynamic maps suggest the presence of a secondary merging process that could be responsible for the peculiar radio halo morphology. This hypothesis is supported by the comparison between the X-ray and radio surface brightnesses, which shows a broad intrinsic scatter and a series of outliers. The global NuSTAR spectrum can be explained by purely thermal gas emission, and there is no convincing evidence that an IC component is needed. The $3σ$ upper limit on the IC flux in the 20-80 keV band is in the range $\left[2.2 - 4.0\right] \times 10^{-13} \, \mathrm{erg} \, \mathrm{s}^{-1} \, \mathrm{cm}^{-2}$, implying a lower limit on the magnetic field strength in the range $B > [0.23 - 0.31] \, μG$. Locally, we looked for IC emission in the central region of the cluster radio halo finding a $3σ$ upper limit on the 20-80 keV non-thermal flux of $3.17 \times 10^{-14} \, \mathrm{erg} \, \mathrm{s}^{-1} \, \mathrm{cm}^{-2}$, corresponding to a lower limit on the magnetic field strength of $B \gtrsim 0.81 \, μG$.

astro-ph.HE↗
arXiv2019

A radio ridge connecting two galaxy clusters in a filament of the cosmic web

Galaxy clusters are the most massive gravitationally bound structures in the Universe. They grow by accreting smaller structures in a merging process that produces shocks and turbulence in the intra-cluster gas. We observed a ridge of radio emission connecting the merging galaxy clusters Abell 0399 and Abell 0401 with the Low Frequency Array (LOFAR) at 140 MHz. This emission requires a population of relativistic electrons and a magnetic field located in a filament between the two galaxy clusters. We performed simulations to show that a volume-filling distribution of weak shocks may re-accelerate a pre-existing population of relativistic particles, producing emission at radio wavelengths that illuminates the magnetic ridge.

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arXiv2018

Evidence of jet-clump interaction: a flip of the radio jet head of 3C~84

Radio jets in active galaxies have been expected to interact with circumnuclear environments in their early phase evolutions. By performing the multi-epoch monitoring observation with the KVN and VERA Array (KaVA) at 43~GHz, we investigate the kinematics of the notable newborn bright component C3 located at the tip of the recurrent jet of 3C~84. During 2015 August-September, we discover the flip of C3 and the amount of the flip is about 0.4~milli-arcsecond in angular scale, which corresponds to 0.14 parsec in physical scale. After the flip of C3, it wobbled at the same location for a few months and then it restarted to propagate towards the southern direction. The flux density of C3 coherently showed the monotonic increase during the observation period. The flip is in good agreement with hydrodynamical simulations of jets in clumpy ambient medium. We estimate the number density of the putative clump based on the momentum balance between the jet thrust and the ram pressure from the clump and it is about $10^{3-5}~{\rm cm^{-3}}$. We briefly discuss possible origins of the clump.

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arXiv2018

Observations of a nearby filament of galaxy clusters with the Sardinia Radio Telescope

We report the detection of diffuse radio emission which might be connected to a large-scale filament of the cosmic web covering a 8deg x 8deg area in the sky, likely associated with a z~0.1 over-density traced by nine massive galaxy clusters. In this work, we present radio observations of this region taken with the Sardinia Radio Telescope. Two of the clusters in the field host a powerful radio halo sustained by violent ongoing mergers and provide direct proof of intra-cluster magnetic fields. In order to investigate the presence of large-scale diffuse radio synchrotron emission in and beyond the galaxy clusters in this complex system, we combined the data taken at 1.4 GHz obtained with the Sardinia Radio Telescope with higher resolution data taken with the NRAO VLA Sky Survey. We found 28 candidate new sources with a size larger and X-ray emission fainter than known diffuse large-scale synchrotron cluster sources for a given radio power. This new population is potentially the tip of the iceberg of a class of diffuse large-scale synchrotron sources associated with the filaments of the cosmic web. In addition, we found in the field a candidate new giant radio galaxy.

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arXiv2018

A wide and collimated radio jet in 3C 84 on the scale of a few hundred gravitational radii

Understanding the launching, acceleration, and collimation of jets powered by active galactic nuclei remains an outstanding problem in relativistic astrophysics. This is partly because observational tests of jet formation models suffer from the limited angular resolution of ground-based very long baseline interferometry that has thus far been able to probe the transverse jet structure in the acceleration and collimation zone of only two sources. Here we report radio interferometric observations of 3C 84 (NGC 1275), the central galaxy of the Perseus cluster, made with an array including the orbiting radio telescope of the RadioAstron mission. The obtained image transversely resolves the edge-brightened jet in 3C 84 only 30 microarcseconds from the core, which is ten times closer to the central engine than what has been possible in previous ground-based observations, and it allows us to measure the jet collimation profile from ~ 100 to ~10000 gravitational radii from the black hole. The previously found, almost cylindrical jet profile on scales larger than a few thousand r_g is now seen to continue at least down to a few hundred r_g from the black hole and we find a broad jet with a transverse radius larger than about 250 r_g at only 350 r_g from the core. If the bright outer jet layer is launched by the black hole ergosphere, it has to rapidly expand laterally on scales smaller than 100 r_g. If this is not the case, then this jet sheath is likely launched from the accretion disk.

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arXiv2017

Enhanced Polarized Emission from the One-Parsec-Scale Hotspot of 3C 84 as a Result of the Interaction with Clumpy Ambient Medium

We present Very Long Baseline Array polarimetric observations of the innermost jet of 3C$\sim$84 (NGC$\sim$1275) at 43$\sim$GHz. A significant polarized emission is detected at the hotspot of the innermost re-started jet, which is located $\sim$1 pc south from the radio core. While the previous report presented a hotspot at the southern end of the western limb, the hotspot location has been moved to the southern end of the eastern limb. Faraday rotation is detected within an entire bandwidth of the 43-GHz band. The measured rotation measure (RM) is at most (6.3$\pm$1.9)$\times10^{5}$$\sim$rad$\sim$m$^{-2}$ and might be slightly time variable on the timescale of a month by a factor of a few. Our measured RM and the RM previously reported by the CARMA and SMA observations cannot be consistently explained by the spherical accretion flow with a power-law profile. We propose that a clumpy/inhomogeneous ambient medium is responsible for the observed rotation measure. Using equipartition magnetic field, we derive the electron density of $2\times10^{4}$$\sim$cm$^{-3}$. Such an electron density is consistent with the cloud of narrow line emission region around the central engine. We also discuss the magnetic field configuration from black hole scale to pc scale and the origin of low polarization.

astro-ph.HE↗
arXiv2017

Observations of the galaxy cluster CIZA J2242.8+5301 with the Sardinia Radio Telescope

We observed the galaxy cluster CIZA J2242.8+5301 with the Sardinia Radio Telescope to provide new constraints on its spectral properties at high frequency. We conducted observations in three frequency bands centred at 1.4 GHz, 6.6 GHz and 19 GHz, resulting in beam resolutions of 14$^{\prime}$, 2.9$^{\prime}$ and 1$^{\prime}$ respectively. These single-dish data were also combined with archival interferometric observations at 1.4 and 1.7 GHz. From the combined images, we measured a flux density of ${\rm S_{1.4GHz}=(158.3\pm9.6)\,mJy}$ for the central radio halo and ${\rm S_{1.4GHz}=(126\pm8)\,mJy}$ and ${\rm S_{1.4GHz}=(11.7\pm0.7)\,mJy}$ for the northern and the southern relic respectively. After the spectral modelling of the discrete sources, we measured at 6.6 GHz ${\rm S_{6.6GHz}=(17.1\pm1.2)\,mJy}$ and ${\rm S_{6.6GHz}=(0.6\pm0.3)\,mJy}$ for the northern and southern relic respectively. Assuming simple diffusive shock acceleration, we interpret measurements of the northern relic with a continuous injection model represented by a broken power-law. This yields an injection spectral index ${\rm α_{inj}=0.7\pm0.1}$ and a Mach number ${\rm M=3.3\pm0.9}$, consistent with recent X-ray estimates. Unlike other studies of the same object, no significant steepening of the relic radio emission is seen in data up to 8.35 GHz. By fitting the southern relic spectrum with a simple power-law (${\rm S_ν\proptoν^{-α}}$) we obtained a spectral index ${\rm α\approx1.9}$ corresponding to a Mach number (${\rm M\approx1.8}$) in agreement with X-ray estimates. Finally, we evaluated the rotation measure of the northern relic at 6.6 GHz. These results provide new insights on the magnetic structure of the relic, but further observations are needed to clarify the nature of the observed Faraday rotation.

astro-ph.CO↗
arXiv2017

Sardinia Radio Telescope observations of Abell 194 - the intra-cluster magnetic field power spectrum

We study the intra-cluster magnetic field in the poor galaxy cluster Abell 194 by complementing radio data, at different frequencies, with data in the optical and X-ray bands. We analyze new total intensity and polarization observations of Abell 194 obtained with the Sardinia Radio Telescope (SRT). We use the SRT data in combination with archival Very Large Array observations to derive both the spectral aging and Rotation Measure (RM) images of the radio galaxies 3C40A and 3C40B embedded in Abell 194. The optical analysis indicates that Abell 194 does not show a major and recent cluster merger, but rather agrees with a scenario of accretion of small groups. Under the minimum energy assumption, the lifetimes of synchrotron electrons in 3C40B measured from the spectral break are found to be 157 Myrs. The break frequency image and the electron density profile inferred from the X-ray emission are used in combination with the RM data to constrain the intra-cluster magnetic field power spectrum. By assuming a Kolmogorov power law power spectrum, we find that the RM data in Abell 194 are well described by a magnetic field with a maximum scale of fluctuations of Lambda_max=64 kpc and a central magnetic field strength of =1.5 microG. Further out, the field decreases with the radius following the gas density to the power of eta=1.1. Comparing Abell 194 with a small sample of galaxy clusters, there is a hint of a trend between central electron densities and magnetic field strengths.

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arXiv2016

Discovery of off-axis jet structure of TeV blazar Mrk 501 with mm-VLBI

High-resolution millimeter wave very-long-baseline interferometry (mm-VLBI) is an ideal tool for probing the structure at the base of extragalactic jets in detail. The TeV blazar Mrk 501 is one of the best targets among BL Lac objects for studying the nature of off-axis jet structures because it shows different jet position angles at different scales. The aim of this study is to investigate the properties of the off-axis jet structure through high-resolution mm-VLBI images at the jet base and physical parameters such as kinematics, flux densities, and spectral indices. We performed Very Long Baseline Array (VLBA) observations over six epochs from 2012 February to 2013 February at 43 GHz. Quasi-simultaneous Global Millimeter VLBI Array (GMVA) observations at 86 GHz were performed in May 2012. We discover a new jet component at the northeast direction from the core in all the images at 43 and 86 GHz. The new component shows the off-axis location from the persistent jet extending to the southeast. The 43 GHz images reveal that the scattering of the positions of the NE component is within ~0.2 mas. The 86 GHz data reveals a jet component located 0.75 mas southeast of the radio core. We also discuss the spectral indices between 43 and 86 GHz, where the northeast component has steeper spectral index and the southeast component has comparable or flatter index than the radio core does.

astro-ph.HE↗
arXiv2015

A multiwavelength view of the galaxy cluster Abell 523 and its peculiar diffuse radio source

We study the structure of the galaxy cluster Abell 523 (A523) at z=0.104 using new spectroscopic data for 132 galaxies acquired at the Telescopio Nazionale Galileo, new photometric data from the Isaac Newton Telescope, and X-ray and radio data from the Chandra and Very Large Array archives. We estimate the velocity dispersion of the galaxy population, sigmaV=949 km/s, and the X-ray temperature of the hot intracluster medium, kT=5.3 keV. We infer that A523 is a massive system: M200 about 7-9 x10E14 Msun. The analysis of the optical data confirms the presence of two subclusters, 0.75 Mpc apart, tracing the SSW-NNE direction and dominated by the two brightest cluster galaxies (BCG1 and BCG2). The X-ray surface brightness is strongly elongated towards the NNE direction, and its peak is clearly offset from both the BCGs. We confirm the presence of a 1.3 Mpc large radio halo, elongated in the ESE-WNW direction and perpendicular to the optical/X-ray elongation. We detect a significant radio/X-ray offset and radio polarization, two features which might be the result of a magnetic field energy spread on large spatial scales. A523 is found consistent with most scaling relations followed by clusters hosting radio haloes, but quite peculiar in the Pradio-LX relation: it is underluminous in the X-rays or overluminous in radio. A523 can be described as a binary head--on merger caught after a collision along the SSW-NNE direction. However, minor optical and radio features suggest a more complex cluster structure, with A523 forming at the crossing of two filaments along the SSW-NNE and ESE-WNW directions.

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arXiv2015

Gamma-ray light curve and VLBI polarization connection in Mrk 421

We present Fermi-LAT and multi-frequency, multi-epoch VLBA data for the TeV blazar Mrk421. We collected the data during a long and intensive multi-frequency campaign in 2011. We study the gamma-ray light curve, the photon index evolution and their connection to the radio data on sub-parsec scales, including total intensity, polarized flux density, polarization angle, spectral index, and rotation measure both in the core and the jet region. The VLBA data were obtained at 15 and 24 GHz for 12 epochs and at 43 GHz for 23 epochs, thus providing the best temporal and spatial coverage in the radio band ever achieved for a TeV blazar. We provide significant constraints on the jet Doppler factor, the presence of proper motion, the magnetic field configuration, and an intriguing connection between variability in the radio data and the gamma-ray light curve: the total intensity and polarized core emission reach a peak simultaneously to the main gamma-ray peak, followed by a rotation of the polarization angle at low frequency. Opacity-related, long wavelength polarization swings are also detected later in the year, possibly related to secondary peaks in the gamma-ray light curve, setting constraints on the physics of the gamma-ray zone.

astro-ph.HE↗
arXiv2015

A strong radio brightening at the jet base of M87 in the period of the elevated TeV gamma-ray state in 2012

The nearby radio galaxy M87 offers a unique opportunity for exploring the connection between gamma-ray production and jet formation at an unprecedented linear resolution. However, the origin and location of the gamma-rays in this source is still elusive. Based on previous radio/TeV correlation events, the unresolved jet base (radio core) and the peculiar knot HST-1 at 120 pc from the nucleus are proposed as candidate site(s) of gamma-ray production. Here we report our intensive, high-resolution radio monitoring observations of the M87 jet with the VLBI Exploration of Radio Astrometry (VERA) and the European VLBI Network (EVN) from February 2011 to October 2012, together with contemporaneous high-energy gamma-ray light curves obtained by the Fermi Large Area Telescope. During this period, an elevated level of the M87 flux is reported at TeV with VERITAS. We detected a remarkable flux increase in the radio core with VERA at 22/43 GHz coincident with the VHE activity. Meanwhile, HST-1 remained quiescent in terms of its flux density and structure in the radio band. These results strongly suggest that the TeV gamma-ray activity in 2012 originates in the jet base within 0.03 pc (projected) from the central black hole.

astro-ph.HE↗
arXiv2015

The connection between radio and high energy emission in black hole powered systems in the SKA era

Strong evidence exists for a highly significant correlation between the radio flux density and gamma-ray energy flux in blazars revealed by Fermi. However, there are central issues that need to be clarified in this field: what are the counterparts of the about 30% of gamma-ray sources that are as yet unidentified? Are they just blazars in disguise or they are something more exotic, possibly associated with dark matter? How would they fit in the radio-gamma ray connection studied so far? With their superb sensitivity, SKA1-MID and SKA1-SUR will help to resolve all of these questions. Even more, while the radio-MeV/GeV connection has been firmly established, a radio-VHE connection has been entirely elusive so far. The advent of CTA in the next few years and the expected CTA-SKA1 synergy will offer the chance to explore this connection, even more intriguing as it involves the opposite ends of the electromagnetic spectrum and the acceleration of particles up to the highest energies. We are already preparing to address these questions by exploiting data from the various SKA pathfinders and precursors. We have obtained 18 cm European VLBI Network observations of E>10 GeV sources, with a detection rate of 83%. Moreover, we are cross correlating the Fermi catalogs with the MWA commissioning survey: when faint gamma-ray sources are considered, pure positional coincidence is not significant enough for selecting counterparts and we need an additional physical criterion to pinpoint the right object. It can be radio spectral index, variability, polarization, or compactness, needing high angular resolution in SKA1-MID; timing studies can also reveal pulsars, which are often found from dedicated searches of unidentified gamma-ray sources. SKA will be the ideal instrument for investigating these characteristics in conjunction with CTA. (abridged)

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arXiv2015

Probing Precise Location of Radio Core in TeV Blazar Mrk 501 with VERA at 43 GHz

We investigate the position of the radio core in a blazar by multi-epoch astrometric observations at 43 GHz. Using the VLBI Exploration of Radio Astrometry (VERA), we have conducted four adjacent observations in February 2011 and another four in October 2011, and succeeded in measuring the position of the radio core in the TeV blazar Mrk 501 relative to a distant compact quasar NRAO 512. During our observations, we find that (1) there is no positional change within ~0.2 mas or ~2.0 pc de-projected with $\pm1σ$ error for the weighted-mean phase-referenced positions of Mrk 501 core relative to NRAO 512 over four adjacent days, and (2) there is an indication of position change for 3C 345 core relative to NRAO 512. By applying our results to the standard internal shock model for blazars, we constrain the bulk Lorenz factors of the ejecta.

astro-ph.HE↗
arXiv2015

Cluster magnetic fields through the study of polarized radio halos in the SKA era

Galaxy clusters are unique laboratories to investigate turbulent fluid motions and large scale magnetic fields. Synchrotron radio halos at the center of merging galaxy clusters provide the most spectacular and direct evidence of the presence of relativistic particles and magnetic fields associated with the intracluster medium. The study of polarized emission from radio halos is extremely important to constrain the properties of intracluster magnetic fields and the physics of the acceleration and transport of the relativistic particles. However, detecting this polarized signal is a very hard task with the current radio facilities.We use cosmological magneto-hydrodynamical simulations to predict the expected polarized surface brightness of radio halos at 1.4 GHz. We compare these expectations with the sensitivity and the resolution reachable with the SKA1. This allows us to evaluate the potential for studying intracluster magnetic fields in the surveys planned for SKA1.

astro-ph.CO↗
arXiv2015

Unravelling the origin of large-scale magnetic fields in galaxy clusters and beyond through Faraday Rotation Measures with the SKA

We investigate the possibility for the SKA to detect and study the magnetic fields in galaxy clusters and in the less dense environments surrounding them using Faraday Rotation Measures. To this end, we produce 3-dimensional magnetic field models for galaxy clusters of different masses and in different stages of their evolution, and derive mock rotation measure observations of background radiogalaxies. According to our results, already in phase I, we will be able to infer the magnetic field properties in galaxy clusters as a function of the cluster mass, down to $10^{13}$ solar-masses. Moreover, using cosmological simulations to model the gas density, we have computed the expected rotation measure through shock-fronts that occur in the intra-cluster medium during cluster mergers. The enhancement in the rotation measure due to the density jump will permit to constraint the magnetic field strength and structure after the shock passage. SKA observations of polarised sources located behind galaxy clusters will answer several questions about the magnetic field strength and structure in galaxy clusters, and its evolution with cosmic time.

astro-ph.CO↗
arXiv2014

A strong radio brightening at the jet base of M87 during the elevated very-high-energy gamma-ray state in 2012

The nearby radio galaxy M87 offers a unique opportunity for exploring the connection between gamma-ray production and jet formation at an unprecedented linear resolution. However, the origin and location of the gamma-rays in this source is still elusive. Based on previous radio/TeV correlation events, the unresolved jet base (radio core) and the peculiar knot HST-1 at >120 pc from the nucleus are proposed as candidate site(s) of gamma-ray production. Here we report our intensive, high-resolution radio monitoring observations of the M87 jet with the VLBI Exploration of Radio Astrometry (VERA) and the European VLBI Network (EVN) from February 2011 to October 2012. During this period, an elevated level of the M87 flux is reported at TeV with VERITAS. We detected a remarkable flux increase in the radio core with VERA at 22/43 GHz coincident with the VHE activity. Meanwhile, HST-1 remained quiescent in terms of its flux density and structure at radio. These results strongly suggest that the TeV gamma-ray activity in 2012 originates in the jet base within 0.03 pc (projected) from the central supermassive black hole.

astro-ph.HE↗