(abridged) We present a detailed reanalysis of the Chandra data for the galaxy cluster Abell 4059... more (abridged) We present a detailed reanalysis of the Chandra data for the galaxy cluster Abell 4059 and its central radio galaxy, PKS2354-35. We also present new 1.4GHz and 4.7GHz CnB-array radio data from the Very Large Array (VLA), as well as a short archival WFPC2 image from the Hubble Space Telescope. The presence of a strong interaction between this radio galaxy and the intracluster medium (ICM) was suggested by Huang & Sarazin (1998) on the basis of a short observation by the High Resolution Imager on ROSAT, and confirmed in our preliminary analysis of the Chandra/ACIS-S data. In particular, X-ray imaging clearly shows two cavities within the ICM that are approximately aligned with the radio-galaxy axis. However, using our new radio maps we fail to find a detailed correspondence between the 1 arcmin scale radio-lobes and the ICM cavities. This suggests that the cavities are ``ghosts'' of a previous burst of powerful activity by PKS 2354-35. We also examine the nature of the central asymmetric ridge (or bar) of X-ray emission extending for 30kpc south-west of the cluster center that has been noted in these previous analyzes. We find the ridge to be denser and cooler than, but probably in pressure balance with, its surroundings. The thermal evolution of this structure seems to be dominated by radiative cooling, possibly enhanced by the radio-galaxy ICM interaction. We discuss several possible models for the formation of this SW ridge and find none of them to be entirely satisfactory.
I will review how current X-ray spectral and timing observations are probing astrophysics within ... more I will review how current X-ray spectral and timing observations are probing astrophysics within strong gravity around neutron stars and black holes. After a critical discussion about the extent to which such observations can probe GR, I shall discuss future prospects.
I will review how current X-ray spectral and timing observations are probing astrophysics within ... more I will review how current X-ray spectral and timing observations are probing astrophysics within strong gravity around neutron stars and black holes. After a critical discussion about the extent to which such observations can probe GR, I shall discuss future prospects.
Monthly Notices of the Royal Astronomical Society, Apr 11, 1997
I present a study of the X-ray spectral properties of a sample containing 24 type 1 active galact... more I present a study of the X-ray spectral properties of a sample containing 24 type 1 active galactic nuclei using the medium spectral resolution of ASCA. The sample consists of 20 radio-quiet objects (18 Seyfert 1 galaxies and two radio-quiet quasars) and four radio-loud objects (three broad-line radio galaxies and one radio-loud quasar). A simple power-law continuum absorbed by Galactic material provides a very poor description of the spectra of most objects. Deviations from the power-law form are interpreted in terms of X-ray reprocessing/absorption processes. In particular, at least half of the objects show K-shell absorption edges of warm oxygen (Ovii and Oviii) characteristic of optically thin, photoionized material along the line of sight to the central engine, the so-called warm absorber. The amount and presence of this absorption are found to depend on either the luminosity or radio properties of the objects: luminous and/or radio-loud objects are found to possess less ionized absorption. This ambiguity exists because the radio-loud objects are also amongst the most luminous of the sample. It is also found that objects with significant optical reddening display deep Ovii edges. The converse is true with two possible exceptions (NGC 3783 and NGC 3516). Coupled with other evidence resulting from detailed study of particular objects, this suggests the existence of dusty warm plasma. A radiatively driven outflow originating from the molecular torus is probably the source of this plasma. Rapid variability of the warm absorber also points to there being another component closer to the central source and probably situated within the broad-line region (BLR). Independent evidence for such an optically thin, highly ionized BLR component comes from detailed optical/UV studies. Spectral features at energies characteristic of cold iron Kalpha emission are common. Such emission is expected to arise from the fluorescence of cold iron in optically thick material when illuminated by the primary X-ray continuum. Radio-quiet objects have iron emission well described as originating either from the inner regions of an accretion disc or, in a small number of cases, from the molecular torus. Two of the radio-loud objects (3C 120 and 3C 382) have a much broader feature which presents problems for the relativistic disc model. The presence of radio jets may be important in forming this spectral feature.
Monthly Notices of the Royal Astronomical Society, Apr 11, 1997
I present a study of the X-ray spectral properties of a sample containing 24 type 1 active galact... more I present a study of the X-ray spectral properties of a sample containing 24 type 1 active galactic nuclei using the medium spectral resolution of ASCA. The sample consists of 20 radio-quiet objects (18 Seyfert 1 galaxies and two radio-quiet quasars) and four radio-loud objects (three broad-line radio galaxies and one radio-loud quasar). A simple power-law continuum absorbed by Galactic material provides a very poor description of the spectra of most objects. Deviations from the power-law form are interpreted in terms of X-ray reprocessing/absorption processes. In particular, at least half of the objects show K-shell absorption edges of warm oxygen (Ovii and Oviii) characteristic of optically thin, photoionized material along the line of sight to the central engine, the so-called warm absorber. The amount and presence of this absorption are found to depend on either the luminosity or radio properties of the objects: luminous and/or radio-loud objects are found to possess less ionized absorption. This ambiguity exists because the radio-loud objects are also amongst the most luminous of the sample. It is also found that objects with significant optical reddening display deep Ovii edges. The converse is true with two possible exceptions (NGC 3783 and NGC 3516). Coupled with other evidence resulting from detailed study of particular objects, this suggests the existence of dusty warm plasma. A radiatively driven outflow originating from the molecular torus is probably the source of this plasma. Rapid variability of the warm absorber also points to there being another component closer to the central source and probably situated within the broad-line region (BLR). Independent evidence for such an optically thin, highly ionized BLR component comes from detailed optical/UV studies. Spectral features at energies characteristic of cold iron Kalpha emission are common. Such emission is expected to arise from the fluorescence of cold iron in optically thick material when illuminated by the primary X-ray continuum. Radio-quiet objects have iron emission well described as originating either from the inner regions of an accretion disc or, in a small number of cases, from the molecular torus. Two of the radio-loud objects (3C 120 and 3C 382) have a much broader feature which presents problems for the relativistic disc model. The presence of radio jets may be important in forming this spectral feature.
Accreting black holes are prodigious sources of hard X-rays, supporting the notion that these obj... more Accreting black holes are prodigious sources of hard X-rays, supporting the notion that these objects support powerful coronae. I will discuss the constraints on the nature of these coronae that arise from modern spectral and timing X-ray observations. A particularly powerful probe are the relativistically broadened iron fluorescent lines observed in these systems; observations of these spectral features suggest the existence of extremely compact coronae very close to the black hole event horizon. Evidence for disk-corona coupling from observations of quasi-periodic oscillations will also be discussed.
Accreting black holes are prodigious sources of hard X-rays, supporting the notion that these obj... more Accreting black holes are prodigious sources of hard X-rays, supporting the notion that these objects support powerful coronae. I will discuss the constraints on the nature of these coronae that arise from modern spectral and timing X-ray observations. A particularly powerful probe are the relativistically broadened iron fluorescent lines observed in these systems; observations of these spectral features suggest the existence of extremely compact coronae very close to the black hole event horizon. Evidence for disk-corona coupling from observations of quasi-periodic oscillations will also be discussed.
Accreting black holes are prodigious sources of hard X-rays, supporting the notion that these obj... more Accreting black holes are prodigious sources of hard X-rays, supporting the notion that these objects support powerful coronae. I will discuss the constraints on the nature of these coronae that arise from modern spectral and timing X-ray observations. A particularly powerful probe are the relativistically broadened iron fluorescent lines observed in these systems; observations of these spectral features suggest the existence of extremely compact coronae very close to the black hole event horizon. Evidence for disk-corona coupling from observations of quasi-periodic oscillations will also be discussed.
The existence of the radius of marginal stability means that accretion flows around black holes i... more The existence of the radius of marginal stability means that accretion flows around black holes invariably undergo a transition from a magnetohydrodynamic (MHD) turbulent disklike flow to an inward-plunging flow. We argue that the plunging inflow can greatly enhance the trapping of large-scale magnetic fields on the black hole, and therefore may increase the importance of the Blandford-Znajek (BZ) effect relative to previous estimates that ignore the plunge region. We support this hypothesis by constructing and analyzing a toy model of the dragging and trapping of a large-scale field by a black hole disk, revealing a strong dependence of this effect on the effective magnetic Prandtl number of the MHD turbulent disk. Furthermore, we show that the enhancement of the BZ effect depends on the geometric thickness of the accretion disk. This may be, at least in part, the physical underpinnings of the empirical relation between the inferred geometric thickness of a black hole disk and the presence of a radio jet.
The existence of the radius of marginal stability means that accretion flows around black holes i... more The existence of the radius of marginal stability means that accretion flows around black holes invariably undergo a transition from a magnetohydrodynamic (MHD) turbulent disklike flow to an inward-plunging flow. We argue that the plunging inflow can greatly enhance the trapping of large-scale magnetic fields on the black hole, and therefore may increase the importance of the Blandford-Znajek (BZ) effect relative to previous estimates that ignore the plunge region. We support this hypothesis by constructing and analyzing a toy model of the dragging and trapping of a large-scale field by a black hole disk, revealing a strong dependence of this effect on the effective magnetic Prandtl number of the MHD turbulent disk. Furthermore, we show that the enhancement of the BZ effect depends on the geometric thickness of the accretion disk. This may be, at least in part, the physical underpinnings of the empirical relation between the inferred geometric thickness of a black hole disk and the presence of a radio jet.
Bulletin of the American Astronomical Society, Dec 1, 2005
Some black hole systems display a relativistically broadened iron emission line in their spectrum... more Some black hole systems display a relativistically broadened iron emission line in their spectrum, presumably due to fluorescence in the surface layers of the inner regions of the accretion disk. Constellation-X will allow us to observe variability in this line due to the orbital motions of inhomogenieties in the disk as well as reverberation effects (i.e., the light echo of short/intense flares across the accretion disk). We will show how these phenomena can be used as a quantitative probe of black hole mass, spin and (possibly) the validity of General Relativity.
Bulletin of the American Astronomical Society, Dec 1, 2005
Some black hole systems display a relativistically broadened iron emission line in their spectrum... more Some black hole systems display a relativistically broadened iron emission line in their spectrum, presumably due to fluorescence in the surface layers of the inner regions of the accretion disk. Constellation-X will allow us to observe variability in this line due to the orbital motions of inhomogenieties in the disk as well as reverberation effects (i.e., the light echo of short/intense flares across the accretion disk). We will show how these phenomena can be used as a quantitative probe of black hole mass, spin and (possibly) the validity of General Relativity.
Bulletin of the American Astronomical Society, May 1, 1998
Observations of the bright Seyfert 1 galaxy MCG-6-30-15 with the Rossi X-ray Timing Explorer clea... more Observations of the bright Seyfert 1 galaxy MCG-6-30-15 with the Rossi X-ray Timing Explorer clearly show evidence for the broad fluorescent iron line, and the Compton reflection continuum at higher energies. A comparison of the iron line and the reflection continuum has enabled us to begin placing constraints on the reflective fraction versus elemental abundance relationship in the accretion disk. Temporal studies with our 50ks RXTE observation provide evidence that spectral variability may be due to both intrinsic changes in the source itself as well as the amount of reprocessing. ASCA observations have shown strong evidence for a variable iron K emission line; our recent 400ks joint ASCA + RXTE observations will allow for study of the variability of the reflection continuum in detail for the first time.
Bulletin of the American Astronomical Society, May 1, 1998
Observations of the bright Seyfert 1 galaxy MCG-6-30-15 with the Rossi X-ray Timing Explorer clea... more Observations of the bright Seyfert 1 galaxy MCG-6-30-15 with the Rossi X-ray Timing Explorer clearly show evidence for the broad fluorescent iron line, and the Compton reflection continuum at higher energies. A comparison of the iron line and the reflection continuum has enabled us to begin placing constraints on the reflective fraction versus elemental abundance relationship in the accretion disk. Temporal studies with our 50ks RXTE observation provide evidence that spectral variability may be due to both intrinsic changes in the source itself as well as the amount of reprocessing. ASCA observations have shown strong evidence for a variable iron K emission line; our recent 400ks joint ASCA + RXTE observations will allow for study of the variability of the reflection continuum in detail for the first time.
(abridged) We present a detailed reanalysis of the Chandra data for the galaxy cluster Abell 4059... more (abridged) We present a detailed reanalysis of the Chandra data for the galaxy cluster Abell 4059 and its central radio galaxy, PKS2354-35. We also present new 1.4GHz and 4.7GHz CnB-array radio data from the Very Large Array (VLA), as well as a short archival WFPC2 image from the Hubble Space Telescope. The presence of a strong interaction between this radio galaxy and the intracluster medium (ICM) was suggested by Huang & Sarazin (1998) on the basis of a short observation by the High Resolution Imager on ROSAT, and confirmed in our preliminary analysis of the Chandra/ACIS-S data. In particular, X-ray imaging clearly shows two cavities within the ICM that are approximately aligned with the radio-galaxy axis. However, using our new radio maps we fail to find a detailed correspondence between the 1 arcmin scale radio-lobes and the ICM cavities. This suggests that the cavities are ``ghosts'' of a previous burst of powerful activity by PKS 2354-35. We also examine the nature of the central asymmetric ridge (or bar) of X-ray emission extending for 30kpc south-west of the cluster center that has been noted in these previous analyzes. We find the ridge to be denser and cooler than, but probably in pressure balance with, its surroundings. The thermal evolution of this structure seems to be dominated by radiative cooling, possibly enhanced by the radio-galaxy ICM interaction. We discuss several possible models for the formation of this SW ridge and find none of them to be entirely satisfactory.
I will review how current X-ray spectral and timing observations are probing astrophysics within ... more I will review how current X-ray spectral and timing observations are probing astrophysics within strong gravity around neutron stars and black holes. After a critical discussion about the extent to which such observations can probe GR, I shall discuss future prospects.
I will review how current X-ray spectral and timing observations are probing astrophysics within ... more I will review how current X-ray spectral and timing observations are probing astrophysics within strong gravity around neutron stars and black holes. After a critical discussion about the extent to which such observations can probe GR, I shall discuss future prospects.
Monthly Notices of the Royal Astronomical Society, Apr 11, 1997
I present a study of the X-ray spectral properties of a sample containing 24 type 1 active galact... more I present a study of the X-ray spectral properties of a sample containing 24 type 1 active galactic nuclei using the medium spectral resolution of ASCA. The sample consists of 20 radio-quiet objects (18 Seyfert 1 galaxies and two radio-quiet quasars) and four radio-loud objects (three broad-line radio galaxies and one radio-loud quasar). A simple power-law continuum absorbed by Galactic material provides a very poor description of the spectra of most objects. Deviations from the power-law form are interpreted in terms of X-ray reprocessing/absorption processes. In particular, at least half of the objects show K-shell absorption edges of warm oxygen (Ovii and Oviii) characteristic of optically thin, photoionized material along the line of sight to the central engine, the so-called warm absorber. The amount and presence of this absorption are found to depend on either the luminosity or radio properties of the objects: luminous and/or radio-loud objects are found to possess less ionized absorption. This ambiguity exists because the radio-loud objects are also amongst the most luminous of the sample. It is also found that objects with significant optical reddening display deep Ovii edges. The converse is true with two possible exceptions (NGC 3783 and NGC 3516). Coupled with other evidence resulting from detailed study of particular objects, this suggests the existence of dusty warm plasma. A radiatively driven outflow originating from the molecular torus is probably the source of this plasma. Rapid variability of the warm absorber also points to there being another component closer to the central source and probably situated within the broad-line region (BLR). Independent evidence for such an optically thin, highly ionized BLR component comes from detailed optical/UV studies. Spectral features at energies characteristic of cold iron Kalpha emission are common. Such emission is expected to arise from the fluorescence of cold iron in optically thick material when illuminated by the primary X-ray continuum. Radio-quiet objects have iron emission well described as originating either from the inner regions of an accretion disc or, in a small number of cases, from the molecular torus. Two of the radio-loud objects (3C 120 and 3C 382) have a much broader feature which presents problems for the relativistic disc model. The presence of radio jets may be important in forming this spectral feature.
Monthly Notices of the Royal Astronomical Society, Apr 11, 1997
I present a study of the X-ray spectral properties of a sample containing 24 type 1 active galact... more I present a study of the X-ray spectral properties of a sample containing 24 type 1 active galactic nuclei using the medium spectral resolution of ASCA. The sample consists of 20 radio-quiet objects (18 Seyfert 1 galaxies and two radio-quiet quasars) and four radio-loud objects (three broad-line radio galaxies and one radio-loud quasar). A simple power-law continuum absorbed by Galactic material provides a very poor description of the spectra of most objects. Deviations from the power-law form are interpreted in terms of X-ray reprocessing/absorption processes. In particular, at least half of the objects show K-shell absorption edges of warm oxygen (Ovii and Oviii) characteristic of optically thin, photoionized material along the line of sight to the central engine, the so-called warm absorber. The amount and presence of this absorption are found to depend on either the luminosity or radio properties of the objects: luminous and/or radio-loud objects are found to possess less ionized absorption. This ambiguity exists because the radio-loud objects are also amongst the most luminous of the sample. It is also found that objects with significant optical reddening display deep Ovii edges. The converse is true with two possible exceptions (NGC 3783 and NGC 3516). Coupled with other evidence resulting from detailed study of particular objects, this suggests the existence of dusty warm plasma. A radiatively driven outflow originating from the molecular torus is probably the source of this plasma. Rapid variability of the warm absorber also points to there being another component closer to the central source and probably situated within the broad-line region (BLR). Independent evidence for such an optically thin, highly ionized BLR component comes from detailed optical/UV studies. Spectral features at energies characteristic of cold iron Kalpha emission are common. Such emission is expected to arise from the fluorescence of cold iron in optically thick material when illuminated by the primary X-ray continuum. Radio-quiet objects have iron emission well described as originating either from the inner regions of an accretion disc or, in a small number of cases, from the molecular torus. Two of the radio-loud objects (3C 120 and 3C 382) have a much broader feature which presents problems for the relativistic disc model. The presence of radio jets may be important in forming this spectral feature.
Accreting black holes are prodigious sources of hard X-rays, supporting the notion that these obj... more Accreting black holes are prodigious sources of hard X-rays, supporting the notion that these objects support powerful coronae. I will discuss the constraints on the nature of these coronae that arise from modern spectral and timing X-ray observations. A particularly powerful probe are the relativistically broadened iron fluorescent lines observed in these systems; observations of these spectral features suggest the existence of extremely compact coronae very close to the black hole event horizon. Evidence for disk-corona coupling from observations of quasi-periodic oscillations will also be discussed.
Accreting black holes are prodigious sources of hard X-rays, supporting the notion that these obj... more Accreting black holes are prodigious sources of hard X-rays, supporting the notion that these objects support powerful coronae. I will discuss the constraints on the nature of these coronae that arise from modern spectral and timing X-ray observations. A particularly powerful probe are the relativistically broadened iron fluorescent lines observed in these systems; observations of these spectral features suggest the existence of extremely compact coronae very close to the black hole event horizon. Evidence for disk-corona coupling from observations of quasi-periodic oscillations will also be discussed.
Accreting black holes are prodigious sources of hard X-rays, supporting the notion that these obj... more Accreting black holes are prodigious sources of hard X-rays, supporting the notion that these objects support powerful coronae. I will discuss the constraints on the nature of these coronae that arise from modern spectral and timing X-ray observations. A particularly powerful probe are the relativistically broadened iron fluorescent lines observed in these systems; observations of these spectral features suggest the existence of extremely compact coronae very close to the black hole event horizon. Evidence for disk-corona coupling from observations of quasi-periodic oscillations will also be discussed.
The existence of the radius of marginal stability means that accretion flows around black holes i... more The existence of the radius of marginal stability means that accretion flows around black holes invariably undergo a transition from a magnetohydrodynamic (MHD) turbulent disklike flow to an inward-plunging flow. We argue that the plunging inflow can greatly enhance the trapping of large-scale magnetic fields on the black hole, and therefore may increase the importance of the Blandford-Znajek (BZ) effect relative to previous estimates that ignore the plunge region. We support this hypothesis by constructing and analyzing a toy model of the dragging and trapping of a large-scale field by a black hole disk, revealing a strong dependence of this effect on the effective magnetic Prandtl number of the MHD turbulent disk. Furthermore, we show that the enhancement of the BZ effect depends on the geometric thickness of the accretion disk. This may be, at least in part, the physical underpinnings of the empirical relation between the inferred geometric thickness of a black hole disk and the presence of a radio jet.
The existence of the radius of marginal stability means that accretion flows around black holes i... more The existence of the radius of marginal stability means that accretion flows around black holes invariably undergo a transition from a magnetohydrodynamic (MHD) turbulent disklike flow to an inward-plunging flow. We argue that the plunging inflow can greatly enhance the trapping of large-scale magnetic fields on the black hole, and therefore may increase the importance of the Blandford-Znajek (BZ) effect relative to previous estimates that ignore the plunge region. We support this hypothesis by constructing and analyzing a toy model of the dragging and trapping of a large-scale field by a black hole disk, revealing a strong dependence of this effect on the effective magnetic Prandtl number of the MHD turbulent disk. Furthermore, we show that the enhancement of the BZ effect depends on the geometric thickness of the accretion disk. This may be, at least in part, the physical underpinnings of the empirical relation between the inferred geometric thickness of a black hole disk and the presence of a radio jet.
Bulletin of the American Astronomical Society, Dec 1, 2005
Some black hole systems display a relativistically broadened iron emission line in their spectrum... more Some black hole systems display a relativistically broadened iron emission line in their spectrum, presumably due to fluorescence in the surface layers of the inner regions of the accretion disk. Constellation-X will allow us to observe variability in this line due to the orbital motions of inhomogenieties in the disk as well as reverberation effects (i.e., the light echo of short/intense flares across the accretion disk). We will show how these phenomena can be used as a quantitative probe of black hole mass, spin and (possibly) the validity of General Relativity.
Bulletin of the American Astronomical Society, Dec 1, 2005
Some black hole systems display a relativistically broadened iron emission line in their spectrum... more Some black hole systems display a relativistically broadened iron emission line in their spectrum, presumably due to fluorescence in the surface layers of the inner regions of the accretion disk. Constellation-X will allow us to observe variability in this line due to the orbital motions of inhomogenieties in the disk as well as reverberation effects (i.e., the light echo of short/intense flares across the accretion disk). We will show how these phenomena can be used as a quantitative probe of black hole mass, spin and (possibly) the validity of General Relativity.
Bulletin of the American Astronomical Society, May 1, 1998
Observations of the bright Seyfert 1 galaxy MCG-6-30-15 with the Rossi X-ray Timing Explorer clea... more Observations of the bright Seyfert 1 galaxy MCG-6-30-15 with the Rossi X-ray Timing Explorer clearly show evidence for the broad fluorescent iron line, and the Compton reflection continuum at higher energies. A comparison of the iron line and the reflection continuum has enabled us to begin placing constraints on the reflective fraction versus elemental abundance relationship in the accretion disk. Temporal studies with our 50ks RXTE observation provide evidence that spectral variability may be due to both intrinsic changes in the source itself as well as the amount of reprocessing. ASCA observations have shown strong evidence for a variable iron K emission line; our recent 400ks joint ASCA + RXTE observations will allow for study of the variability of the reflection continuum in detail for the first time.
Bulletin of the American Astronomical Society, May 1, 1998
Observations of the bright Seyfert 1 galaxy MCG-6-30-15 with the Rossi X-ray Timing Explorer clea... more Observations of the bright Seyfert 1 galaxy MCG-6-30-15 with the Rossi X-ray Timing Explorer clearly show evidence for the broad fluorescent iron line, and the Compton reflection continuum at higher energies. A comparison of the iron line and the reflection continuum has enabled us to begin placing constraints on the reflective fraction versus elemental abundance relationship in the accretion disk. Temporal studies with our 50ks RXTE observation provide evidence that spectral variability may be due to both intrinsic changes in the source itself as well as the amount of reprocessing. ASCA observations have shown strong evidence for a variable iron K emission line; our recent 400ks joint ASCA + RXTE observations will allow for study of the variability of the reflection continuum in detail for the first time.
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