arXiv subjects

T. Yaqoob

Publications and source records attributed to T. Yaqoob.

At least 55 records · Page 3Linked to original sources

arXiv2003

A warm hot intergalactic medium towards 3C 120?

We observed the Seyfert I active galaxy/broad line radio galaxy 3C120 with the Chandra high energy transmission gratings and present an analysis of the soft X-ray spectrum. We identify the strongest absorption feature (detected at >99.9% confidence) with O VIII Lya (FWHM=1010^{+295}_{-265} km/s), blueshifted by -5500 +/- 140 km/s from systemic velocity. The absorption may be due to missing baryons in warm/hot intergalactic medium (WHIGM) along the line-of-sight to 3C 120 at z=0.0147 +/- 0.0005, or it could be intrinsic to the jet of 3C 120. Assuming metallicities of 0.1 solar, we estimate an ionic column density of N_{O VIII}>3.4 \times 10^{16} cm^{-2} for WHIGM and a filament depth of <19 h^{-1}_{70} Mpc. We find a baryon overdensity >56 relative to the critical density of a $Λ$-dominated cold dark matter universe, which is in reasonable agreement with WHIGM simulations. We detect, at marginal significance, absorption of O VIII Lya at z\sim 0 due to a hot medium in the Local Group. We also detect an unidentified absorption feature at \sim 0.71 keV. Absorption features which might be expected along with O VIII Lya, were not significant statistically. Relative abundances of metals in the WHIGM and local absorbers may therefore be considerably different from solar.

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arXiv2003

Fe XXV and Fe XXVI Diagnostics of the Black Hole and Accretion Disk in Active Galaxies: Chandra Time-Resolved Spectroscopy of NGC 7314

We report the detection of Fe XXV and Fe XXVI $Kα$ emission lines from a Chandra HETGS observation of the Seyfert~1 galaxy NGC 7314, made simultaneously with RXTE. The lines are redshifted (cz ~ 1500 km/s) relative to the systemic velocity and unresolved. We argue that the lines originate in a near face-on (<7 degrees) disk having a radial line emissivity flatter than r^-2. Line emission from ionization states of Fe in the range ~Fe I up to Fe XXVI is observed. The ionization balance of Fe responds to continuum variations on timescales less than 12.5 ks, supporting an origin of the lines close to the X-ray source. We present additional, detailed diagnostics from this rich data set. These results identify NGC 7314 as a key source to study in the future if we are to pursue reverberation mapping of space-time near black-hole event horizons. This is because it is first necessary to understand the ionization structure of accretion disks and the relation between the X-ray continuum and Fe K line emission. However, we also describe how our results are suggestive of a means of measuring black-hole spin without a knowledge of the relation between the continuum and line emission. Finally, these data emphasize that one {\it can} study strong gravity with narrow (as opposed to very broad) disk lines. In fact narrow lines offer higher precision, given sufficient energy resolution.

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arXiv2003

A Puzzling X-Ray Source Found in the chandra Deep Field South

In this letter we report the detection of an extremely strong X-ray emission line in the 940ks chandra ACIS-I spectrum of CXO CDFS J033225.3-274219. The source was identified as a Type1 AGN at redshift of z = 1.617, with 2.0 -- 10.0 keV rest frame X-ray luminosity of ~ 10^44 ergs s^-1. The emission line was detected at 6.2^{+0.2}_{-0.1} keV, with an equivalent width (EW) of 4.4^{+3.2}_{-1.4} keV, both quantities referring to the observed frame. In the rest frame, the line is at 16.2^{+0.4}_{-0.3} keV with an EW of 11.5^{+8.3}_{-3.7} keV. An X-ray emission line at similar energy (~ 17 keV, rest frame) in QSO PKS 2149-306 was discovered before using ASCA data. We reject the possibility that the line is due to a statistical or instrumental artifact. The line is most likely due to blueshifted Fe-K emission from an relativistic outflow, probably an inner X-ray jet, with velocities of the order of ~ 0.6-0.7c. Other possible explanations are also discussed.

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arXiv2003

Simultaneous UV and X-ray Spectroscopy of the Seyfert 1 Galaxy NGC 5548. I. Physical Conditions in the UV Absorbers

We present new UV spectra of the nucleus of the Seyfert 1 galaxy NGC 5548, which we obtained with the Space Telescope Imaging Spectrograph at high spectral resolution, in conjunction with simultaneous Chandra X-ray Observatory spectra. Taking advantage of the low UV continuum and broad emission-line fluxes, we have determined that the deepest UV absorption component covers at least a portion of the inner, high-ionization narrow-line region (NLR). We find nonunity covering factors in the cores of several kinematic components, which increase the column density measurements of N V and C IV by factors of 1.2 to 1.9 over the full-covering case; however, the revised columns have only a minor effect on the parameters derived from our photoionization models. For the first time, we have simultaneous N V and C IV columns for component 1 (at -1040 km/s), and find that this component cannot be an X-ray warm absorber, contrary to our previous claim based on nonsimultaneous observations. We find that models of the absorbers based on solar abundances severely overpredict the O VI columns previously obtained with the Far Ultraviolet Spectrograph, and present arguments that this is not likely due to variability. However, models that include either enhanced nitrogen (twice solar) or dust, with strong depletion of carbon in either case, are successful in matching all of the observed ionic columns. These models result in substantially lower ionization parameters and total column densities compared to dust-free solar-abundance models, and produce little O VII or O VIII, indicating that none of the UV absorbers are X-ray warm absorbers.

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arXiv2003

The Kinematics and Physical Conditions of the Ionized Gas in NGC 4593 from Chandra High-Energy Grating Spectroscopy

We observed the Seyfert 1 galaxy NGC 4593 with the Chandra high energy transmission gratings and present a detailed analysis of the soft X-ray spectrum. We measure strong absorption lines from He-like O, Ne, Mg, Si, H-like N, O, Ne, Mg, Si and highly ionized Fe XIX-XXV. Only the NLya, OLya and MgLya lines appear to be marginally resolved. We identify a spectral feature at $\sim 0.707$ keV with a neutral Fe L edge, which might suggest that there is dust along the line-of-sight to \ngc. Neutral Si absorption, which might also be expected from absorption due to dust is present (though not significantly) in the form of a weak neutral Si edge. We also detect, at marginal signficance, NLya and O VII (r) 1s-2p ($λ21.602 $) absorption at $z \sim 0$, due to a hot medium in our Local Group. The soft X-ray spectrum of NGC 4593 is adequately described by a simple, single-zone photoionized absorber with an equivalent Hydrogen column density of $5.37^{+1.45}_{-0.79} \times 10^{21} \ \rm{cm}^{-2}$ and an ionization parameter of $\log ξ= 2.52^{+0.06}_{-0.04}$ ergs cm $\rm{s}^{-1}$ although there remain some features which are not identified. Although the photoionized gas almost certainly is comprised of matter in more than one ionization state and may consist of several kinematic components, data with better signal-to-noise ratio and better spectral resolution are required to justify a more complex model.

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arXiv2002

New Chandra/HST-STIS results on Seyfert I AGN: Photoionized Outflows

We present soft X-ray results from observations with the Chandra HETGS of the Seyfert I AGN NGC 4593 and Mrk 509. We discuss the photoionized outflows associated with Seyfert I AGN in terms of their absorption spectral signatures and discuss their kinematics, column density and ionization state. We discuss the link between UV and X-ray absorbers in Mrk 509 (which was simultaneously observed with Chandra and HST/STIS. We also briefly discuss the possibility of absorption due to neutral Fe embedded in the warm absorber of NGC 4593 versus an interpretation of the data in terms soft X-ray relativistic emission lines. We conclude with a summary of what is being learnt about warm absorbers in type I AGN from high resolution spectroscopy.

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arXiv2002

New {\it Chandra} Results on Seyfert I galaxies: Fe-K lines

We present measurements of the Fe-K line for nine Seyfert I galaxies using Chandra High Energy Transmission Grating Spectrometer (HETGS) data. The centroid energies are narrowly dispersed (6.403 +/- 0.062 keV) and indicate an origin of the line cores in cold matter. If {\it all} the lines in this sample were the peaks of a relativistically broadened disk line, it would require unrealistically fine tuning. However, at least three of the nine AGN clearly show a complex Fe-K line with an underlying broad component, possibly from a disk. In the others, an apparently narrow Fe-K line (even if it is resolved by Chandra) may still be due to the peak of a disk line. To distinguish this scenario from an origin in distant matter requires variability information.

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arXiv2002

Chandra Grating Observations of AGN

The highest spectral resolution data for the Fe-K lines in type I AGN, as observed with the Chandra High Energy Grating (HEG), reveal a variety of line shapes. However, the energies of the most prominent peak are all clustered tightly around 6.4 keV (weighted mean $6.403 \pm 0.062$ keV). If all the peaks were part of single, relativistically broadened disk line, this would require unrealistically fine tuning. Thus, some of the cores must originate in distant matter (e.g. BLR, NLR, torus). On the other hand, in at least two AGN, the emission at 6.4 keV has been seen to vanish on short timescales, indicating an origin close to the central engine. For one of these (Mrk 509) this is puzzling because the HEG and simultaneous RXTE time-averaged spectra indicate only a narrow line was present at that time. Simultaneous HEG/RXTE observations for NGC 4593 indicate a broad and narrow Fe-K complex, all originating in neutral Fe, and for F9 show a narrow, neutral Fe-K component plus a broad, He-like component. The latter signature has been observed in three other AGN by XMM and may be quite common.

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arXiv2002

The Kinematics and Physical Conditions of the Ionized Gas in Markarian 509. I. Chandra High-Energy Grating Spectroscopy

We observed the Seyfert 1 galaxy Mrk 509 simultaneously with Chandra/HETG, HST/STIS and RXTE. We measure strong absorption lines from He-like Ne and Mg, and from H-like N, O, and Ne. The lines are unresolved except for Ne Lya and Ne IX(r). The absorption-line profiles are blueshifted by ~200 km/s with respect to the systemic velocity of Mrk 509, and there is a hint of another component near systemic. The soft X-ray spectrum can be described in remarkable detail with a simple, single-zone photoionized absorber with a column density of 2.06 (+0.39,-0.45) x 10^21 cm^-2 and an ionization parameter of log{xi} = 1.76 (+0.13,-0.14) (or log{U}=0.27). The UV data, on the other hand, can easily detect eight kinematic components, covering roughly the same velocities as the X-ray absorption profiles. Even though the X-ray and UV absorbers share the same velocity space, the UV absorbers have a much smaller column density and ionization state. We show that models of the X-ray data do not predict significant UV absorption and are therefore consistent with the UV data. The UV absorbers could be embedded in the X-ray absorber, as a multiphase medium. Finally, we do not detect any soft X-ray emission lines.

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arXiv2002

The Kinematics and Physical Conditions pf the Ionized Gas in Markarian 509. II. STIS Echelle Observations

We present observations of the UV absorption lines in the luminous Seyfert 1 galaxy Mrk 509, obtained with the medium resolution (lambda/Delta-lambda ~ 40,000) echelle gratings of the Space Telescope Imaging Spectrograph on the Hubble Space Telescope. The spectra reveal the presence of eight kinematic components of absorption in Ly-alpha, C IV, and N V, at radial velocities of -422, -328, -259, -62, -22, +34, +124, and +210 km s^-1 with respect to an emission-line redshift of z = 0.03440, seven of which were detected in an earlier Far Ultraviolet Spectrographic Explorer (FUSE) spectrum. The component at -22 km s^-1 also shows absorption by Si IV. The covering factor and velocity width of the Si IV lines were lower than those of the higher ionization lines for this component, which is evidence for two separate absorbers at this velocity. We have calculated photoionization models to match the UV column densities in each of these components. Using the predicted O VI column densities, we were able to match the O VI profiles observed in the FUSE spectrum. Based on our results, none of the UV absorbers can produce the X-ray absorption seen in simultaneous Chandra observations; therefore, there must be more highly ionized gas in the radial velocity ranges covered by the UV absorbers.

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arXiv2002

Narrow Components within the Fe Kalpha Profile of NGC 3516: Evidence for the Importance of General Relativistic Effects?

We present results from a simultaneous Chandra HETG and XMM-Newton observation of NGC 3516. We find evidence for several narrow components of Fe Kalpha along with a broad line. We consider the possibility that the lines arise in an blob of material ejected from the nucleus with velocity ~0.25c. We also consider an origin in a neutral accretion disk, suffering enhanced illumination at 35 and 175 gravitational radii, perhaps due to magnetic reconnection. The presence of these narrow features indicates there is no Comptonizing region along the line-of-sight to the nucleus. This in turn is compelling support for the hypothesis that broad Fe Kalpha components are, in general, produced by strong gravity.

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arXiv2001

The Effect of Changes in the ASCA Calibration on the Fe-Kalpha Lines in Active Galaxies

The ASCA calibration has evolved considerably since launch and indeed, is still evolving. There have been concerns in the literature that changes in the ASCA calibration have resulted in the Fe-Kalpha lines in active galaxies (AGN) now being systematically narrower than was originally thought. If this were true, a large body of ASCA results would be impacted. In particular, it has been claimed that the broad red wing (when present) of the Fe-Kalpha line has been considerably weakened by changes in the ASCA calibration. We demonstrate explicitly that changes in the ASCA calibration over a period of about eight years have a negligible effect on the width, strength, or shape of the Fe-Kalpha lines. The reduction in both width and equivalent width is only ~8% or less. We confirm this with simulations and individual sources, as well as sample average profiles. The average profile for type 1 AGN is still very broad, with the red wing extending down to ~4 keV. The reason for the claimed, apparently large, discrepancies is that in some sources the \fekalfa line is complex, and a single-Gaussian model, being an inadequate description of the line profile, picks up different portions of the profile with different calibration. Single-Gaussian fits do not therefore model all of the line emission in some sources, in which case they do not compare old and current calibration since the models do not then describe the data.

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arXiv2001

The Detection of Circumnuclear X-ray Emission from the Seyfert Galaxy NGC 3516

We present the first high-resolution, X-ray image of the circumnuclear regions of the Seyfert 1 galaxy NGC 3516, using the Chandra X-ray Observatory (CXO). All three of the CXO observations reported were performed with one of the two grating assemblies in place, and here we restrict our analysis to undispersed photons (i.e. those detected in the zeroth-order). A previously-unknown X-ray source is detected approx 6 arcsec (1.1 h_{75}^{-1} kpc) NNE of the nucleus (position angle approx 29 degrees) which we designate CXOU 110648.1+723412. Its spectrum can be characterized as a power law with a photon index Gamma ~ 1.8--2.6, or as thermal emission with a temperature kT~0.7--3 keV. Assuming a location within NGC 3516, isotropic emission implies a luminosity L ~ 2--8e39 h_{75}^{-2} erg s^{-1} in the 0.4--2 keV band. If due to a single point source, the object is super-Eddington for a 1.4 M_sun neutron star. However, multiple sources or a small, extended source cannot be excluded using the current data. Large-scale extended X-ray emission is also detected out to ~10 arcsec (~1.7 h_{75}^{-1} kpc) from the nucleus to the NE and SW, and is approximately aligned with the morphologies of the radio emission and extended narrow emission line region (ENLR). The mean luminosity of this emission is 1--5e37 h_{75}^{-2} erg s^{-1} arcsec^{-2}, in the 0.4--2 keV band. Unfortunately the current data cannot usefully constrain its spectrum. These results are consistent with earlier suggestions of circumnuclear X-ray emission in NGC 3516. If the extended emission is due to scattering of the nuclear X-ray continuum, then the pressure in the X-ray emitting gas is at least two orders of magnitude too small to provide the confining medium for the ENLR clouds.

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arXiv2001

The Spectral Energy Distribution of the Seyfert galaxy Ton S180

We present spectral results from a multi-satellite, broad-band campaign on the Narrow-line Seyfert 1 galaxy Ton S180 performed at the end of 1999. We discuss the spectral-energy distribution of the source, combining simultaneous Chandra, ASCA and EUVE data with contemporaneous FUSE, HST, and ground-based optical and infra-red data. The resulting SED shows that most of the energy is emitted in the 10 -- 100 eV regime, which must be dominated by the primary energy source. No spectral turnover is evident in the UV regime. This, the strong soft X-ray emission, and the overall shape of the SED indicate that emission from the accretion disk peaks between 15 and 100 eV. High resolution FUSE spectra showing UV absorption due to O VI and the lack of detectable X-ray absorption in the Chandra spectrum demonstrate the presence of a low column density of highly ionized gas along our line-of-sight. The highly-ionized state of the circumnuclear gas is most likely linked to the high luminosity and steep spectrum of the active nucleus. Given the strong ionizing flux in Ton S180, it is possible that the clouds within a few tens of light days of the central source are too highly ionized to produce much line emission. Thus the narrow width of the emission lines in Ton S180 is due to the emission arising from large radii.

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arXiv2001

Subtleties in Measuring Iron K Lines in AGN

Chandra and XMM observations are showing that the Fe-K emission lines in type 1 AGN are composite, in general consisting of a narrow and a broad component. We review the latest Chandra HETG results and compare the line profiles with those measured by ASCA. The narrow Fe-K line components necessitate re-modeling of the line profiles measured previously and revision of the parameters of the relativistic broad components, as well re-interpretation of variability studies. We also explicitly demonstrate the effect of changes in the ASCA calibration on the measurements of the Fe-K lines in AGN. We find that the differences in measured parameters are insignificant. Both the intrinsic width and EW change by less than 8%. Fe-K line studies which claim much larger differences do not adequately model the line emission. We point out that it is incorrect to compare different calibrations, and indeed make astrophysical inferences, using models which do not describe the data. We then present some new results for the highest S/N broad Fe-K line profile in the entire ASCA archive, from NGC 4151.

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arXiv2001

Chandra Grating Observations of Seyfert 1 Galaxies

We present new results from a Chandra HETG observation of NGC 5548 and give a comparison of absorption and emission features found in other Seyfert 1 galaxies using Chandra grating observations. Deep soft X-ray edges are seen in Mkn 509 and NGC 3783, consistent with ASCA data. In NGC 5548 and NGC 4501 the edges are weak but consistent with the low column densities. We show that the detection of a narrow, probably non-disk, component of the iron line is very common. However, just as importantly, it is not detected in some cases. We show the effect of removing this narrow component from the ASCA Fe-K line profile in NGC 4151, revealing the underlying true shape of the relativistic Fe-K line component.

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arXiv2001

Quasi-Periodic Oscillation in Seyfert galaxies: Significance levels. The Case of Mrk 766

We discuss methods to compute significance levels for the existence of quasi-periodic oscillations (QPOs) in Active Galactic Nuclei (AGN) which take the red-noise character of the X-ray lightcurves of these objects into account. Applying epoch folding and periodogram analysis to the XMM-Newton observation of the Seyfert galaxy Mrk 766, a possible QPO at a timescale of 4200s has been reported. Our computation of the significance of this QPO, however, shows that the 4200s peak is not significant at the 95% level. We conclude that the 4200s feature is an artifact of the red-noise process and not the result of a physical process within the Active Galactic Nuclei.

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arXiv2001

The Future of Fe-K Line Diagnostics for Probing Strong Gravity

We review what we have learnt with ASCA from studying the Fe-K lines in AGN and describe a program to deconvolve the narrow, non-disk components of the lines with Chandra. This is necessary to derive the correct profiles of the broad, relativistic lines obtained using data from XMM and other high-throughput instruments. Since reverberation techniques are now not looking promising, we present Constellation-X simulations showing an alternative way we might be able to measure black-hole mass and spin.

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