arXiv subjects

F. Massaro

Publications and source records attributed to F. Massaro.

At least 109 records · Page 6Linked to original sources

arXiv2011

An Experiment to Locate the Site of TeV Flaring in M87

We describe a Chandra X-ray ToO project designed to isolate the site of TeV flaring in the radio galaxy M87. To date, we have triggered the Chandra observations only once (2010 April) and by the time of the first of our 9 observations, the TeV flare had ended. However, we found that the X-ray intensity of the unresolved nucleus was at an elevated level for our first observation. Of the more than 60 Chandra observations we have made of the M87 jet covering 9 years, the nucleus was measured at a comparably high level only 3 times. Two of these occasions can be associated with TeV flaring, and at the time of the third event, there were no TeV monitoring activities. From the rapidity of the intensity drop of the nucleus, we infer that the size of the emitting region is of order a few light days times the unknown beaming factor; comparable to the same sort of estimate for the TeV emitting region. We also find evidence of spectral evolution in the X-ray band which seems consistent with radiative losses affecting the non-thermal population of the emitting electrons within the unresolved nucleus.

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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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arXiv2011

Large Scale Extragalactic Jets in the Chandra era I: data reduction and analysis

In this paper we report the first stages of an investigation into the X-ray properties of extragalactic jets. Our approach is to subject all sources for which X-ray emission has been detected by Chandra to uniform reduction procedures. Using Chandra archival data for 106 such sources, we measure X-ray fluxes in three bands and compare these to radio fluxes. We discuss the sample, the reduction methods, and present first results for the ratio of X-ray to radio flux for jet knots and hotspots. In particular, we apply statistical tests to various distributions of key observational parameters to evaluate differences between the different classes of sources. Subsequent papers will deal with various ramifications such as considerations of how well the observational data fulfill expectations of the different radiation processes proposed for the knots of FRI radio galaxies and quasars.

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arXiv2011

Spectral curvature behavior during X-ray flares in GRB afterglow emission

One of the most impressive recent discovery of SWIFT is the evidence that X-ray flares occurring during the GRB afterglows are quite common, being observed in roughly 50% of the afterglows. These X-ray flares range fluences comparable with the GRB prompt emission and could be also repetitive. Several pictures have been proposed on their origin and among them the most accepted regards the internal shock scenario, interpreting the X-ray flares as late time activity of the GRB central engine. We propose to describe the spectral shape of the X-ray flares adopting the same physical model recently used to interpret the GRB prompt emission: the log-parabolic function. In particular, we show that their spectral energy distribution (SED) is remarkably curved, while no significant curvature appears in the underlying X-ray afterglow emission. In addition, the log-parabolic function is statistically favored with respect to other proposed spectral models. By using a time resolved spectral analysis, we show the evolution of the peak energy and the curvature parameters of the SED during the X-ray flares in two of the brightest GRBs afterglows observed by SWIFT. We found that in the X-ray flares there is an anti-correlation between the peak energy and the curvature, as expected in a stochastic acceleration scenario.

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arXiv2011

Spectral evolution of long Gamma Ray Burst prompt emission: electrostatic acceleration and adiabatic expansion

Despite the great variation in the light curves of Gamma Ray Burst (GRB) prompt emission, their spectral energy distribution is generally curved and broadly peaked. In particular, their spectral evolution is well described by the hardness-intensity correlation during a single pulse decay phase, when the SED peak height S_p decreases as its peak energy E_p decreases. We propose an acceleration scenario, based on electrostatic acceleration, to interpret the E_p distribution peak at ~ 0.25 MeV. We show that during the decay phase of individual pulses in the long GRB light curve, the adiabatic expansion losses likely dominate the synchrotron cooling effects. The energy loss as due to adiabatic expansion can also be used to describe the spectral evolution observed during their decay phase. The spectral evolution predicted by our scenario is consistent with that observed in single pulses of long BATSE GRBs.

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arXiv2011

GRBs in the SWIFT and Fermi era: a new view of the prompt emission

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arXiv2011

Fueling lobes of radio galaxies: statistical particle acceleration and the extragalactic gamma-ray background

The recent discovery of the gamma-ray emission from the lobes of the closest radio galaxy Centaurus A by Fermi implies the presence of high-energy electrons at least up to gamma ~ 10^5 - 10^6. These high-energy electrons are required to interpret the observed gamma-ray radiation in terms of inverse Compton emission off the cosmic microwave background (IC/CMB); the widely accepted scenario to describe the X-ray emission of radio galaxy lobes. In this Letter, we consider the giant radio lobes of FR II radio galaxies showing that it is possible to maintain electrons at energies gamma ~ 10^5 - 10^6, assuming an acceleration scenario (driven by turbulent magnetic fields) that compensates the radiative losses. In addition, we consider the contribution to the diffuse extragalactic gamma-ray background due to the IC/CMB emission of FR IIs' lobes showing its relevance in the keV to MeV energy range.

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arXiv2010

VERITAS 2008 - 2009 monitoring of the variable gamma-ray source M87

M87 is a nearby radio galaxy that is detected at energies ranging from radio to VHE gamma-rays. Its proximity and its jet, misaligned from our line-of-sight, enable detailed morphological studies and extensive modeling at radio, optical, and X-ray energies. Flaring activity was observed at all energies, and multi-wavelength correlations would help clarify the origin of the VHE emission. In this paper, we describe a detailed temporal and spectral analysis of the VERITAS VHE gamma-ray observations of M87 in 2008 and 2009. In the 2008 observing season, VERITAS detected an excess with a statistical significance of 7.2 sigma from M87 during a joint multi-wavelength monitoring campaign conducted by three major VHE experiments along with the Chandra X-ray Observatory. In February 2008, VERITAS observed a VHE flare from M87 occurring over a 4-day timespan. The peak nightly flux above 250GeV was 7.7% of the Crab Nebula flux. M87 was marginally detected before this 4-day flare period, and was not detected afterwards. Spectral analysis of the VERITAS observations showed no significant change in the photon index between the flare and pre-flare states. Shortly after the VHE flare seen by VERITAS, the Chandra X-ray Observatory detected the flux from the core of M87 at a historical maximum, while the flux from the nearby knot HST-1 remained quiescent. Acciari et al. (2009) presented the 2008 contemporaneous VHE gamma-ray, Chandra X-ray, and VLBA radio observations which suggest the core as the most likely source of VHE emission, in contrast to the 2005 VHE flare that was simultaneous with an X-ray flare in the HST-1 knot. In 2009, VERITAS continued its monitoring of M87 and marginally detected a 4.2 sigma excess corresponding to a flux of ~1% of the Crab Nebula. No VHE flaring activity was observed in 2009.

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arXiv2010

Gamma Ray Bursts in the Fermi era: the spectral energy distribution of the prompt emission

Gamma Ray Bursts (GRBs) show evidence of different light curves, duration, afterglows, host galaxies and they explode within a wide redshift range. However, their spectral energy distributions (SEDs) appear to be very similar showing a curved shape. Band et al. (1993) proposed a phenomenological description of the integrated spectral shape for the GRB prompt emission, the so called Band function. In this letter we suggest an alternative scenario to explain the curved shape of GRB SEDs: the log-parabolic model. In comparison with the Band spectral shape our model is statistically favored because it fits the GRB spectra with one parameter less than the Band function and it is motivated by a theoretical acceleration scenario. The new Fermi observations of GRBs will be crucial to disentangle between these two models.

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arXiv2010

A Flare in the Jet of Pictor A

A Chandra X-ray imaging observation of the jet in Pictor A showed a feature that appears to be a flare that faded between 2000 and 2002. The feature was not detected in a follow-up observation in 2009. The jet itself is over 150 kpc long and a kpc wide, so finding year-long variability is surprising. Assuming a synchrotron origin of the observed high-energy photons and a minimum energy condition for the outflow, the synchrotron loss time of the X-ray emitting electrons is of order 1200 yr, which is much longer than the observed variability timescale. This leads to the possibility that the variable X-ray emission arises from a very small sub-volume of the jet, characterized by magnetic field that is substantially larger than the average over the jet.

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arXiv2010

Chandra Observations of 3C Radio Sources with z<0.3: Nuclei, Diffuse Emission, Jets and Hotspots

We report on our Chandra Cycle 9 program to observe half of the 60 (unobserved by Chandra) 3C radio sources at z<0.3 for 8 ksec each. Here we give the basic data: the X-ray intensity of the nuclei and any features associated with radio structures such as hot spots and knots in jets. We have measured fluxes in soft, medium and hard bands and are thus able to isolate sources with significant intrinsic column density. For the stronger nuclei, we have applied the standard spectral analysis which provides the best fit values of X-ray spectral index and column density. We find evidence for intrinsic absorption exceeding a column density of 10^{22} cm^{-2} for one third of our sources.

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arXiv2010

The Classification of Extragalactic X-ray Jets

The overall classification of X-ray jets has clung to that prevalent in the radio: FRI vs. FRII (including quasars). Indeed, the common perception is that X-ray emission from FRI's is synchrotron emission whereas that from FRII's may be IC/CMB and/or synchrotron. Now that we have a sizable collection of sources with detected X-ray emission from jets and hotspots, it seems that a more unbiased study of these objects could yield additional insights on jets and their X-ray emission. The current contribution is a first step in the process of analyzing all of the relevant parameters for each detected component for the sources collected in the XJET website. This initial effort involves measuring the ratio of X-ray to radio fluxes and evaluating correlations with other jet parameters. For single zone synchrotron X-ray emission, we anticipate that larger values of fx/fr should correlate inversely with the average magnetic field strength (if the acceleration process is limited by loss time equals acceleration time). Beamed IC/CMB X-rays should produce larger values of fx/fr for smaller values of the angle between the jet direction and the line of sight but will also be affected by the low frequency radio spectral index.

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arXiv2009

Fermi Large Area Telescope Gamma-Ray Detection of the Radio Galaxy M87

We report the Fermi-LAT discovery of high-energy (MeV/GeV) gamma-ray emission positionally consistent with the center of the radio galaxy M87, at a source significance of over 10 sigma in ten-months of all-sky survey data. Following the detections of Cen A and Per A, this makes M87 the third radio galaxy seen with the LAT. The faint point-like gamma-ray source has a >100 MeV flux of 2.45 (+/- 0.63) x 10^-8 ph cm^-2 s^-1 (photon index = 2.26 +/- 0.13) with no significant variability detected within the LAT observation. This flux is comparable with the previous EGRET upper limit (< 2.18 x 10^-8 ph cm^-2 s^-1, 2 sigma), thus there is no evidence for a significant MeV/GeV flare on decade timescales. Contemporaneous Chandra and VLBA data indicate low activity in the unresolved X-ray and radio core relative to previous observations, suggesting M87 is in a quiescent overall level over the first year of Fermi-LAT observations. The LAT gamma-ray spectrum is modeled as synchrotron self-Compton (SSC) emission from the electron population producing the radio-to-X-ray emission in the core. The resultant SSC spectrum extrapolates smoothly from the LAT band to the historical-minimum TeV emission. Alternative models for the core and possible contributions from the kiloparsec-scale jet in M87 are considered, and can not be excluded.

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arXiv2009

X-ray emission from the extended emission-line region of the powerful radio galaxy 3C171

We present Chandra X-ray observations of the powerful radio galaxy 3C171, which reveal an extended region of X-ray emission spatially associated with the well-known 10-kpc scale optical emission-line region around the radio jets. We argue that the X-ray emission comes from collisionally ionized material, originally cold gas that has been shock-heated by the passage of the radio jet, rather than being photoionized by nuclear radiation. This hot plasma is also responsible for the depolarization at low frequencies of the radio emission from the jet and hotspots, which allows us to estimate the magnetic field strength in the external medium. We show that it is likely that both the cold emission-line gas and the hot plasma in which it is embedded are being driven out of the host galaxy of 3C171 at supersonic speeds. A significant fraction of the total energy budget of the central AGN must have been expended in driving this massive outflow. We argue that 3C171, with its unusual radio morphology and the strong relation between the jet and large amounts of outflowing material, is a member of a class of radio galaxies in which there is strong interaction between the radio jets and cold material in the host galaxy; such objects may have been very much more common in the early universe.

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arXiv2009

SSC radiation in BL Lac sources, the end of the tether

The synchrotron-self Compton (SSC) radiation process is widely held to provide a close representation of the double peaked spectral energy distributions from BL Lac Objects (BL Lacs), which are marked by non-thermal beamed radiations, highly variable on timescales of days or less. Their outbursts in the gamma ray relative to the optical/X rays might be surmised to be enhanced in BL Lacs as these photons are upscattered via the inverse Compton (IC) process. From the observed correlations among the spectral parameters during optical/X-ray variations we aim at predicting corresponding correlations in the gamma-ray band, and the actual relations between the gamma-ray and the X-ray variability consistent with the SSC emission process. We start from the homogeneous single-zone SSC source model, with log-parabolic energies distributions of emitting electron as required by the X-ray data of many sources. We find relations among spectral parameters of the IC radiation in both the Thomson (for Low energy BL Lacs) and the Klein-Nishina regimes (mainly for High energy BL Lacs) and we compute how variability is driven by a smooth increase of key source parameters, primarily the root mean square electron energy. The single component SSC source model in the Thomson regime turns out to be adequate for many LBL sources. However, the simple model meets its limits with the fast/strong flares recently reported for a few sources in the TeV range; these require sudden accelerations of emitting electrons in a second source component.

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arXiv2009

The Jet of 3C 17 and the Use of Jet Curvature as a Diagnostic of the X-ray Emission Process

We report on the X-ray emission from the radio jet of 3C 17 from Chandra observations and compare the X-ray emission with radio maps from the VLA archive and with the optical-IR archival images from the Hubble Space Telescope. X-ray detections of two knots in the 3C 17 jet are found and both of these features have optical counterparts. We derive the spectral energy distribution for the knots in the jet and give source parameters required for the various X-ray emission models, finding that both IC/CMB and synchrotron are viable to explain the high energy emission. A curious optical feature (with no radio or X-ray counterparts) possibly associated with the 3C 17 jet is described. We also discuss the use of curved jets for the problem of identifying inverse Compton X-ray emission via scattering on CMB photons.

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arXiv2009

Extended X-ray emission in radio galaxies: the peculiar case of 3C 305

Extended X-ray structures are common in Active Galactic Nuclei (AGNs). Here we present the first case of a Compact Steep Spectrum (CSS) radio galaxy, 3C 305, in which the X-ray radiation appears to be associated with the optical emission line region, dominated by the [O III]5007. On the basis of a morphological study, performed using the comparison between the X-rays, the optical and the radio band, we argue that the high energy emission has a thermal nature and it is not directly linked to the radio jet and hotspots of this source. Finally, we discuss the origin of the extended X-ray structure connected with the optical emission line region following two different interpretations: as due to the interaction between matter outflows and shock-heated environment gas, or as due to gas photoionized by nuclear emission.

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arXiv2009

Swift observations of IBL and LBL objects

BL Lacs are an enigmatic class of active galactic nuclei (AGNs), characterized by the non-thermal continuum typically attributed to synchrotron and inverse Compton emission. Depending on the frequency location of the maxima of these components, they are subdivided into three subclasses LBLs, IBLs, and HBLs. We present the results of a set of observations of eight BL Lac objects of LBL and IBL type performed by the XRT and UVOT detectors onboard the Swift satellite between January 2005 and November 2006. We are mainly interested in measuring the spectral parameters, and particularly the steepness between the UV and the X-ray band, useful for determining the classification of these sources. We compare the behavior of these sources with previous XMM-Newton, BeppoSAX obser- vations and with historical data in the X-ray and in the optical band. We are also interested in classifying the sources in our sample on the basis of the observations and comparing them with their classification presented in literature. We performed X-ray spectral analysis of observed BL Lac objects using a simple powerlaw and in a few cases the log-parabolic model. We also combined the UV emission with the low energy X-ray data to We used observational data to classify sources in our sample and derived parameters of their spectral energy distribution. We found that for the IBLs X-rays low states show features of the high energy component, usually interpreted as due to inverse Compton emission. Sources in our sample exhibit a range of temporal UV and X-ray behaviors, some objects having clear and neat correlated UV and X-ray variations (e.g. ON231) and other objects showing no clear (e.g. AO 0235+164) UV and X-ray correlation. Finally, we also note that our estimates of spectral curvature are in the range of that measured for the HBLs.

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