Edward L. Robinson

Near-Infrared Spectroscopy of Low Mass X-ray Binaries : Accretion Disk Contamination and Compact Object Mass Determination in V404 Cyg and Cen X-4

We present near-infrared (NIR) broadband (0.80–2.42 µm) spectroscopy of two low mass X-ray binaries: V404 Cyg and Cen X-4. One important parameter required in the determination of the mass of the compact objects in these systems is the binary inclination. We can determine the inclination by modeling the ellipsoidal modulations of the Roche-lobe filling donor star, but the contamination of the donor star light from other components of the binary, particularly the accretion disk, must be taken into account. To this end, we determined the donor star contribution to the infrared flux by comparing the spectra of V404 Cyg and Cen X-4 to those of various field K-stars of known spectral type. For V404 Cyg, we determined that the donor star has a spectral type of K3 III. We determined the fractional donor contribution to the NIR flux in the H- and K-bands as 0.98 ± .05 and 0.97 ± .09, respectively. We remodeled the H-band light curve from Sanwal et al. (1996) after correcting for the donor star contribution to obtain a new value for the binary inclination. From this, we determined the mass of

The Crab pulsar in the visible and ultraviolet with 20 microsecond effective time resolution

Observations of PSR 0531+21 with the High Speed Photometer on the HST in the visible in October 1991 and in the UV in January 1992 are presented. The time resolution of the instrument was 10.74 microsec; the effective time resolution of the light curves folded modulo the pulsar period was 21.5 microsec. The main pulse arrival time is the same in the UV as in the visible and radio to within the accuracy of the establishment of the spacecraft clock, +/- 1.05 ms. The peak of the main pulse is resolved in time. Corrected for reddening, the intensity spectral index of the Crab pulsar from 1680 to 7400 A is 0.11 +/- 0.13. The pulsed flux has an intensity less than 0.9 percent of the peak flux just before the onset of the main pulse. The variations in intensity of individual main and secondary pulses are uncorrelated, even within the same rotational period.

THE ULTRAVIOLET PULSATIONS OF THE CATACLYSMIC VARIABLE AE AQUARII AS OBSERVED WITH THE HUBBLE SPACE TELESCOPE

We present the results of high time resolution UV spectroscopy and simultaneous high-speed UBVR photometry of AE Aqr. The UV spectra were obtained with the Faint Object Spectrograph aboard the Hubble Space Telescope (HST), and the photometry was carried out with the 82 sec telescope at McDonald Observatory. Our study focuses on the coherent 33 sceond oscillations, whose amplitude is found to be very large in the UV (40% of the mean quiescent level). The mean pulse profile has two broad unequal peaks spaced by half an oscillation cycle. The pulse profiles in the UV and optical bands appear quite similar in shape, with no discernible shifts. The orbital delay curve of the UV pulses establishes the white dwarf as their origin. The (UV+optical) spectrum of the pulsations is well described by a white dwarf atmosphere model with a temperature of about 26,000 K. We find no oscillations in the UV emission-line fluxes, nor in their velocities, down to a limit of 800 km/s. Based on the properties of the UV and optical pulsations we suggest that they originate in the X-ray heated magnetic polar caps of the white dwarf. Under this assumption we produce maximum entropy maps of the brightness distribution of the white dwarf surface. Using this model we are able to reproduce the observed mean pulse profile and interpret fluctuations in the oscillation amplitude as small fluctuations in the accretion rate. We find that the amplitudes and profiles of the pulses are not strongly affectd by the large aperiodic flares exhibited by the system. This suggests that the large flares are not related to the process of depositing material onto the white dwarf and argues against models that place their origin at the white dwarf magnetosphere.

Correlated X-Ray and Optical Variability in V404 Cygni in Quiescence

We report simultaneous X-ray and optical observations of V404 Cyg in quiescence. The X-ray flux varied dramatically by a factor of 20 during a 60 ks observation. X-ray variations were well correlated with those in Hα, although the latter include an approximately constant component as well. Correlations can also be seen with the optical continuum, although these are less clear. We see no large lag between X-ray and optical line variations; this implies they are causally connected on short timescales. As in previous observations, Hα flares exhibit a double-peaked profile suggesting emission distributed across the accretion disk. The peak separation is consistent with material extending outward to at least the circularization radius. The prompt response in the entire Hα line confirms that the variability is powered by X-ray (and/or EUV) irradiation.

Mode Identification of Pulsating White Dwarfs Using the Hubble Space Telescope

We have obtained time-resolved ultraviolet spectroscopy for the pulsating DAV stars G226-29 and G185-32 and for the pulsating DBV star PG 1351+489 with the Hubble Space Telescope Faint Object Spectrograph to compare the ultraviolet to the optical pulsation amplitude and determine the pulsation indices. We find that for essentially all observed pulsation modes, the amplitude rises to the ultraviolet as the theoretical models predict for l = 1 nonradial g-modes. We do not find any pulsation mode visible only in the ultraviolet, nor any modes whose phase flips by 180° in the ultraviolet, as would be expected if high l pulsations were excited. We find one periodicity in the light curve of G185-32, at 141 s, which does not fit theoretical models for the change of amplitude with wavelength of g-mode pulsations.

Low Carbon Abundance in Type Ia Supernovae

We investigate the quantity and composition of unburned material in the outer layers of three normal Type Ia supernovae (SNe Ia): 2000dn, 2002cr, and 2004bw. Pristine matter from a white dwarf progenitor is expected to be a mixture of oxygen and carbon in approximately equal abundance. Using near-infrared (NIR, 0.7-2.5 μm) spectra, we find that oxygen is abundant, while carbon is severely depleted with low upper limits in the outer third of the ejected mass. Strong features from the O I line at λrest = 0.7773 μm are observed through a wide range of expansion velocities ≈ × 103 km s-1. This large velocity domain corresponds to a physical region of the supernova with a large radial depth. We show that the ionization of C and O will be substantially the same in this region. C I lines in the NIR are expected to be 7-50 times stronger than those from O I, but there is only marginal evidence of C I in the spectra and none of C II. We deduce that for these three normal SNe Ia, oxygen is more abundant than carbon by factors of 102-103. Mg II is also detected in a velocity range similar to that of O I. The presence of O and Mg combined with the absence of C indicates that for these SNe Ia, nuclear burning has reached all but the extreme outer layers; any unburned material must have expansion velocities greater than 18 × 103 km s-1. This result favors deflagration to detonation transition (DD) models over pure deflagration models for SNe Ia.

High-Dispersion Spectroscopy of the X-Ray Transient RXTE J0421+560 (=CI Camelopardalis) during Outburst*

We obtained high-dispersion spectroscopy of CI Cam, the optical counterpart of XTE J0421+560, 2 weeks after the peak of its short outburst in 1998 April. The optical counterpart is a supergiant B[e] star that is emitting a two-component wind, a cool, low-velocity wind and a hot, high-velocity wind. The cool wind, which is the source of narrow emission lines of neutral and ionized metals, has a velocity of 32 km s-1 and a temperature near 8000 K. It is dense, roughly spherical, fills the space around the sgB[e] star, and, based on the size of an infrared-emitting dust shell around the system, extends to a radius between 13 and 50 AU. It carries away mass at a high rate, > 10-6 M☉ yr-1. The hot wind has a velocity in excess of 2500 km s-1 and a temperature of 1.7 ± 0.3 × 104 K. From an ultraviolet spectrogram of CI Cam obtained in 2000 March with Hubble Space Telescope, we derive a differential extinction E(B-V) = 0.85 ± 0.05. We show that the distance to CI Cam is greater than 5 kpc. Based on this revised distance, the X-ray luminosity at the peak of the outburst was L(2-25 keV) > 3.0 × 1038 ergs s-1, making CI Cam one of the most luminous X-ray transients. The ratio of quiescent luminosity to peak luminosity in the 2-25 keV band is Lq/Lp < 1.7 × 10-6. The compact star in CI Cam is immersed in the dense circumstellar wind from the sgB[e] star and burrows through the wind, producing little X-ray emission except for rare transient outbursts. This picture, a compact star traveling in a wide orbit through the dense circumstellar envelope of a sgB[e] star, occasionally producing transient X-ray outbursts, makes CI Cam unique among the known X-ray binaries. There is strong circumstantial evidence that the compact object is a black hole, not a neutron star. We speculate that the X-ray outburst was short because the accretion disk around the compact star is fed from a stellar wind and is smaller than disks fed by Roche lobe overflow.

Short-term emission line and continuum variations in mrk110

We present results of a variability campaign of Mrk 110 performed with the 9.2-m Hobby-Eberly Telescope (HET) at McDonald Observatory. The high

Further Evidence for Variable Synchrotron Emission in XTE J1118+480 in Outburst

S/N

On the mass of the compact object in the black hole binary A0620-00

spectra cover most of the optical range. They were taken from 1999 November through 2000 May. The average interval between the observations was 7.3 days and the median interval was only 3.0 days. Mrk 110 is a narrow-line Seyfert 1 galaxy. During our campaign the continuum flux was in a historically low stage. Considering the delays of the emission lines with respect to the continuum variations we could verify an ionization stratification of the BLR. We derived virial masses of the central black hole from the radial distances of the different emission lines and from their widths. The calculated central masses agree within 20% . Furthermore, we identified optical Hei singlet emission lines emitted in the broad-line region. The observed line fluxes agree with theoretical predictions. We show that a broad wing on the red side of the [Oiii] λ 5007 line is caused by the Hei singlet line at 5016 A.