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ABSTRACT: Black hole masses for samples of active galactic nuclei (AGN) are currently
estimated from single-epoch optical spectra using scaling relations anchored in
reverberation mapping results. In particular, the two quantities needed for
calculating black hole masses, namely, the velocity and the radial distance of
the orbiting gas are derived from the widths of the Balmer hydrogen broad
emission lines and the optical continuum luminosity, respectively. We have
recently presented a near-infrared (near-IR) relationship for estimating AGN
black hole masses based on the widths of the Paschen hydrogen broad emission
lines and the total 1 micron continuum luminosity. The near-IR offers several
advantages over the optical: it suffers less from dust extinction, the AGN
continuum is observed only weakly contaminated by the host galaxy and the
strongest Paschen broad emission lines Pa alpha and Pa beta are unblended. Here
we improve the calibration of the near-IR black hole mass relationship by
increasing the sample from 14 to 23 reverberation-mapped AGN using additional
spectroscopy obtained with the Gemini Near-Infrared Spectrograph (GNIRS). The
additional sample improves the number statistics in particular at the high
luminosity end.
03/2013;
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ABSTRACT: We demonstrate a new technique for determining the physical conditions of the
broad line emitting gas in quasars, using near-infrared hydrogen emission
lines. Unlike higher ionisation species, hydrogen is an efficient line emitter
for a very wide range of photoionisation conditions, and the observed line
ratios depend strongly on the density and photoionisation state of the gas
present. A locally optimally emitting cloud model of the broad emission line
region was compared to measured emission lines of four nearby ($z\approx0.2$)
quasars that have optical and NIR spectra of sufficient signal-to-noise to
measure their Paschen lines. The model provides a good fit to three of the
objects, and a fair fit to the fourth object, a ULIRG. We find that low
incident ionising fluxes ($\phih<10^{18}$\cmsqs), and high gas densities
($\nh>10^{12}$\cmcu) are required to reproduce the observed hydrogen emission
line ratios. This analysis demonstrates that the use of composite spectra in
photoionisation modelling is inappropriate; models must be fitted to the
individual spectra of quasars.
06/2012;
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Hermine Landt
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ABSTRACT: The large majority of sources detected by the Fermi Gamma-ray Space Telescope
are blazars, belonging in particular to the blazar subclass of BL Lacertae
objects (BL Lacs). BL Lacs often have featureless optical spectra, which make
it difficult and sometimes impossible to determine their redshifts. This
presents a severe impediment for the use of BL Lacs to measure the spectrum of
the extragalactic background light through its interaction with high-energy
gamma-ray photons. I present here high-quality optical spectroscopy of two of
the brightest gamma-ray blazars, namely, PKS 0447-439 and PMN J0630-24. The
medium-resolution and high signal-to-noise ratio optical spectra show clear
absorption lines, which place these BL Lacs at relatively high redshifts of
z>=1.246 for PKS 0447-439 and z>=1.238 for PMN J0630-24.
03/2012;
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ABSTRACT: Black hole masses for samples of active galactic nuclei (AGN) are currently
estimated from single-epoch optical spectra. In particular, the size of the
broad-line emitting region needed to compute the black hole mass is derived
from the optical or ultraviolet continuum luminosity. Here we consider the
relationship between the broad-line region size, R, and the near-infrared
(near-IR) AGN continuum luminosity, L, as the near-IR continuum suffers less
dust extinction than at shorter wavelengths and the prospects for separating
the AGN continuum from host-galaxy starlight are better in the near-IR than in
the optical. For a relationship of the form R propto L^alpha, we obtain for a
sample of 14 reverberation-mapped AGN a best-fit slope of alpha=0.5+/-0.1,
which is consistent with the slope of the relationship in the optical band and
with the value of 0.5 naively expected from photoionisation theory. Black hole
masses can then be estimated from the near-IR virial product, which is
calculated using the strong and unblended Paschen broad emission lines (Pa
alpha or Pa beta).
03/2011;
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ABSTRACT: We use quasi-simultaneous near-infrared (near-IR) and optical spectroscopy
from four observing runs to study the continuum around 1 micron in 23
well-known broad-emission line active galactic nuclei (AGN). We show that,
after correcting the optical spectra for host galaxy light, the AGN continuum
around this wavelength can be approximated by the sum of mainly two emission
components, a hot dust blackbody and an accretion disc. The accretion disc
spectrum appears to dominate the flux at ~1 micron, which allows us to derive a
relation for estimating AGN black hole masses based on the near-IR virial
product. This result also means that a near-IR reverberation programme can
determine the AGN state independent of simultaneous optical spectroscopy. On
average we derive hot dust blackbody temperatures of ~1400 K, a value close to
the sublimation temperature of silicate dust grains, and relatively low hot
dust covering factors of ~7%. Our preliminary variability studies indicate that
in most sources the hot dust emission responds to changes in the accretion disc
flux with the expected time lag, however, a few sources show a behaviour that
can be attributed to dust destruction.
01/2011;
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ABSTRACT: We use photometric and spectroscopic infrared observations obtained with the Spitzer Space Telescope of 12 radio-loud active galactic nuclei (AGN) to investigate the dust geometry. Our approach is to look at the change of the infrared spectral energy distribution (SED) and the strength of the 10 micron silicate feature with jet viewing angle. We find that (i) a combination of three or four blackbodies fits well the infrared SED; (ii) the sources viewed closer to the jet axis appear to have stronger warm (~300 - 800 K) and cold (~150 - 250 K) dust emissions relative to the hot component; and (iii) the silicate features are always in emission and strongly redshifted. We test clumpy torus models and find that (i) they approximate well the mid-infrared part of the SED, but significantly underpredict the fluxes at both near- and far-infrared wavelengths; (ii) they can constrain the dust composition (in our case to that of the standard interstellar medium); (iii) they require relatively large (~10%-20% the speed of light) redward displacements; and (iv) they give robust total mass estimates, but are insensitive to the assumed geometry. Comment: 17 pages, 12 figures, accepted by MNRAS
07/2010;
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ABSTRACT: X-shaped radio galaxies are defined by their peculiar large-scale radio morphology. In addition to the classical double-lobed structure they have a pair of low-luminosity wings that straddles the nucleus at almost right angles to the active lobes, thus giving the impression of an 'X'. In this paper we study for the first time the optical spectral properties of this object class using a large sample (~50 sources). We find that the X-shaped radio population is composed roughly equally of sources with weak and strong emission line spectra, which makes them, in combination with the well-known fact that they preferentially have radio powers intermediate between those of Fanaroff-Riley type I (FR I) and type II (FR II) radio galaxies, the archetypal transition population. We do not find evidence in support of the proposition that the X-shape is the result of a recent merger: X-shaped radio sources do not have unusually broad emission lines, their nuclear environments are in general not dusty, and their host galaxies do not show signs of enhanced star formation. Instead, we observe that the nuclear regions of X-shaped radio sources have relatively high temperatures. This finding favours models, which propose that the X-shape is the result of an overpressured environment. Comment: 12 pages, 8 figures, accepted by MNRAS
06/2010;
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ABSTRACT: We present high-quality (high signal-to-noise ratio and moderate spectral resolution) near-infrared (near-IR) spectroscopic observations of 23 well-known broad emission line active galactic nuclei (AGNs). In addition, we obtained simultaneous (within 2 months) optical spectroscopy of similar quality. The near-IR broad emission line spectrum of AGNs is dominated by permitted transitions of hydrogen, helium, oxygen, and calcium, and by the rich spectrum of singly ionized iron. In this paper we present the spectra, line identifications, and measurements, and we address briefly some of the important issues regarding the physics of AGN broad emission line regions. In particular, we investigate the excitation mechanism of neutral oxygen and confront for the first time theoretical predictions of the near-IR iron emission spectrum with observations.
The Astrophysical Journal Supplement Series 12/2008; 174(2):282. · 13.46 Impact Factor
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ABSTRACT: This paper presents XMM-Newton and Chandra X-ray spectroscopy of 10 flat-spectrum radio quasars (FSRQ) that are candidates to have an X-ray spectrum dominated by jet synchrotron emission. In all these FSRQ, which are less strongly relativistically beamed than blazars, a considerable contribution from a power-law component similar to that present in radio-quiet quasars is required to explain the observed X-ray fluxes and X-ray spectral slopes. As in radio-quiet quasars, their relatively high optical/UV fluxes can be accounted for by a significant contribution from thermal accretion disk emission. The lack of success in finding radio quasars with synchrotron X-rays is attributed to the adopted selection criteria, which were based on the multiwavelength flux ratios of BL Lacertae objects. A refined selection technique, which also involves radio imaging, is proposed to search for these important candidates with the Gamma Ray Large Area Space Telescope (GLAST). On the other hand, the discovered FSRQ with their strong accretion disk signatures are expected to be important probes for studying the poorly understood accretion disk-jet connection.
The Astrophysical Journal 12/2008; 676(1):87. · 6.02 Impact Factor
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ABSTRACT: We report optical spectroscopic observations of X-shaped radio sources with the Hobby-Eberly Telescope and Multiple-Mirror Telescope, focused on the sample of candidates from the FIRST survey presented in Paper I (Cheung 2007). A total of 27 redshifts were successfully obtained, 21 of which are new, including that of a newly identified candidate source of this type which is presented here. With these observations, the sample of candidates from Paper I is over 50% spectroscopically identified. Two new broad emission-lined X-shaped radio sources are revealed, while no emission lines were detected in about one third of the observed sources; a detailed study of the line properties is deferred to a future paper. Finally, to explore their relation to the Fanaroff-Riley division, the radio luminosities and host galaxy absolute magnitudes of a spectroscopically identified sample of 50 X-shaped radio galaxies are calculated to determine their placement in the Owen-Ledlow plane. Comment: emulateapj style, 10 pages, 4 figures, 2 tables; ApJS accepted with minor revision from submitted version (v1)
11/2008;
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ABSTRACT: We present deep radio images at 1.4 GHz of a large and complete sample of BL Lacertae objects (BL Lacs) selected from the Deep X-ray Radio Blazar Survey (DXRBS). We have observed 24 northern sources with the Very Large Array (VLA) in both its A and C configurations and 15 southern sources with the Australia Telescope Compact Array (ATCA) in its largest configuration. We find that in the DXRBS, as in the 1-Jy survey, which has a radio flux limit roughly ten times higher than the DXRBS, a considerable number (about a third) of BL Lacs can be identified with the relativistically beamed counterparts of Fanaroff-Riley type II (FR II) radio galaxies. We attribute the existence of FR II-BL Lacs, which is not accounted for by current unified schemes, to an inconsistency in our classification scheme for radio-loud active galactic nuclei (AGN). Taking the extended radio power as a suitable measure of intrinsic jet power, we find similar average values for low- (LBL) and high-energy peaked BL Lacs (HBL), contrary to the predictions of the blazar sequence. Comment: 21 pages, 10 figures, accepted by MNRAS
09/2008;
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ABSTRACT: We present the first deep VLA radio images of flat-spectrum radio quasars (FSRQ) with multiwavelength emission properties similar to those of BL Lacs with synchrotron X-rays. Our observations of twenty-five of these sources show that their radio morphologies are similar to those of other radio quasars. However, their range of extended powers is more similar to that of BL Lacertae objects (BL Lacs) and extends down to the low values typical of FR I radio galaxies. Five out of our nine lobe-dominated sources have extended radio powers in the range typical of both FR I and FR II radio galaxies, but their extended radio structure is clearly FR II-like. Therefore, we have not yet found a large population of radio quasars hosted by FR Is. Two thirds of our sources have a core-dominated radio morpholgy and thus X-rays likely dominated by the jet. We find that their ratios of radio core to total X-ray luminosity are low and in the regime indicative of synchrotron X-rays. This result shows that also blazars with strong emission lines can produce jets of high-energy synchrotron emission and undermines at least in part the ``blazar sequence'' scenario which advocates that particle Compton cooling by an external radiation field governs the frequency of the synchrotron emission peak. Comment: 26 pages, 33 figures. Accepted for publication in ApJ
09/2005;
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ABSTRACT: We present a new population of radio quasars whose X-ray band, unlike previously known sources which have (flat) inverse Compton
radiation, is characterized by (steep) synchrotron emission, with a broad-band spectral energy distribution similar to that
of BL Lacs with high energy synchrotron peaks. We discuss how this new class was discovered, the class properties, and the
implications of its existence for our understanding of jets and active galactic nuclei in general.
Astrophysics and Space Science 10/2004; 294(1):71-78. · 1.69 Impact Factor
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ABSTRACT: Blazars are currently separated into BL Lacertae objects (BL Lacs) and flat spectrum radio quasars based on the strength of their emission lines. This is performed rather arbitrarily by defining a diagonal line in the Ca H&K break value–equivalent width plane, following Marchã et al. We readdress this problem and put the classification scheme for blazars on firm physical grounds. We study ∼100 blazars and radio galaxies from the Deep X-ray Radio Blazar Survey (DXRBS) and 2-Jy radio survey and find a significant bimodality for the narrow emission line [O iii]λ5007. This suggests the presence of two physically distinct classes of radio-loud active galactic nuclei (AGN). We show that all radio-loud AGN, blazars and radio galaxies, can be effectively separated into weak- and strong-lined sources using the [O iii]λ5007–[O ii]λ3727 equivalent width plane. This plane allows one to disentangle orientation effects from intrinsic variations in radio-loud AGN. Based on DXRBS, the strongly beamed sources of the new class of weak-lined radio-loud AGN are made up of BL Lacs at the ∼75 per cent level, whereas those of the strong-lined radio-loud AGN include mostly (∼97 per cent) quasars.
Monthly Notices of the Royal Astronomical Society 05/2004; 351(1):83 - 100. · 4.90 Impact Factor
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ABSTRACT: The relation between emission from the accretion disc (thermal) and the jet (non-thermal) in blazars is still a mystery as, typically, the beamed jet emission swamps the disc emission even in the ultraviolet band where disc emission peaks. In this paper we estimate the accretion disc component for 136 flat-spectrum radio quasars selected from the Deep X-ray Radio Blazar Survey. We do this by deriving the accretion disc spectrum from the mass and accretion rate on to the central black hole for each object, estimated using the emission linewidths and the power emitted from the broad-line region. We find that non-thermal emission dominates the optical/UV band of our sources. The thermal component is in fact on average ∼15 per cent of the total and ≳ 90 per cent of the objects in the sample have a thermal component <0.5 of the total luminosity. We then estimate the integrated disc and kinetic jet powers and find that on average the disc luminosity is ∼1 to 20 times the jet power (depending on the uncertainties in the estimation of the latter quantity). A comparison with previous, independent results favours a scenario in which jet and disc powers are of the same order of magnitude. Extraction of energy from a rotating black hole via the ‘Blandford–Znajek’ mechanism fails to explain the estimated jet power in the majority of our sources. Finally, we find that the typical masses for our sources are ∼5 × 108 M⊙ and that, contrary to previous claims, about one quarter of our radio quasars have relatively small (<3 × 108 M⊙) black hole mass.
Monthly Notices of the Royal Astronomical Society 02/2003; 339(4):1081 - 1094. · 4.90 Impact Factor
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ABSTRACT: We investigate why BL Lacertae objects (BL Lacs) have values of the Ca H&K break (a stellar absorption feature) lower than low-power radio galaxies, and whether its use is justified to separate the two classes. For this purpose we relate this parameter to the radio and optical core emission, as well as to the X-ray powers, for a sample of ∼90 radio sources. We find that the Ca H&K break value decreases with increasing jet power, and that it also anticorrelates with the radio core dominance parameter but not with extended radio emission. Based on this we conclude that the Ca H&K break value of BL Lacs and radio galaxies is a suitable indicator of orientation. From the luminosity ratios between objects with low and high Ca H&K break values we constrain the average Lorentz factors for BL Lacs and low-power radio galaxies in the radio and X-ray band to Γ∼ 2–4 and derive average viewing angles for the galaxies. Our values are in agreement with results from independent methods. We find that the correlations between Ca H&K break and radio core and X-ray luminosity hold independently for low- (LBL) and high-energy peaked BL Lacs (HBL). We derive average viewing angles for their parent populations, which turn out to be similar to the ones for our entire sample, and compare for the first time the luminosities of LBL and HBL at different orientations.
Monthly Notices of the Royal Astronomical Society 10/2002; 336(3):945 - 956. · 4.90 Impact Factor
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ABSTRACT: We have searched the archived, pointed ROSAT Position Sensitive Proportional Counter data for blazars by correlating the WGACAT X-ray data base with several publicly available radio catalogues, restricting our candidate list to serendipitous X-ray sources with a flat radio spectrum (αr≤0.70, where S∝ν−α). This makes up the Deep X-ray Radio Blazar Survey (DXRBS). Here we present new identifications and spectra for 106 sources, including 86 radio-loud quasars, 11 BL Lacertae objects, and nine narrow-line radio galaxies. Together with our previously published objects and already-known sources, our sample now contains 298 identified objects: 234 radio-loud quasars [181 flat-spectrum quasars: FSRQ (αr≤0.50) and 53 steep-spectrum quasars: SSRQ], 36 BL Lacs and 28 narrow-line radio galaxies. Redshift information is available for 96 per cent of these. Thus our selection technique is ∼90 per cent efficient at finding radio-loud quasars and BL Lacs. Reaching 5-GHz radio fluxes ∼50 mJy and 0.1–2.0 keV X-ray fluxes a few ×10−14 erg cm−2 s−1, DXRBS is the faintest and largest flat-spectrum radio sample with nearly complete (∼85 per cent) identification. We review the properties of the DXRBS blazar sample, including redshift distribution and coverage of the X-ray-radio–power plane for quasars and BL Lacs. Additionally, we touch upon the expanded multiwavelength view of blazars provided by DXRBS. By sampling for the first time the faint end of the radio and X-ray luminosity functions, this sample will allow us to investigate the blazar phenomenon and the validity of unified schemes down to relatively low powers.
Monthly Notices of the Royal Astronomical Society 04/2002; 323(3):757 - 784. · 4.90 Impact Factor
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ABSTRACT: The recent Deep X-ray Radio Blazar Survey (DXRBS) has identified a sample of BL Lac ob-jects spanning the intermediate range of spectral energy distributions between "classical" X-ray selected and radio-selected samples of BL Lacs. Detailed studies of such samples are needed to answer some of the currently open questions regarding the nature of BL Lacs and their place in a unified model of AGN. High-resolution radio imaging provides direct information on jet evo-lution and beaming parameters. We present some preliminary results from EVN & MERLIN observations of sources in the DXRBS BL Lac sample for which little or no high-resolution radio data were previously available.
