arXiv subjects

M. Giroletti

Publications and source records attributed to M. Giroletti.

At least 217 records · Page 12Linked to original sources

arXiv2010

Flux variability in the FSRQ 1510-089. A radio-gamma perspective

The knowledge of the physical conditions occurring in the relativistic jet of radio-loud active galactic nuclei (AGNs) is important to understand the mechanisms at the basis of their multiband emission. From parsec-scale radio observations of blazar objects it has been suggested a connection between the ejection of new jet features and strong gamma-ray flares. We present results from multi-epoch Very Long Baseline Interferometer (VLBI) and Space-VLBI observations of the Flat Spectrum Radio Quasar (FSRQ) PKS 1510-089. The comparison of the radio structure observed at different epochs shows the presence of jet features moving with highly superluminal apparent velocity. Radio flux density variability and changes in the source structure and in the polarization properties are then compared with the information on the gamma-ray emission in order to find a possible connection between radio and gamma-ray emission.

astro-ph.CO↗

arXiv2010

The jet in M87 from EVN observations

One of the most intriguing open questions of today's astrophysics is the jet physical properties and the location and the mechanisms for the production of MeV, GeV, and TeV gamma-rays in AGN jets. M87 is a privileged laboratory for a detailed study of the properties of jets, owing to its proximity, its massive black hole, and its conspicuous emission at radio wavelengths and above. We started on November 2009 a monitoring program with the e-EVN at 5 GHz. We present here results of these multi-epoch observations and discuss the two episodes of activity at energy E>100 GeV that occured in this period. One of these observations was obtained at the same day of the first high energy flare. We added to our results literature data obtained with the VLBI and VLA. A clear change in the proper motion velocity of HST-1 is present at the epoch ~ 2005.5. In the time range 1998 -- 2005.5 the apparent velocity is subluminal, and superluminal (~ 2.7c) after 2005.5.

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arXiv2010

The Jet in M87 from e-EVN Observations

One of the most intriguing open questions of today's astrophysics is the one concerning the location and the mechanisms for the production of MeV, GeV, and TeV gamma-rays in AGN jets. M87 is a privileged laboratory for a detailed study of the properties of jets, owing to its proximity, its massive black hole, and its conspicuous emission at radio wavelengths and above. We started on November 2009 a monitoring program with the e-EVN at 5 GHz, during which two episodes of activity at energy E > 100 GeV have occured. We present here results of these multi-epoch observations. The inner jet and HST-1 are both detected and resolved in our datasets. One of these observations was obtained at the same day of the first high energy flare. A clear change in the proper motion velocity of HST-1 is present at the epoch ~2005.5. In the time range 2003 -- 2005.5 the apparent velocity is subluminal, and superluminal (~ 2.7c) after 2005.5.

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arXiv2010

Parsec-scale Properties of Brightest Cluster Galaxies

We present new VLBI observations at 5 GHz of a complete sample of Brightest Cluster Galaxies (BCGs) in nearby Abell Clusters (distance class <3). Combined with data from the literature, this provides parsec-scale information for 34 BCGs. Our analysis of their parsec scale radio emission and cluster X-ray properties shows a possible dichotomy between BCGs in cool core clusters and those in non cool core clusters. Among resolved sources, those in cool core clusters tend to have two-sided parsec-scale jets, while those in less relaxed clusters have predominantly one-sided parsec-scale jets. We suggest that this difference could be the result of interplay between the jets and the surrounding medium. The one-sided structure in non cool core clusters could be due to Doppler boosting effects in relativistic, intrinsically symmetric jets; two-sided morphology in cool core clusters is likely related to the presence of heavy and mildly relativistic jets slowed down on the parsec-scale. Evidence of recurrent activity are also found in BCGs in cool core clusters.

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arXiv2010

Multiwavelength observations of 3C 454.3. III. Eighteen months of AGILE monitoring of the "Crazy Diamond"

We report on 18 months of multiwavelength observations of the blazar 3C 454.3 (Crazy Diamond) carried out in July 2007-January 2009. We show the results of the AGILE campaigns which took place on May-June 2008, July-August 2008, and October 2008-January 2009. During the May 2008-January 2009 period, the source average flux was highly variable, from an average gamma-ray flux F(E>100MeV) > 200E-8 ph/cm2/s in May-June 2008, to F(E>100MeV)~80E-8 ph/cm2/s in October 2008-January 2009. The average gamma-ray spectrum between 100 MeV and 1 GeV can be fit by a simple power law (Gamma_GRID ~ 2.0 to 2.2). Only 3-sigma upper limits can be derived in the 20-60 keV energy band with Super-AGILE. During July-August 2007 and May-June 2008, RXTE measured a flux of F(3-20 keV)= 8.4E-11 erg/cm2/s, and F(3-20 keV)=4.5E-11 erg/cm2/s, respectively and a constant photon index Gamma_PCA=1.65. Swift/XRT observations were carried out during all AGILE campaigns, obtaining a F(2-10 keV)=(0.9-7.5)E-11 erg/cm2/s and a photon index Gamma_XRT=1.33-2.04. BAT measured an average flux of ~5 mCrab. GASP-WEBT monitored 3C 454.3 during the whole 2007-2008 period from the radio to the optical. A correlation analysis between the optical and the gamma-ray fluxes shows a time lag of tau=-0.4 days. An analysis of 15 GHz and 43 GHz VLBI core radio flux observations shows an increasing trend of the core radio flux, anti- correlated with the higher frequency data. The modeling SEDs, and the behavior of the long-term light curves in different energy bands, allow us to compare the jet properties during different emission states, and to study the geometrical properties of the jet on a time-span longer than one year.

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arXiv2010

The Connection between Radio and Gamma-ray Emission in Active Galactic Nuclei

Radio and gamma-ray emission from active galactic nuclei (AGN) are thought to share a common origin, related to the ejection phenomena in the vicinity of supermassive black holes. Thanks to its sensitivity, surveying capability, and broad energy range, the Large Area Telescope (LAT) onboard the Fermi Gamma-ray Space Telescope has permitted us to discover and characterize a huge number of extragalactic gamma-ray sources. Similarly to what was found by EGRET, these sources are typically associated with blazars, characterized by significant radio emission and flat spectrum. The radio luminosity distribution is extended over 7 orders of magnitudes, with flat spectrum radio quasars clustered at higher powers and BL Lacs more scattered; the average spectral index is consistent with alpha=0, although a few remarkable sources have alpha>0.5. A comparison of the radio flux density and the gamma-ray photon flux is presented, although claims on its significance require a detailed discussion and Monte Carlo simulations which will be presented in a future paper.

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arXiv2009

VLBI observations of Brightest Cluster Galaxies: are cooling and parsec scale morphology correlated?

We present a statistical study on parsec scale properties of a sample of Brigthest Cluster Galaxies (BCGs) in Abell Clusters. These data show a possible difference between BCGs in cool core clusters (two-sided parsec scale jets) and in non cool core clusters (one-sided parsec scale jet). We suggest that the two-sided morphology in cool core clusters could be due to the presence of mildly relativistic jets slowed down already at mas scale as consequence of the jet interaction with a dense surrounding medium.

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arXiv2009

The Bologna complete sample of nearby radio sources: II -- phase referenced observations of faint nuclear sources

To study statistical properties of different classes of sources, it is necessary to observe a sample that is free of selection effects. To do this, we initiated a project to observe a complete sample of radio galaxies selected from the B2 Catalogue of Radio Sources and the Third Cambridge Revised Catalogue (3CR), with no selection constraint on the nuclear properties. We named this sample "the Bologna Complete Sample" (BCS). We present new VLBI observations at 5 and 1.6 GHz for 33 sources drawn from a sample not biased toward orientation. By combining these data with those in the literature, information on the parsec-scale morphology is available for a total of 76 of 94 radio sources with a range in radio power and kiloparsec-scale morphologies. The fraction of two-sided sources at milliarcsecond resolution is high (30%), compared to the fraction found in VLBI surveys selected at centimeter wavelengths, as expected from the predictions of unified models. The parsec-scale jets are generally found to be straight and to line up with the kiloparsec-scale jets. A few peculiar sources are discussed in detail.

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arXiv2009

Multiwavelength Observations of Mrk 501 in 2008

The well-studied VHE (E >100 GeV) blazar Mrk 501 was observed between March and May 2008 as part of an extensive multiwavelength observation campaign including radio, optical, X-ray and VHE gamma-ray instruments. Mrk 501 was in a low state of activity during the campaign, with a low VHE flux of about 20% the Crab Nebula flux. Nevertheless, significant flux variations could be observed in X-rays as well as $γ$-rays. Overall Mrk 501 showed increased variability when going from radio to gamma-ray energies. The broadband spectral energy distribution during the two different emission states of the campaign was well described by a homogeneous one-zone synchrotron self-Compton model. The high emission state was satisfactorily modeled by increasing the amount of high energy electrons with respect to the low emission state. This parameterization is consistent with the energy-dependent variability trend observed during the campaign.

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arXiv2009

The parsec-scale properties of the radio galaxy 4C 26.42 in the dense cooling core cluster A1795

The aim of the present work is to study the radio emission on the parsec scale of 4C 26.42, the Brightest cluster galaxy in Abell 1795, in the framework of radiosources in a dense cool core cluster. We present Very Long Baseline Array (VLBA) observations at 1.6, 5, 8.4 and 22 GHz. We performed a spectral index and multiepoch analysis. The source appears two-sided with a well defined and symmetric Z-structure at ~5 mas from the core. The kiloparsec-scale morphology is similar to the parsec-scale structure, but reversed in P.A., with symmetric 90 deg. bends at about 2 arcsec from the nuclear region. Comparing data obtained at 3 different epochs we derive a 3$σ$ limit to the apparent proper motion of $β_a$ < 0.04. We suggest that the parsec-scale jets are sub-relativistic, in contrast with the high velocities found for most low-power radio galaxies. The origin of the unusual radio morphology remains a puzzle. We suggest that the identification of the parent galaxy with the central cD in a cooling cluster plays an important role in the properties and structure of the jets.

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arXiv2009

Samples and statistics of CSS and GPS sources

Several samples have been proposed in the last years in order to study the properties of intrinsically small sources. In this paper, we review the properties of the main samples that are currently available, both selected on the basis of spectral index and of morphology. As a result of the work in this area, large numbers of intrinsically small sources have been found. We summarize the present status of hot spot advance measurements, listing 18 sources with available VLBI data. The mean hot spot separation velocity is v_{sep} = (0.19 +/- 0.11)h^{-1}c and the kinematic ages span the range from 20 to 3000 years. Finally, we present a brief outlook on the use of future instrumentation in order to improve our understanding of radio source evolution. Prospects for VSOP2, e-VLA, e-MERLIN, LOFAR, ALMA, and Fermi are suggested.

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arXiv2009

New results on the parsec-scale properties of the Bologna Complete Sample

We present an update of the parsec scale properties of the Bologna Complete Sample consisting of 95 radio sources with z $<$ 0.1 from the B2 Catalog of Radio Sources and the Third Cambridge Revised Catalog (3CR). Thanks to new data obtained in phase reference mode, we have now parsec scale images for 76 sources of the sample. Most of them show a one-sided jet structure but we find a higher fraction of two-sided sources in comparison with previous flux-limited VLBI surveys. Results for two peculiar sources, 3C 293 and 3C 310, are presented and discussed in more detail.

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arXiv2009

Parsec Scale Properties of Brightest Cluster Galaxies

We present new VLBI observations of a complete sample of Brigthest Cluster Galaxies (BCGs) in nearby Abell Clusters. These data show a possible difference between BCGs in cool core clusters (two-sided parsec scale jets) and in non cool core clusters (one-sided parsec scale jet). We suggest that this difference could be due to the jet interaction with the surrounding medium. More data are necessary to discuss whether parsec-scale properties of BCGs are influenced by their peculiar morphology and position at the center of rich galaxy clusters.

astro-ph.CO↗

arXiv2008

VLBI observations of nearby radio loud Active Galactic Nuclei

We present an update of the parsec scale properties of the Bologna Complete Sample consisting of 95 radio sources from the B2 Catalog of Radio Sources and the Third Cambridge Revised Catalog (3CR), with z < 0.1. Thanks to recent new data we have now parsec scale images for 76 sources of the sample. Most of them show a one-sided jet structure but we find a higher fraction of two-sided sources in comparison with previous flux-limited VLBI surveys. A few peculiar sources are presented and discussed in more detail.

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arXiv2008

The two INTEGRAL X-ray transients IGR J17091--3624 and IGR J17098--3628: a multi-wavelength long term campaign

IGR J17091-3624 and IGR J17098-3628 are two X-ray transients discovered by INTEGRAL and classified as possible black hole candidates (BHCs). We present here the results obtained from the analysis of multi-wavelength data sets collected by different instruments from 2005 until the end of 2007 on both sources. IGR J17098-3628 has been regularly detected by INTEGRAL and RXTE over the entire period of the observational campaign; it was also observed with pointed observations by XMM and Swift/XRT in 2005 and 2006 and exhibited flux variations not linked with the change of any particular spectral features. IGR J17091-3624 was initially in quiescence (after a period of activity between 2003 April and 2004 April) and it was then detected again in outburst in the XRT field of view during a Swift observation of IGR J17098--3628 on 2007 July 9. The observations during quiescence provide an upper limit to the 0.2-10 keV luminosity, while the observations in outburst cover the transition from the hard to the soft state. Moreover, we obtain a refined X-ray position for IGR J17091-3624 from the Swift/XRT observations during the outburst in 2007. The new position is inconsistent with the previously proposed radio counterpart. We identify in VLA archive data a compact radio source consistent with the new X-ray position and propose it as the radio counterpart of the X-ray transient.

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arXiv2008

The jet of Markarian 501 from millions of Schwarzschild radii down to a few hundreds

Aims: The TeV BL Lac object Markarian 501 is a complex, core dominated radio source, with a one sided, twisting jet on parsec scales. In the present work, we attempt to extend our understanding of the source physics to regions of the radio jet which have not been accessed before. Methods: We present new observations of Mrk 501 at 1.4 and 86 GHz. The 1.4 GHz data were obtained using the Very Large Array (VLA) and High Sensitivity Array (HSA) in November 2004, in full polarization, with a final r.m.s. noise of 25 microJy/beam in the HSA total intensity image; the 86 GHz observations were performed in October 2005 with the Global Millimeter VLBI Array (GMVA), providing an angular resolution as good as 110 x 40 microarcseconds. Results: The sensitivity and resolution provided by the HSA make it possible to detect the jet up to ~700 milliarcseconds (corresponding to a projected linear size of ~500 pc) from its base, while the superior resolution of the 86 GHz GMVA observations probes the innermost regions of the jet down to ~200 Schwarzschild radii. The brightness temperature at the jet base is in excess of 6e10 K. We find evidence of limb brightening on physical scales from <1 pc to ~40 pc. Polarization images and fits to the trend of jet width and brightness vs. distance from the core reveal a magnetic field parallel to the jet axis.

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arXiv2008

Parsec Scale Properties of Brightest Cluster Galaxies

We present new VLBI observations of Brightest Cluster Galaxies in eight nearby Abell clusters. These data show a possible difference between Brightest Cluster Galaxies in cool core clusters (two-sided pc scale jets) and in non cool core clusters (one-sided pc scale jets). We suggest that this difference could be due to the jet interaction with the surrounding medium. More data are necessary to discuss if pc-scale properties of Brightest Cluster Galaxies are influenced by their peculiar morphology and position in the center of rich clusters of galaxies.

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arXiv2007

Broadband X-ray spectrum of the newly discovered broad line radio galaxy IGR J21247+5058

In this paper we present radio and high energy observations of the INTEGRAL source IGR J21247+5058, a broad line emitting galaxy obscured by the Galactic plane. Archival VLA radio data indicate that IGR J21247+5058 can be classified as an FRII Broad Line Radio Galaxy. The spectrum between 610 MHz and 15 GHz is typical of synchrotron self-absorbed radiation with a peak at 8 GHz and a low energy turnover; the core fraction is 0.1 suggestive of a moderate Doppler boosting of the base of the jet. The high energy broad-band spectrum was obtained by combining XMM-Newton and Swift/XRT observation with INTEGRAL/IBIS data. The 0.4-100 keV spectrum is well described by a power law, with slope $Γ$=1.5, characterised by complex absorption due to two layers of material partially covering the source and a high energy cut-off around 70-80 keV. Features such as a narrow iron line and a Compton reflection component, if present, are weak, suggesting that reprocessing of the power law photons in the accretion disk plays a negligible role in the source.

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