J. Patterson

The remarkable rapid X-ray, ultraviolet, optical and infrared variability in the black hole XTE J1118+480

The transient black-hole binary XTE J1118+480 exhibited dramatic rapid variability at all wavelengths which were suitably observed during its 2000 April–July outburst. We examine time-resolved X-ray, ultraviolet, optical and infrared data spanning the plateau phase of the outburst. We find that both X-ray and infrared bands show large amplitude variability. The ultraviolet and optical variability is more subdued, but clearly correlated with that seen in the X-rays. The ultraviolet, at least, appears to be dominated by the continuum, although the lines are also variable. Using the X-ray variations as a reference point, we find that the ultraviolet (UV) variability at long wavelengths occurs later than that at short wavelengths. Uncertainty in the Hubble Space Telescope timing prohibits a determination of the absolute lag with respect to the X-rays, however. The transfer function is clearly not a delta-function, exhibiting significant repeatable structure. For the main signal we can rule out an origin in reprocessing on the companion star – the lack of variation in the lags is not consistent with this, given a relatively high orbital inclination. Weak reprocessing from the disc and/or companion star may be present, but is not required, and another component must dominate the variability. This could be variable synchrotron emission correlated with X-ray variability, consistent with our earlier interpretation of the infrared (IR) flux as due to synchrotron emission rather than thermal disc emission. In fact, the broad-band energy distribution of the variability from IR to X-rays is consistent with expectations of optically thin synchrotron emission. We also follow the evolution of the low-frequency quasi-periodic oscillation in X-rays, UV, and optical. Its properties at all wavelengths are similar, indicating a common origin.

SUPERHUMPS IN CATACLYSMIC BINARIES. XI. V603 AQUILAE REVISITED

After a decade of near stability at P=0.146 d, the photometric superhump periodicity of the old nova V603 Aquilae experienced a remarkable change between 1991 and 1992. Observation in 1992--1994 indicates that the dominant signal was then at a period in the range 0.1338-0.1345 d, 3% shorter than the orbital period. Like its predecessor, the new signal also wanders in period on a timescale of a few months. The full amplitude in 1994 was 0.20 mag, more than twice as great as the superhump displayed during 1980--1991. An intensive observing campaign in 1994 revealed that the old superhump at 0.146 d was still present with approximately undiminished amplitude (averaging 0.07 mag). In a precession model, the simultaneous presence of superhumps above and below the orbital period strongly suggests identification with two independent types of precessional sideband. The observed periods and period changes are consistent with a simple hypothesis: that the longer period (positive superhump) arises from the prograde motion of the line of apsides, and the shorter period (negative superhump) arises from the retrograde motion of the line of nodes. A detailed account of how a fluid disk manages to maintain such well-organized motions is sorely needed.

A photometric study of the recurrent nova WZ Sagittae at minimum light

We have acquired a new set of high-speed photometric observations of the recurrent nova WZ Sge; using these observations, we develop a quantitative model for the system. WZ Sge is a binary system with an orbital period of 81/sup m/38/sup s/ and a separation of 3.2 x 10/sup 10/ cm consisting of an unevolved star with a mass of 0.019 M/sub sun/ orbiting a white dwarf with a mass of 0.38 M/sub sun/. Mass is being transferred from the late-type star to the white dwarf in a stream through the inner Lagrangian point at a rate of 1 x 10/sup 16/ g s/sup -1/. The transferred material forms an accretion disk with a radius of 9.7 x 10/sup 9/ cm around the white dwarf. The accretion disk is optically thick and is the sole source of the broad hydrogen absorption lines seen in the spectrum of WZ Sge. At the point where the stream of transferred material impacts the disk, a bright shock front (bright spot) is formed. The spot has a radius of 1.0 x 10/sup 9/ cm and an effective temperature near 1.6 x 10/sup 4/ K, and contributes about 15% of the total optical luminosity of themorexa0» system. In addition, we detect very stable rapid coherent oscillations in the light curve of WZ Sge with periods of 28.98 s and 27.87 s, making WZ Sge the first recurrent nova in which such oscillations have been found. On one occasion both oscillations were present simultaneously in the light curve, and we use this fact to conclude that the rapid oscillations must be pulsations, that the pulsations cannot be radial pulsations, and that the pulsations are probably nonradial g-mode pulsations.«xa0less

The non‐radially pulsating primary of the cataclysmic variable GW Librae

ABSTRACT The dwarf nova GW Librae is the first cataclysmic variable dis covered to have a primary in awhite dwarf instability strip, making it the first multi-mode, nonradially-pulsatingstar knownto be accreting. The primaries of CVs, embedded in hot, bright accretion discs, are difficultto study directly. Applying the techniques of asteroseismology to GW Librae could thereforegive us an unprecedented look at a white dwarf that has undergone ∼ 10 9 years of accretion.However, an accreting white dwarf may have characteristics sufficiently different from thoseof single pulsating white dwarfs to render the standard models of white dwarf pulsations in-valid for its study.This paper presents amplitudespectra of GW Lib from a series of observingcampaignsconductedduring1997,1998and 2001.We find that t he dominantpulsationmodescluster at periods near 650, 370 and 230 s, which also appear in linear combinations with eachother. GW Lib’s pulsation spectrum is highly unstable on time-scales of months, and exhibitsclusters of signals very closely spaced in frequency, with separations on the order of 1 µHz.Key words: Stars: cataclysmic variables, white dwarfs, oscillations – Individual: GW Librae

33 second X-ray pulsations in AE Aquarii

The discovery of 33-s pulsations in the 0.1-4.0 keV light curve of the nova-like variable AE Aquarii is reported. These pulsations agree in period and phase with the optical pulsations. The periodicity probably originated from an accretion-induced hot spot on a rapidly rotating, magnetized white dwarf. It is possible that transient pulsations at nearby periods, similar to those seen in the optical light curve, are also present in the X-ray light curve.

The System Parameters of DW Ursae Majoris

We present new constraints on the system parameters of the SW Sextantis star DW Ursae Majoris, based on ultraviolet (UV) eclipse observations with the Hubble Space Telescope. Our data were obtained during a low state of the system, in which the UV light was dominated by the hot white dwarf (WD) primary. The duration of the WD eclipse allows us to set a firm lower limit on the mass ratio, q = M2/M1 > 0.24; if q 71°. We have also been able to determine the duration of WD ingress and egress from our data. This allows us to constrain the masses and radii of the system components and the distance between them to be 0.67 ≤ M1/M☉ ≤ 1.06, 0.008 ≤ R1/R☉ ≤ 0.014, M2/M☉ > 0.16, R2/R☉ > 0.28, and a/R☉ > 1.05. If the secondary follows Smith & Dhillons mass-period relation for CV secondaries, our estimates for the system parameters become M1/M☉ = 0.77 ± 0.07, R1/R☉ = 0.012 ± 0.001, M2/M☉ = 0.30 ± 0.10, R2/R☉ = 0.34 ± 0.04, q = 0.39 ± 0.12, i = 82° ± 4°, and a/R☉ = 1.14 ± 0.06. We have also obtained time-resolved I- and K-band photometry of DW UMa during the same low state. Using Bessells spectral type versus (I-K) color calibration, we estimate the spectral type of the donor star to be M3.5 ± 1.0. This latter result helps us to estimate the distance toward the system via Baileys method as d = 930 ± 160 pc. Finally, we have repeated Knigge and coworkers WD model atmosphere fit to the low-state UV spectrum of DW UMa in order to account for the higher surface gravity indicated by our eclipse analysis. The best-fit model with surface gravity fixed at log g = 8 has an effective temperature of Teff = 50,000 ± 1000 K. The normalization of the fit also yields a second distance estimate, d = 590 ± 100 pc. If we adopt this distance and assume that the mideclipse K-band flux is entirely due to the donor star, we obtain a second estimate for the spectral type of the secondary in DW UMa, M7 ± 2.0. After discussing potential sources of systematic errors in both methods, we conclude that the true value for the distance and spectral type will probably be in between the values obtained by the two methods.

Rapid oscillations in cataclysmic variables. I. the 71 second oscillation of DQ Herculis

We present a new photometric study of the 71 s oscillation in the old nova DQ Herculis. All timings of the oscillation maxima may be phased with a simple ephemeris; the most likely choice has a large P term, which will, if continued, halt the speedup of the oscillation in approx.40 years.The oscillation shows a wealth of fascinating detail in its phase and amplitude variations around the binary orbit. We confirm the oscillation phase shift in eclipse found by Warner et al., and provide a more accurate description. We find that the pulses arrive systematically late by approx.6 s for a 1 hour interval around orbital phase 0.7. Multicolor observations establish that the visible oscillation arises from the reprocessing of the white dwarfs pulsed ultraviolet flux by the accretion disk.We also report new eclipse timings, and show that a sinusoidal modulation with a period of approx.14 years is present in all the timings sine 1954. This effect cannot be explained by the presence of a distant companion or by classical apsidal motion.

X-ray Variability of the Magnetic Cataclysmic Variable V1432 Aql and the Seyfert Galaxy NGC 6814

V1432 Aquilae (=RX J1940.2-1025) is the X-ray-bright, eclipsing magnetic cataclysmic variable ~37 away from the Seyfert galaxy NGC 6814. Because of a 0.3% difference between the orbital (12,116.3 s) and the spin (12,150 s) periods, the accretion geometry changes over the ~50 day beat period. Here we report the results of a Rossi X-Ray Timing Explorer campaign to observe the eclipse 25 times, as well as of archival observations with ASCA and BeppoSAX. Having confirmed that the eclipse is indeed caused by the secondary, we use the eclipse timings and profiles to map the accretion geometry as a function of the beat phase. We find that the accretion region is compact and that it moves relative to the center of white dwarf on the beat period. The amplitude of this movement suggests a low-mass white dwarf, in contrast to the high mass previously estimated from its X-ray spectrum. The size of the X-ray emission region appears to be larger than in other eclipsing magnetic cataclysmic variables. We also report on the RXTE data as well as the long-term behavior of NGC 6814, indicating flux variability by a factor of at least 10 on timescales of years.

CONSTRAINING THE ANGULAR MOMENTUM EVOLUTION OF V455 ANDROMEDAE

Time-series photometry on the cataclysmic variable V455 Andromedae (hereafter V455 And, HS 2331+3905) reveals a rotation period shorter than the orbital period, implying the presence of a magnetic field. We expect that this magnetic field channels the accreted matter from the disk toward the white dwarf poles, classifying it as an Intermediate Polar. The two polar spinning emission areas are visible in the lightcurves at the rotation period of 67.61970396 ± 0.00000072 s, and its harmonic. Using photometric observations of V455 And obtained from 2007 October to 2015, we derive 3σ upper limits to the rate of change of the spin harmonic (SH) with time to be dPSH/dt ≤ −7.5 × 10−15 s s−1 employing the O–C method, and −5.4 × 10−15 s s−1 with a direct nonlinear least squares fit. There is no significant detection of a changing spin period for the duration of 2007 October–2015. The 3σ upper limit for the rate of change of spin period with time is dPspin/dt ≤ −10.8 × 10−15 s s−1 or −0.34 μs yr−1. V455 And underwent a large-amplitude dwarf nova outburst in 2007 September. The pre-outburst data reflect a period 4.8 ± 2.2 μs longer than the best-fit post-outburst spin period. The angular momentum gained by the white dwarf from matter accreted during outburst and its slight subsequent shrinking should both cause the star to spin slightly faster after the outburst. We estimate that the change in spin period due to the outburst should be 5 μs, consistent with the empirical determination of 4.8 ± 2.2 μs (3σ upper limit of 11.4 μs).

GW Librae: a unique laboratory for pulsations in an accreting white dwarf

Non-radial pulsations have been identified in a number of accreting white dwarfs in cataclysmic variables. These stars offer insight into the excitation of pulsation modes in atmospheres with mixed compositions of hydrogen, helium, and metals, and the response of these modes to changes in the white dwarf temperature. Among all pulsating cataclysmic variable white dwarfs, GW Librae stands out by having a well-established observational record of three independent pulsation modes that disappeared when the white dwarf temperature rose dramatically following its 2007 accretion outburst. Our analysis of HST ultraviolet spectroscopy taken in 2002, 2010 and 2011, showed that pulsations produce variations in the white dwarf effective temperature as predicted by theory. Additionally in May 2013, we obtained new HST/COS ultraviolet observations that displayed unexpected behaviour: besides showing variability at 275 s, which is close to the post-outburst pulsations detected with HST in 2010 and 2011, the white dwarf exhibits high-amplitude variability on a 4.4 h time-scale. We demonstrate that this variability is produced by an increase of the temperature of a region on white dwarf covering up to 30 per cent of the visible white dwarf surface. We argue against a short-lived accretion episode as the explanation of such heating, and discuss this event in the context of non-radial pulsations on a rapidly rotating star.