arXiv subjects

A. C. Fabian

Publications and source records attributed to A. C. Fabian.

At least 325 records · Page 18Linked to original sources

arXiv2013

Dissecting X-ray-emitting Gas around the Center of our Galaxy

Most supermassive black holes (SMBHs) are accreting at very low levels and are difficult to distinguish from the galaxy centers where they reside. Our own Galaxy's SMBH provides a uniquely instructive exception, and we present a close-up view of its quiescent X-ray emission based on 3 mega-second of Chandra observations. Although the X-ray emission is elongated and aligns well with a surrounding disk of massive stars, we can rule out a concentration of low-mass coronally active stars as the origin of the emission based on the lack of predicted Fe Kalpha emission. The extremely weak H-like Fe Kalpha line further suggests the presence of an outflow from the accretion flow onto the SMBH. These results provide important constraints for models of the prevalent radiatively inefficient accretion state.

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arXiv2013

NuSTAR Spectroscopy of GRS 1915+105: Disk Reflection, Spin, and Connections to Jets

We report on the results of spectral fits made to a NuSTAR observation of the black hole GRS 1915+105 in a "plateau" state. This state is of special interest because it is similar to the "low/hard" state seen in other black holes, especially in that compact, steady jets are launched in this phase. The 3-79 keV bandpass of NuSTAR, and its ability to obtain moderate-resolution spectra free from distortions such as photon pile-up, are extremely well suited to studies of disk reflection in X-ray binaries. In only 15 ks of net exposure, an extraordinarily sensitive spectrum of GRS 1915+105 was measured across the full bandpass. Ionized reflection from a disk around a rapidly-spinning black hole is clearly required to fit the spectra; even hybrid Comptonization models including ionized reflection from a disk around a Schwarzschild black hole proved inadequate. A spin parameter of a = 0.98 +/- 0.01 (1-sigma statistical error) is measured via the best-fit model; low spins are ruled out at a high level of confidence. This result suggests that jets can be launched from a disk extending to the innermost stable circular orbit. A very steep inner disk emissivity profile is also measured, consistent with models of compact coronae above Kerr black holes. These results support an emerging association between the hard X-ray corona and the base of the relativistic jet.

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arXiv2013

An XMM-Newton view of the merging activity in the Centaurus cluster

We report the results of XMM-Newton observations of the regions around the core of the Centaurus cluster where evidence for merging activity between the subgroup Cen 45 and the main Centaurus cluster has previously been observed using ASCA and ROSAT data. We confirm the ASCA findings of a temperature excess surrounding Cen 45. We find that this temperature excess can be explained using simple shock heating given the large line of sight velocity difference between Cen 45 and the surrounding main Centaurus cluster. We find that there is a statistically significant excess in metallicity around Cen 45, showing that Cen 45 has managed to retain its gas as it has interacted with the main Centaurus cluster. There is a pressure excess to the east in the direction of the merger, and there is also an entropy excess around the central galaxy of Cen 45. The metallicity between 50-100 kpc to the north of NGC 4696 is higher than to the south, which may be the result of the asymmetric distribution of metals due to previous sloshing of the core, or which may be associated with the filamentary structure we detect between NGC 4696 and NGC 4696B.

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arXiv2013

A multi-wavelength view of cooling vs. AGN heating in the X-ray luminous cool-core of Abell 3581

We report the results of a multi-wavelength study of the nearby galaxy group, Abell 3581 (z=0.0218). This system hosts the most luminous cool core of any nearby group and exhibits active radio mode feedback from the super-massive black hole in its brightest group galaxy, IC 4374. The brightest galaxy has suffered multiple active galactic nucleus outbursts, blowing bubbles into the surrounding hot gas, which have resulted in the uplift of cool ionised gas into the surrounding hot intragroup medium. High velocities, indicative of an outflow, are observed close to the nucleus and coincident with the radio jet. Thin dusty filaments accompany the uplifted, ionised gas. No extended star formation is observed, however, a young cluster is detected just north of the nucleus. The direction of rise of the bubbles has changed between outbursts. This directional change is likely due to sloshing motions of the intragroup medium. These sloshing motions also appear to be actively stripping the X-ray cool core, as indicated by a spiraling cold front of high metallicity, low temperature, low entropy gas.

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arXiv2013

Azimuthally Resolved X-Ray Spectroscopy to the Edge of the Perseus Cluster

We present the results from extensive, new observations of the Perseus Cluster of galaxies, obtained as a Suzaku Key Project. The 85 pointings analyzed span eight azimuthal directions out to 2 degrees = 2.6 Mpc, to and beyond the virial radius r_200 ~ 1.8 Mpc, offering the most detailed X-ray observation of the intracluster medium (ICM) at large radii in any cluster to date. The azimuthally averaged density profile for r>0.4r_200 is relatively flat, with a best-fit power-law index of 1.69+/-0.13 significantly smaller than expected from numerical simulations. The entropy profile in the outskirts lies systematically below the power-law behavior expected from large-scale structure formation models which include only the heating associated with gravitational collapse. The pressure profile beyond ~0.6r_200 shows an excess with respect to the best-fit model describing the SZ measurements for a sample of clusters observed with Planck. The inconsistency between the expected and measured density, entropy, and pressure profiles can be explained primarily by an overestimation of the density due to inhomogeneous gas distribution in the outskirts; there is no evidence for a bias in the temperature measurements within the virial radius. We find significant differences in thermodynamic properties of the ICM at large radii along the different arms. Along the cluster minor axis, we find a flattening of the entropy profiles outside ~0.6r_200, while along the major axis, the entropy rises all the way to the outskirts. Correspondingly, the inferred gas clumping factor is typically larger along the minor than along the major axis.

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arXiv2013

Thermodynamics of the Coma Cluster Outskirts

We present results from a large mosaic of Suzaku observations of the Coma Cluster, the nearest and X-ray brightest hot, dynamically active, non-cool core system, focusing on the thermodynamic properties of the ICM on large scales. For azimuths not aligned with an infalling subcluster towards the southwest, our measured temperature and X-ray brightness profiles exhibit broadly consistent radial trends, with the temperature decreasing from about 8.5 keV at the cluster center to about 2 keV at a radius of 2 Mpc, which is the edge of our detection limit. The SW merger significantly boosts the surface brightness, allowing us to detect X-ray emission out to ~2.2 Mpc along this direction. Apart from the southwestern infalling subcluster, the surface brightness profiles show multiple edges around radii of 30-40 arcmin. The azimuthally averaged temperature profile, as well as the deprojected density and pressure profiles, all show a sharp drop consistent with an outward propagating shock front located at 40 arcmin, corresponding to the outermost edge of the giant radio halo observed at 352 MHz with the WSRT. The shock front may be powering this radio emission. A clear entropy excess inside of r_500 reflects the violent merging events linked with these morphological features. Beyond r_500, the entropy profiles of the Coma Cluster along the relatively relaxed directions are consistent with the power-law behavior expected from simple models of gravitational large-scale structure formation. The pressure is also in agreement at these radii with the expected values measured from SZ data from the Planck satellite. However, due to the large uncertainties associated with the Coma Cluster measurements, we cannot yet exclude an entropy flattening in this system consistent with that seen in more relaxed cool core clusters.

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arXiv2013

Weak Hard X-ray Emission from Two Broad Absorption Line Quasars Observed with NuSTAR: Compton-thick Absorption or Intrinsic X-ray Weakness?

We present NuSTAR hard X-ray observations of two X-ray weak broad absorption line (BAL) quasars, PG 1004+130 (radio loud) and PG 1700+518 (radio quiet). Many BAL quasars appear X-ray weak, probably due to absorption by the shielding gas between the nucleus and the accretion-disk wind. The two targets are among the optically brightest BAL quasars, yet they are known to be significantly X-ray weak at rest-frame 2-10 keV (16-120 times fainter than typical quasars). We would expect to obtain ~400-600 hard X-ray (>10 keV) photons with NuSTAR, provided that these photons are not significantly absorbed (NH<1E24 cm^{-2}). However, both BAL quasars are only detected in the softer NuSTAR bands (e.g., 4-20 keV) but not in its harder bands (e.g., 20-30 keV), suggesting that either the shielding gas is highly Compton-thick or the two targets are intrinsically X-ray weak. We constrain the column densities for both to be NH~7E24 cm^{-2} if the weak hard X-ray emission is caused by obscuration from the shielding gas. We discuss a few possibilities for how PG 1004+130 could have Compton-thick shielding gas without strong Fe K? line emission; dilution from jet-linked X-ray emission is one likely explanation. We also discuss the intrinsic X-ray weakness scenario based on a coronal-quenching model relevant to the shielding gas and disk wind of BAL quasars. Motivated by our NuSTAR results, we perform a Chandra stacking analysis with the Large Bright Quasar Survey BAL quasar sample and place statistical constraints upon the fraction of intrinsically X-ray weak BAL quasars; this fraction is likely 17-40%.

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arXiv2013

The origin of blue-shifted absorption features in the X-ray spectrum of PG 1211+143: Outflow or disc?

In some radio-quiet active galaxies (AGN), high-energy absorption features in the x-ray spectra have been interpreted as Ultrafast Outflows (UFOs) -- highly ionised material (e.g. Fe XXV and Fe XXVI) ejected at mildly relativistic velocities. In some cases, these outflows can carry energy in excess of the binding energy of the host galaxy. Needless to say, these features demand our attention as they are strong signatures of AGN feedback and will influence galaxy evolution. For the same reason, alternative models need to be discussed and refuted or confirmed. Gallo & Fabian proposed that some of these features could arise from resonance absorption of the reflected spectrum in a layer of ionised material located above and corotating with the accretion disc. Therefore, the absorbing medium would be subjected to similar blurring effects as seen in the disc. A priori, the existence of such plasma above the disc is as plausible as a fast wind. In this work, we highlight the ambiguity by demonstrating that the absorption model can describe the ~7.6 keV absorption feature (and possibly other features) in the quasar PG 1211+143, an AGN that is often described as a classic example of an UFO. In this model, the 2-10 keV spectrum would be largely reflection dominated (as opposed to power law dominated in the wind models) and the resonance absorption would be originating in a layer between about 6 and 60 gravitational radii. The studies of such features constitutes a cornerstone for future X-ray observatories like Astro-H and Athena+. Should our model prove correct, or at least important in some cases, then absorption will provide another diagnostic tool with which to probe the inner accretion flow with future missions.

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arXiv2013

Discovery of high-frequency iron K lags in Ark 564 and Mrk 335

We use archival XMM-Newton observations of Ark 564 and Mrk 335 to calculate the frequency dependent time-lags for these two well-studied sources. We discover high-frequency Fe K lags in both sources, indicating that the red wing of the line precedes the rest frame energy by roughly 100 s and 150 s for Ark 564 and Mrk 335, respectively. Including these two new sources, Fe K reverberation lags have been observed in seven Seyfert galaxies. We examine the low-frequency lag-energy spectrum, which is smooth, and shows no feature of reverberation, as would be expected if the low-frequency lags were produced by distant reflection off circumnuclear material. The clear differences in the low and high frequency lag-energy spectra indicate that the lags are produced by two distinct physical processes. Finally, we find that the amplitude of the Fe K lag scales with black hole mass for these seven sources, consistent with a relativistic reflection model where the lag is the light travel delay associated with reflection of continuum photons off the inner disc.

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arXiv2013

The Hot and Energetic Universe: AGN feedback in galaxy clusters and groups

Mechanical feedback via Active Galactic Nuclei (AGN) jets in the centres of galaxy groups and clusters is a crucial ingredient in current models of galaxy formation and cluster evolution. Jet feedback is believed to regulate gas cooling and thus star formation in the most massive galaxies, but a robust physical understanding of this feedback mode is currently lacking. The large collecting area, excellent spectral resolution and high spatial resolution of Athena+ will provide the breakthrough diagnostic ability necessary to develop this understanding, via: (1) the first kinematic measurements on relevant spatial scales of the hot gas in galaxy, group and cluster haloes as it absorbs the impact of AGN jets, and (2) vastly improved ability to map thermodynamic conditions on scales well-matched to the jets, lobes and gas disturbances produced by them. Athena+ will therefore determine for the first time how jet energy is dissipated and distributed in group and cluster gas, and how a feedback loop operates in group/cluster cores to regulate gas cooling and AGN fuelling. Athena+ will also establish firmly the cumulative impact of powerful radio galaxies on the evolution of baryons from the epoch of group/cluster formation to the present day.

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arXiv2013

X-ray Outflows and Super-Eddington Accretion in the Ultraluminous X-ray Source Holmberg IX X-1

Studies of X-ray continuum emission and flux variability have not conclusively revealed the nature of ultra-luminous X-ray sources (ULXs) at the high-luminosity end of the distribution (those with Lx > 1e40 erg/s). These are of particular interest because the luminosity requires either super-Eddington accretion onto a black hole of mass ~10 Msun, or more standard accretion onto an intermediate-mass black hole. Super-Eddington accretion models predict strong outflowing winds, making atomic absorption lines a key diagnostic of the nature of extreme ULXs. To search for such features, we have undertaken a long, 500 ks observing campaign on Holmberg IX X-1 with Suzaku. This is the most sensitive dataset in the iron K bandpass for a bright, isolated ULX to date, yet we find no statistically significant atomic features in either emission or absorption; any undetected narrow features must have equivalent widths less than 15-20 eV at 99% confidence. These limits are far below the >150 eV lines expected if observed trends between mass inflow and outflow rates extend into the super-Eddington regime, and in fact rule out the line strengths observed from disk winds in a variety of sub-Eddington black holes. We therefore cannot be viewing the central regions of Holmberg IX X-1 through any substantial column of material, ruling out models of spherical super-Eddington accretion. If Holmberg IX X-1 is a super-Eddington source, any associated outflow must have an anisotropic geometry. Finally, the lack of iron emission suggests that the stellar companion cannot be launching a strong wind, and that Holmberg IX X-1 must primarily accrete via roche-lobe overflow.

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arXiv2013

Searching for the missing iron in the core of the Centaurus cluster

We re-analyse a combined 198 ks Chandra observation of NGC4696, the brightest galaxy of the Centaurus cluster. We extract temperature and metallicity profiles from the data, and we confirm the presence of a sharp drop in iron abundance, from ~1.8 Zsolar to ~0.4 Zsolar, within the central 5 kpc of the cluster. We estimate that this abundance drop corresponds to a total "missing" iron mass of 1.4e06 Msolar. We propose that part of this missing iron is locked up in cool (~19 K) far-IR emitting dust, as found by Spitzer and Herschel observations. This can occur if the iron injected by stellar mass loss in the central region is in grains, which remain in that form as the injected dusty cold gas mixes and joins the cold dusty filamentary nebula observed within the same region. The bubbling feedback process observed in the cluster core then drags filaments outward and dumps them at 10-20 kpc radius, where the metallicity is high.

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arXiv2013

VLT/FORS2 observations of four high-luminosity ULX candidates

We obtained VLT/FORS2 spectra of the optical counterparts of four high-luminosity (L_X >= 10^40 erg/s) ULX candidates from the catalog of Walton (2011). We first determined accurate positions for the X-ray sources from archival Chandra observations and identified counterparts in archival optical observations that are sufficiently bright for spectroscopy with an 8 meter telescope. From the spectra we determine the redshifts to the optical counterparts and emission line ratios. One of the candidate ULXs, in the spiral galaxy ESO 306-003, appears to be a bona fide ULX in an HII region. The other three sources, near the elliptical galaxies NGC 533 and NGC 741 and in the ring galaxy AM 0644-741, turn out to be background AGN with redshifts of 1.85, 0.88 or 1.75 and 1.40 respectively. Our findings confirm the trend of a high probability of finding background AGN for systems with a ratio of log(F_X/F_opt) in the range of -1 to 1.

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arXiv2013

Cosmological growth and feedback from supermassive black holes

We develop a simple evolutionary scenario for the growth of supermassive black holes (BHs), assuming growth due to accretion only, to learn about the evolution of the BH mass function from $z=3$ to 0 and from it calculate the energy budgets of different modes of feedback. We tune the parameters of the model by matching the derived X-ray luminosity function (XLF) with the observed XLF of active galactic nuclei. We then calculate the amount of comoving kinetic and bolometric feedback as a function of redshift, derive a kinetic luminosity function and estimate the amount of kinetic feedback and $PdV$ work done by classical double Fanaroff-Riley II (FR II) radio sources. We also derive the radio luminosity function for FR IIs from our synthesized population and set constraints on jet duty cycles. Around 1/6 of the jet power from FR II sources goes into $PdV$ work done in the expanding lobes during the time the jet is on. Anti hierarchical growth of BHs is seen in our model due to addition of an amount of mass being accreted on to all BHs independent of the BH mass. The contribution to the total kinetic feedback by active galaxies in a low accretion, kinetically efficient mode is found to be the most significant at $z<1.5$. FR II feedback is found to be a significant mode of feedback above redshifts $z\sim 1.5$, which has not been highlighted by previous studies.

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arXiv2013

1ES 1927+654: a bare Seyfert 2

1ES 1927+654 is an active galactic nucleus (AGN) that appears to defy the unification model. It exhibits a type-2 optical spectrum, but possesses little X-ray obscuration. XMM-Newton and Suzaku observations obtained in 2011 are used to study the X-ray properties of 1ES 1927+654. The spectral energy distribution derived from simultaneous optical-to-X-ray data obtained with XMM-Newton shows the AGN has a typical Eddington ratio (L/L_Edd = 0.014-0.11). The X-ray spectrum and rapid variability are consistent with originating from a corona surrounding a standard accretion disc. Partial covering models can describe the x-ray data; however, the narrow Fe Ka emission line predicted from standard photoelectric absorption is not detected. Ionized partial covering also favours a high-velocity outflow (v ~ 0.3c), which requires the kinetic luminosity of the wind to be >30 per cent of the bolometric luminosity of the AGN. Such values are not unusual, but for 1ES 1927+654 it requires the wind is launched very close to the black hole (~ 10 Rg). Blurred reflection models also work well at describing the spectral and timing properties of 1ES 1927+654 if the AGN is viewed nearly edge-on, implying that an inner accretion disc must be present. The high inclination is intriguing as it suggests 1ES 1927+654 could be orientated like a Seyfert 2, in agreement with its optical classification, but viewed through a tenuous torus.

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arXiv2013

X-ray emission and absorption features during an energetic thermonuclear X-ray burst from IGR J17062-6143

Type-I X-ray bursts are thermonuclear explosions occurring in the surface layers of accreting neutron stars. These events are powerful probes of the physics of neutron stars and their surrounding accretion flow. We analyze a very energetic type-I X-ray burst from the neutron star low-mass X-ray binary IGR J17062-6143 that was detected with Swift on 2012 June 25. The light curve of the ~18 min long X-ray burst tail shows an episode of ~10 min during which the intensity is strongly fluctuating by a factor of ~3 above and below the underlying decay trend, on a time scale of seconds. The X-ray spectrum reveals a highly significant emission line around ~1 keV, which can be interpreted as a Fe-L shell line caused by irradiation of cold gas. We also detect significant absorption lines and edges in the Fe-K band, which are strongly suggestive of the presence of hot, highly ionized gas along the line of sight. None of these features are present in the persistent X-ray spectrum of the source. The time scale of the strong intensity variations, the velocity width of the Fe-L emission line (assuming Keplerian motion), and photoionization modeling of the Fe-K absorption features each independently point to gas at a radius of ~1E3 km as the source of these features. The unusual X-ray light curve and spectral properties could have plausibly been caused by a disruption of the accretion disk due to the super-Eddington fluxes reached during the X-ray burst.

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arXiv2013

X-ray exploration of the outskirts of the nearby Centaurus cluster using Suzaku and Chandra

We present Suzaku observations of the Centaurus cluster out to 0.95r200, taken along a strip to the north west. We have also used congruent Chandra observations of the outskirts to resolve point sources down to a threshold flux around 7 times lower than that achievable with just Suzaku data, considerably reducing the systematic uncertainties in the cosmic X-ray background emission in the outskirts. We find that the temperature decreases by a factor of 2 from the peak temperature to the outskirts. The entropy profile demonstrates a central excess (within 0.5r200) over the baseline entropy profile predicted by simulations of purely gravitational hierarchical structure formation. In the outskirts the entropy profile is in reasonable agreement with the baseline entropy profile from Voit et al., but lies slightly below it. We find that the pressure profile agrees with the universal pressure profile of Arnaud et al. but lies slightly above it in the outskirts. The excess pressure and decrement in entropy in the outskirts appear to be the result of an excess in the measured gas density, possible due to gas clumping biasing the density measurements high. The gas mass fraction rises and reaches the mean cosmic baryon fraction at the largest radius studied. The clumping corrected gas mass fraction agrees with the expected hot gas fraction and with the simulations of Young et al. We further the analysis of Walker et al. which studied the shapes of the entropy profiles of the clusters so far explored in the outskirts with Suzaku. When scaled by the self similar entropy the Suzaku entropy profiles demonstrate a central excess over the baseline entropy profile, and are consistent with it at around r500 . However outside r500 the entropy profiles tend to lie below the baseline entropy profile.

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arXiv2013

Discovery of Fe K-alpha X-ray reverberation around the black holes in MCG-5-23-16 and NGC 7314

Several X-ray observations have recently revealed the presence of reverberation time delays between spectral components in AGN. Most of the observed lags are between the power-law Comptonization component, seen directly, and the soft excess produced by reflection in the vicinity of the black hole. NGC 4151 was the first object to show these lags in the iron K band. Here, we report the discovery of reverberation lags in the Fe K band in two other sources: MCG-5-23-16 and NGC 7314. In both objects, the 6-7 keV band, where the Fe K line peaks, lags the bands at lower and higher energies with a time delay of ~ 1 kilo-seconds. These lags are unlikely to be due to the narrow Fe K line. They are fully consistent with reverberation of the relativistically-broadened iron K line. The measured lags, their time-scale and spectral modeling, indicate that most of the radiation is emitted at ~ 5 and 24 gravitational radii for MCG-5-23-16 and NGC 7314 respectively.

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