Kendrah D. Murphy

Skidmore College, Saratoga Springs, New York, United States

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Publications (15)41.1 Total impact

  • Source
    Kendrah D. Murphy, Tahir Yaqoob
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    ABSTRACT: The central engines of both type 1 and type 2 active galactic nuclei are thought to harbour a toroidal structure that absorbs and reprocesses high-energy photons from the central X-ray source. Unique features in the reprocessed spectra can provide powerful physical constraints on the geometry, column density, element abundances and orientation of the circumnuclear matter. If the reprocessor is Compton-thick, the calculation of emission-line and continuum spectra that are suitable for direct fitting to X-ray data is challenging because the reprocessed emission depends on the spectral shape of the incident continuum, which may not be directly observable. We present new Monte Carlo calculations of Green's functions for a toroidal reprocessor that provide significant improvements over currently available models. The Green's function approach enables the construction of X-ray spectral fitting models that allow arbitrary incident spectra as part of the fitting process. The calculations are fully relativistic and have been performed for column densities that cover the Compton-thin to Compton-thick regime, for incident photon energies up to 500 keV. The Green's function library can easily be extended cumulatively to provide models that are valid for higher input energies and a wider range of element abundances and opening angles of the torus. The reprocessed continuum and fluorescent line emission due to Fe Kα, Fe Kβ and Ni Kα are treated self-consistently, eliminating the need for ad hoc modelling that is currently common practice. We find that the spectral shape of the Compton-thick reflection spectrum in both the soft and hard X-ray bands in our toroidal geometry is different compared with that obtained from disc models. A key result of our study is that a Compton-thick toroidal structure that subtends the same solid angle at the X-ray source as a disc can produce a reflection spectrum that is ~6 times weaker than that from a disc. This highlights the widespread and erroneous interpretation of the so-called `reflection-fraction' as a solid angle, obtained from fitting disc-reflection models to Compton-thick sources without regard for proper consideration of geometry.
    Monthly Notices of the Royal Astronomical Society 08/2011; 415(4):3962-3965. DOI:10.1111/j.1365-2966.2011.18744.x · 5.23 Impact Factor
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    Tahir Yaqoob, Kendrah D. Murphy
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    ABSTRACT: We present new results from Monte Carlo calculations of the flux and equivalent width (EW) of the Ni Kalpha fluorescent emission line in the toroidal X-ray reprocessor model of Murphy & Yaqoob. In the Compton-thin regime, the EW of the Ni Kalpha line is a factor of ˜22 less than that of the Fe Kalpha line but this factor can be as low as ˜6 in the Compton-thick regime. We show that the optically thin limit for this ratio depends only on the Fe to Ni abundance ratio, it being independent of the geometry and covering factor of the reprocessor, and also independent of the shape of the incident X-ray continuum. We give some useful analytic expressions for the absolute flux and the EW of the Ni Kalpha line in the optically thin limit. When the reprocessor is Compton thick and the incident continuum is a power law with a photon index of 1.9, the Ni Kalpha EW has a maximum value of ˜3 and ˜250 eV for non-intercepting and intercepting lines of sight, respectively. Larger EWs are obtained for flatter continua. We have also studied the Compton shoulder of the Ni Kalpha line and find that the ratio of scattered to unscattered flux in the line has a maximum value of 0.26, less than the corresponding maximum for the Fe Kalpha line. However, we find that the shape of the Compton shoulder profile for a given column density and inclination angle of the torus is similar to the corresponding profile for the Fe Kalpha line. Our results will be useful for interpreting X-ray spectra of active galactic nuclei (AGNs) and X-ray binary systems in which the system parameters are favourable for the Ni Kalpha line to be detected.
    Monthly Notices of the Royal Astronomical Society 04/2011; 412(3):1765-1770. DOI:10.1111/j.1365-2966.2010.18012.x · 5.23 Impact Factor
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    Tahir Yaqoob, Kendrah D. Murphy
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    ABSTRACT: Heavily obscured active galactic nuclei (AGNs) play an important role in contributing to the cosmic X-ray background (CXRB). However, the AGNs found in deep X-ray surveys are often too weak to allow direct measurement of the column density of obscuring matter. One method adopted in recent years to identify heavily obscured, Compton-thick AGNs under such circumstances is to use the observed mid-infrared to X-ray luminosity ratio as a proxy for the column density. This is based on the supposition that the amount of energy lost by the illuminating X-ray continuum to the obscuring matter and reprocessed into infrared emission is directly related to the column density and that the proxy is not sensitive to other physical parameters of the system (aside from contamination by dust emission from, for example, star-forming regions). Using Monte Carlo simulations, we find that the energy losses experienced by the illuminating X-ray continuum in the obscuring matter are far more sensitive to the shape of the X-ray continuum and to the covering factor of the X-ray reprocessor than they are to the column density of the material. Specifically we find that it is possible for the infrared to X-ray luminosity ratio for a Compton-thin source to be just as large as that for a Compton-thick source even without any contamination from dust. Since the intrinsic X-ray continuum and covering factor of the reprocessor are poorly constrained from deep X-ray survey data, we conclude that the mid-infrared to X-ray luminosity ratio is not a reliable proxy for the column density of obscuring matter in AGNs even when there is no other contribution to the mid-infrared luminosity aside from X-ray reprocessing. This conclusion is independent of the geometry of the obscuring matter. Comment: Accepted for publication in MNRAS. 12 pages, 7 figures
    Monthly Notices of the Royal Astronomical Society 10/2010; 412(2). DOI:10.1111/j.1365-2966.2010.17942.x · 5.23 Impact Factor
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    Tahir Yaqoob, Kendrah D. Murphy
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    ABSTRACT: We present new, high signal-to-noise ratio results from a Monte Carlo study of the properties of the Compton shoulder of the Fe Kalpha emission line in the toroidal X-ray reprocessor model of Murphy & Yaqoob (2009, MNRAS, 397, 1549). The model comprehensively covers the Compton-thin to Compton-thick regimes and we find that the variety of Compton shoulder profiles is greater than that for both (centrally-illuminated) spherical and disk geometries. Our Monte Carlo simulations were done with a statistical accuracy that is high enough to reveal, for the case of an edge-on, Compton-thick torus, a new type of Compton shoulder that is not present in the spherical or disk geometries. Such a Compton shoulder is dominated by a narrow back-scattering feature at ~6.24 keV. Our results also reveal a dependence of the shape of the Compton shoulder (and its magnitude relative to the Fe Kalpha line core) on the spectral shape of the incident X-ray continuum. We also show the effects of velocity broadening on the Fe Kalpha line profile and find that if either the velocity width or instrument resolution is greater than a FWHM of ~2000 km/s, the Compton shoulder begins to become blended with the line core and the characteristic features of the Compton shoulder become harder to resolve. In particular, at a FWHM of ~7000 km/s the Compton shoulder is NOT resolved at all, its only signature being a weak asymmetry in the blended line profile. Thus, CCD X-ray detectors cannot unambiguously resolve the Compton shoulder. Our results are freely available in a format that is suitable for direct spectral-fitting of the continuum and line model to real data. Comment: Accepted for publication in MNRAS. 12 pages, 6 figures. Abstract is abridged
    Monthly Notices of the Royal Astronomical Society 10/2010; 412(1). DOI:10.1111/j.1365-2966.2010.17902.x · 5.23 Impact Factor
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    ABSTRACT: We developed an instrument design capable of measuring linear X-ray polarization over a broad-band using conventional spectroscopic optics, using a method previously described by Marshall (2008) involving laterally graded, multilayer-coated flat mirrors. We present possible science investigations with such an instrument and two possible configurations. This instrument could be used in a small orbiting mission or scaled up for the International X-ray Observatory. Laboratory work has begun that would demonstrate the capabilities of key components.
    Proceedings of SPIE - The International Society for Optical Engineering 07/2010; DOI:10.1117/12.857443 · 0.20 Impact Factor
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    ABSTRACT: Multilayer-coated optics can strongly polarize X-rays and are central to a new design of a broad-band, soft X-ray polarimeter. We have begun laboratory work to verify the performance of components that could be used in future soft X-ray polarimetric instrumentation. We have reconfigured a 17 meter beamline facility, originally developed for testing transmission gratings for Chandra, to include a polarized X-ray source, an X-ray-dispersing transmission grating, and a multilayer-coated optic that illuminates a CCD detector. The X-rays produced from a Manson Model 5, multi-anode source are polarized by a multilayer-coated flat mirror. The current configuration allows for a 180 degree rotation of the source in order to rotate the direction of polarization. We will present progress in source characterization and system modulation measurements as well as null and robustness tests.
    Proceedings of SPIE - The International Society for Optical Engineering 07/2010; DOI:10.1117/12.857574 · 0.20 Impact Factor
  • Kendrah D. Murphy, T. Yaqoob
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    ABSTRACT: Obscured AGN are widely recognized to play a critical role in AGN evolution and the accretion history of the universe. These objects are furthermore thought to make a significant contribution to the cosmic X-ray background (CXRB), although their contribution relative to unobscured AGN, especially with respect to a possible "hidden" population of obscured AGN, remains a topic of debate. The infrared to X-ray luminosity ratio is often used as an indicator of Compton-thick AGN, especially for weak sources in deep surveys. We present new theoretical energy-loss calculations that allow us to investigate the relationship between this luminosity ratio due to X-ray reprocessing, and the column density of the reprocessor. We will discuss the reliability of the infrared to X-ray luminosity ratio as a diagnostic of the column density. We will furthermore discuss observed-to-intrinsic X-ray luminosity ratio calculations, showing that even hard X-ray surveys are biased against detecting obscured Compton-thick AGN. However, if a source is viewed through an unobscured line-of-sight, the reprocessing material out of the line-of-sight can significantly enhance the apparent luminosity, if the Compton depth is greater than 1. X-ray surveys will therefore favor detections of unobscured AGN with Compton-thick reprocessors rather than obscured Compton-thick sources.
  • Source
    Kendrah D. Murphy, Tahir Yaqoob
    [Show abstract] [Hide abstract]
    ABSTRACT: The central engines of both type 1 and type 2 active galactic nuclei are thought to harbour a toroidal structure that absorbs and reprocesses high-energy photons from the central X-ray source. Unique features in the reprocessed spectra can provide powerful physical constraints on the geometry, column density, element abundances and orientation of the circumnuclear matter. If the reprocessor is Compton-thick, the calculation of emission-line and continuum spectra that are suitable for direct fitting to X-ray data is challenging because the reprocessed emission depends on the spectral shape of the incident continuum, which may not be directly observable. We present new Monte Carlo calculations of Green's functions for a toroidal reprocessor that provide significant improvements over currently available models. The Green's function approach enables the construction of X-ray spectral fitting models that allow arbitrary incident spectra as part of the fitting process. The calculations are fully relativistic and have been performed for column densities that cover the Compton-thin to Compton-thick regime, for incident photon energies up to 500 keV. The Green's function library can easily be extended cumulatively to provide models that are valid for higher input energies and a wider range of element abundances and opening angles of the torus. The reprocessed continuum and fluorescent line emission due to Fe Kα, Fe Kβ and Ni Kα are treated self-consistently, eliminating the need for ad hoc modelling that is currently common practice. We find that the spectral shape of the Compton-thick reflection spectrum in both the soft and hard X-ray bands in our toroidal geometry is different compared with that obtained from disc models. A key result of our study is that a Compton-thick toroidal structure that subtends the same solid angle at the X-ray source as a disc can produce a reflection spectrum that is ∼6 times weaker than that from a disc. This highlights the widespread and erroneous interpretation of the so-called ‘reflection-fraction’ as a solid angle, obtained from fitting disc-reflection models to Compton-thick sources without regard for proper consideration of geometry.
    Monthly Notices of the Royal Astronomical Society 08/2009; 397(3):1549 - 1562. DOI:10.1111/j.1365-2966.2009.15025.x · 5.23 Impact Factor
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    ABSTRACT: Future X-ray instrumentation is expected to allow us to significantly improve the constraints derivedfrom the Fe K lines in AGN, such as the black-hole angular momentum (spin) and the inclination angle of the putative accretion disk. We consider the possibility that measurements of the persistent, time-averaged Fe K line emission from the disk could be supplemented by the observation of a localized flare, or "hotspot", orbiting close to the black hole. Although observationally challenging, such measurements would recover some of the information loss that is inherent to the radially-integrated line profiles. We present calculations for this scenario to assess the extent to which, in principle, black-hole spin may be measured. We quantify the feasibility of this approach using realistic assumptions about likely measurement uncertainties. Comment: 7 pages, 7 figures. Accepted for publication in ApJ
    The Astrophysical Journal 06/2009; DOI:10.1088/0004-637X/701/1/635 · 6.28 Impact Factor
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    Tahir Yaqoob, Kendrah D. Murphy, Yuichi Terashima
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    ABSTRACT: The Seyfert 1.9 galaxy NGC 2992 has been observed by all X-ray astronomy missions since HEAO-1 for over a quarter of a century, including a one-year RXTE monitoring campaign in 2005/2006 and three Suzaku observations in 2005. The source exhibited over a factor of 20 variability in the hard X-ray luminosity over the ˜25 year period and over an order-of-magnitude variability in the luminosity during the RXTE monitoring period. The luminosity changes are accompanied by spectral variability. In particular there is complex variability in the Fe K emission-line profile which consists of an accretion-disk component and a distant-matter component. The relativistic disk line component shows variability that likely corresponds to changes in the spatial illumination of the disk and the Suzaku data are able to decouple the disk line component from the distant-matter component. In one of the historical BeppoSAX observations an emission-line component from highly ionized Fe was also apparent. In this contribution we illustrate how the unique X-ray properties of NGC 2992 make it one of best supermassive black-hole candidates for testing accretion disk theories since any model must be able to satisfy all of the short and long timescale observational constraints. Future observations need to be planned to exploit the rich behavior to provide even more stringent constraints. NGC 2992 is also a good candidate for constraining AGN unification models.
    Proceedings of the International Astronomical Union 08/2007; 238:123-126. DOI:10.1017/S1743921307004826
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    Kendrah D. Murphy, Tahir Yaqoob, Yuichi Terashima
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    ABSTRACT: We present the results of a one year monitoring campaign of the Seyfert 1.9 galaxy NGC 2992 with RXTE. Historically, the source has been shown to vary dramatically in 2-10 keV flux over timescales of years and was thought to be slowly transitioning between periods of quiescence and active accretion. Our results show that in one year the source continuum flux covered almost the entire historical range, making it unlikely that the low-luminosity states correspond to the accretion mechanism switching off. During flaring episodes we found that a highly redshifted Fe K line appears, implying that the violent activity is occurring in the inner accretion disk, within ~100 gravitational radii of the central black hole. We also found that the spectral index of the X-ray continuum remained approximately constant during the large amplitude variability. These observations make NGC 2992 well-suited for future multi-waveband monitoring, as a test-bed for constraining accretion models. Comment: Abstract is abridged. Accepted for publication in the September 2007 issue of ApJ. 39 pages, 14 figures, 3 tables
    The Astrophysical Journal 05/2007; 666(1). DOI:10.1086/520039 · 6.28 Impact Factor
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    ABSTRACT: We present detailed time-averaged X-ray spectroscopy in the 0.5-10keV band of the Seyfert 1.9 galaxy NGC 2992 with the Suzaku X-ray Imaging Spectrometer (XIS). There is an Fe K line emission complex that we model with broad and narrow lines and we show that the intensities of the two components are decoupled at a confidence level >3σ The broad line (EW = 118-61+32eV) could originate in an accretion disk (inclined at > 30°). The narrow Fe Kα line (EW = 163-26+47eV) is unresolved by the XIS at 99% confidence and likely originates in distant matter. A significant (narrow) Fe Kβ line is also detected and we describe a new robust method to constrain the ionization state of Fe in the distant line emitter (e.g. the putative obscuring torus). The method does not require any knowledge of possible gravitational and Doppler energy shifts and we deduce that the predominant ionization state of Fe in the distant matter is lower than Fe VIII (at 99% confidence), conservatively taking into account residual calibration uncertainties and theoretical and experimental uncertainties in the Fe K fluorescent line energies. We argue that the narrow Fe Kα and Fe Kβ lines likely originate in a Compton-thin structure.
    Publications- Astronomical Society of Japan 12/2006; 59:283-299. DOI:10.1093/pasj/59.sp1.S283 · 2.01 Impact Factor
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    ABSTRACT: We present detailed time-averaged X-ray spectroscopy in the 0.5--10 keV band of the Seyfert~1.9 galaxy NGC 2992 with the Suzaku X-ray Imaging Spectrometers (XIS). We model the complex continuum in detail. There is an Fe K line emission complex that we model with broad and narrow lines and we show that the intensities of the two components are decoupled at a confidence level >3sigma. The broad Fe K line has an EW of 118 (+32,-61) eV and could originate in an accretion disk (with inclination angle greater than ~30 degrees). The narrow Fe Kalpha line has an EW of 163 (+47,-26) eV and is unresolved FWHM <4090 km/s) and likely originates in distant matter. The absolute flux in the narrow line implies that the column density out of the line-of-sight could be much higher than measured in the line-of-sight, and that the mean (historically-averaged) continuum luminosity responsible for forming the line could be a factor of several higher than that measured from the data. We also detect the narrow Fe Kbeta line with a high signal-to-noise ratio and describe a new robust method to constrain the ionization state of Fe responsible for the Fe Kalpha and Fe Kbeta lines that does not require any knowledge of possible gravitational and Doppler energy shifts affecting the line energies. For the distant line-emitting matter (e.g. the putative obscuring torus) we deduce that the predominant ionization state is lower than Fe VIII (at 99% confidence), conservatively taking into account residual calibration uncertainties in the XIS energy scale and theoretical and experimental uncertainties in the Fe K fluorescent line energies. From the limits on a possible Compton-reflection continuum it is likely that the narrow Fe Kalpha and Fe Kbeta lines originate in a Compton-thin structure.
  • Kendrah D. Murphy, T. Yaqoob, Y. Teramshima
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    ABSTRACT: Over a quarter of a century of X-ray observations of the Seyfert 1.9 galaxy NGC 2992 show that it is a promising test-bed for severely constraining accretion disk models. We present the results of a 1 year monitoring campaign of the Seyfert 1.9 galaxy NGC 2992 with RXTE that were quasi-simultaneous with three Suzaku observations. The X-ray continuum varied by more than an order of magnitude on a timescale of weeks. The previous interpretation of the historical activity of NGC 2992 in terms of the accretion disk slowly becoming dormant over many years and then `re-building' itself is not supported by the new results. During the large-amplitude flares the centroid energy of the Fe K emission-line complex became significantly redshifted, indicating that the violent activity was occurring close to the putative central black hole where gravitational energy shifts can be sufficiently large. Moreover, the Suzaku data are able to deconvolve the persistent disk-line emission from the distant-matter Fe K line emission and we discuss the implied constraints on the disk and circumnuclear matter. Even though the line-of-sight column density in NGC 2992 has a Thomson depth
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    Tahir Yaqoob, Kendrah D Murphy

Publication Stats

169 Citations
41.10 Total Impact Points

Institutions

  • 2010–2011
    • Skidmore College
      • Department of Physics
      Saratoga Springs, New York, United States
    • Massachusetts Institute of Technology
      Cambridge, Massachusetts, United States
  • 2007–2009
    • Johns Hopkins University
      • Department of Physics and Astronomy
      Baltimore, Maryland, United States
  • 2006
    • Nagoya University
      • Division of Cell Science
      Nagoya, Aichi, Japan