Hermine Landt

Durham University, Durham, England, United Kingdom

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Publications (48)154.34 Total impact

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    ABSTRACT: We present the second extensive study of the coronal line variability in an active galaxy. Our data set for the well-studied Seyfert galaxy NGC 5548 consists of five epochs of quasisimultaneous optical and near-infrared spectroscopy spanning a period of about five years and three epochs of X-ray spectroscopy overlapping in time with it. Whereas the broad emission lines and hot dust emission varied only moderately, the coronal lines varied strongly. However, the observed high variability is mainly due to a flux decrease. Using the optical [Fe VII] and X-ray OVII emission lines we estimate that the coronal line gas has a relatively low density of ne ~ 103 cm-3 and a relatively high ionisation parameter of log U ~ 1. The resultant distance of the coronal line gas from the ionizing source of about eight light-years places this region well beyond the hot inner face of the dusty torus. These results imply that the coronal line region is an independent entity. We find again support for the X-ray heated wind scenario of Pier & Voit; the increased ionizing radiation that heats the dusty torus also increases the cooling efficiency of the coronal line gas, most likely due to a stronger adiabatic expansion. The much stronger coronal line variability of NGC 5548 relative to that of NGC 4151 can also be explained within this picture. NGC 5548 has much stronger coronal lines relative to the low-ionization lines than NGC 4151 indicating a stronger wind, in which case a stronger adiabatic expansion of the gas and so fading of the line emission is expected. © 2015 The Authors. Published by Oxford University Press on behalf of the Royal Astronomical Society.
    No preview · Article · Dec 2015 · Monthly Notices of the Royal Astronomical Society
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    ABSTRACT: We perform a spectral analysis of a sample of 11 medium redshift (1.5 ≲ z ≲ 2.2) quasars. Our sample all have optical spectra from the SDSS, infrared spectra from GNIRS and TripleSpec, and X-ray spectra from XMM–Newton. We first analyse the Balmer broad emission line profiles which are shifted into the IR spectra to constrain black hole masses. Then we fit an energy-conserving, three component accretion model of the broad-band spectral energy distribution (SED) to our multiwavelength data. 5 out of the 11 quasars show evidence of an SED peak, allowing us to constrain their bolometric luminosity from these models and estimate their mass accretion rates. Based on our limited sample, we suggest that estimating bolometric luminosities from $L_{5100 \,\rm \mathring{A}}$ and L2-10 keV may be unreliable, as has been also noted for a low-redshift, X-ray selected active galactic nucleus sample.
    Preview · Article · Feb 2015 · Monthly Notices of the Royal Astronomical Society
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    ABSTRACT: We present the first extensive study of the coronal line variability in an active galaxy. Our data set for the nearby source NGC 4151 consists of six epochs of quasi-simultaneous optical and near-infrared spectroscopy spanning a period of about 8 yr and five epochs of X-ray spectroscopy overlapping in time with it. None of the coronal lines showed the variability behaviour observed for the broad emission lines and hot dust emission. In general, the coronal lines varied only weakly, if at all. Using the optical [Fe vii] and X-ray O vii emission lines we estimate that the coronal line gas has a relatively low density of ne ∼ 103 cm−3 and a relatively high ionization parameter of log U ∼ 1. The resultant distance of the coronal line gas from the ionizing source is about two light years, which puts this region well beyond the hot inner face of the obscuring dusty torus. The high ionization parameter implies that the coronal line region is an independent entity rather than part of a continuous gas distribution connecting the broad and narrow emission line regions. We present tentative evidence for the X-ray heated wind scenario of Pier & Voit. We find that the increased ionizing radiation that heats the dusty torus also increases the cooling efficiency of the coronal line gas, most likely due to a stronger adiabatic expansion.
    Full-text · Article · Jan 2015 · Monthly Notices of the Royal Astronomical Society
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    Anthony M. Brown · Paula M. Chadwick · Hermine Landt
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    ABSTRACT: In this paper, we report the Fermi Large Area Telescope (LAT) detection of Very High Energy (VHE; Eγ > 100 GeV) γ-ray emission from the BL Lac object RBS 0679. 5.3 years of LAT observations revealed the presence of three VHE photon events within 0 $_{.}^{\circ}$1 of RBS 0679, with a subsequent unbinned likelihood analysis finding RBS 0679 to be a source of VHE photons at 6.9 standard deviations (σ). An unbinned likelihood analysis of the 0.1–100 GeV data, binned in 28-d periods, finds both the flux and spectral index to be variable, with a ‘softer-when-brighter’ trend in the global γ-ray characteristics. On the other hand, the 28-d periods in which the VHE photons were detected have spectral indices that are consistent with the 5.3 year average suggesting that the observed VHE emission is not associated with a spectral hardening event. The discovery of RBS 0679 in the 100–300 GeV energy range, combined with the non-detection above 390 GeV with the H.E.S.S. telescope array, suggest RBS 0679 to be an intriguing source that requires further follow-up observations with ground-based γ-ray observatories.
    Full-text · Article · Oct 2014 · Monthly Notices of the Royal Astronomical Society
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    ABSTRACT: Here we present observational evidence that the broad emission line region (BELR) of active galactic nuclei (AGN) generally has an outer boundary. This was already clear for sources with an obvious transition between the broad and narrow components of their emission lines. We show that the narrow component of the higher-order Paschen lines is absent in all sources, revealing a broad emission line profile with a broad, flat top. This indicates that the BELR is kinematically separate from the narrow emission line region. We use the virial theorem to estimate the BELR outer radius from the flat top width of the unblended profiles of the strongest Paschen lines, Paα and Paβ, and find that it scales with the ionizing continuum luminosity roughly as expected from photoionization theory. The value of the incident continuum photon flux resulting from this relationship corresponds to that required for dust sublimation. A flat-topped broad emission line profile is produced by both a spherical gas distribution in orbital motion and an accretion disc wind if the ratio between the BELR outer and inner radius is assumed to be less than ∼100–200. On the other hand, a pure Keplerian disc can be largely excluded, since for most orientations and radial extents of the disc the emission line profile is double-horned.
    Full-text · Article · Jan 2014 · Monthly Notices of the Royal Astronomical Society
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    ABSTRACT: We report the serendipitous discovery of a bright point source flare in the Abell cluster A1795 with archival EUVE and Chandra observations. Assuming the EUVE emission is associated with the Chandra source, the X-ray 0.5-7 keV flux declined by a factor of ~2300 over a time span of 6 yr, following a power-law decay with index ~2.44 ± 0.40. The Chandra data alone vary by a factor of ~20. The spectrum is well fit by a blackbody with a constant temperature of kT ~ 0.09 keV (~106 K). The flare is spatially coincident with the nuclear region of a faint, inactive galaxy with a photometric redshift consistent at the 1σ level with the cluster (z = 0.062476). We argue that these properties are indicative of a tidal disruption of a star by a black hole (BH) with log (M BH/M ☉) ~ 5.5 ± 0.5. If so, such a discovery indicates that tidal disruption flares may be used to probe BHs in the intermediate mass range, which are very difficult to study by other means.
    Full-text · Article · Nov 2013 · The Astrophysical Journal
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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 μm 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 Paα and Paβ 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. The additional sample improves the number statistics in particular at the high-luminosity end.
    Full-text · Article · Mar 2013 · Monthly Notices of the Royal Astronomical Society
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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 ionization species, hydrogen is an efficient line emitter for a very wide range of photoionization conditions, and the observed line ratios depend strongly on the density and photoionization 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 ? 0.2) 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, an ultraluminous infrared galaxy. We find that low-incident-ionizing fluxes (?H < 1018?cm?2 s?1) and high gas densities (n H > 1012?cm?3) are required to reproduce the observed hydrogen emission line ratios. This analysis demonstrates that the use of composite spectra in photoionization modeling is inappropriate; models must be fitted to the individual spectra of quasars.
    Full-text · Article · Jun 2012 · The Astrophysical Journal
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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.
    Preview · Article · Mar 2012 · Monthly Notices of the Royal Astronomical Society
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    ABSTRACT: Black hole masses for samples of active galactic nuclei (AGNs) 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∝Lα, we obtain for a sample of 14 reverberation-mapped AGN a best-fitting slope of α= 0.5 ± 0.1, which is consistent with the slope of the relationship in the optical band and with the value of 0.5 naïvely expected from photoionization 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 (Paα or Paβ).
    Full-text · Article · Mar 2011 · Monthly Notices of the Royal Astronomical Society
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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 μm 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 μm, 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 per cent. 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.
    Full-text · Article · Jan 2011 · Monthly Notices of the Royal Astronomical Society

  • No preview · Article · Jan 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 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-μm 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 per cent the speed of light) redward displacements and (iv) they give robust total mass estimates, but are insensitive to the assumed geometry.
    Preview · Article · Jul 2010 · Monthly Notices of the Royal Astronomical Society
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    Hermine Landt · Chi C. Cheung · Stephen E. Healey
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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.
    Preview · Article · Jun 2010 · Monthly Notices of the Royal Astronomical Society
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    ABSTRACT: 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.
    Full-text · Article · Apr 2010 · The Astrophysical Journal Letters
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    ABSTRACT: We have obtained a deep Chandra X-ray image of the z=0.72 quasar 4C 19.44 (PKS 1354+195). Our objectives included measuring the magnetic field and relativistic bulk velocity at many independent spatial elements along the jet. We assume minimum energy in the jet rest frame, and that the X-rays are produced by inverse Compton scattering on the cosmic microwave background. This approach is problematic because there are only two relations among the three relevant quantities: bulk Lorentz factor Gamma the Doppler factor delta=1/(Gamma (1-beta cos (theta))), and the angle theta to the line of sight. Generally authors have assumed either Gamma = delta or given a fixed fiducial value to Gamma . We explore the consequences of a different assumption: that the kinetic energy flux carried along the jet is constant. For our calculation this lowers the kinetic energy flux about a factor of two, to 1047 ergs per s, while defining a mean angle to the line of sight of 6 degrees, within +/- 1 degree.
    No preview · Article · Dec 2009
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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.
    Full-text · Article · Dec 2008 · The Astrophysical Journal Supplement Series
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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.
    Preview · Article · Dec 2008 · The Astrophysical Journal
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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)
    Full-text · Article · Nov 2008 · The Astrophysical Journal Supplement Series
  • Hayley Bignall · Cormac Reynolds · Hermine Landt
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    ABSTRACT: We request 24 hours of eVLBI at 1.4 GHz to observe a sample of 15 southern BL Lac objects for which no high resolution radio data are currently available. The sources, identified in the ``Deep X-ray Radio Blazar Survey (DXRBS)'' span the intermediate range of spectral energy distributions between ``classical'' X-ray selected and radio-selected samples of BL Lacs. Detailed studies of such an intermediate sample are needed to answer some of the currently open questions regarding the origin of different observed spectral energy distributions in BL Lacs, and their place in a unified model of AGN. High-resolution radio imaging is essential in order to disentangle the effects of different viewing angles, intrinsic jet power and environment of the AGN host galaxies, all of which influence the observed broad-band properties of BL Lacs. Radio core dominance is an indication of viewing angle, while the extended flux density distribution provides an estimate of intrinsic jet power. The proposed LBA observations, combined with ATCA data, will allow study of the compact, sub-arcsecond "core" and extended flux density distribution, complementing existing data on the northern DXBRS BL Lacs from the VLA and MERLIN.
    No preview · Article · Oct 2008

Publication Stats

650 Citations
154.34 Total Impact Points

Institutions

  • 2012-2015
    • Durham University
      • Department of Physics
      Durham, England, United Kingdom
    • Western Michigan University
      • Department of Physics
      Kalamazoo, Michigan, United States
  • 2011
    • Georgia State University
      • Department of Physics and Astronomy
      Atlanta, Georgia, United States
  • 2010-2011
    • University of Melbourne
      • School of Physics
      Melbourne, Victoria, Australia
    • Massachusetts Institute of Technology
      • Kavli Institute for Astrophysics and Space Research
      Cambridge, MA, United States
  • 2008
    • Harvard University
      Cambridge, Massachusetts, United States
  • 2005-2008
    • Harvard-Smithsonian Center for Astrophysics
      • Smithsonian Astrophysical Observatory
      Cambridge, Massachusetts, United States
  • 2002-2004
    • Space Telescope Science Institute
      Baltimore, Maryland, United States