arXiv subjects

Michael Eracleous

Publications and source records attributed to Michael Eracleous.

At least 145 records · Page 8Linked to original sources

arXiv2006

Electromagnetic Signatures of Massive Black Hole Binaries

We model the electromagnetic emission signatures of massive black hole binaries (MBHBs) with an associated gas component. The method comprises numerical simulations of relativistic binaries and gas coupled with calculations of the physical properties of the emitting gas. We calculate the accretion powered UV/X-ray and Halpha light curves and the Halpha emission line profiles. The simulations have been carried out with a modified version of the parallel tree SPH code Gadget. The heating, cooling, and radiative processes for the solar metallicity gas have been calculated with the photoionization code Cloudy. We investigate gravitationally bound, sub-parsec binaries which have not yet entered the gravitational radiation phase. The results from the first set of calculations, carried out for a coplanar binary and gas disk, suggest that the outbursts in the X-ray light curve are pronounced during pericentric passages and can serve as a fingerprint for this type of binaries if periodic outbursts are a long lived signature of the binary. The Halpha emission-line profiles also offer strong indications of a binary presence and may be used as a criterion for selection of MBHB candidates for further monitoring from existing archival data. The orbital period and mass ratio of a binary could be determined from the Halpha light curves and profiles of carefully monitored candidates. Although systems with the orbital periods studied here are not within the frequency band of the Laser Interferometer Space Antenna (LISA), their discovery is important for understanding of the merger rates of MBHBs and the evolution of such binaries through the last parsec and towards the detectable gravitational wave window.

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arXiv2005

Radiatively Inefficient Accretion Flow in the Nucleus of NGC 1097

We present a model for the accretion flow around the supermassive black hole in the LINER nucleus of NGC 1097 which fits the optical to X-ray spectral energy distribution (SED). The X-ray segment of the SED is based on observations with the Chandra X-Ray Observatory, which are reported here for the first time. The inner part of the flow is modeled as a radiatively inefficient accretion flow (RIAF) and the outer part as a standard thin disk. The value of the transition radius (~225 Schwarzschild radii) between the RIAF and outer thin disk was obtained from our previous fitting of the double-peaked Balmer emission line profile, which originates in the thin disk. The black hole mass was inferred from measurements of the stellar velocity dispersion in the host galaxy. When these parameters are used in the accretion flow model, the SED can be successfully reproduced, which shows that the line profile model and the accretion flow model are consistent with each other. A small remaining excess in the near-UV is accounted by the contribution of an obscured starburst located within 9 pc from the nucleus, as we reported in an earlier paper. The radio flux is consistent with synchrotron emission of a relativistic jet modeled by means of the internal shock scenario. In an appendix we also analyze the Chandra X-ray observations of the ~1 kpc circumnuclear star-forming ring and of an ultraluminous compact X-ray source located outside the ring.

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arXiv2005

Observational Signature of Tidal Disruption of a Star by a Massive Black Hole

We have modeled the time-variable profiles of the Balmer alpha emission from the nonaxisymmetric disk and debris tail created in the tidal disruption of a solar-type star by a million solar mass black hole. Two tidal disruption events were simulated using a three dimensional relativistic smoothed particle hydrodynamic code to describe the early evolution of the debris during the first 50-90 days. We have calculated the physical conditions and radiative processes in the debris using the photoionization code CLOUDY. We model the emission-line profiles in the period immediately after the accretion rate onto the black hole becomes significant. We find that the line profiles at these very early stages of the evolution of the postdisruption debris do not resemble the double-peaked profiles expected from a rotating disk, since the debris has not yet settled into such a stable structure. As a result of the uneven distribution of the debris and the existence of a ``tidal tail'' (the stream of returning debris), the line profiles depend sensitively on the orientation of the tail relative to the line of sight. Moreover, the predicted line profiles vary on fairly short time scales (of order hours to days). Given the accretion rate onto the black hole we also model the Balmer alpha light curve from the debris.

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arXiv2005

Time-Variable Complex Metal Absorption Lines in the Quasar HS1603+3820

We present a new spectrum of the quasar HS1603+3820 taken 1.28 years (0.36 years in the quasar rest frame) after a previous observation with Subaru+HDS. The new spectrum enables us to search for time variability as an identifier of intrinsic narrow absorption lines (NALs). This quasar shows a rich complex of C IV NALs within 60,000 km/s of the emission redshift. Based on covering factor analysis, Misawa et al. found that the C IV NAL system at z_abs= 2.42--2.45 (System A, at a shift velocity of v_sh = 8,300--10,600 km/s relative to the quasar) was intrinsic to the quasar. With our new spectrum, we perform time variability analysis as well as covering factor analysis to separate intrinsic NALs from intervening NALs for 8 C IV systems. Only System A, which was identified as an intrinsic system in the earlier paper by Misawa et al., shows a strong variation in line strength (W_obs ~ 10.4A -> 19.1A). We speculate that a broad absorption line (BAL) could be forming in this quasar. We illustrate the plausibility of this suggestion with the help of a simulation. Under the assumption that a change of ionization state causes the variability, a lower limit can be placed on the electron density (n_e > 3x10^4 cm^-3) and an upper limit on the distance from the continuum source (r < 6 kpc). On the other hand, if the motion of clumpy gas causes the variability (a more likely scenario), the crossing velocity and the distance from the continuum source are estimated to be v_cross > 8,000 km/s and r < 3 pc. In this case, the absorber does not intercept any flux from the broad emission line region, but only flux from the UV continuum source. If we adopt the dynamical model of Murray et al., we can obtain a much more strict constraint on the distance of the gas parcel from the continuum source, r < 0.2 pc.

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arXiv2005

Extreme X-ray Behavior of the Low-Luminosity Active Nucleus in NGC 4395

We present the results of a 17 ks Chandra ACIS-S observation of the nearby dwarf spiral galaxy NGC 4395. Chandra affords the first high-quality, broadband X-ray detection of the active nucleus of this object that is uncontaminated by nearby sources in the field. We find the nuclear X-ray source to be unresolved and confirm the rapid, large-amplitude variability reported in previous studies. The light curve appears to show ~ 11 cycles of a quasi-periodic oscillation with a period of ~ 400 s. If associated with an orbital feature near the innermost stable orbit of the accretion disk, this period would constrain the black-hole mass to be M < 9e5 M_sun. The X-ray spectrum indicates absorption by an ionized medium, and the spectral shape appears to vary over the course of our observation. Contrary to prior reports, however, the spectral variations are uncorrelated with changes in the hard X-ray flux. It is possible that the short-term spectral variability we observe results from column density fluctuations in the ionized absorber. A power-law fit to the spectrum above 1 keV yields a photon index of Gamma ~ 0.6, much flatter than that typically observed in the spectra of Seyfert 1 galaxies. We have ruled out photon pile-up as the cause of the flat spectrum. Even when complex spectral features are considered, the photon index is constrained to be Gamma < 1.25 (90% confidence). Comparing our results with previous determinations of the photon index (Gamma = 1.46 and 1.72), we conclude that the slope of the primary continuum varies significantly on time scales of a year or less. The extreme flatness and dramatic long-term variability of the X-ray spectrum are unprecedented among active galactic nuclei.

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arXiv2004

A Simultaneous RXTE and XMM-Newton Observation of the Broad-Line Radio Galaxy 3C 111

We present the results of simultaneous XMM-Newton and RXTE observations of the Broad-Line Radio Galaxy 3C 111. We find that the Compton reflection bump is extremely weak, however, broad residuals are clearly present in the spectrum near the Fe Kalpha emission line region. When fitted with a Gaussian emission line,the feature has an equivalent width of 40-100eV and full-width at half maximum of greater than 20,000 km/s, however the exact properties of this weak line are highly dependent upon the chosen continuum model. The width of the line suggests an origin in the inner accretion disk, which is, however, inconsistent with the lack of Compton reflection. We find that much of the broad residual emission can be attributed to continuum curvature. The data are consistent with a model in which the primary powerlaw continuum is reprocessed by an accretion disk which is truncated as small radii. Alternatively, the primary source could be partially covered by a dense absorber. The latter model is less attractive than the former because of the small inclination angle of the jet of 3C 111 to the line of sight. We consider it likely that the curved continuum of the partial covering model is fortuitously similar to the continuum shape of the reprocessing model. In both models, the fit is greatly improved by the addition of an unresolved Fe K alpha emission line, which could arise either in a Compton-thin obscuring torus or dense clouds lying along the line of sight. We also find that there are unacceptable residuals at low energies in the MOS data in particular, which were modeled as a Gaussian with an energy of ~1.5 keV; we attribute these residuals to calibration uncertainties of the MOS detectors.

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arXiv2004

Long-Term Profile Variability of Double-Peaked Emmission Lines in AGNs

An increasing number of AGNs exhibit broad, double-peaked Balmer emission lines, which arise from the outer regions of the accretion disk which fuels the AGN. The line profiles vary on timescales of 5--10 years. Our group has monitored a set of 20 double-peaked emitters for the past 8 years (longer for some objects). Here we describe a project to characterize the variability patterns of the double-peaked H alpha line profiles and compare with those of two simple models: a circular disk with a spiral arm and an elliptical disk.

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arXiv2004

Tidal Disruption of a Star By a Black Hole : Observational Signature

We have modeled the time-variable profiles of the Halpha emission line from the non-axisymmetric disk and debris tail created in the tidal disruption of a solar-type star by a million solar mass black hole. Two tidal disruption event simulations were carried out using a three dimensional relativistic smooth-particle hydrodynamic code, to describe the early evolution of the debris during the first fifty to ninety days. We have calculated the physical conditions and radiative processes in the debris using the photoionization code CLOUDY. We model the emission line profiles in the period immediately after the accretion rate onto the black hole became significant. We find that the line profiles at these very early stages of the evolution of the post-disruption debris do not resemble the double peaked profiles expected from a rotating disk since the debris has not yet settled into such a stable structure. As a result of the uneven distribution of the debris and the existence of a ``tidal tail'' (the stream of returning debris), the line profiles depend sensitively on the orientation of the tail relative to the line of sight. Moreover, the predicted line profiles vary on fairly short time scales (of order hours to days). Given the accretion rate onto the black hole we also model the Halpha light curve from the debris and the evolution of the Halpha line profiles in time.

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arXiv2004

Observational Signature of Tidal Disruption of a Star by a Massive Black Hole

We have modeled the time-variable profiles of the Halpha emission line from the non-axisymmetric disk and debris tail created in the tidal disruption of a solar-type star by a million solar mass black hole. We find that the line profiles at these very early stages of the evolution of the post-disruption debris do not resemble the double peaked profiles expected from a rotating disk since the debris has not yet settled into such a stable structure. The predicted line profiles vary on fairly short time scales (of order hours to days). As a result of the uneven distribution of the debris and the existence of a ``tidal tail'' (the stream of returning debris), the line profiles depend sensitively on the orientation of the tail relative to the line of sight. Given the illuminating UV/X-ray light curve, we also model the Halpha light curve from the debris.

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arXiv2004

A Study of QSO Evolution in the X-Ray Band with the Aid of Gravitational Lensing

We present results from a mini-survey of relatively high redshift (1.7 < z < 4) gravitationally lensed radio-quiet quasars observed with the Chandra X-ray Observatory and with XMM-Newton. The lensing magnification effect allows us to search for changes in quasar spectroscopic and flux variability properties with redshift over three orders of magnitude in intrinsic X-ray luminosity. It extends the study of quasar properties to unlensed X-ray flux levels as low as a few times 10^{-15} erg cm^{-2} s^{-1} in the observed 0.4-8 keV band. We find a possible correlation between the X-ray powerlaw photon index and X-ray luminosity of the gravitationally lensed radio-quiet quasar sample. The X-ray spectral slope steepens as the X-ray luminosity increases. This correlation is still significant when we combine our data with other samples of radio-quiet quasars with z > 1.5, especially in the low luminosity range between 10^{43}-10^{45.5} erg s^{-1}. This result is surprising considering that such a correlation is not found for quasars with redshifts below 1.5. We suggest that this correlation can be understood in the context of the hot-corona model for X-ray emission from quasar accretion disks, under the hypothesis that the quasars in our sample accrete very close to their Eddington limits and the observed luminosity range is set by the range of black hole masses (this hypothesis is consistent with recent predictions of semi-analytic models for quasar evolution). The upper limits of X-ray variability of our relatively high redshift sample of lensed quasars are consistent with the known correlation between variability and luminosity observed in Seyfert 1s when this correlation is extrapolated to the larger luminosities of our sample.

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arXiv2003

Active Galaxies With Double-Peaked Emission Lines and What They Imply About the "Broad-Line Region"

I review the distinguishing observational characteristics of active galaxies with double-peaked emission lines and their implications for the nature of the line-emitting region. Since double-peaked lines most likely originate in the outer parts of the accretion disk, they can be used to study the structure and dynamics of the disk and the associated wind. Such studies lead to general inferences about the broad-line regions of all AGNs. To this end, I describe the results of recent UV spectroscopy of double-peaked emitters that probes the disk-wind relation. I also summarize efforts to exploit the variability of the lines to study dynamical and thermal phenomena in the disk.

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arXiv2003

Long Term Profile Variability of Double-Peaked Emission Lines in AGNs

An increasing number of AGNs exhibit broad, double-peaked Balmer emission lines, which are thought to arise from the outer regions of the accretion disk which fuels the AGN. The line profiles are observed to vary on a characteristic timescales of 5-10 years. The variability is not a reverberation effect; it is a manifestation of physical changes in the disk. Our group has monitored a set of 20 double-peaked emitters for the past 8 years (longer for some objects). Here, we characterize the variability of the double-peaked H alpha line profiles in five objects from our sample. By experimenting with simple models, we find that disks with a single precessing spiral arm are able to reproduce many of the variability trends that are seen in the data.

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arXiv2003

Accurate Redshifts and Classifications for 110 Radio-Loud AGNs

We report accurate redshifts of 110 active galaxies (mostly radio-loud objects at z<0.4) observed in the course of a survey to find broad, double-peaked emission lines. These redshifts are measured from the narrow emission lines of these objects and are accurate to at least one part in 10^4 . For each object we determine a redshift from high- and low-ionization lines separately, as well as an average redshift from all the available lines. We find that in about 15% of cases, the low-ionization lines yield a slightly higher redshift than the high-ionization lines; the average redshift difference amounts to a velocity difference of approximately 80 km/s. In addition to the redshift measurements we also report revised redshifts for two objects as well as new classifications for three narrow-line objects.

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arXiv2003

Emission-Line Diagnostics of the Central Engines of Weak-Line Radio Galaxies

A handful of well-studied Weak-Line Radio Galaxies (WLRGs) have been traditionally classified as Low Ionization Nuclear Emission-line Regions (LINERs), suggesting that these two groups of AGNs might be related. In this paper, we present new optical emission-line measurements for twenty Weak-Line Radio Galaxies which we supplement with measurements for an additional four from the literature. Classifying these objects by their emission-line ratios, we find that 50% of the objects are robustly classified as LINERs while an additional 25% are likely to be LINERs. Photoionization calculations show that the Spectral Energy Distribution of the well-studied WLRG 3C 270 (NGC 4261) is able to produce the observed emission-line ratios, but only if the UV emission seen by the narrow emission-line gas is significantly higher than that observed, implying A_V = 2.5-4.2 magnitudes along our line of sight to the nucleus. From the photoionization calculations, we find that the emission-line gas must have an ionization parameter between 10^-3.5 and 10^-4.0 and a wide range in hydrogen density (10^2-10^6 cm^-3) to reproduce the measured emission-line ratios, similar to the properties inferred for the emission-line gas in LINERs. Thus, we find that properties of the emission-line gas as well as the underlying excitation mechanism are indeed similar in LINERs and WLRGs. By extension, the central engines of accretion-powered LINERs and WLRGs, which do host an accreting black hole, may be qualitatively similar.

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arXiv2002

Feeding the active nucleus of NGC 1097

We discuss the signatures of an evolving accretion disk around the nucleus of NGC 1097, based on observations of its double-peaked H-alpha emission-line profile which cover a time span of 10 yrs. The monitoring of the profile has allowed us to better constrain the inner radius of the accretion disk, and to propose a scenario in which the ionizing source is getting dimmer and making the peak of the disk emissivity to drift inwards.

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arXiv2001

Emission Line Diagnostics of Accretion Flows in Weak-Line Radio Galaxies

In recent surveys of Radio-loud AGN, a new sub-class of objects, known as Weak-Line Radio Galaxies (WLRGs) has emerged. These radio galaxies have only weak, low-ionization optical emission lines. In the X-ray band, these objects are much fainter and have flatter spectra than broad-line and narrow-line radio galaxies. In these respects, WLRGs are reminiscent of Low Ionization Nuclear Emission Regions (LINERs). We have begun a multi-wavelength study of WLRGs to better understand their possible connection to LINERs and the structure of the accretion flow in both these systems. Here, we present new optical spectra of a sample of WLRGs. We find that 81% of the objects have optical emission-line properties that are similar to LINERs, indicating that these two classes of AGN may be related. Future high resolution X-ray observations of WLRGs will be critical in determining the true nature of the accretion flow in these objects.

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arXiv2001

Variable UV Absorption in the Spectrum of MRC 2251-178

We present an ultraviolet spectrum of MRC 2251-178 taken with HST/STIS-G230L. The observation is part of a snap shot program of QSOs to systematically search for intrinsic absorption lines through variability. The sample consists of all QSOs observed with HST/FOS which showed associated narrow absorption lines. The FOS spectrum of MRC 2251-178, taken in 1996, showed an associated C IV doublet with an C IV \lambda1548 equivalent width 1.09+/-0.09 A (Monier et al. 2001). It is not detected in the STIS spectrum taken four years later, down to a 3-sigma threshold of 0.19 A. In addition to its other accolades, these observations make MRC 2251-178 the very first QSO at low redshift in which the associated absorption is shown to be truly intrinsic. We discuss the implications of this and suggest courses for future study.

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arXiv2001

NGC 3065: A Certified LINER With Broad, Variable Balmer Lines

Motivated by the X-ray properties of the galaxy NGC 3065, we have obtained new optical spectra which reveal that it has a low-ionization nuclear emission-line region (LINER) as well as broad Balmer emission lines, establishing it as an active galactic nucleus. We also examined an older spectrum from the CfA Redshift Survey which, lacking broad Balmer lines, indicates that they appeared some time after 1980. Thus NGC 3065 joins the set of LINERs with broad, variable Balmer lines, which includes such well-known galaxies as NGC 1097 and M81. Inspired by the sometimes double-peaked profiles of the variable Balmer lines in other LINERs, we speculate that the broad Balmer lines of NGC 3065 also come from an accretion disk. We illustrate the plausibility of this hypothesis by fitting a disk model to the observed H$α$ profile. We also estimate the mass of the central black hole as (9+/-4) 10^7 M_sol from the properties of the host galaxy, which leads to the conclusion that the accretion rate is only 2 10^{-4} times the Eddington value, a property that appears to be common among LINERs. At such a low relative accretion rate the inner accretion disk can turn into a vertically-extended ion torus, which can illuminate the outer, thin disk and power the broad-line emission. The reason for the sudden appearance of broad Balmer lines is an open question, although we suggest two possible explanations: tidal disruption of a star or a sudden transition in the structure of the accretion disk.

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