Subscribe to the gold package and get unlimited access to Shamra Academy
Register a new userFar-ultraviolet Spectroscopy of Magnetic Cataclysmic Variables
107
0
0.0
(
0
)
Added by
Sofia Araujo-Betancor
Publication date
2004
fields
Physics
and research's language is
English
Authors
S. Araujo-Betancor
Ask ChatGPT about the research
No Arabic abstract
We have obtained HST/STIS data for a total of eleven polars as part of a program aimed to compile a homogeneous database of high-quality far-ultraviolet (FUV) spectra for a large number of cataclysmic variables (CVs). Of the eleven polars, eight were found in a state of low accretion activity (V347Pav, VVPup, V834Cen, BLHyi, MRSer, STLMi, RXJ1554.2+2721 and V895Cen) and three in a state of high activity (CDInd, ANUMa and UWPic). 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 10800K to 14200K for log g = 8.0. Comparing the WD temperatures of polars to those of non-magnetic CVs, we find that at any given orbital period the WDs in polars are colder than those in non-magnetic 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 non-magnetic CVs are significantly larger above the period gap, suggesting that magnetic braking in polars might be reduced by the strong field of the primary.
rate research
Read More
Time-resolved HST UV eclipse spectrophotometry is presented for the magnetic CVs V1309 Ori, MN Hya, V2301 Oph, and V1432 Aql. Separation of the light curves into wavebands allows the multiple emission components to be distinguished. Photospheric hot spots are detected in V1309 Ori and V2301 Oph. The emission- line spectra of V1309 Ori and MN Hya are unusual, with the strength of N V 1240 and N IV 1718 suggesting an overabundance of nitrogen. Three epochs of observation of the asynchronous V1432 Aql cover ~1/3 of a 50-day lap cycle between the white dwarf spin and binary orbit. The light curves vary from epoch to epoch and as a function of waveband. The dereddened UV spectrum is extremely bright and the spectral energy distribution coupled with the duration of eclipse ingress indicate that the dominant source of energy is a hot (T~35,000K) white dwarf. Undiminished line emission through eclipse indicates that the eclipse is caused by the accretion stream, not the secondary star. The hot white dwarf, combined with its current asynchronous nature and rapid timescale for relocking, suggests that V1432 Aql underwent a nova eruption in the past 75-150 yr. The reversed sense of asynchronism, with the primary star currently spinning up toward synchronism, is not necessarily at odds with this scenario, if the rotation of the magnetic white dwarf can couple to the ejecta during the wind phase of the eruption.
The increasing number of synoptic surveys made by small robotic telescopes, such as the photometric Catalina Real-Time Transient Survey (CRTS), represents a unique opportunity for the discovery of variable sources and improves the statistical samples of such classes of objects. Our goal is the discovery of magnetic Cataclysmic Variables (mCVs). These are rare objects, which probe interesting accretion scenarios controlled by the white dwarf magnetic field. In particular, improved statistics of mCVs would help to address open questions on their formation and evolution. We performed an optical spectroscopy survey to search for signatures of magnetic accretion in 45 variable objects selected mostly from the CRTS. In this sample we found 32 CVs, 22 being mCV candidates from which 13 are previously unreported as such. If the proposed classifications are confirmed, it would represent an increase of 4% in the number of known polars and 12% in the number of known IPs. A fraction of our initial sample was classified as extragalactic sources or other types of variable stars by the inspection of the identification spectra. Despite the inherent complexity in identifying a source as a mCV, variability-based selection followed by spectroscopic snapshot observations has proved to be an efficient strategy for their discoveries, being a relatively inexpensive approach in terms of telescope time.
We present a reanalysis of far-ultraviolet (FUV) observations of the globular cluster NGC 2808 obtained with the Hubble Space Telescope. These data were first analyzed by Brown and coworkers, with an emphasis on the bright, blue horizontal branch (HB) stars in this cluster. Here, our focus is on the population of fainter FUV sources, which include white dwarfs (WDs), blue stragglers (BSs) and cataclysmic variables (CVs). We have therefore constructed the deepest FUV-NUV colour-magnitude diagram of NGC 2808 and also searched for variability among our FUV sources. Overall, we have found approx. 40 WD, approx. 60 BS and approx. 60 CV candidates; three of the BSs and two of the CV candidates are variable. We have also recovered a known RR Lyrae star in the core of NGC 2808, which exhibits massive (approx. 4 mag) FUV variability. We have investigated the radial distribution and found that our CV and BS candidates are more centrally concentrated than the HBs and WD candidates. This might be an effect of mass segregation, but could as well be due to the preferential formation of such dynamically-formed objects in the dense cluster core. For one of our CV candidates we found a counterpart in WFPC2 optical data published by Piotto and coworkers.
The NSFs Karl G. Jansky Very Large Array (VLA) is used to observe 122 magnetic cataclysmic variables (MCVs) during three observing semesters (13B, 15A, and 18A). We report radio detections of 33 stars with fluxes in the range 6--8031 uJy. Twenty-eight stars are new radio sources, increasing the number of radio detected MCVs to more that 40. A surprising result is that about three-quarters (24 of 33 stars) of the detections show highly circularly polarized radio emission of short duration, which is characteristic of electron cyclotron maser emission. We argue that this emission originates from the lower corona of the donor star, and not from a region between the two stars. Maser emission enables a more direct estimate of the mean coronal magnetic field of the donor star, which we estimate to be 1--4 kG assuming a magnetic filling factor of 50%. A two-sample Kolmogorov-Smirnov test supports the conclusion that the distribution function of radio detected MCVs with orbital periods between 1.5-5 hours is similar to that of all MCVs. This result implies that rapidly-rotating (Pspin < 10 days), fully convective stars can sustain strong magnetic dynamos. These results support the model of Taam & Spruit (1989) that the change in angular momentum loss across the fully convective boundary at Porb = ~3 hours is due to a change in the magnetic field structure of the donor star from a low-order to high-order multipolar field.
Magnetic cataclysmic variables are complex accreting binary systems with short orbital periods. Here we present circular polarimetry of five magnetic cataclysmic variable candidates. 1RXS J161008.0+035222, V1432 Aql, and 1RXS J231603.9-052713 have cyclotron emission, which confirms them as AM Her systems. Our data are consistent with zero values for the circular polarization of 1RXS J042555.8-194534 and FIRST J102347.6+003841 imposing some constraints to the polar classification of these objects.
Log in to be able to interact and post comments
comments
Fetching comments
Sign in to be able to follow your search criteria
