arXiv subjects

Paola Grandi

Publications and source records attributed to Paola Grandi.

At least 19 recordsLinked to original sources

arXiv2026

The twin-jet system in the FRII radio galaxy 3C 452: A sub-parsec scale VLBI study

We present a comprehensive multifrequency VLBI analysis of the FRII, high-excitation radio galaxy 3C 452, aiming to resolve and analyze for the first time its twin-jet structure on sub-parsec scales. Our data set comprises High Sensitivity Array (HSA) observations at 4.9, 8.4, 15.4, 23.6, and 43.2 GHz. Through fitting methods performed in both the visibility and the image plane, we trace the jet expansion from scales of a few thousand to nearly $10^5$ Schwarzschild radii ($R_S$) on both the approaching and receding jets. Additionally, we derive the core brightness temperatures and Doppler factors to constrain the jet's orientation and intrinsic speed. Our study provides the first detailed description of the twin-jet system in 3C 452 on VLBI scales, confirming it as a rare FRII source with jets detected down to millimeter wavelengths. We resolve both jet and counter-jet down to scales of a few thousand $R_S$, revealing a symmetric, parabolically expanding structure with power-law indices $k \approx 0.66$ (jet) and $k \approx 0.47$ (counter-jet). The brightness temperature analysis yields low Doppler factors ($δ\sim 0.03$-$0.83$), indicative of Doppler de-boosting due to the large viewing angle ($θ\approx 70^\circ$) and/or a magnetically dominated jet base. A spectral index analysis reveals a strongly inverted core spectrum ($α> 2$) with additional absorption at the highest frequencies, followed by a sharp steepening ($α\sim -2.5$) to optically thin values in the innermost jet. Finally, a comparison between broad- and narrow-line high-excitation radio galaxies shows that jets in narrow-line sources such as 3C 452 and Cygnus A complete collimation at $\leq 10^5 R_S$, whereas broad-line sources exhibit shape transitions at $10^6$-$10^7 R_S$, suggesting that orientation plays an important role in the observed collimation scales.

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arXiv2022

Detection of a radio-filled X-ray cavity within the interstellar medium of NGC 5141

We present the first Chandra detection of a single X-ray cavity within the interstellar medium of the small Fanaroff-Riley type I (FRI) radio galaxy NGC 5141. The X-ray surface brightness depression, located $\approx 4$ kpc away from the galaxy center, is projected on the northern radio lobe, which is completely contained within the galaxy. The thermal gas surrounding the cavity, which extends to $\approx$ 20 kpc, has a bolometric X-ray luminosity (0.1 - 100 keV) of L${_X}\approx2\times10^{40}$ erg s$^{-1}$ and a temperature of $kT\approx0.8$ keV. We calculated the total energy (E$_{cav} = 4PV \approx 10^{55}$ erg) required to inflate the cavity and its age ($t_{cav}\approx 9$ Myrs), assuming that it is filled with relativistic particles and rises buoyantly. The inferred total cavity power is as low as P$_{cav}=E_{cav}/t_{cav}\approx6\times10^{40}$ erg s$^{-1}$, which is the lowest one among the radio-filled systems. Comparing $P_{cav}$ to the bolometric X-ray luminosity (i.e., the cooling luminosity), we conclude that NGC 5141's central active galactic nucleus can heat the interstellar medium and balance its cooling luminosity, confirming that the $P_{cav}-L_{cool}$ relation, mainly tested on groups and clusters, also works for such a low-power system.

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arXiv2021

The central FR0 in the sloshing cluster Abell 795: Indications of mechanical feedback from Chandra data

We present a detailed study of the galaxy cluster Abell 795 and of its central Fanaroff-Riley Type 0 (FR0) radio galaxy. From an archival Chandra observation, we found a dynamically disturbed environment with evidences for sloshing of the intracluster medium. We argue that the environment alone cannot explain the compactness of the radio galaxy, as similar conditions are also found around extended sources. We identified a pair of putative X-ray cavities in the proximity of the center: These could have been created in a past outburst of the FR0, and dragged away by the large-scale gas movement. The presence of X-ray cavities associated with a FR0 could open a new window on the study of jet power and feedback properties of this recently discovered class of compact radio galaxies.

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arXiv2021

A Chandra study of Abell 795 -- a sloshing cluster with a FR0 radio galaxy at its center

We present the first X-ray dedicated study of the galaxy cluster A795 and of the Fanaroff-Riley Type 0 hosted in its brightest cluster galaxy. Using an archival 30 ks \textit{Chandra} observation we study the dynamical state and cooling properties of the intracluster medium, and we investigate whether the growth of the radio galaxy is prevented by the surrounding environment. We discover that A795 is a weakly cool core cluster, with an observed mass deposition rate $\lessapprox 14\,$ M$_{\odot}$yr$^{-1}$ in the cooling region (central $\sim$66 kpc). In the inner $\sim$ 30 kpc we identify two putative X-ray cavities, and we unveil the presence of two prominent cold fronts at $\sim$60 kpc and $\sim$178 kpc from the center, located along a cold ICM spiral feature. The central galaxy, which is offset by 17.7 kpc from the X-ray peak, is surrounded by a multi-temperature gas with an average density of $n_{\text{e}} = 2.14 \times 10^{-2}$ cm$^{-3}$. We find extended radio emission at 74-227 MHz centered on the cluster, exceeding the expected flux from the radio galaxy extrapolated at low frequency. We propose that sloshing is responsible for the spiral morphology of the gas and the formation of the cold fronts, and that the environment alone cannot explain the compactness of the radio galaxy. We argue that the power of the two cavities and the sloshing kinetic energy can reduce and offset cooling. Considering the spectral and morphological properties of the extended radio emission, we classify it as a candidate radio mini-halo.

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arXiv2021

A new distant Giant Radio Galaxy in the Boötes field serendipitously detected by Chandra

Giant Radio Galaxies (GRGs) are the largest single structures in the Universe. Exhibiting extended radio morphology, their projected sizes range from 0.7 Mpc up to 4.9 Mpc. LOFAR has opened a new window on the discovery and investigation of GRGs and, despite the hundreds that are today known, their main growth catalyst is still debated. One natural explanation for the exceptional size of GRGs is their old age. In this context, hard X-ray selected GRGs show evidence of restarting activity, with the giant radio lobes being mostly disconnected from the nuclear source, if any. In this paper, we present the serendipitous discovery of a distant ($z=0.629$), medium X-ray selected GRG in the Boötes field. High-quality, deep Chandra and LOFAR data allow a robust study of the connection between the nucleus and the lobes, at a larger redshift so far inaccessible to coded-mask hard X-ray instruments. The radio morphology of the GRG presented in this work does not show evidence for restarted activity, and the nuclear radio core spectrum does not appear to be GPS-like. On the other hand, the X-ray properties of the new GRG are perfectly consistent with the ones previously studied with Swift/BAT and INTEGRAL at lower redshift. In particular, the bolometric luminosity measured from the X-ray spectrum is a factor of six larger than the one derived from the radio lobes, although the large uncertainties make them formally consistent at $1σ$. Finally, the moderately dense environment around the GRG, traced by the spatial distribution of galaxies, supports recent findings that the growth of GRGs is not primarily driven by underdense environments.

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arXiv2021

Jet-accretion system in the nearby mJy Radio Galaxies

It is generally thought that FRII Radio Galaxies host thin optically thick disks, while FRIs are powered by Advected Dominated Accretion Flows. The sources with an efficient engine are optically classified as High Excitation Radio Galaxies (HERGs) and those with an inefficient motor as Low Excitation Radio Galaxies (LERGs). Recently, the study of Radio Galaxies down to mJy fluxes has cast serious doubts on the LERG-FRI and HERG-FRII correspondence, revealing that many LERGs show FRII radio morphologies. The FR catalogs recently compiled by Capetti et al. (2017a,b) and Baldi et al. (2018) have allowed us to explore this issue in the local ($z\le 0.15$) mJy Universe. Our statistical study shows that the majority of nearby mJy objects are in a late stage of their life. FRII-LERGs appear more similar to the old FRI-LERGs than to the young FRII-HERGs. FRII-LERGs may be aged HERGs that, exhausted the cold fuel, have changed their accretion regime or a separate LERG class particularly efficient in launching jets. Exploiting the empirical relations which convert L$_{\rm [OIII]}$ and L$_{\rm 1.4~GHz}$ into accretion power and jet kinetic power, respectively, we observed that LERGs with similar masses and accretion rates seem to expel jets of different power. We speculate that intrinsic differences related to the black hole properties (spin and magnetic field at its horizon) can determine the observed spread in jet luminosity. In this view, FRII-LERGs should have the fastest spinning black holes and/or the most intense magnetic fluxes. On the contrary, compact LERGs (i.e. FR0s) should host extremely slow black holes and/or weak magnetic fields.

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arXiv2018

Observatory science with eXTP

In this White Paper we present the potential of the enhanced X-ray Timing and Polarimetry (eXTP) mission for studies related to Observatory Science targets. These include flaring stars, supernova remnants, accreting white dwarfs, low and high mass X-ray binaries, radio quiet and radio loud active galactic nuclei, tidal disruption events, and gamma-ray bursts. eXTP will be excellently suited to study one common aspect of these objects: their often transient nature. Developed by an international Consortium led by the Institute of High Energy Physics of the Chinese Academy of Science, the eXTP mission is expected to be launched in the mid 2020s.

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arXiv2018

X-ray study of a sample of FR0 radio galaxies: unveiling the nature of the central engine

FR0s are compact radio sources that represent the bulk of the Radio-Loud (RL) AGN population, but they are still poorly understood. Pilot studies on these sources have been already performed at radio and optical wavelengths: here we present the first X-ray study of a sample of 19 FR0 radio galaxies selected from the SDSS/NVSS/FIRST sample of Best & Heckman (2012), with redshift $\leq$ 0.15, radio size $\leq$ 10 kpc and optically classified as low-excitation galaxies (LEG). The X-ray spectra are modeled with a power-law component absorbed by Galactic column density with, in some cases, a contribution from thermal extended gas. The X-ray photons are likely produced by the jet as attested by the observed correlation between X-ray (2-10 keV) and radio (5 GHz) luminosities, similar to FRIs. The estimated Eddington-scaled luminosities indicate a low accretion rate. Overall, we find that the X-ray properties of FR0s are indistinguishable from those of FRIs, thus adding another similarity between AGN associated with compact and extended radio sources. A comparison between FR0s and low luminosity BL Lacs, rules out important beaming effects in the X-ray emission of the compact radio galaxies. FR0s have different X-ray properties with respect to young radio sources (e.g. GPS/CSS sources), generally characterized by higher X-ray luminosities and more complex spectra. In conclusion, the paucity of extended radio emission in FR0s is probably related to the intrinsic properties of their jets that prevent the formation of extended structures, and/or to intermittent activity of their engines.

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arXiv2016

Multiband Observations of the Quasar PKS 2326-502 during Active and Quiescent Gamma-Ray States in 2010-2012

Quasi-simultaneous observations of the Flat Spectrum Radio Quasar PKS 2326-502 were carried out in the gamma-ray, X-ray, UV, optical, near-infrared, and radio bands. Thanks to these observations we are able to characterize the spectral energy distribution of the source during two flaring and one quiescent gamma-ray states. These data were used to constrain one-zone leptonic models of the spectral energy distributions of each flare and investigate the physical conditions giving rise to them. While modeling one flare only required changes to the electron spectrum, the other flare needed changes in both the electron spectrum and the size of the emitting region with respect to the quiescent state. These results are consistent with an emerging pattern of two broad classes of flaring states seen in blazars.

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arXiv2015

Discovery of a FR0 radio galaxy emitting at $γ$-ray energies

We present supporting evidence for the first association of a Fermi source, 3FGLJ1330.0-3818, with the FR0 radio galaxy Tol1326-379. FR0s represent the majority of the local radio loud AGN population but their nature is still unclear. They share the same properties of FRIs from the point of view of the nuclear and host properties, but they show a large deficit of extended radio emission. Here we show that FR0s can emit photons at very high energies. Tol1326-379 has a GeV luminosity of $L_{>1~{\rm GeV}} \sim 2\times10^{42}$ erg s$^{-1}$, typical of FRIs, but with a steeper $γ$-ray spectrum ($Γ=2.78\pm 0.14$). This could be related to the intrinsic jet properties but also to a different viewing angle.

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arXiv2015

Real-Time Analysis sensitivity evaluation of the Cherenkov Telescope Array

The Cherenkov Telescope Array (CTA), the new generation very high-energy gamma-ray observatory, will improve the flux sensitivity of the current Cherenkov telescopes by an order of magnitude over a continuous range from about 10 GeV to above 100 TeV. With tens of telescopes distributed in the Northern and Southern hemispheres, the large effective area and field of view coupled with the fast pointing capability make CTA a crucial instrument for the detection and understanding of the physics of transient, short-timescale variability phenomena (e.g. Gamma-Ray Bursts, Active Galactic Nuclei, gamma-ray binaries, serendipitous sources). The key CTA system for the fast identification of flaring events is the Real-Time Analysis (RTA) pipeline, a science alert system that will automatically detect and generate science alerts with a maximum latency of 30 seconds with respect to the triggering event collection and ensure fast communication to/from the astrophysics community. According to the CTA design requirements, the RTA search for a true transient event should be performed on multiple time scales (from minutes to hours) with a sensitivity not worse than three times the nominal CTA sensitivity. Given the CTA requirement constraints on the RTA efficiency and the fast response ability demanded by the transient science, we perform a preliminary evaluation of the RTA sensitivity as a function of the CTA high-level technical performance (e.g. effective area, point spread function) and the observing time. This preliminary approach allows the exploration of the complex parameter space defined by the scientific and technological requirements, with the aim of defining the feasibility range of the input parameters and the minimum background rejection capability of the RTA pipeline.

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arXiv2015

The On-Site Analysis of the Cherenkov Telescope Array

The Cherenkov Telescope Array (CTA) observatory will be one of the largest ground-based very high-energy gamma-ray observatories. The On-Site Analysis will be the first CTA scientific analysis of data acquired from the array of telescopes, in both northern and southern sites. The On-Site Analysis will have two pipelines: the Level-A pipeline (also known as Real-Time Analysis, RTA) and the level-B one. The RTA performs data quality monitoring and must be able to issue automated alerts on variable and transient astrophysical sources within 30 seconds from the last acquired Cherenkov event that contributes to the alert, with a sensitivity not worse than the one achieved by the final pipeline by more than a factor of 3. The Level-B Analysis has a better sensitivity (not be worse than the final one by a factor of 2) and the results should be available within 10 hours from the acquisition of the data: for this reason this analysis could be performed at the end of an observation or next morning. The latency (in particular for the RTA) and the sensitivity requirements are challenging because of the large data rate, a few GByte/s. The remote connection to the CTA candidate site with a rather limited network bandwidth makes the issue of the exported data size extremely critical and prevents any kind of processing in real-time of the data outside the site of the telescopes. For these reasons the analysis will be performed on-site with infrastructures co-located with the telescopes, with limited electrical power availability and with a reduced possibility of human intervention. This means, for example, that the on-site hardware infrastructure should have low-power consumption. A substantial effort towards the optimization of high-throughput computing service is envisioned to provide hardware and software solutions with high-throughput, low-power consumption at a low-cost.

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arXiv2015

The connection between the radio jet and the gamma-ray emission in the radio galaxy 3C 120

We present the analysis of the radio jet evolution of the radio galaxy 3C 120 during a period of prolonged gamma-ray activity detected by the Fermi satellite between December 2012 and October 2014. We find a clear connection between the gamma-ray and radio emission, such that every period of gamma-ray activity is accompanied by the flaring of the mm-VLBI core and subsequent ejection of a new superluminal component. However, not all ejections of components are associated with gamma-ray events detectable by Fermi. Clear gamma-ray detections are obtained only when components are moving in a direction closer to our line of sight.This suggests that the observed gamma-ray emission depends not only on the interaction of moving components with the mm-VLBI core, but also on their orientation with respect to the observer. Timing of the gamma-ray detections and ejection of superluminal components locate the gamma-ray production to within almost 0.13 pc from the mm-VLBI core, which was previously estimated to lie about 0.24 pc from the central black hole. This corresponds to about twice the estimated extension of the broad line region, limiting the external photon field and therefore suggesting synchrotron self Compton as the most probable mechanism for the production of the gamma-ray emission. Alternatively, the interaction of components with the jet sheath can provide the necessary photon field to produced the observed gamma-rays by Compton scattering.

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arXiv2013

XIPE: the X-ray Imaging Polarimetry Explorer

X-ray polarimetry, sometimes alone, and sometimes coupled to spectral and temporal variability measurements and to imaging, allows a wealth of physical phenomena in astrophysics to be studied. X-ray polarimetry investigates the acceleration process, for example, including those typical of magnetic reconnection in solar flares, but also emission in the strong magnetic fields of neutron stars and white dwarfs. It detects scattering in asymmetric structures such as accretion disks and columns, and in the so-called molecular torus and ionization cones. In addition, it allows fundamental physics in regimes of gravity and of magnetic field intensity not accessible to experiments on the Earth to be probed. Finally, models that describe fundamental interactions (e.g. quantum gravity and the extension of the Standard Model) can be tested. We describe in this paper the X-ray Imaging Polarimetry Explorer (XIPE), proposed in June 2012 to the first ESA call for a small mission with a launch in 2017 but not selected. XIPE is composed of two out of the three existing JET-X telescopes with two Gas Pixel Detectors (GPD) filled with a He-DME mixture at their focus and two additional GPDs filled with pressurized Ar-DME facing the sun. The Minimum Detectable Polarization is 14 % at 1 mCrab in 10E5 s (2-10 keV) and 0.6 % for an X10 class flare. The Half Energy Width, measured at PANTER X-ray test facility (MPE, Germany) with JET-X optics is 24 arcsec. XIPE takes advantage of a low-earth equatorial orbit with Malindi as down-link station and of a Mission Operation Center (MOC) at INPE (Brazil).

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arXiv2012

Exploring the FRI/FRII radio dichotomy with the Fermi satellite

Misaligned Active Galactic Nuclei (MAGNs), i.e., radio galaxies and quasars with the jet not directly pointing at the observer, are a new class of GeV emitters. In low power radio galaxies (i.e., FRIs), gamma-rays are mainly produced in compact jet regions, although in at least one case, Centaurus A, high energy photons from the radio lobes have been also observed. The first localization of the gamma-ray dissipation zone in a high power radio galaxy (i.e., FRII) excludes major contributions from extended regions. The study of the FRII source 3C111 indicates that gamma-ray photons are produced in the jet. The site, coincident with the radio core, is estimated to be at a distance <~0.3 pc from the black hole. Although the place where high energy photons are produced is probably similar in FRIs and FRIIs, high power radio galaxies are rarer in the GeV sky. Our study of all the radio sources belonging to four complete radio catalogs (3CR, 3CRR, MS4, 2Jy) disfavors the idea that the paucity of FRIIs is due to their larger distance (and therefore to their faintness) and supports other possibilities, pointing to beaming/jet structural differences between FRIs and FRIIs.

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arXiv2012

The gamma-ray emission region in the FRII Radio Galaxy 3C 111

The Broad Line Radio Galaxy 3C 111, characterized by a Fanaroff-Riley II (FRII) radio morphology, is one of the sources of the Misaligned Active Galactic Nuclei sample, consisting of Radio Galaxies and Steep Spectrum Radio Quasars, recently detected by the Fermi-Large Area Telescope. Our analysis of the 24-month gamma-ray light curve shows that 3C 111 was only occasionally detected at high energies. It was bright at the end of 2008 and faint, below the Fermi-Large Area Telescope sensitivity threshold, for the rest of the time. A multifrequency campaign of 3C~111, ongoing in the same period, revealed an increase of the mm, optical and X-ray fluxes in 2008 September-November, interpreted by Chatterjee et al. (2011) as due to the passage of a superluminal knot through the jet core. The temporal coincidence of the mm-optical-X-ray outburst with the GeV activity suggests a co-spatiality of the events, allowing, for the first time, the localization of the gamma-ray dissipative zone in a FRII jet. We argue that the GeV photons of 3C 111 are produced in a compact region confined within 0.1 pc and at a distance of about 0.3 pc from the black hole.

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arXiv2011

Gamma-Rays from Radio Galaxies: Fermi-Lat Observations

We review the high energy properties of Misaligned AGNs associated with gamma-ray sources detected by Fermi in 24 months of survey. Most of them are nearby emission low power radio galaxies (i.e FRIs) which probably have structured jets. On the contrary, high power radio sources (i.e FRIIs) with GeV emission are rare. The small number of FRIIs does not seem to be related to their higher redshifts. Assuming proportionality between the radio core flux and the gamma-ray flux, several of them are expected to be bright enough to be detected above 100 MeV in spite of their distance. We suggest that beaming/jet structural differences are responsible for the detection rate discrepancy observed between FRIs and FRIIs.

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arXiv2010

The broad-band X-ray spectrum of the Seyfert 1 galaxy, MCG+8-11-11

We present a long (100 ks) Suzaku observation of one of the X-ray brightest AGN, MCG+8-11-11. These data were complemented with the 54-month Swift BAT spectrum, allowing us to perform a broad-band fit in the 0.6-150 keV range. The fits performed in the 0.6-10 keV band give consistent results with respect to a previous XMM-Newton observation, i.e. the lack of a soft excess, warm absorption along the line of sight, a large Compton reflection component (R~1) and the absence of a relativistic component of the neutral iron K$α$ emission line. However, when the PIN and Swift BAT data are included, the reflection amount drops significantly (R~0.2-0.3), and a relativistic iron line is required, the latter confirmed by a phenomenological analysis in a restricted energy band (3-10 keV). When a self-consistent model is applied to the whole broadband data, the observed reflection component appears to be all associated to the relativistic component of the iron K$α$ line. The resulting scenario, though strongly model-dependent, requires that all the reprocessing spectral components from Compton-thick material must be associated to the accretion disc, and no evidence for the classical pc-scale torus is found. The narrow core of the neutral iron K$α$ line is therefore produced in a Compton-thin material, like the BLR, similarly to what found in another Seyfert galaxy, NGC7213, but with the notable difference that MCG+8-11-11 presents spectral signatures from an accretion disc. The very low accretion rate of NGC7213 could explain the lack of relativistic signatures in its spectrum, but the absence of the torus in both sources is more difficult to explain, since their luminosities are comparable, and their accretion rates are completely different.

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