[Show abstract][Hide abstract] ABSTRACT: We present $\gamma$-ray, X-ray, ultraviolet, optical, and near-infrared light
curves of 33 $\gamma$-ray bright blazars over four years that we have been
monitoring since 2008 August with multiple optical, ground-based telescopes and
the Swift satellite, and augmented by data from the Fermi Gamma-ray Space
Telescope and other publicly available data from Swift. The sample consists of
21 flat-spectrum radio quasars (FSRQs) and 12 BL Lac objects (BL Lacs). We
identify quiescent and active states of the sources based on their $\gamma$-ray
behavior. We derive $\gamma$-ray, X-ray, and optical spectral indices,
$\alpha_\gamma$, $\alpha_X$, and $\alpha_o$, respectively
($F_\nu\propto\nu^\alpha$), and construct spectral energy distributions (SEDs)
during quiescent and active states. We analyze the relationships between
different spectral indices, blazar classes, and activity states. We find (i)
significantly steeper $\gamma$-ray spectra of FSRQs than for BL Lacs during
quiescent states, but a flattening of the spectra for FSRQs during active
states while the BL Lacs show no significant change; (ii) a small difference of
$\alpha_X$ within each class between states, with BL Lac X-ray spectra
significantly steeper than in FSRQs; (iii) a highly peaked distribution of
X-ray spectral slopes of FSRQs at $\sim-$0.60, but a very broad distribution of
$\alpha_X$ of BL Lacs during active states; (iv) flattening of the optical
spectra of FSRQs during quiescent states, but no statistically significant
change of $\alpha_o$ of BL Lacs between states; and (v) a positive correlation
between optical and $\gamma$-ray spectral slopes of BL Lacs, with similar
values of the slopes. We discuss the findings with respect to the relative
prominence of different components of high-energy and optical emission as the
flux state changes.
The Astrophysical Journal 06/2014; 789(2). · 6.28 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We analyze the multifrequency behavior of the quasar 3C 454.3 during three
prominent \gamma-ray outbursts: 2009 Autumn, 2010 Spring, and 2010 Autumn. The
data reveal a repeating pattern, including a triple flare structure, in the
properties of each \gamma-ray outburst, which implies similar mechanism(s) and
location for all three events. The multi-frequency behavior indicates that the
lower frequency events are co-spatial with the \gamma-ray outbursts, although
the \gamma-ray emission varies on the shortest timescales. We determine that
the variability from UV to IR wavelengths during an outburst results from a
single synchrotron component whose properties do not change significantly over
the different outbursts. Despite a general increase in the degree of optical
linear polarization during an outburst, the polarization drops significantly at
the peak of the \gamma-ray event, which suggests that both shocks and turbulent
processes are involved. We detect two disturbances (knots) with superluminal
apparent speeds in the parsec-scale jet associated with the outbursts in 2009
Autumn and 2010 Autumn. The kinematic properties of the knots can explain the
difference in amplitudes of the \gamma-ray events, while their millimeter-wave
polarization is related to the optical polarization during the outbursts. We
interpret the multi-frequency behavior within models involving either a system
of standing conical shocks or magnetic reconnection events located in the
parsec-scale millimeter-wave core of the jet. We argue that \gamma-ray
outbursts with variability timescales as short as ~ 3 hr can occur on parsec
scales if flares take place in localized regions such as turbulent cells.
The Astrophysical Journal 07/2013; 773(2). · 6.28 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: This paper completes the series of cataclysmic variables (CVs) identified from the Sloan Digital Sky Survey (SDSS) I/II. The coordinates, magnitudes, and spectra of 33 CVs are presented. Among the 33 are eight systems known prior to SDSS (CT Ser, DO Leo, HK Leo, IR Com, V849 Her, V405 Peg, PG1230+226, and HS0943+1404), as well as nine objects recently found through various photometric surveys. Among the systems identified since the SDSS are two polar candidates, two intermediate polar candidates, and one candidate for containing a pulsating white dwarf. Our follow-up data have confirmed a polar candidate from Paper VII and determined tentative periods for three of the newly identified CVs. A complete summary table of the 285 CVs with spectra from SDSS I/II is presented as well as a link to an online table of all known CVs from both photometry and spectroscopy that will continue to be updated as future data appear.
The Astronomical Journal 10/2011; 142(6):181. · 4.97 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: This paper completes the series of cataclysmic variables (CVs) identified
from the Sloan Digital Sky Survey I/II. The coordinates, magnitudes and spectra
of 33 CVs are presented. Among the 33 are eight systems known previous to SDSS
(CT Ser, DO Leo, HK Leo, IR Com, V849 Her, V405 Peg, PG1230+226 and
HS0943+1404), as well as nine objects recently found through various
photometric surveys. Among the systems identified since the SDSS are two polar
candidates, two intermediate polar candidates and one candidate for containing
a pulsating white dwarf. Our followup data have confirmed a polar candidate
from Paper VII and determined tentative periods for three of the newly
identified CVs. A complete summary table of the 285 CVs with spectra from SDSS
I/II is presented as well as a link to an online table of all known CVs from
both photometry and spectroscopy that will continue to be updated as future
[Show abstract][Hide abstract] ABSTRACT: We present observations of a major outburst at centimeter, millimeter,
optical, X-ray, and gamma-ray wavelengths of the BL Lacertae object AO
0235+164. We analyze the timing of multi-waveband variations in the flux and
linear polarization, as well as changes in Very Long Baseline Array (VLBA)
images at 7mm with 0.15 milliarcsecond resolution. The association of the
events at different wavebands is confirmed at high statistical significance by
probability arguments and Monte-Carlo simulations. A series of sharp peaks in
optical linear polarization, as well as a pronounced maximum in the 7 mm
polarization of a superluminal jet knot, indicate rapid fluctuations in the
degree of ordering of the magnetic field. These results lead us to conclude
that the outburst occurred in the jet both in the quasi-stationary "core" and
in the superluminal knot, both parsecs downstream of the supermassive black
hole. We interpret the outburst as a consequence of the propagation of a
disturbance, elongated along the line of sight by light-travel time delays,
that passes through a standing recollimation shock in the core and propagates
down the jet to create the superluminal knot. The multi-wavelength light curves
vary together on long time-scales (months/years), but the correspondence is
poorer on shorter time-scales. This, as well as the variability of the
polarization and the dual location of the outburst, agrees with the
expectations of a multi-zone emission model in which turbulence plays a major
role in modulating the synchrotron and inverse Compton fluxes.
The Astrophysical Journal Letters 05/2011; 735(1). · 5.60 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Recent theoretical work has suggested that Lyalpha nebulae could be substantially polarized in the Lyalpha emission line, depending on the geometry, kinematics, and powering mechanism at work. Polarization observations can therefore provide a useful constraint on the source of ionization in these systems. In this Letter, we present the first Lyalpha polarization measurements for a giant Lyalpha nebula at z≈ 2.656. We do not detect any significant linear polarization of the Lyalpha emission: P Lyalpha = 2.6% ± 2.8% (corrected for statistical bias) within a single large aperture. The current data also do not show evidence for the radial polarization gradient predicted by some theoretical models. These results rule out singly scattered Lyalpha (e.g., from the nearby active galactic nucleus, AGN) and may be inconsistent with some models of backscattering in a spherical outflow. However, the effects of seeing, diminished signal-to-noise ratio, and angle averaging within radial bins make it difficult to put strong constraints on the radial polarization profile. The current constraints may be consistent with higher density outflow models, spherically symmetric infall models, photoionization by star formation within the nebula or the nearby AGN, resonant scattering, or non-spherically symmetric cold accretion (i.e., along filaments). Higher signal-to-noise ratio data probing to higher spatial resolution will allow us to harness the full diagnostic power of polarization observations in distinguishing between theoretical models of giant Lyalpha nebulae.
The Astrophysical Journal Letters 04/2011; 730(2). · 5.60 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We combine time-dependent multi-waveband flux and linear polarization observations with submilliarcsecond-scale polarimetric images at λ = 7 mm of the BL Lacertae type blazar OJ287 to locate the γ-ray emission in prominent flares in the jet of the source >14 pc from the central engine. We demonstrate a highly significant correlation between the strongest γ-ray and millimeter-wave flares through Monte Carlo simulations. The two reported γ-ray peaks occurred near the beginning of two major millimeter-wave outbursts, each of which is associated with a linear polarization maximum at millimeter wavelengths. Our very long baseline array observations indicate that the two millimeter-wave flares originated in the second of two features in the jet that are separated by >14 pc. The simultaneity of the peak of the higher-amplitude γ-ray flare and the maximum in polarization of the second jet feature implies that the γ-ray and millimeter-wave flares are cospatial and occur >14 pc from the central engine. We also associate two optical flares, accompanied by sharp polarization peaks, with the two γ-ray events. The multi-waveband behavior is most easily explained if the γ-rays arise from synchrotron self-Compton scattering of optical photons from the flares. We propose that flares are triggered by interaction of moving plasma blobs with a standing shock. The γ-ray and optical emission is quenched by inverse Compton losses as synchrotron photons from the newly shocked plasma cross the emission region. The millimeter-wave polarization is high at the onset of a flare, but decreases as the electrons emitting at these wavelengths penetrate less polarized regions.
The Astrophysical Journal Letters 12/2010; 726(1):L13. · 5.60 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We analyze the behavior of the parsec-scale jet of the quasar 3C~454.3 during pronounced flaring activity in 2005-2008. Three major disturbances propagated down the jet along different trajectories with Lorentz factors $\Gamma>$10. The disturbances show a clear connection with millimeter-wave outbursts, in 2005 May/June, 2007 July, and 2007 December. High-amplitude optical events in the $R$-band light curve precede peaks of the millimeter-wave outbursts by 15-50 days. Each optical outburst is accompanied by an increase in X-ray activity. We associate the optical outbursts with propagation of the superluminal knots and derive the location of sites of energy dissipation in the form of radiation. The most prominent and long-lasting of these, in 2005 May, occurred closer to the black hole, while the outbursts with a shorter duration in 2005 Autumn and in 2007 might be connected with the passage of a disturbance through the millimeter-wave core of the jet. The optical outbursts, which coincide with the passage of superluminal radio knots through the core, are accompanied by systematic rotation of the position angle of optical linear polarization. Such rotation appears to be a common feature during the early stages of flares in blazars. We find correlations between optical variations and those at X-ray and $\gamma$-ray energies. We conclude that the emergence of a superluminal knot from the core yields a series of optical and high-energy outbursts, and that the mm-wave core lies at the end of the jet's acceleration and collimation zone. Comment: 57 pages, 23 figures, 8 tables (submitted to ApJ)
[Show abstract][Hide abstract] ABSTRACT: As part of our comprehensive long-term multi-waveband monitoring of 34 blazars, we followed the activity in the jet of the blazar PKS 1510-089 during major outbursts during the first half of 2009. The most revealing event was a two-month long outburst that featured a number of gamma-ray flares. During the outburst, the position angle of optical linear polarization rotated by about 720 degrees, which implies that a single emission feature was responsible for all of the flares during the outburst. At the end of the rotation, a new superluminal knot (~ 22c) passed through the "core" seen on 43 GHz VLBA images at essentially the same time as an extremely sharp, high-amplitude gamma-ray and optical flare occurred. We associate the entire multi-flare outburst with this knot. The ratio of gamma-ray to synchrotron integrated flux indicates that some of the gamma-ray flares resulted from inverse Compton scattering of seed photons outside the ultra-fast spine of the jet. Because many of the flares occurred over time scales of days or even hours, there must be a number of sources of IR-optical-UV seed photons -- probably synchrotron emission -- surrounding the spine, perhaps in a slower sheath of the jet. Comment: 6 pages, 3 figures, in 2009 Fermi Symposium, eConf Proceedings C091122
[Show abstract][Hide abstract] ABSTRACT: We present results from monitoring the multi-waveband flux, linear polarization, and parsec-scale structure of the quasar PKS 1510-089, concentrating on eight major gamma-ray flares that occurred during the interval 2009.0-2009.5. The gamma-ray peaks were essentially simultaneous with maxima at optical wavelengths, although the flux ratio of the two wavebands varied by an order of magnitude. The optical polarization vector rotated by 720 degrees during a 5-day period encompassing six of these flares. This culminated in a very bright, roughly 1 day, optical and gamma-ray flare as a bright knot of emission passed through the highest-intensity, stationary feature (the "core") seen in 43 GHz Very Long Baseline Array images. The knot continued to propagate down the jet at an apparent speed of 22c and emit strongly at gamma-ray energies as a months-long X-ray/radio outburst intensified. We interpret these events as the result of the knot following a spiral path through a mainly toroidal magnetic field pattern in the acceleration and collimation zone of the jet, after which it passes through a standing shock in the 43 GHz core and then continues downstream. In this picture, the rapid gamma-ray flares result from scattering of infrared seed photons from a relatively slow sheath of the jet as well as from optical synchrotron radiation in the faster spine. The 2006-2009.7 radio and X-ray flux variations are correlated at very high significance; we conclude that the X-rays are mainly from inverse Compton scattering of infrared seed photons by 20-40 MeV electrons. Comment: 10 pages of text + 5 figures, to be published in Astrophysical Journal Letters in 2010
The Astrophysical Journal Letters 01/2010; · 5.60 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We identify a sample of 74 high-redshift quasars (z > 3) with weak emission lines from the Fifth Data Release of the Sloan Digital Sky Survey and present infrared, optical, and radio observations of a subsample of four objects at z > 4. These weak emission-line quasars (WLQs) constitute a prominent tail of the Lyα + N v equivalent width distribution, and we compare them to quasars with more typical emission-line properties and to low-redshift active galactic nuclei with weak/absent emission lines, namely BL Lac objects. We find that WLQs exhibit hot (T ~ 1000 K) thermal dust emission and have rest-frame 0.1-5 μm spectral energy distributions that are quite similar to those of normal quasars. The variability, polarization, and radio properties of WLQs are also different from those of BL Lacs, making continuum boosting by a relativistic jet an unlikely physical interpretation. The most probable scenario for WLQs involves broad-line region properties that are physically distinct from those of normal quasars.
The Astrophysical Journal 06/2009; 699(1):782. · 6.28 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Coordinates, magnitudes, and spectra are presented for 39 cataclysmic variables (CVs) found in Sloan Digital Sky Survey (SDSS) spectra that were primarily obtained in 2006. Of these, 13 were CVs identified prior to the SDSS spectra (AK Cnc, GY Cnc, GO Com, ST LMi, NY Ser, MR Ser, QW Ser, EU UMa, IY UMa, HS1340+1524, RXJ1610.1+0352, Boo 1, Leo 5). Follow-up spectroscopic observations of seven systems (including one from year 2005 and another from year 2004) were obtained, resulting in estimates of the orbital periods for three objects. The new CVs include two candidates for high inclination, eclipsing systems, four new polars, and three systems whose spectra clearly reveal atmospheric absorption lines from the underlying white dwarf.
The Astronomical Journal 03/2009; 137(4):4011. · 4.97 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We identify the luminous soft X-ray source AR UMa as a magnetic cataclysmic variable containing a white dwarf with the highest field yet detected in an accreting binary. IUE and optical spectroscopy, optical photometry, and circular polarimetry and spectropolarimetry define remarkably distinct accretion states of this binary. Circular polarization is nearly absent in the high state, but the low state exhibits values which vary between 2% and 5% on the orbital period of 1.932 hr. The UV continuum contains a broad absorption feature near 1300 Å, while optical spectropolarimetry during the low state reveals a number of strongly polarized dips. These are interpreted as Zeeman components of hydrogen Lyα and another atmospheric species, possibly He I, in a photospheric magnetic field of ~ 230 MG.The radial velocity curve of the low-state optical emission lines shares the period of the optical photometry and polarimetry and is phased appropriately for an origin on the irradiated secondary star. While the high state exhibits prominent UV line emission typical of the magnetic variables, the strength of the UV continuum does not vary appreciably with a change in accretion state. This, combined with the high soft X-ray luminosity and lack of circular polarization, indicates that accretion occurs largely in the form of dense filaments which avoid a standoff shock and thermalize their kinetic energy below the white dwarf photosphere. We suggest that these phenomena may play a role in the apparent lack of high-field systems with easily detectable circular polarization during high-accretion states.
The Astrophysical Journal 01/2009; 473(1):483. · 6.28 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: AR UMa is the highest field example known of the magnetic cataclysmic variables, with a polar field Bp = 230 MG. We report here the results of a long program of photometry, spectroscopy, and spectropolarimetry of the object that was undertaken with the aim of gaining a better understanding of the role that the white dwarf field plays in shaping the properties of the magnetic systems. Only three accretion episodes, reaching mV ≤ 15 and covering a total of 9 months, were detected during the 4 yr of available photometry. Fortunately, the onset of one episode coincided with a week-long observing run, resulting in spectrophotometry and linear and circular spectropolarimetry that document the event. The long intervals of quiescence have enabled a determination of the orbital ephemeris that is now sufficiently precise to phase observations 20 yr old. The high-state observations lead to a number of conclusions regarding the system geometry. The white dwarf magnetic axis is inclined rather little to the spin axis, but the dipole is twisted in azimuth such that it lies nearly perpendicular to the stellar line of centers. Tomographic analysis of a wide variety of atomic species indicates that the white dwarf likely accretes in twin funnels that split off a ballistic gas stream reaching 30%-50% of the distance between the stars. A narrow high-velocity (KNHV ≈ 700 km s-1) emission-line component offset in phase from the main features suggests the existence of an additional gas stream, but the interpretation of this is not yet resolved. The system is viewed from a moderate inclination, which places the disfavored (retrograde) magnetic pole on the observed hemisphere at all times. We have investigated the possible effect that the white dwarf magnetosphere might have in restricting mass flow through L1, and therefore in giving rise to the unique, protracted low-accretion states of this object. However, the mechanisms considered are too weak and/or would give rise to consequences that are at odds with low-state observations. At the same time, the high magnetic field in AR UMa has yielded a new, powerful observational tool: phase-resolved Zeeman spectroscopy of the emission lines produced in the accretion stream(s). Future high-quality observations and sophisticated modeling of these features hold promise for three-dimensional reconstructions of the gas flow in high-field magnetic variables.
The Astrophysical Journal 01/2009; 525(1):407. · 6.28 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The star LHS 3250 is found to be a white dwarf at a distance of 30 pc. Its absolute magnitudes (MV = 15.72, Mbol = 16.2) put it among the least luminous white dwarfs known. Its optical spectrum shows no features, indicating that it has a DC classification, and it shows no detectable polarization, indicating that it does not have a very strong magnetic field. However, its broadband colors show it to have a unique spectral energy distribution, and it stands out from all other stars in BVI and other broadband photometric surveys. We discuss these properties and conclude that LHS 3250 must be an extremely cool white dwarf with strong collision-induced absorption at red-infrared wavelengths from molecular hydrogen, in accord with models for very cool white dwarf atmospheres. If so, it is the first such star known and the first star to provide observational evidence supporting these models. It suggests that other very cool white dwarfs, both halo white dwarfs and the oldest disk white dwarfs, also may have colors affected by similar absorption. The atmospheric composition of LHS 3250 is not known, and therefore its temperature is poorly determined. It is likely to be a helium-core star with a mass 0.3 < M < 0.45 M☉ and a product of mass transfer in a close binary system. However, until its temperature is better known, its mass and age remain uncertain.
The Astrophysical Journal 01/2009; 524(2):1000. · 6.28 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We present the combined results of optical polarization surveys of QSOs showing broad absorption lines (BALQSOs) conducted at the Steward and McDonald Observatories. The merged list of 53 objects provides the first statistical justification for claims of the tendency of BALQSOs to show stronger than average polarization, with a typical BALQSO being polarized a factor 2.4 times greater than a QSO from an optical survey selected without regard for absorption lines. Spectropolarimetry of sufficient quality to distinguish the polarization of emission lines versus absorption troughs versus continuum is also presented for six objects from the survey. When taken together with published data on other sources, the results lead to several significant correlations that can be used to gain insight into the structure of a BALQSO. These include: (1) a reduced degree of polarization in the broad emission lines, (2) enhanced polarization in the absorption troughs, and (3) an increase in the degree of polarization toward shorter wavelengths. In addition, BALQSOs with more prominent absorption systems tend to be more strongly polarized, and there is evidence that the subclass of low-ionization absorbers is more strongly polarized than other BALQSOs. If the increased polarization of BALQSOs is due to attenuation of direct, as opposed to scattered, lines of sight to the nucleus, absorption-line objects are underrepresented in optical surveys, and their true incidence is 20%-30% that of all UV-bright QSOs. Nevertheless, BALQSOs on average are not as highly polarized as their more obscured radio-quiet cousins, the IRAS QSOs/hyperluminous IR galaxies. We are led to a consistent picture in which broad absorption is observed in BALQSOs because they are inclined at intermediate inclinations, where our line of sight passes through gas clouds located near the surface of a dusty torus.
The Astrophysical Journal 01/2009; 512(1):125. · 6.28 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Systematic follow-up of high proper-motion stars has identified a new cool magnetic white dwarf that displays a series of spectacular absorption bands in the range 4200-6500 Å. The spectrum bears a striking resemblance to that of LP 790-29, a magnetic DQ star dominated by what are apparently Zeeman-shifted Swan bands of C2. However, key differences in the detailed spectra, polarization, and temperature of the two stars indicate that instead LHS 2229 may represent the first case of a magnetic "peculiar" DQ white dwarf, where absorption in the optical is produced by C2H or another carbon-hydrogen compound. Crude arguments suggest that the field strength on LHS 2229 is in the neighborhood of 108 G. BVI photometry proves to be effective in identifying such peculiar stars, since they lie well outside the main white dwarf sequence in a color-color diagram.
The Astrophysical Journal 01/2009; 512(2):916. · 6.28 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Hubble Space Telescope Wide-Field Planetary Camera 2 imaging polarimetry of the hyperluminous infrared galaxy (and misdirected QSO) IRAS P09104+4109 reveals a highly polarized (p≈20%) giant (~5 kpc) bipolar reflection nebula centered on the nucleus. This, together with our previous detection of broad, polarized Mg II and newly detected broad, polarized Balmer emission lines in our ground-based spectropolarimetry, confirms that the lobes of the nebula are dominated by scattered light from the misdirected QSO and that the object would be indistinguishable from typical luminous QSOs if viewed from either pole. Comparison with previously published narrowband images in the light of [O III] λλ4959, 5007 and [O II] λ3727 shows that the northern lobe of the nebula is coincident with the ionization cone, thus the same light that impinges on the scattering material also ionizes the narrow-line gas. The biconical structure and high polarization suggest that the central UV continuum source is surrounded by a dusty torus of half-opening angle ~23° inclined ~37° with respect to our line of sight. The radio structure of this radio-intermediate object also indicates a long-lived axisymmetry to the central power source, but with a different axis relative to the scattering bicone. We propose that this difference in axes betrays the history of a cataclysmic event that altered the fundamental orientation of the central engine—the bicone defines the current axis of the system. Radio-emitting plasma is beginning to move outward along this new axis, depriving the old radio lobes of power. IRAS P09104+4109 is the first radio-quiet/intermediate object of QSO luminosity and hidden broad lines to show direct evidence that the axisymmetric torus inferred to exist in many Seyfert nuclei is also present in objects of high luminosity.
The Astrophysical Journal 01/2009; 512(1):145. · 6.28 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The highest field polar AR UMa was observed with ASCA during a low state, and with simultaneous EUVE, RXTE, and ground-based optical photometry during a high state. The marginal detection at the low state places a limit on the hard X-ray emission, which is a factor of 5 below the high-state flux limit. The high-state EUV light curves are highly modulated with peak flux at phase 0.9, but the flux never entirely disappears, implying some view of a heated area at all times of the orbit. The spectra during bright phases suggest a temperature of 265,000 K while the fainter phases are cooler, ~215,000 K, and the spectrum is hottest (320,000 K) during the stream dip phases. However, neither a blackbody nor a standard stellar atmosphere model provides a good fit to the data. In order for the EUV modulation to be consistent with geometrical parameters derived from polarimetry and spectroscopy, the primary heated area on the white dwarf must be geometrically extended from the southern magnetic pole while the northern pole is constantly in view. The temperatures of the heated regions are typical of polars, but the projected areas are small, which could be due to the lack of a good view of the main pole at the low inclination of the system. The RXTE light curve shows no modulation over the orbit and only a marginal detection in hard X-rays, implying a weak bremsstrahlung component that is typical for the highest magnetic field polars. In the optical, a low-amplitude sinusoidal modulation peaking at phase 0.5 is consistent with an origin from the irradiated secondary.
The Astrophysical Journal 01/2009; 520(2):841. · 6.28 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We present a detailed study of the nuclear structure of the highly polarized broad absorption line quasar, Mrk 231, through a multiwavelength campaign of Chandra observations, optical spectroscopy, optical spectropolarimetry, and imaging polarimetry. This campaign was designed to extend the 40 ks Chandra study of Gallagher and coworkers and the optical and UV spectropolarimetric study of Smith and coworkers to probe variability on multiple timescales. As direct emission from the nucleus is heavily obscured at optical through X-ray wavelengths, the detailed study of scattered emission has lead to insights into the stratified and complex central region of this active galaxy. Although significant continuum variability is not detected in any of the three new 40 ks Chandra observations, we investigate Fe Kα emission features in the individual and combined spectra. Comparing echelle spectra of the Na I D absorption lines with the literature, we show that one system disappeared in 2000 only to reappear in later epochs. Notably, we detect a large decrease in polarization across the entire optical spectrum of Mrk 231 from 1995 to 2003. Although the polarization fraction fell, e.g., from 4% to 3% at λobs ~ 6000 Å, the polarization position angle spectrum remained unchanged. The optical polarization behavior is consistent with a decrease in the flux scattered by circumnuclear material on spatial scales where the broad-line region is resolved by the scattering material. Ultraviolet imaging polarimetry of Mrk 231 by the Hubble Space Telescope sets an upper limit on the distance between the active nucleus and the scattering regions of <20 pc. We present a detailed study of the nuclear structure of the highly polarized broad absorption line quasar, Mrk 231, through a multiwavelength campaign of Chandra observations, optical spectroscopy, optical spectropolarimetry, and imaging polarimetry. This campaign was designed to extend the 40 ks Chandra study of Gallagher and coworkers and the optical and UV spectropolarimetric study of Smith and coworkers to probe variability on multiple timescales. As direct emission from the nucleus is heavily obscured at optical through X-ray wavelengths, the detailed study of scattered emission has lead to insights into the stratified and complex central region of this active galaxy. Although significant continuum variability is not detected in any of the three new 40 ks Chandra observations, we investigate Fe Kα emission features in the individual and combined spectra. Comparing echelle spectra of the Na I D absorption lines with the literature, we show that one system disappeared in 2000 only to reappear in later epochs. Notably, we detect a large decrease in polarization across the entire optical spectrum of Mrk 231 from 1995 to 2003. Although the polarization fraction fell, e.g., from 4% to 3% at λobs ~ 6000 Å, the polarization position angle spectrum remained unchanged. The optical polarization behavior is consistent with a decrease in the flux scattered by circumnuclear material on spatial scales where the broad-line region is resolved by the scattering material. Ultraviolet imaging polarimetry of Mrk 231 by the Hubble Space Telescope sets an upper limit on the distance between the active nucleus and the scattering regions of <20 pc.
The Astrophysical Journal 12/2008; 633(1):71. · 6.28 Impact Factor