A multichannel fiber optic photometer present performance and future developments

A three channel photometer for simultaneous multicolor observations was designed with the aim of making possible highly efficient photometry of fast variable objects like cataclysmic variables. Experiences with this instrument over a period of three years are presented. Aspects of the special techniques applied are discussed with respect to high precision photometry. In particular, the use of fiber optics is critically analyzed. Finally, the development of a new photometer concept is discussed

Advection-Dominated Accretion and Black Hole Event Horizons

The defining characteristic of a black hole is that it possesses an event horizon through which matter and energy can fall in but from which nothing escapes. Soft X-ray transients (SXTs), a class of X-ray binaries, appear to confirm this fundamental property of black holes. SXTs that are thought to contain accreting black holes display a large variation of luminosity between their bright and faint states, while SXTs with accreting neutron stars have a smaller variation. This difference is predicted if the former stars have horizons and the latter have normal surfaces.Comment: 11 pages, including 2 tables and 2 figures. To appear in The Astrophysical Journal Letter

Hubble Space Telescope Far Ultraviolet Spectroscopy of the Dwarf Nova VW Hyi in Superoutburst

We obtained three consecutive HST spectroscopic observations of a single superoutburst of the dwarf nova VW Hydri. The spectra cover the beginning, middle, and end of the superoutburst. All of the spectra are dominated by strong absorption lines due to CIII (1175 \AA), Lyman alpha (1216 \AA), NIV (1238 \AA, 1242 \AA), SII (1260-65 \AA), SIII (1300 \AA), CII (1335 \AA), SIV (1394 \AA, 1402 \AA) and CIV (1548 \AA, 1550 \AA). We discuss the evolution of the far UV energy distribution and line structure during the superoutburst. We note the absence of any P Cygni line structure in the STIS spectra. Using state of the art accretion disk models by Wade and Hubeny, we have determined accretion rates for all three spectra, for two white dwarf masses, 0.55 M_{\sun} and 0.8 M-{\sun}. For both white dwarf masses the accretion rate during superoutburst decreased by a factor of two from early to late in the superoutburst. The average accretion rate during superoutburst is $3 - 6 \times 10^{-9}$M_{\sun}/yr depending on the white dwarf mass.Comment: 2007, PASP, in pres

Chandra Observations of the Dwarf Nova WX Hyi in Quiescence

We report Chandra observations of the dwarf nova WX Hyi in quiescence. The X-ray spectrum displays strong and narrow emission lines of N, O, Mg, Ne, Si, S and Fe. The various ionization states implied by the lines suggest that the emission is produced within a flow spanning a wide temperature range, from T ~ 10^6 K to T >~ 10^8 K. Line diagnostics indicate that most of the radiation originates from a very dense region, with n ~ 10^{13}-10^{14} cm^{-3}. The Chandra data allow the first tests of specific models proposed in the literature for the X-ray emission in quiescent dwarf novae. We have computed the spectra for a set of models ranging from hot boundary layers, to hot settling flows solutions, to X-ray emitting coronae. WX Hyi differs from other dwarf novae observed at minimum in having much stronger low temperature lines, which prove difficult to fit with existing models, and possibly a very strong, broad O VII line, perhaps produced in a wind moving at a few x 10^3 km/s. The accretion rate inferred from the X-rays is lower than the value inferred from the UV. The presence of high-velocity mass ejection could account for this discrepancy while at the same time explaining the presence of the broad O VII line. If this interpretation is correct, it would provide the first detection of a wind from a dwarf nova in quiescence.Comment: accepted to ApJ; 19 pages, 3 figures, 1 tabl

Far Ultraviolet Observations of the Dwarf Nova VW Hyi in Quiescence

We present a 904-1183 A spectrum of the dwarf nova VW Hydri taken with the Far Ultraviolet Spectroscopic Explorer during quiescence, eleven days after a normal outburst, when the underlying white dwarf accreter is clearly exposed in the far ultraviolet. However, model fitting show that a uniform temperature white dwarf does not reproduce the overall spectrum, especially at the shortest wavelengths. A better approximation to the spectrum is obtained with a model consisting of a white dwarf and a rapidly rotating ``accretion belt''. The white dwarf component accounts for 83% of the total flux, has a temperature of 23,000K, a v sin i = 400 km/s, and a low carbon abundance. The best-fit accretion belt component accounts for 17% of the total flux, has a temperature of about 48,000-50,000K, and a rotation rate Vrot sin i around 3,000-4,000 km/s. The requirement of two components in the modeling of the spectrum of VW Hyi in quiescence helps to resolve some of the differences in interpretation of ultraviolet spectra of VW Hyi in quiescence. However, the physical existence of a second component (and its exact nature) in VW Hyi itself is still relatively uncertain, given the lack of better models for spectra of the inner disk in a quiescent dwarf nova.Comment: 6 figures, 10 printed page in the journal, to appear in APJ, 1 Sept. 2004 issue, vol. 61

Simulations of spectral lines from an eccentric precessing accretion disc

Two dimensional SPH simulations of a precessing accretion disc in a q=0.1 binary system (such as XTE J1118+480) reveal complex and continuously varying shape, kinematics, and dissipation. The stream-disc impact region and disc spiral density waves are prominent sources of energy dissipation.The dissipated energy is modulated on the period P_{sh} = ({P_{orb}}^{-1}-{P_{prec}}^{-1}^{-1} with which the orientation of the disc relative to the mass donor repeats. This superhump modulation in dissipation energy has a variation in amplitude of ~10% relative to the total dissipation energy and evolves, repeating exactly only after a full disc precession cycle. A sharp component in the light curve is associated with centrifugally expelled material falling back and impacting the disc. Synthetic trailed spectrograms reveal two distinct "S-wave" features, produced respectively by the stream gas and the disc gas at the stream-disc impact shock. These S-waves are non-sinusoidal, and evolve with disc precession phase. We identify the spiral density wave emission in the trailed spectrogram. Instantaneous Doppler maps show how the stream impact moves in velocity space during an orbit. In our maximum entropy Doppler tomogram the stream impact region emission is distorted, and the spiral density wave emission is uppressed. A significant radial velocity modulation of the whole line profile occurs on the disc precession period. We compare our SPH simulation with a simple 3D model: the former is appropriate for comparison with emission lines while the latter is preferable for skewed absorption lines from precessing discs.Comment: See http://physics.open.ac.uk/FHMR/ for associated movie (avi) files. The full paper is in MNRAS press. Limited disk space limit of 650k, hence low resolution figure file

Dwarf Nova Oscillations and Quasi-Periodic Oscillations in Cataclysmic Variables: III. A New Kind of Dwarf Nova Oscillation, and Further Examples of the Similarities to X-Ray Binaries

We present measurements of the periods of Dwarf Nova Oscillations (DNOs) and Quasi-Periodic Oscillations (QPOs) in Cataclysmic Variable stars (CVs), many culled from published literature, but also others newly observed (in VZ Pyx, CR Boo, OY Car, Z Cha, AQ Eri, TU Men, HX Peg, CN Ori, V893 Sco, WX Hyi and EC2117-54). These provide data for 26 systems. We show that in general P_QPO ~ 15 P_DNO and that the correlation for CVs extends by three orders of magnitude lower in frequency the similar relationship found for X-Ray binaries. In addition, we have found that there is a second type of DNO, previously overlooked, which have periods ~ 4 times those of the regular DNOs (As well as those mined from publications, we have observed them in VW Hyi, OY Car, AQ Eri, V803 Cen, CR Boo, VZ Pyx, HX Peg and EC2117-54). Often both types of DNO coexist. Unlike the standard DNOs, the periods of the new type, which we refer to as longer period DNOs (lpDNOs), are relatively insensitive to accretion luminosity and can even appear in quiescence of dwarf novae. We interpret them as magnetically channelled accretion onto the differentially rotating main body of the white dwarf primary, rather than onto a rapidly slipping equatorial belt as in the case of the standard DNOs. This is supported by published measurements of v sin(i) for some of the primaries. Some similarities of the DNOs, lpDNOs and QPOs in CVs to the three types of QPO in X-Ray binaries (burst pulsation, high and low frequency QPOs) are noted.Comment: 19 pages, 30 figures. To appear in MNRA

Photometry and Spectroscopy of KS Ursae Majoris during Superoutburst

We report photometric and spectroscopic observations of the SU UMa-type dwarf novae, KS Ursae Majoris, during its 2003 February superoutburst. Modulations with a period of $0.07017\pm0.00021$ day, which is 3.3% larger than the orbital period, have been found during the superoutburst and may be positive superhumps. A maximum trough-to-peak amplitude of around 0.3 magnitude is determined for this superhump. The spectra show broad, absorption-line profiles. The lines display blue and red troughs which alternate in depth. The radial velocity curve of the absorption wings of H$\beta$ has an amplitude of $40\pm11$ km s$^{-1}$ and a phase offset of $0.12\pm0.03$. The $\gamma$ velocity of the binary is $3\pm9$ km s$^{-1}$ and varies on an order of 50 km s$^{-1}$ from day to day. From another clear evidence for a precessing eccentric disk, we obtain a solution to an eccentric outer disk consistent with theoretical works, which demonstrates the validity of the relation between superhumps and tidal effects. The inner part of the disk is also eccentric as evidenced by asymmetric and symmetric wings in the lines. Therefore, the whole disk is eccentric and the variation of $\gamma$ velocity and the evolutionary asymmetric line profiles could be criterions for an precessing eccentric accretion disk.Comment: 12 pages, 8 figures, accpeted for publication in A

Pulsations, Boundary Layers, and Period Bounce in the Cataclysmic Variable RE J1255+266

We report time-series photometry of the quiescent optical counterpart of the EUV transient RE J1255+266. The star appears as a DA white dwarf with bright emission lines and a complex spectrum of periodic signals in its light curve. A signal at 0.0829 d is likely to be the orbital period of the underlying cataclysmic binary (probably a dwarf nova). Characteristic periods of 1344, 1236, and 668 s are seen, and a host of weaker signals. We interpret these noncommensurate signals as (nonradial) pulsation periods of the white dwarf. The donor star is unseen at all wavelengths, and the accretion rate is very low. We estimate a distance of 180+-50 pc, and M_v=14.6+-1.3 for the accretion light. The binary probably represents a very late stage of evolution, with the donor star whittled down to M_2<0.05 M_sol. Such binaries stubbornly resist discovery due to their faintness and reluctance to erupt, but are probably a very common type of cataclysmic variable. If the signal at 0.0829 d is indeed the orbital period, then the binary is an excellent candidate as a "period bouncer". Plausible colleagues in this club include four dwarf novae and the (so far) noneruptive stars GD 552 and 1RXS J105010.3-140431 (hereafter RX 1050). The 1994 EUV eruption implies a soft X-ray/EUV luminosity of 10^(34-35) erg/s, greater than that of any other dwarf nova. We attribute that to a favorable blend of properties: a high-mass white dwarf; a very transparent line of sight (the "Local Chimney"); and a low binary inclination. The first maximizes the expected temperature and luminosity of boundary-layer emission; the other two increase the likelihood that soft X-rays can survive their perilous passage through an accretion-disk wind and the interstellar medium.Comment: PDF, 43 pages, 5 tables, 10 figures, 1 appendix; accepted, in press, to appear May 2005, PASP; more info at http://cba.phys.columbia.edu

Spitzer IRS Spectroscopy of Intermediate Polars: Constraints on Mid-Infrared Cyclotron Emission

We present Spitzer Infrared Spectrograph (IRS) observations of eleven intermediate polars (IPs). Spectra covering the wavelength range from 5.2 to 14 $\mu$m are presented for all eleven objects, and longer wavelength spectra are presented for three objects (AE Aqr, EX Hya, and V1223 Sgr). We also present new, moderate resolution (R $\sim$ 2000) near-infrared spectra for five of the program objects. We find that, in general, the mid-infrared spectra are consistent with simple power laws that extend from the optical into the mid-infrared. There is no evidence for discrete cyclotron emission features in the near- or mid-infrared spectra for any of the IPs investigated. If cyclotron emission is occurring in the 5.2 to 14.0 $\mu$m bandpass it constitutes less than 1% of the bolometric luminosity of any of the IPs. We discuss our results in the context of the standard model for IPs.Comment: 43 pages, including 17 figures, accepted by the Astrophysical Journa