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[show abstract]
[hide abstract]
ABSTRACT: Recently, a new method to constrain the distance of blazars with unknown
redshift using combined observations in the GeV and TeV regimes has been
developed, with the underlying assumption that the Very High Energy (VHE)
spectrum corrected for the absorption of TeV photons by the Extragalactic
Background Light (EBL) via photon-photon interaction should still be softer
than the gamma-ray spectrum observed by Fermi/LAT. The constraints found are
related to the real redshifts by a simple linear relation, that has been used
to infer the unknown distance of blazars. The sample will be revised with the
up-to-date spectra in both TeV and GeV bands, the method tested with the more
recent EBL models and finally applied to the unknown distance blazars detected
at VHE.
01/2011;
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[show abstract]
[hide abstract]
ABSTRACT: A new method to constrain the distance of blazars with unknown redshift using
combined observations in the GeV and TeV regimes will be presented. The
underlying assumption is that the Very High Energy (VHE) spectrum corrected for
the absorption of TeV photons by the Extragalactic Background Light (EBL) via
photon-photon interaction should still be softer than the extrapolation of the
gamma-ray spectrum observed by Fermi/LAT. Starting from the observed spectral
data at VHE, the EBL-corrected spectra are derived as a function of the
redshift z and fitted with power laws. Comparing the redshift dependent VHE
slopes with the power law fits to the LAT data an upper limit to the source
redshift can be derived. The method is applied to all TeV blazars detected by
LAT with known distance and an empirical law describing the relation between
the upper limits and the true redshifts is derived. This law can be used to
estimate the distance of unknown redshift blazars: as an example, the distance
of PKS 1424+240 is inferred.
01/2011;
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Megan Urry,
Aldo Treves, Laura Maraschi,
L. Marshall,
Tsuneo Kii,
Greg Madejski,
Steve Penton,
E. Pesce,
Elena Pian,
Annalisa Celotti,
R. Fujimoto,
F. Makino,
C. Otani,
M. Sambruna,
K. Sasaki,
M. Shull,
S. Smith,
T. Takahashi,
and M. Tashiro
[show abstract]
[hide abstract]
ABSTRACT: In 1994 May, the BL Lac object PKS 2155-304 was observed continuously for ~10 days with the International Ultraviolet Explorer and the Extreme Ultraviolet Explorer and for 2 days with ASCA, as well as with ROSAT and with ground-based radio, infrared, and optical telescopes. The light curves show a well-defined X-ray flare followed by a broader, lower amplitude extreme-ultraviolet flare ~1 day later and a broad, low-amplitude UV flare ~2 days later. X-ray fluxes obtained at three well-separated times the preceding week indicate at least one previous flare of comparable amplitude or perhaps ongoing stochastic X-ray variations, and additional rapid variability was seen at the beginning of the IUE observation, when extremely sharp changes in UV flux occurred. The X-ray flux observed with ASCA flared by a factor of ~2 in about half a day and decayed roughly as fast. In contrast, the subsequent UV flare had an amplitude of only ~35% and lasted longer than 2 days. Assuming that the X-ray, EUV, and UV events are associated, the lags, the decrease of amplitude with wavelength, and the broadening of the temporal profile with wavelength are all qualitatively as expected for synchrotron emission from an inhomogeneous, relativistic jet. Because of the high quality of the data, we can rule out that the observed flares were caused by either a Fermi-type shock acceleration event or a pair cascade in a homogeneous synchrotron-emitting region. A homogeneous region is still possible if there was an instantaneous (t hours) injection of high-energy electrons that emit first at X-ray energies. Alternatively, the data are consistent with a compression wave or other disturbance crossing a region with stratified particle energy distributions. This kind of situation is expected to occur behind a shock front and/or in an inhomogeneous jet. The present light curves are in sharp contrast to the multiwavelength variability observed in 1991 November, when the amplitude was wavelength independent and the UV lagged the X-rays by less than ~3 hr. This means that the origin of rapid multiwavelength variability in this blazar is complex, involving at least two different modes.
The Astrophysical Journal 01/2009; 486(2):799. · 6.02 Impact Factor
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Elena Pian,
Megan Urry,
Aldo Treves, Laura Maraschi,
Steve Penton,
Michael Shull,
E. Pesce,
Paola Grandi,
Tsuneo Kii,
I. Kollgaard,
Greg Madejski,
L. Marshall,
Willem Wamsteker,
Annalisa Celotti,
J.-L. Courvoisier,
Renato Falomo,
H. Fink,
M. George,
and Gabriele Ghisellini
[show abstract]
[hide abstract]
ABSTRACT: PKS 2155-304, the brightest BL Lac object in the ultraviolet sky, was monitored with the International Ultraviolet Explorer satellite at ~1 hr time resolution for 10 nearly uninterrupted days in 1994 May. The campaign, which was coordinated with Extreme Ultraviolet Explorer, ROSAT, and ASCA monitoring, along with optical and radio observations from the ground, yielded the largest set of spectra and the richest short-timescale variability information ever gathered for a blazar at UV wavelengths. The source flared dramatically during the first day, with an increase by a factor of ~2.2 in an hour and a half. In subsequent days, the flux maintained a nearly constant level for ~5 days, then flared with ~35% amplitude for 2 days. The same variability was seen in both short- and long-wavelength IUE light curves, with zero formal lag (2 hr), except during the rapid initial flare, when the variations were not resolved. Spectral index variations were small and not clearly correlated with flux. The flux variability observed in the present monitoring is so rapid that, for the first time, based on the UV emission alone, the traditional ΔL/Δt limit indicating relativistic beaming is exceeded. The most rapid variations, under the likely assumption of synchrotron radiation, lead to a lower limit of 1 G on the magnetic field strength in the UV-emitting region. These results are compared with earlier intensive monitoring of PKS 2155-304 with IUE in 1991 November, when the UV flux variations had completely different characteristics.
The Astrophysical Journal 01/2009; 486(2):784. · 6.02 Impact Factor
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Elena Pian,
Giuseppe Vacanti,
Gianpiero Tagliaferri,
Gabriele Ghisellini, Laura Maraschi,
Aldo Treves,
C. Megan Urry,
Fabrizio Fiore,
Paolo Giommi,
Eliana Palazzi,
Lucio Chiappetti,
and Rita M. Sambruna
[show abstract]
[hide abstract]
ABSTRACT: The BL Lacertae object Markarian 501, one of only three extragalactic sources (with Mrk 421 and 1ES 2344+514) so far detected at TeV energies, was observed with the BeppoSAX satellite in 1997 April 7, 11, and 16 during a phase of high activity at TeV energies, as monitored with the Whipple, HEGRA, and CAT Cherenkov telescopes. Over the whole 0.1-200 keV range, the spectrum was exceptionally hard (α≤1, with Fν ν−α), indicating that the X-ray power output peaked at (or above) ~100 keV. This represents a shift of at least 2 orders of magnitude with respect to previous observations of Mrk 501, a behavior never seen before in this or any other blazar. The overall X-ray spectrum hardens with increasing intensity, and at each epoch it is softer at larger energies. The correlated variability from soft X-rays to the TeV band points to models in which the same population of relativistic electrons produces the X-ray continuum via synchrotron radiation and the TeV emission by inverse Compton scattering of the synchrotron photons or other seed photons. For the first time in any blazar, the synchrotron power is observed to peak at hard X-ray energies. The large shift of the synchrotron peak frequency with respect to previous observations of Mrk 501 implies that intrinsic changes in the relativistic electron spectrum caused the increase in emitted power. Due to the very high electron energies, the inverse Compton process is limited by the Klein-Nishina regime. This implies a quasi-linear (as opposed to quadratic) relation of the variability amplitude in the TeV and hard X-ray ranges (for the synchrotron self-Compton model) and an increase of the inverse Compton peak frequency smaller than that of the synchrotron peak frequency.
The Astrophysical Journal 01/2009; 492(1):L17. · 6.02 Impact Factor
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[show abstract]
[hide abstract]
ABSTRACT: The XMM-Newton satellite has revealed extended X-ray emission from the eastern radio lobe of the Fanaroff-Riley II radio galaxy Pictor A. The X-ray spectrum, accumulated on a region covering about half the entire radio lobe, is well described by both a thermal (kT = 5 keV) model and a power law with an energy index αX = 0.6 ± 0.2. The X-ray emission could be thermal and produced by circumgalactic gas shocked by the expanding radio lobe or, alternatively, by inverse Compton (IC) scattering of cosmic microwave background photons by relativistic electrons in the lobe. The latter possibility seems to be supported by the good agreement between the lobe-averaged synchrotron radio index (αradio = 0.8) and the X-ray energy slope αX. However, if this is the case, the magnetic field (BIC ~ 1-2 μG), as deduced from the comparison of the IC X-ray and radio fluxes, is more than a factor of 2 below the equipartition value estimated in the same X-ray region.
The Astrophysical Journal 12/2008; 586(1):123. · 6.02 Impact Factor
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Fabrizio Nicastro,
Luigi Piro,
Alessandra De Rosa,
Marco Feroci,
Paola Grandi,
Fabrizio Fiore,
Martin Elvis,
Francesco Haardt,
Jelle Kaastra,
Angela Malizia, Laura Maraschi,
Giorgio Matt,
G. Cesare Perola,
and Pierre Olivier Petrucci
[show abstract]
[hide abstract]
ABSTRACT: NGC 5548 was observed by BeppoSAX in a single long (8 day) observation from 0.2 to 200 keV. We find (1) spectral variation of the source produced by a change of the intrinsic power-law slope; (2) a high-energy cutoff at Ec = 115 keV with a hint of change of Ec with flux; and (3) O VII and O VIII absorption K edges and a possible blended O VII-O VIII Kα,β emission feature at 0.54 keV, inconsistent with a purely photoionized gas in equilibrium. We propose that the temperature of the absorbing and emitting gas is ~106 K so that both collisional ionization and photoionization contribute.
The Astrophysical Journal 12/2008; 536(2):718. · 6.02 Impact Factor
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[show abstract]
[hide abstract]
ABSTRACT: We constrain the mean kinetic efficiency of radio-loud active galactic nuclei by using an optically selected sample for which both the optical and the radio luminosity functions (LFs) have been determined; the former traces the bolometric luminosity L, while the latter traces the kinetic power L_k, empirically correlated to the radio emission. Thus in terms of the ratio g_k=L_k/L, we can convert the optical LF of the sample into a radio one. This computed LF is shown to match the directly observed LF for the same sample if g_k=0.10^{+0.05}_{-0.01} holds, with a scatter \sigma=0.38^{+0.04}_{-0.09} dex; with these values we also match a number of independent correlations between L_k, L and radio emission, that we derive through Monte Carlo simulations. We proceed to translate the value of g_k into a constraint on the kinetic efficiency for the production of radio jets or winds, namely, \epsilon_k=L_k/(Mdot*c^2)~0.01 in terms of the rate Mdot of mass accretion onto the central black hole. Then, on assuming that on average the radio sources share the same kinetic efficiency, we compute a solid lower limit of about 25% on the contribution of radio sources to the local black hole mass density. Comment: ApJ, accepted, 7 pages, 6 figures
12/2007;
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[show abstract]
[hide abstract]
ABSTRACT: We present temporal and spectral analysis of the X-ray bright blazar PKS 2155-304 observed with BeppoSAX. Power density spectra show strong red noise feature with steep slope of
~ 2-3\sim 2-3
. Structure functions suggest typical timescale of
~ 0.5\sim 0.5
days. Inter-band time lags differ from flare to flare. Peak energies of synchrotron component increase with increasing flux,
and complexities of spectral evolution are detected. The implications of our results are discussed in the context of synchrotron
cooling model of relativistic electrons accelerated through internal shocks taking place in the jet.
02/2006: pages 304-306;
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[show abstract]
[hide abstract]
ABSTRACT: A synthesis of the present knowledge on Gamma‐ray blazars is presented, focusing on the best studied “protoypical” objects. Basic results and concepts are introduced following a historical path. Multifrequency correlations and spectral variability are discussed as probes of our understanding of physical processes in the emission regions of relativistic jets. Finally an overall scheme for unification of GeV and TeV blazars is described. © 2005 American Institute of Physics
AIP Conference Proceedings. 02/2005; 745(1):129-139.
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[show abstract]
[hide abstract]
ABSTRACT: ▪ Abstract A large collective effort to study the variability of active galactic nuclei (AGN) over the past decade has led to a number of fundamental results on radio-quiet AGN and blazars. In radio-quiet AGN, the ultraviolet (UV) bump in low-luminosity objects is thermal emission from a dense medium, very probably an accretion disk, irradiated by the variable X-ray source. The validity of this model for high-luminosity radio-quiet AGN is unclear because the relevant UV and X-ray observations are lacking. The broad-line gas kinematics appears to be dominated by virialized motions in the gravity field of a black hole, whose mass can be derived from the observed motions. The “accretion disk plus wind” model explains most of the variability (and other) data and appears to be the most appropriate model at present. Future investigations are outlined. In blazars, rapid variability at the highest energies (gamma-rays) implies that the whole continuum is relativistically boosted along the line of sight. The gener...
11/2003; 35:445-502.
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[show abstract]
[hide abstract]
ABSTRACT: The XMM-Newton satellite has revealed extended X-ray emission from the eastern radio lobe of the Fanaroff-Riley II Radio Galaxy Pictor A. The X-ray spectrum, accumulated on a region covering about half the entire radio lobe, is well described by both a thermal model and a power law. The X-ray emission could be thermal and produced by circum-galactic gas shocked by the expanding radio lobe or, alternatively, by Inverse Compton (IC) of cosmic microwave background photons by relativistic electrons in the lobe. The latter possibility seems to be supported by the good agreement between the lobe-average synchrotron radio index and the X-ray energy slope. However, if this is the case, the magnetic field, as deduced from the comparison of the IC X-ray and radio fluxes, is more than a factor 2 below the equipartition value estimated in the same X-ray region. Comment: 8 pages, 5 figures; accepted for publication in ApJ
11/2002;
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[show abstract]
[hide abstract]
ABSTRACT: We discuss the relation between the power carried by relativistic jets and the nuclear power provided by accretion, for a group of blazars including FSRQs and BL Lac objects. They are characterized by good quality broad band X-ray data provided by the Beppo SAX satellite. The jet powers are estimated using physical parameters determined from uniformly modelling their spectral energy distributions (SEDs). Our analysis indicates that for Flat Spectrum Radio Quasars the total jet power is of the same order as the accretion power. We suggest that blazar jets are likely powered by energy extraction from a rapidly spinning black hole through the magnetic field provided by the accretion disk. FSRQs must have large BH masses (10^8 - 10^9 solar masses) and high, near Eddington accretion rates. For BL Lac objects the jet luminosity is larger than the disk luminosity. This can be understood within the same scenario if BL Lac objects have masses similar to FSRQ but accrete at largely subcritical rates, whereby the accretion disk radiates inefficiently. Thus the ``unification'' of the two classes into a single blazar population, previously proposed on the basis of a spectral sequence governed by luminosity, finds a physical basis. Comment: 27 pages, 3 figures, ApJ in press
05/2002;
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Laura Maraschi
[show abstract]
[hide abstract]
ABSTRACT: Advances in the capabilities of X-ray, gamma-ray and TeV telescopes have brought new information on the physics of relativistic jets, which are responsible for the blazar "phenomenon". In particular the broad band sensitivity of the BeppoSAX satellite, extending up to 100 KeV has allowed unprecedented studies of their hard X-ray spectra. I summarize here some basic results and present a unified view of the blazar population, whereby all sources contain essentially similar jets despite diversities in other properties, like the presence or absence of emission lines in their optical spectra. Blazars with emission lines are of particular interest in that it is possible to estimate both the luminosity of the jet and the luminosity of the accretion disk. Implications for the origin of the power carried by relativistic jets, possibly involving rapidly spinning supermassive black holes are discussed. We suggest that emission line blazars are accreting at near critical rates, while BL lacs, where emission lines are weak or absent are highly subcritical. Comment: 11 pages, 7 figures, presented at the 20th Texas Symposium, 10-15 Dec. 2000
07/2001;
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[show abstract]
[hide abstract]
ABSTRACT: We present a detailed X-ray study of the Broad Line Radio Galaxy 3C382, observed with the BeppoSAX satellite in a very bright state. The continuum emission is well modeled with a power law that steepens at high energies, with an e-folding energy of about 120 keV. At soft energies a clear excess of emission is detected, which can not be explained solely by the extended thermal halo seen in a ROSAT HRI image. A second, more intense soft X-ray component, possibly related to an accretion disk, is required by the data. Both a reflection component (R=0.3) and an iron line (EW \sim 50) are detected, at levels much weaker than in Seyfert galaxies, suggesting a common origin. Combining our measurements with results from the literature we find that the iron line has remained approximately constant over 9 years while the continuum varied by a factor of 5. Thus the fluorescent gas does not respond promptly to the variations of the X-ray primary source, suggesting that the reprocessing site is located away, likely at parsec distances. While the continuum shape indicates that X-rays derive from a thermal Comptonization process, the weakness of other spectral features implies that either the upper layers of the optically thick accretion disk are completely ionized or the corona above the disk is outflowing with mildly relativistic velocity. Comment: 11 pages, 4 figures. Accepted for publication in the Astrophysical Journal
03/2001;
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[show abstract]
[hide abstract]
ABSTRACT: We briefly review BeppoSAX observations of X-ray bright radio-galaxies. Their X-ray spectra are quite varied, and perhaps surprisingly, any similarity between radio-loud AGN and Seyfert galaxies is the exception rather than the rule. When detected, reprocessing features (iron line and reflection) are generally weak, suggesting two possible scenarios: either: (1) non-thermal (jet?) radiation dilutes the X-ray emission from the disk in radio-loud objects, or (2) the solid angle subtended by the X-ray reprocessing material is smaller in radio-loud than in radio-quiet AGN due to different characteristics of the accretion disk itself. Comment: 6 pages, to appear in `Life Cycles of Radio Galaxies', ed. J. Biretta et al., New Astronomy Reviews
04/2000;
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[show abstract]
[hide abstract]
ABSTRACT: The contribution of an astrophysics group based in Milan to the science with the IUE satellite during its almost 20 years lifetime has focussed on high energy sources, of both galactic (LMXRB, HMXRB, and black hole candidates) and extragalactic (AGN) nature. The results of this long term research and in particular of the latest multiwavelength campaigns conducted simultaneously with IUE are reviewed here.
12/1998;
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[show abstract]
[hide abstract]
ABSTRACT: Only ~10 blazars have been detected in soft gamma-rays by OSSE and the PDS, often in flaring state. We investigate the impact of the imaging and spectral capabilities of the IBIS instrument on board INTEGRAL on the study of blazars. The objectives are: 1) to remove ambiguities in the identification of some blazars as soft gamma-ray emitters; 2) to determine the spectral shape of "extreme synchrotron" blazars, namely sources with high F_X/F_radio ratios, for which the synchrotron component peaks at far UV-soft X-ray frequencies. We expect that in these sources the synchrotron peak shifts to higher energies during outbursts, making them strong soft gamma-ray emitters, and therefore possible targets for INTEGRAL wide band X- and gamma-ray spectroscopy.
12/1998;
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Elena Pian,
Eliana Palazzi,
Lucio Chiappetti, Laura Maraschi,
Fabrizio Tavecchio,
Gabriele Ghisellini,
Gianpiero Tagliaferri,
Giovanni Fossati,
Aldo Treves,
Claudia Megan Urry,
Giuseppe Vacanti
[show abstract]
[hide abstract]
ABSTRACT: The BL Lac object Mkn 501 was observed with the BeppoSAX satellite at three epochs in April-May 1998, simultaneously with the Whipple and HEGRA Cherenkov telescopes. The X-ray spectrum is well detected up to 70 keV and it exhibits, at all epochs, a continuous curvature, which is here modeled with three power-laws of increasingly steeper index at larger energies. In the nu*f_nu representation the spectrum exhibits a peak at ~20 keV, which is interpreted as the maximum of the synchrotron emission. This implies that the synchrotron peak energy has lowered by an order of magnitude with respect to the powerful X-ray outburst observed in April 1997. The simultaneous TeV flux was comparable to the lowest levels observed for Mkn 501, possibly suggesting that the peak of the inverse Compton radiation had also shifted toward lower energies.
11/1998;
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Elena Pian,
C. Megan Urry,
Aldo Treves, Laura Maraschi,
Steve Penton,
J. Michael Shull,
Joseph E. Pesce,
Paola Grandi,
Tsuneo Kii,
Ron I. Kollgaard,
Greg Madejski,
Herman Marshall,
Willem Wamsteker,
Annalisa Celotti,
Thierry J. -L. Courvoisier,
Renato Falomo,
Henner H. Fink,
Ian M. George,
Gabriele Ghisellini
[show abstract]
[hide abstract]
ABSTRACT: PKS 2155-304, the brightest BL Lac object in the ultraviolet sky, was monitored with the IUE satellite at ~1 hour time-resolution for ten nearly uninterrupted days in May 1994. The campaign, which was coordinated with EUVE, ROSAT, and ASCA monitoring, along with optical and radio observations from the ground, yielded the largest set of spectra and the richest short time scale variability information ever gathered for a blazar at UV wavelengths. The source flared dramatically during the first day, with an increase by a factor ~2.2 in an hour and a half. In subsequent days, the flux maintained a nearly constant level for ~5 days, then flared with ~35% amplitude for two days. The same variability was seen in both short- and long-wavelength IUE light curves, with zero formal lag (~<2 hr), except during the rapid initial flare, when the variations were not resolved. Spectral index variations were small and not clearly correlated with flux. The flux variability observed in the present monitoring is so rapid that for the first time, based on the UV emission alone, the traditional Delta L/Delta t limit indicating relativistic beaming is exceeded. The most rapid variations, under the likely assumption of synchrotron radiation, lead to a lower limit of 1 G on the magnetic field strength in the UV emitting region. These results are compared with earlier intensive monitoring of PKS 2155-304 with IUE in November 1991, when the UV flux variations had completely different characteristics. Comment: 45 pages, Latex, 11 PostScript figures, to appear in The Astrophysical Journal
06/1997;
