AN UMa Research Articles

Ultraviolet Spectroscopic Investigation of HU Aqr and AN UMa with the Data from HST and IUE

Abstract We present ultraviolet spectroscopic study of two polar systems, HU Aqr and AN UMa, observed with the Hubble Space Telescope’s Space Telescope Imaging Spectrograph (STIS) and Faint Object Spectrograph (FOS) and with the International Ultraviolet Explorer (IUE) during the period 1979–2003, to diagnose the ultraviolet fluxes of CIV 1550 Å and He II 1640 Å emission lines originating in an accretion stream during different orbital phases. The reddening of two systems is determined from the 2200 Å absorption feature. Different spectra for both systems, showing the variations in line fluxes at different orbital phases, are presented. We concentrated on calculating the line fluxes of CIV and He II emission lines. From HST and IUE data, we derived accretion luminosities and accretion rates for the two systems. The average temperature of the accretion stream for HU Aqr and AN UMa is about 5000 K and 6000 K, respectively. Our results show that there are variations in line fluxes, accretion luminosities and accretion rates with time for the two systems. These modulations are attributed to the variations of both density and temperature as a result of a changing rate of mass transfer from the secondary star to the primary star. These results from the HST and IUE observations support the irradiation model producing sufficient ultraviolet flux for orbital modulations.

Short and long time-scale variability in magnetic cataclysmic variables: long-term monitoring of polars

We present long-term observations of the magnetic cataclysmic variables AM Her, AN UMa, AR UMa, DP Leo and V1309 Ori. Analyses of both short- and long-period light variations are presented. For the first time, as far as we know, long-period variations of AN UMa and AR UMa have been studied and multiple frequencies obtained. Fourier analysis indicates 170-, 218- and 180-d variations for AM Her, AN UMa and AR UMa, respectively. These periodicities may be due to modulation of the mass-transfer rate resulting from magnetic cycles in the secondary stars. In addition, we collect the physical parameters of polars from the literature and estimate their mass-transfer rates and orbital period variation and give a rough estimation of the donor magnetic fields.

Long-Term Monitoring of Polars

AbstractWe present long-term observations of magnetic cataclysmic variables AM Her, AN UMa, AR UMa, DP Leo, and V1309 Ori obtained with the ROTSEIIId telescope. All data have been analysed and preliminary results indicate periods of 170 days, 217 days, and 180 days for AM Her, AN UMa, and AR UMa, respectively.

Cyclotron Modeling Phase‐Resolved Infrared Spectroscopy of Polars. II. EQ Ceti, AN Ursa Majoris, and VV Puppis

We present phase-resolved low-resolution infrared spectra of the polars EQ Cet, AN UMa, and VV Pup obtained with SPEX on the IRTF. In addition, we supplement our IRTF observations for VV Pup with optical data sets. The infrared spectra cover the wavelength range 0.8 μ m ⩽ λ ⩽ 2.4 μm and show cyclotron emission at all orbital phases for the three objects. We use a constant-lambda prescription to attempt to model the changing cyclotron features seen in the spectra. We model the EQ Cet data using two different approaches, yielding final parameters of B≃ 34.0 MG, kT≃ 3.5 keV, and log Λ ≃ 4.1. For AN UMa we find orbitally averaged models of B = 32.1 MG, kT = 5.0 keV, and log Λ ≃ 4.3. We modeled phase-resolved spectra for VV Pup at four different epochs. We find the following range in parameters: 30.0 MG ⩽ B⩽ 31.9 MG, 4.0 keV ⩽ kT ⩽ 15.0 keV, and 3.7 ⩽ log Λ ⩽ 6.5. Finally, we find that in B-kT space, the four epochs are well separated based on the visual brightness of the system at the time of observation.

Cataclysmic Variables from Sloan Digital Sky Survey. V. The Fifth Year (2004)

This paper identifies the cataclysmic variables that appear in spectra obtained in 2004 as part of the Sloan Digital Sky Survey. Spectra of 41 objects, including seven systems that were previously known (CC Cnc, DW Cnc, PQ Gem,AR UMa,AN UMa,RXJ1131.3+4322,andUMa6)and34new cataclysmicvariablesarepresented.The positions and ugriz photometry of all 41 systems are given, as well as additional follow-up spectroscopic, photometric, and/or polarimetric observations of eight of the new systems. The new objects include three eclipsing systems, six with prominent He ii emission, and six systems that show the underlying white dwarf.

Far‐Ultraviolet Spectroscopy of Magnetic Cataclysmic Variables

We have obtained Hubble Space Telescope (HST) STIS data for a total of 11 polars as part of a program aimed at compiling a homogeneous database of high-quality far-ultraviolet (FUV) spectra for a large number of cataclysmic variables (CVs). Of the 11 polars, eight were found in a state of low accretion activity (V347 Pav, VV Pup, V834 Cen, BL Hyi, MR Ser, ST LMi, RX J1554.2+2721, and V895 Cen) and three in a state of high activity ( CD Ind, AN UMa, and UW Pic). The STIS spectra of the low-state polars unambiguously reveal the photospheric emission of their white dwarf (WD) primaries. We have used pure hydrogen WD models to fit the FUV spectra of the low-state systems (except RX J1554.2+2721, which is a high-field polar) in order to measure the WD effective temperatures. In all cases, the fits could be improved by adding a second component, which is presumably due to residual accretion onto the magnetic pole of the WD. The WD temperatures obtained range from 10,800 to 14,300 K for log g = 8.0. Our analysis more than doubles the number of polars with accurate WD effective temperatures. Comparing the WD temperatures of polars with those of nonmagnetic CVs, we find that at any given orbital period the WDs in polars are colder than those in nonmagnetic CVs. The temperatures of polars below the period gap are consistent with gravitational radiation as the only active angular momentum loss mechanism. The differences in WD effective temperatures between polars and nonmagnetic CVs are significantly larger above the period gap, suggesting that magnetic braking in polars might be reduced by the strong field of the primary. We derive distance estimates to the low-state systems from the flux scaling factors of our WD model fits. Combining these distance measurements with those from the literature, we establish a lower limit on the space density of polars of 1.3 x 10(-6) pc(-3).

The Three-Dimensional Structure of EUV Accretion Regions of AM Her Stars: Analysis of EUVE Light Curves

We present EUV light curves for a number of AM Her systems observed either as guest observer targets or with the EUVE Right Angle Program. We have formed light curves for eight AM Her stars and show in our presentation the similarities and differences present. We draw some conclusions by grouping the systems by inclination, magnetic field strength, and accretion region geometry. In order to understand the physical structures responsible for the EUV emissions, we have developed a software model to generate synthetic light curves. We find that the EUV accretion regions in the systems UZ For, VV Pup, and AM Her cannot be fit with a flat spot confined to the white dwarf surface, regardless of its shape or brightness profile. Rather, a small, symmetric, raised spot is the only shape consistent with the data. The light curves for systems EF Eri, RE1149+28, AN UMa, and V834 Cen show evidence for additional structure that precedes the primary accretion region in phase. Our model indicates that a large portion of the light curve in each system is seen in absorption. Finally, in three systems, we detect a very gradual rise and fall in the EUV flux (<5% of the peak flux) at phases when the spot is completely obscured behind the white dwarf (WD) limb. We attribute this detection to emission from the accretion column that decreases exponentially with distance from the WD and detect emission at heights up to 15% of the WD radius.

ROSAT observations of AN UMa and MR Ser: the status of the soft X-ray excess in AM Her stars

We present the results of ROSAT pointed observations of the AM Her stars AN UMa and MR Ser. AN UMa was in a high-brightness state. We examine its soft X-ray light curve, and fit its spectrum with a two-component model consisting of an absorbed soft blackbody and a harder thermal bremsstrahlung distribution. MR Ser was found to be in a low-brightness state, and had a spectrum which was well fitted with a single-component, thermal bremsstrahlung model.The question of the energy balance in AM Her stars is reviewed with a view to quantifying the incidence of the 'soft X-ray excess'. The excess has been explained as due to the deposition of energy directly into the white dwarf photosphere by the accretion stream, supplementing the energy of photons from the hot accretion column which irradiate the photosphere. We have calculated the energy balance in AN UMa and 16 other AM Her stars observed with ROSAT. We find that, although the majority of AM Her stars exhibit a soft X-ray excess, a number of objects are consistent with a model which includes radiative heating only. We examine possible factors which could determine the value of the soft X-ray excess. We find a correlation of soft X-ray excess with magnetic field strength or, alternatively, the radius at which material couples on to the magnetic field lines.

A survey for QPOs in AM Herculis stars and a detailed study of the QPOs in AN Ursae Majoris

We have searched for quasi-periodic oscillations with frequencies between 0.1 and 2.4 Hz in the light curves of nine AM Her stars using simultaneous multi-colour photometry. Oscillations were not seen in BY Cam ( = H 0538 + 608), EF Eri, AM Her, DP Leo ( = E 1114 + 182), ST LMi ( = CW 1103 + 254), MR Ser ( = PG 1550 + 191) or QQ Vul ( = E 2003 + 225), but were detected in AN UMa and VV Pup. We were unable to find any systematic difference between those systems with and those without quasi-periodic oscillations. When averaged over the orbital period, the amplitude of the oscillations in AN UMa is largest in the B bandpass ( ~ 3.5 per cent). The colour of the oscillations varies systematically with orbital phase, however, and the amplitude is at least as high in the U band as in the B band during the main hump in the light curve. The average coherence time as measured with autocorrelation techniques is 37 ± 2 s for strong oscillations in the B band. We show that the accretion column is divided into 2–45 regions, each of which is oscillating simultaneously but independently. Our observations are consistent with the shock-oscillation model.

The optical emission from oscillating white dwarf radiative shock waves

The hypothesis that quasi-periodic oscillations (QPOs) are due to the oscillatory instability of radiative shock waves discovered by Langer et al. (1981, 1092) is examined. The time-dependent optical spectra of oscillating radiative shocks produced by flows onto magnetic white dwarfs are calculated. The results are compared with the observations of the AM Her QPO sources V834 Cen, AN UMa, EF Eri, and VV Pup. It is found that the shock oscillation model has difficulties with aspects of the observations for each of the sources. For VV Pup, AN UMa, and V834 Cen, the cyclotron luminosities for the observed magnetic fields of these systems, based on our calculations, are large. The strong cyclotron emission probably stabilizes the shock oscillations. For EF Eri, the mass of the white dwarf based on hard X-ray observations is greater than 0.6 solar mass.

Millisecond optical photometry of the DQ Herculis objects

The results of millisecond optical photometry of the DQ Herculis stars EX Hya, H2252-035, V1223 Sgr, and AE Aqr obtained at the Las Campanas Observatory and Cerro Tololo Inter-American Observatory during the years 1985, 1986, and 1987 are reported. The data for coherent and incoherent features are searched for. Coherent features (other than those previously known) were not detected for frequencies between 0.1 and 250 Hz at 4-sigma upper limits of 0.05-0.4 percent. Evidence is not found for 1-3 s, quasi-coherent features. Such features have been detected in the optical emission of the AM Her objects AN UMa, EF Eri, and E1405-451. Because of the similarities between AM Her and DQ Her objects, it was suggested that such quasi-coherent features might also be found in the DQ Her objects.

WHITE DWARFS IN CATACLYSMIC VARIABLES: LOW STATE IUE OBSERVATIONS OF V794 AQUILAE, MR SERPENTIS, AND AN URSAE MAJORIS

ABSTRACT The authors have obtained IUE low dispersion observations of one nova-like system (V794 Aql) and two AM Her systems (MR Ser and AN UMa) during low or intermediate low accretion rate states. The decreased UV emission during these times was used to place limits on the white dwarf temperatures. When these results are added to the existing data on white dwarf temperatures, it appears that the white dwarfs in systems with normal high accretion rates and only occasional low accretion rate states remain hotter than the white dwarfs in systems with low average accretion rates.

High-speed multicolor photometry of an Ursae Majoris

view Abstract Citations (19) References (29) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS High-Speed Multicolor Photometry of an Ursae Majoris Imamura, James N. ; Steiman-Cameron, Thomas Y. Abstract Results are reported of multibandpass optical photometry of the AM Her object, AN UMa Fourier techniques were used to search for variability on time scales ranging from milliseconds to tens of seconds. Longer term behavior was studied using photometric light curves. Middleditch (1982) found that the power spectrum of AN UMa, if averaged over times longer than about 10 minutes, showed 'excess' power over the frequency range 0.4-1 Hz, but he did not study the behavior for individual time intervals shorter than 10 minutes. The broad feature can be resolved into multiple features of width 0.02 Hz or less when power spectra are calculated for time intervals of about 1 minute. This suggests that the noisar feature at a given time is the superposition of many discrete periodicities of limited coherence. The light curves show two distinct minima at polarization phases 0.1 and 0.55, with the possibility of a third minimum near 0.3. The results are consistent with earlier results if there are two or more optical emission regions fixed in the frame of the binary system whose luminosities vary as a function of time. Publication: The Astrophysical Journal Pub Date: December 1986 DOI: 10.1086/164816 Bibcode: 1986ApJ...311..786I Keywords: Magnetic Stars; Stellar Spectrophotometry; Visible Spectrum; White Dwarf Stars; X Ray Binaries; Fourier Analysis; Light Curve; Polarization Characteristics; Power Spectra; Stellar Mass Accretion; Stellar Oscillations; Astrophysics full text sources ADS | data products SIMBAD (2)

Comparison of the X-ray observations of the AM Herculis objects to theory

view Abstract Citations (19) References (39) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Comparison of the X-ray observations of the AM Herculis objects to theory. Imamura, J. N. Abstract The X-ray observations of the original AM Her objects (AM Her, VV Pup, EF Eri, and AN UMa are reexamined. The work differs from that carried out previously in that the observed X-ray spectral properties are compared to theoretical models calculated using a dipolar accretion geometry rather than a spherically symmetric one, and the observed X-ray light curves are compared to theoretical light curves calculated assuming that the X-ray sources are cataclysmic variable-like binaries which contain a strongly magnetic white dwarf. The added constraints offered by the light curve properties restrict the possible kinds of X-ray emission scenarios in some cases. For instance, it is found that the X-ray observations of AM Her can be self-consistently explained by accretion powered radiative shock models. Further, the observations cannot be self-consistently explained by conduction dominated models as has been suggested by previous workers. Publication: The Astrophysical Journal Pub Date: October 1984 DOI: 10.1086/162495 Bibcode: 1984ApJ...285..223I Keywords: Binary Stars; Cataclysmic Variables; Stellar Mass Accretion; Stellar Models; White Dwarf Stars; X Ray Sources; Light Curve; Magnetic Stars; Shock Waves; Stellar Radiation; Stellar Spectra; X Ray Spectra; Astrophysics full text sources ADS | data products SIMBAD (4)

Synchronization of magnetic stars in binary systems

Asynchronous rotation of magnetic stars in close binary systems drives substantial field-aligned electrical currents between the magnetic star and its companion. The resulting magnetohydrodynamic torque is able to account for the heretofore unexplained synchronous rotation of the strongly magnetic degenerate dwarf component in systems like AM Her, VV Pup, AN UMa, and EF Eri as well as the magnetic A type component in systems like HD 98088 and 41 Tauri. The electric fields produced by even a small asynchronism are large and may accelerate some electrons to high energies, producing radio emission. The total energy dissipation rate in systems with degenerate dwarf spin periods as short as 1 minute may reach 10 to the 33rd ergs/s. Total luminosities of this order may be a characteristic feature of such systems.

Radio emission from AM Herculis - The quiescent component and an outburst

The VLA has been used to search for radio emission from the AM Her-type binaries VV Pup, EF Eri, PG 1550 + 191, CW 1103 + 354, and AN UMa, at 4.9 GHz. A remarkable 10-min outburst was detected from AM Her at 4.9 GHz, which was about 20 times more intense than the quiescent emission and was essentially 100 percent circularly polarized. It is suggested that the quiescent emission of AM Her can be accounted for by 500-keV electrons trapped in the magnetosphere of the white dwarf, provided that the electron energy spectrum is quite hard and that the spectral hardness or number density of energetic electrons increases with radius, while the outburst is probably due to an electron-cyclotron maser operating near the surface of the red dwarf companion. The implied existence of a 1000-gauss localized magnetic field and a corona on the red dwarf has consequences for mass transfer, field line interactions, and variable activity.

Two-second variability in AM Herculis binaries

AN UMa and the newly identified AM Her star E1405--451 both show excesses of optical variability in the broad frequency range from 0.4 to 0.8 Hz (2.5--1.25 s). The optical fluxes from AN UMa and E1405--451 must vary by 2.4% and 1.2% rms, respectively, in order to produce the level of noise observed in the power spectra. The noise is not produced during any one section of time or orbital phase but is approximately a constant proportion of the net optical light from E1405--451. The feature is possibly bimodal in AN UMa with a minimum of power at 0.65 Hz. Time series observations of AM Her made during both its ''bright'' and ''faint'' phases show that no such noise feature is present to less than 0.25% and 1.3% rms of the stellar light. The predictions of a model for this phenomenon, recently advanced by Langer, Chanmugam, and Shaviv and based on a thermal instability in the height of a standoff accretion shock, are tested with inconclusive results.

The 1979 minimum state of an Ursae Majoris

view Abstract Citations (33) References (31) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS The 1979 minimum state of an UMa. Liebert, J. ; Tapia, S. ; Bond, H. E. ; Grauer, A. D. Abstract Spectrophotometry, photometry, and circular polarimetry are reported of the magnetic binary AN UMa during February/March 1979, when it reached a very low state of B (approximately 19-20), 2 mag fainter than ever observed previously. The B light curve was generally similar in shape to that for the high state, although only approximately 0.6 of a period was covered. The energy distribution became markedly redder compared with that in higher states; the I magnitude had dropped only to about 17. The low-state I-band circular polarization reached a peak value of -35%, which is considerably stronger than in the high state; the presumed cyclotron funnel source thus remained dominant, at least at red wavelengths. The spectrophotometry at V approximately 17.5-18.5 revealed a red continuous energy distribution; there was weak H-alpha emission, a probable very broad emission feature near H-beta, but no evidence for strong absorption features from the secondary or magnetic primary. Publication: The Astrophysical Journal Pub Date: March 1982 DOI: 10.1086/159727 Bibcode: 1982ApJ...254..232L Keywords: Binary Stars; Circular Polarization; Magnetic Stars; Stellar Magnitude; Stellar Spectrophotometry; Constellations; Cyclotron Radiation; Ephemerides; H Alpha Line; H Beta Line; Harmonic Radiation; Interstellar Extinction; Light Curve; Mass Transfer; Polarimetry; Visual Photometry; Astronomy full text sources ADS | data products SIMBAD (5)

Time Resolved Circular Polarimetry of White Dwarf Pulsars

There are two recognized subsets of cataclysmic binaries that contain white dwarfs: the AM Her stars, whose white dwarf components are sufficiently magnetized to ensure synchronous rotation with the orbital period, and the DQ Her stars, which do not maintain synchronism and probably have been spun up by mass accretion. Both groups of stars, recognized also as X-ray sources, are important as probes of the accretion process, and the radiation mechanisms in the vicinity of the white dwarf. So far, we know five objects belonging to the former group (AM Her, AN UMa, VV Pup, 2A 0311-227, and PG 1550+191), and seven falling into the latter: WZ Sge, AE Aqr, V533 Her, DQ Her, V1223 Sgr, H2252-035, and EX Hya, with the corresponding rotation periods of their white dwarf primaries of 28, 33, 64, 71, 794, 805, and 4020 s, respectively. While the main observational and theoretical efforts have been focussed on the AM Her class, the DQ Her binaries have been investigated to much lesser extent. This is probably because the DQ Her stars have been recognized only very recently as a homogeneous class.

Simultaneous X-ray and optical observations of an Ursae Majoris during a low state

view Abstract Citations (21) References (21) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Simultaneous X-ray and optical observations of an UMa during a low state. Szkody, P. ; Schmidt, E. ; Crosa, L. ; Schommer, R. Abstract In November 1975, AN UMa was detected as a soft X-ray source with a 0.1-0.4 keV flux which was modulated with the orbital cycle. In order to investigate further this orbital dependence and to correlate it with existing photometric and spectroscopic observations to aid in determining the geometry of the system, simultaneous X-ray and optical observations were made throughout several cycles of AN UMa. The system was in a low optical state during the observations. The X-ray observations were made with the Imaging Proportional Counter on the Einstein Observatory, yielding measurements over an available energy range of 0.1-4 keV. Attention is given to total X-ray flux, the X-ray light curve, optical and infrared data, and model implications. Publication: The Astrophysical Journal Pub Date: May 1981 DOI: 10.1086/158915 Bibcode: 1981ApJ...246..223S Keywords: Eclipsing Binary Stars; Stellar Spectrophotometry; Variable Stars; X Ray Astronomy; X Ray Stars; Light Curve; Stellar Magnitude; Ubv Spectra; Astronomy full text sources ADS | data products SIMBAD (4)