arXiv subjects

M. Orienti

Publications and source records attributed to M. Orienti.

At least 109 records · Page 6Linked to original sources

arXiv2013

Multifrequency studies of the narrow-line Seyfert 1 galaxy SBS 0846+513

The narrow-line Seyfert 1 galaxy SBS 0846+513 was first detected by the Large Area Telescope (LAT) on-board Fermi in 2011 June-July when it underwent a period of flaring activity. Since then, as Fermi continues to accumulate data on this source, its flux has been monitored on a daily basis. Two further gamma-ray flaring episodes from SBS 0846+513 were observed in 2012 May and August, reaching a daily peak flux integrated above 100 MeV of (50+/-12)x10^-8 ph/cm^2/s, and (73+/-14)x10^-8 ph/cm^2/s on May 24 and August 7, respectively. Three outbursts were detected at 15 GHz by the Owens Valley Radio Observatory 40-m telescope in 2012 May, 2012 October, and 2013 January, suggesting a complex connection with the gamma-ray activity. The most likely scenario suggests that the 2012 May gamma-ray flare may not be directly related to the radio activity observed over the same period, while the two gamma-ray flaring episodes may be related to the radio activity observed at 15 GHz in 2012 October and 2013 January. The gamma-ray flare in 2012 May triggered Swift observations that confirmed that SBS 0846+513 was also exhibiting high activity in the optical, UV and X-ray bands, thus providing a firm identification between the gamma-ray source and the lower-energy counterpart. We compared the spectral energy distribution (SED) of the flaring state in 2012 May with that of a quiescent state. The two SEDs, modelled as an external Compton component of seed photons from a dust torus, could be fitted by changing the electron distribution parameters as well as the magnetic field. No significant evidence of thermal emission from the accretion disc has been observed. Interestingly, in the 5 GHz radio luminosity vs. synchrotron peak frequency plot SBS 0846+513 seems to lie in the flat spectrum radio quasar part of the so-called `blazar sequence'.

astro-ph.HE↗

arXiv2013

The ordinary life of the gamma-ray emitting narrow-line Seyfert 1 galaxy PKS 1502+036

We report on multifrequency observations of the gamma-ray emitting narrow-line Seyfert 1 galaxy PKS 1502+036 performed from radio to gamma-rays during 2008 August-2012 November by Fermi-LAT, Swift (XRT and UVOT), OVRO, VLBA, and VLA. No significant variability has been observed in gamma-rays, with 0.1-100 GeV flux that ranged between (3-7)x10^-8 ph/cm^2/s using 3-month time bins. The photon index of the LAT spectrum (Gamma=2.60+/-0.06) and the apparent isotropic gamma-ray luminosity, L(0.1-100 GeV)= 7.8x10^45 erg/s, over 51 months are typical of a flat spectrum radio quasar. The radio spectral variability and the one-sided structure, in addition to the observed gamma-ray luminosity, suggest a relativistic jet with a high Doppler factor. In contrast to SBS 0846+513, the VLBA at 15 GHz did not observe superluminal motion for PKS 1502+036. Despite having the optical characteristics typical of a narrow-line Seyfert 1 galaxy, radio and gamma-ray properties of PKS 1502+036 are found to be similar to those of a blazar at the low end of the black hole mass distribution for blazars. This is in agreement with what has been found in the case of the other gamma-ray emitting narrow-line Seyfert 1 SBS 0846+513.

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arXiv2013

A sample of small size compact steep-spectrum radio sources. VLBI images and VLA polarization at 5 GHz

Global VLBI observations at 5 GHz have been performed to study the source morphology in 10 compact steep-spectrum (CSS) sources selected from the Peacock & Wall catalogue with the aim of finding asymmetric structures produced by the interaction with the ambient medium. The combination of these data and earlier 1.7-GHz observations allows the study of the spectral index distribution across the source structure and the unambiguous determination of the nature of each component. In seven sources we detected the core component with a flat or inverted spectrum. In six sources the radio emission has a two-sided morphology and comes mainly from steep-spectrum extended structures, like lobes, jets, and hotspots. Only one source, 0319+121, has a one-sided core-jet structure. In three out of the six sources with a two-sided structure the flux density arising from the lobes is asymmetric, and the brightest lobe is the one closest to the core, suggesting that the jets are expanding in an inhomogeneous ambient medium which may influence the source growth. The interaction between the jet and the environment may slow down the source expansion and enhance the luminosity due to severe radiative losses, likely producing an excess of CSS radio sources in flux density limited samples. The lobes of the other three asymmetric sources have a brighter-when-farther behaviour, in agreement with what is expected by projection and relativistic effects. Simultaneous VLA observations carried out to investigate the polarization properties of the targets detected significant polarized emission (~5.5%) only from the quasar 0319+121.

astro-ph.CO↗

arXiv2013

Four Years of Fermi LAT Observations of Narrow-Line Seyfert 1 Galaxies

Before the launch of the Fermi satellite only two classes of AGN were known to generate relativistic jets and thus emit up to the gamma-ray energy range: blazars and radio galaxies, both hosted in giant elliptical galaxies. The first two years of observations by the Large Area Telescope (LAT) on board Fermi confirmed that these two are the most numerous classes of identified sources in the extragalactic gamma-ray sky, but the discovery of variable gamma-ray emission from 5 radio-loud Narrow-Line Seyfert 1 galaxies (NLSy1s) revealed the presence of a possible emerging third class of AGN with relativistic jets. Considering also that NLSy1s are typically hosted in spiral galaxy, this finding poses intriguing questions about the nature of these objects, the onset of production of relativistic jets, and the cosmological evolution of radio-loud AGN. Here, we report on a preliminary investigation of the properties of this sample of radio-loud NLSy1 at MeV-GeV photon energies, utilizing the four-year accumulation of Fermi LAT data. In addition we briefly discuss some radio-to-gamma-rays properties of the gamma-ray emitting NLSy1 in the context of the blazar scenario.

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arXiv2013

Constraints on the Galactic Halo Dark Matter from Fermi-LAT Diffuse Measurements

We have performed an analysis of the diffuse gamma-ray emission with the Fermi Large Area Telescope in the Milky Way Halo region searching for a signal from dark matter annihilation or decay. In the absence of a robust dark matter signal, constraints are presented. We consider both gamma rays produced directly in the dark matter annihilation/decay and produced by inverse Compton scattering of the e+e- produced in the annihilation/decay. Conservative limits are derived requiring that the dark matter signal does not exceed the observed diffuse gamma-ray emission. A second set of more stringent limits is derived based on modeling the foreground astrophysical diffuse emission using the GALPROP code. Uncertainties in the height of the diffusive cosmic-ray halo, the distribution of the cosmic-ray sources in the Galaxy, the index of the injection cosmic-ray electron spectrum and the column density of the interstellar gas are taken into account using a profile likelihood formalism, while the parameters governing the cosmic-ray propagation have been derived from fits to local cosmic-ray data. The resulting limits impact the range of particle masses over which dark matter thermal production in the early Universe is possible, and challenge the interpretation of the PAMELA/Fermi-LAT cosmic ray anomalies as annihilation of dark matter.

astro-ph.CO↗

arXiv2012

Radio and gamma-ray follow-up of the exceptionally high activity state of PKS 1510-089 in 2011

We investigate the radio and gamma-ray variability of the flat spectrum radio quasar PKS 1510-089 in the time range between 2010 November and 2012 January. In this period the source showed an intense activity, with two major gamma-ray flares detected in 2011 July and October. During the latter episode both the gamma-ray and the radio flux density reached their historical peak. Multiwavelength analysis shows a rotation of about 380 deg of the optical polarization angle close in time with the rapid and strong gamma-ray flare in 2011 July. An enhancement of the optical emission and an increase of the fractional polarization both in the optical and in radio bands is observed about three weeks later, close in time with another gamma-ray outburst. On the other hand, after 2011 September a huge radio outburst has been detected, first in the millimeter regime followed with some time delay at centimeter down to decimeter wavelengths. This radio flare is characterized by a rising and a decaying stage, in agreement with the formation of a shock and its evolution, as a consequence of expansion and radiative cooling. If the gamma-ray flare observed in 2011 October is related to this radio outburst, then this strongly indicates that the region responsible for the gamma-ray variability is not within the broad line, but a few parsecs downstream along the jet.

astro-ph.HE↗

arXiv2012

To be or not to be a blazar. The case of the Narrow-Line Seyfert 1 SBS 0846+513

The presence of a relativistic jet in some radio-loud Narrow-Line Seyfert 1s (NLSy1) galaxies, first suggested by their variable radio emission and the flat radio spectra, is now confirmed by the Fermi-LAT detection of five NLSy1s in gamma rays. In particular, a strong gamma-ray flare from SBS 0846+513 was observed in 2011 June by Fermi-LAT reaching a gamma-ray luminosity (0.1-300 GeV) of about 10^48 erg/s, comparable to that of bright flat spectrum radio quasars. Apparent superluminal velocity in the jet was inferred from 2011-2012 VLBA images, suggesting the presence of a highly relativistic jet. Both the power released by this object during the flaring activity and the apparent superluminal velocity are strong indicators of the presence of a relativistic jet as powerful as those in blazars. In addition, variability and spectral properties in radio and gamma-ray bands indicate a blazar-like behaviour, suggesting that, except for some distinct optical characteristics, SBS 0846+513 could be considered as a young blazar at the low end of the blazar's black hole mass distribution.

astro-ph.HE↗

arXiv2012

PKS 2123-463: a confirmed gamma-ray blazar at high redshift

The flat spectrum radio quasar (FSRQ) PKS 2123-463 was associated in the First Fermi-LAT source catalog with the gamma-ray source 1FGL J2126.1-4603, but when considering the full first two years of Fermi observations, no gamma-ray source at a position consistent with this FSRQ was detected, and thus PKS 2123-463 was not reported in the Second Fermi-LAT source catalog. On 2011 December 14 a gamma-ray source positionally consistent with PKS 2123-463 was detected in flaring activity by Fermi-LAT. This activity triggered radio-to-X-ray observations by the Swift, GROND, ATCA, Ceduna, and KAT-7 observatories. Results of the localization of the gamma-ray source over 41 months of Fermi-LAT operation are reported here in conjunction with the results of the analysis of radio, optical, UV and X-ray data collected soon after the gamma-ray flare. The strict spatial association with the lower energy counterpart together with a simultaneous increase of the activity in optical, UV, X-ray and gamma-ray bands led to a firm identification of the gamma-ray source with PKS 2123-463. A new photometric redshift has been estimated as z = 1.46+/-0.05 using GROND and Swift/UVOT observations, in rough agreement with the disputed spectroscopic redshift of z = 1.67. We fit the broadband spectral energy distribution with a synchrotron/external Compton model. We find that a thermal disk component is necessary to explain the optical/UV emission detected by Swift/UVOT. This disk has a luminosity of about 1.8x10^46 erg/s, and a fit to the disk emission assuming a Schwarzschild (i.e., nonrotating) black hole gives a mass of about 2x10^9 solar masses. This is the first black hole mass estimate for this source.

astro-ph.HE↗

arXiv2012

VLBA monitoring of Mrk 421 at 15 GHz and 24 GHz during 2011

High-resolution radio observations are ideal for constraining the value of physical parameters in the inner regions of active-galactic-nucleus jets and complement results on multiwavelength (MWL) observations. This study is part of a wider multifrequency campaign targeting the nearby TeV blazar Markarian 421 (z=0.031), with observations in the sub-mm (SMA), optical/IR (GASP), UV/X-ray (Swift, RXTE, MAXI), and gamma rays (Fermi-LAT, MAGIC, VERITAS). We investigate the jet's morphology and any proper motions, and the time evolution of physical parameters such as flux densities and spectral index. The aim of our wider multifrequency campaign is to try to shed light on questions such as the nature of the radiating particles, the connection between the radio and gamma-ray emission, the location of the emitting regions and the origin of the flux variability. We consider data obtained with the Very Long Baseline Array (VLBA) over twelve epochs (one observation per month from January to December 2011) at 15 GHz and 24 GHz. We investigate the inner jet structure on parsec scales through the study of model-fit components for each epoch. The structure of Mrk 421 is dominated by a compact (~0.13 mas) and bright component, with a one-sided jet detected out to ~10 mas. We identify 5-6 components in the jet that are consistent with being stationary during the 12-month period studied here. Measurements of the spectral index agree with those of other works: they are fairly flat in the core region and steepen along the jet length. Significant flux-density variations are detected for the core component. From our results, we draw an overall scenario in which we estimate a viewing angle 2° < theta < 5° and a different jet velocity for the radio and the high-energy emission regions, such that the respective Doppler factors are δr ~3 and δh.e. ~14.

astro-ph.HE↗

arXiv2012

Physical properties of the nuclear region in Seyfert galaxies derived from observations with the European VLBI Network

We report on sensitive dual-frequency (1.7 and 5 GHz) European VLBI Network observations of the central region of nine Seyfert galaxies. These sources are among the faintest and least luminous members of a complete sample of nearby (d<22 Mpc) low luminosity AGNs. We detect radio emission on milliarcsecond scale in the nuclei of 4 galaxies, while for the other five sources we set an upper limit of <~100 microJy. In three sources, namely NGC 3227, NGC 3982, and NGC 4138, radio emission is detected at both 1.7 and 5 GHz and it is resolved in two or more components. We describe the structural and spectral properties of these features; we find that in each of these three nuclei there is one component with high brightness temperature (typically T_B >10^7.5 K) and flat/intermediate spectral index (0.3\leq alpha \leq 0.6, S(nu) \sim nu^(-alpha), accompanied by secondary steep spectrum extended components. In these cases, non-thermal emission from jets or outflows is thus the most natural explanation. A faint feature is detected in NGC 4477 at 5 GHz; keeping in mind the modest significance of this detection (~5sigma), we propose the hot corona as the origin of non-thermal emission, on the basis of the unrealistic magnetic field values required by synchrotron self-absorption. Finally, the five non-detected nuclei remain elusive and further observations on intermediate scales will be necessary to investigate their nature.

astro-ph.CO↗

arXiv2012

SBS 0846+513: a new gamma-ray emitting Narrow-Line Seyfert 1 galaxy

We report Fermi-LAT observations of the radio-loud AGN SBS 0846+513 (z=0.5835), optically classified as a Narrow-Line Seyfert 1 galaxy, together with new and archival radio-to-X-ray data. The source was not active at gamma-ray energies during the first two years of Fermi operation. A significant increase in activity was observed during 2010 October-2011 August. In particular a strong gamma-ray flare was observed in 2011 June reaching an isotropic gamma-ray luminosity (0.1-300 GeV) of 1.0x10^48 erg/s, comparable to that of the brightest flat spectrum radio quasars, and showing spectral evolution in gamma rays. An apparent superluminal velocity of (8.2+/-1.5)c in the jet was inferred from 2011-2012 VLBA images, suggesting the presence of a highly relativistic jet. Both the power released by this object during the flaring activity and the apparent superluminal velocity are strong indications of the presence of a relativistic jet as powerful as those of blazars. In addition, variability and spectral properties in radio and gamma-ray bands indicate blazar-like behaviour, suggesting that, except for some distinct optical characteristics, SBS 0846+513 could be considered as a young blazar at the low end of the blazar's black hole mass distribution.

astro-ph.HE↗

arXiv2012

Gamma-ray observations of the Orion Molecular Clouds with the Fermi Large Area Telescope

We report on the gamma-ray observations of giant molecular clouds Orion A and B with the Large Area Telescope (LAT) on-board the Fermi Gamma-ray Space Telescope. The gamma-ray emission in the energy band between \sim100 MeV and \sim100 GeV is predicted to trace the gas mass distribution in the clouds through nuclear interactions between the Galactic cosmic rays (CRs) and interstellar gas. The gamma-ray production cross-section for the nuclear interaction is known to \sim10% precision which makes the LAT a powerful tool to measure the gas mass column density distribution of molecular clouds for a known CR intensity. We present here such distributions for Orion A and B, and correlate them with those of the velocity integrated CO intensity (WCO) at a 1° \times1° pixel level. The correlation is found to be linear over a WCO range of ~10 fold when divided in 3 regions, suggesting penetration of nuclear CRs to most of the cloud volumes. The Wco-to-mass conversion factor, Xco, is found to be \sim2.3\times10^20 cm-2(K km s-1)-1 for the high-longitude part of Orion A (l > 212°), \sim1.7 times higher than \sim1.3 \times 10^20 found for the rest of Orion A and B. We interpret the apparent high Xco in the high-longitude region of Orion A in the light of recent works proposing a non-linear relation between H2 and CO densities in the diffuse molecular gas. Wco decreases faster than the H2 column density in the region making the gas "darker" to Wco.

astro-ph.HE↗

arXiv2012

High resolution radio observations of gamma-ray emitting Narrow-Line Seyfert 1s

The detection by Fermi-LAT of gamma-ray emission from radio-loud Narrow-Line Seyfert 1s (NLS1s) indicates that relativistic jets do not form only in blazars and radio galaxies, but also in other AGN populations. Despite a spectral energy distribution similar to blazars, their physical characteristics are quite different: lower black hole masses, generally higher accretion rates, and possibly hosted in spirals. Furthermore, their radio properties make the interpretation of these objects even more puzzling. The radio emission is very compact, not exceeding the parsec scales, as also found in the population of young radio sources. We present high resolution VLBA observations of three radio-loud NLS1s detected by Fermi-LAT: SBS 0846+513, PKS 1502+036, and PKS 2004-447. The information on the pc-scale morphology will be complemented with studies of flux density and spectral variability from multi-epoch and multifrequency observations, in order to unveil the nature of their radio emission.

astro-ph.HE↗

arXiv2012

VLBA monitoring of Mrk 421 at 15 and 24 GHz during 2011

We present a preliminary analysis of new high resolution radio observations of the nearby TeV blazar Markarian 421 (z=0.031). This study is part of an ambitious multifrequency campaign, with observations in sub-mm (SMA), optical/IR (GASP), UV/X-ray (Swift, RXTE, MAXI), and gamma rays (Fermi-LAT, MAGIC, VERITAS). In this manuscript we consider only data obtained with the Very Long Baseline Array (VLBA) at seven epochs (one observation per month from January to July 2011) at 15 and 23.8 GHz. We investigate the inner jet structure on parsec scales through the study of model-fit components for each epoch. We identified 5-6 components which are consistent with being stationary during the 6-month period reported here. The aim is to try to shed light on questions such as the nature of radiating particles, the connection between radio and gamma-ray emission, the location of emitting regions and the origin of the flux variability.

astro-ph.HE↗

arXiv2012

Parsec-scale morphology and spectral index distribution in faint high frequency peakers

We investigate the parsec-scale structure of 17 high frequency peaking radio sources from the faint HFP sample. VLBA observations were carried out at two adjacent frequencies, 8.4 and 15.3 GHz, both in the optically-thin part of the spectrum, to obtain the spectral index information. We found that 64% of the sources are resolved into subcomponents, while 36% are unresolved even at the highest frequency. Among the resolved sources, 7 have a morphology and a spectral index distribution typical of young radio sources, while in other 4 sources, all optically associated with quasars, the radio properties resemble those of the blazar population. The equipartition magnetic field of the single components are a few tens milliGauss, similar to the values found in the hotspots of young sources with larger sizes. Such high magnetic fields cause severe radiative losses, precluding the formation of extended lobe structures emitting at centimeter wavelengths. The magnetic fields derived in the various components of individual source are usually very different, indicating a non self-similar source evolution, at least during the very first stages of the source growth.

astro-ph.CO↗

arXiv2012

Anisotropies in the diffuse gamma-ray background measured by the Fermi LAT

The contribution of unresolved sources to the diffuse gamma-ray background could induce anisotropies in this emission on small angular scales. We analyze the angular power spectrum of the diffuse emission measured by the Fermi LAT at Galactic latitudes |b| > 30 deg in four energy bins spanning 1 to 50 GeV. At multipoles \ell \ge 155, corresponding to angular scales \lesssim 2 deg, angular power above the photon noise level is detected at >99.99% CL in the 1-2 GeV, 2-5 GeV, and 5-10 GeV energy bins, and at >99% CL at 10-50 GeV. Within each energy bin the measured angular power takes approximately the same value at all multipoles \ell \ge 155, suggesting that it originates from the contribution of one or more unclustered source populations. The amplitude of the angular power normalized to the mean intensity in each energy bin is consistent with a constant value at all energies, C_P/ ^2 = 9.05 +/- 0.84 x 10^{-6} sr, while the energy dependence of C_P is consistent with the anisotropy arising from one or more source populations with power-law photon spectra with spectral index Γ_s = 2.40 +/- 0.07. We discuss the implications of the measured angular power for gamma-ray source populations that may provide a contribution to the diffuse gamma-ray background.

astro-ph.HE↗

arXiv2011

Young radio sources: the duty-cycle of the radio emission and prospects for gamma-ray emission

The evolutionary stage of a powerful radio source originated by an AGN is related to its linear size. In this context, compact symmetric objects (CSOs), which are powerful and intrinsically small objects, should represent the young stage in the individual radio source life. However, the fraction of young radio sources in flux density-limited samples is much higher than what expected from the number counts of large radio sources.This indicates that a significant fraction of young radio sources does not develop to the classical Fanaroff-Riley radio galaxies,suggesting an intermittent jet activity. As the radio jets are expanding within the dense and inhomogeneous interstellar medium,the ambient may play a role in the jet growth, for example slowing down or even disrupting its expansion when a jet-cloud interaction takes place. Moreover, this environment may provide the thermal seed photons that scattered by the lobes' electrons may be responsible for high energy emission, detectable by Fermi-LAT.

astro-ph.HE↗

arXiv2011

Complex particle acceleration processes in the hotspots of 3C105 and 3C445

We investigate the nature of the broad-band emission associated with the low-power radio hotspots 3C105 South and 3C445 South. Both hotspot regions are resolved in multiple radio/optical components. High-sensitivity radio VLA, NIR/optical VLT and HST, and X-ray Chandra data have been used to construct the multi-band spectra of individual hotspot components. The radio-to-optical spectra of both hotspot regions are well fitted by a synchrotron model with steep spectral indices ~0.8 and break frequencies 10^12-10^14 Hz. 3C105 South is resolved in two optical components: a primary one, aligned with the jet direction and possibly marking the first jet impact with the surrounding medium, and a secondary, further out from the jet and extended in a direction perpendicular to it. This secondary region is interpreted as a splatter-spot formed by the deflection of relativistic plasma from the primary hotspot. Radio and optical images of 3C445 South show a spectacular 10-kpc arc-shape structure characterized by two main components, and perpendicular to the jet direction. HST images in I and B bands further resolve the brightest components into thin elongated features. In both 3C105 South and 3C445 South the main hotspot components are enshrouded by diffuse optical emission on scale of several kpcs, indicating that very high energy particles, possibly injected at strong shocks, are continuously re-accelerated in situ by additional acceleration mechanisms. We suggest that stochastic processes, linked to turbulence and instabilities, could provide the required additional re-acceleration.

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