49 research outputs found
Magnetic field and unstable accretion during AM Herculis low states
A study of AM Her low states in September 1990 and 1991 and June-July 1997 is
reported from a coordinated campaign with observations obtained at the
Haute-Provence observatory, at the 6-m telescope of the Special Astrophysical
Observatory and at the 2.6m and 1.25m telescopes of the Crimean observatory.
Spectra obtained at different dates when the source was in low states at a
comparable V magnitude, show the presence of strong Zeeman absorption features
and marked changes in emission lines with a day-to-day reappearance of the HeII
(4686\AA) emission lines in 1991. Despite this variability, the magnetic field
inferred from the fitting of the absorption spectrum with Zeeman hydrogen
splitting, is remarkably constant with a best value of (12.50.5)MG.
Detailed analysis of the UBVRI light curves shows the presence of repetitive
moderate amplitude ( 0.3-0.5 mag) flares predominantly red in colour.
These flares are attributed to small accretion events and are compared to the
large ( 2 mag.) blue flare reported by Shakhovskoy et al. (1993). We
suggest that the general flaring activity observed during the low states is
generated by accretion events. The different characteristics of the flares
(colour and polarization) are the results of different shock geometries
depending on the net mass accretion flux.Comment: accepted in Astronomy & Astrophysics (Main Journal), 10 pages, 6
Figures, Late
V405 Aurigae: A High Magnetic Field Intermediate Polar
Our simultaneous multicolor (UBVRI) circular polarimetry has revealed nearly
sinusoidal variation over the WD spin cycle, and almost symmetric positive and
negative polarization excursions. Maximum amplitudes are observed in the B and
V bands (+-3 %). This is the first time that polarization peaking in the blue
has been discovered in an IP, and suggests that V405 Aur is the highest
magnetic field IP found so far. The polarized flux spectrum is similar to those
found in polars with magnetic fields in the range B ~ 25-50 MG. Our low
resolution circular spectropolarimetry has given evidence of transient features
which can be fitted by cyclotron harmonics n = 6, 7, and 8, at a field of B =
31.5 +- 0.8 MG, consistent with the broad-band polarized flux spectrum. Timings
of the circular polarization zero crossovers put strict upper limits on WD spin
period changes and indicate that the WD in V405 Aur is currently accreting
closely at the spin equilibrium rate, with very long synchronization
timescales, T_s > 10^9 yr. For the observed spin to orbital period ratio,
P_{spin}/P_{orb} = 0.0365, and P_{orb} ~ 4.15 hr, existing numerical accretion
models predict spin equilibrium condition with B ~ 30 MG if the mass ratio of
the binary components is q_1 ~ 0.4. The high magnetic field makes V405 Aur a
likely candidate as a progenitor of a polar.Comment: To appear in The Astrophysical Journal, September 1 Issue (2008), 9
pages, 10 figure
Irregular Mass Transfer in the Polars VV Puppis and V393 Pavonis during the Low State
The polars VV Pup and V393 Pav were observed with XMM-Newton during states of
low accretion rate with peak X-ray luminosities of ~1 x 10^30 and ~1 x 10^31
erg/s, respectively. In both polars, accretion onto the white dwarf was
extremely irregular, and the accretion rate varied by more than 1 order of
magnitude on timescales of ~1 hr. Our observations suggest that this type of
irregular accretion is a common phenomenon in polars during the low state. The
likely cause of the accretion rate fluctuations are coronal mass ejections or
solar flares on the companion star that intermittently increase the mass
transfer into the accretion stream. Our findings demonstrate that the companion
stars in cataclysmic variables possess highly active atmospheres.Comment: Accepted for publication in ApJ, 16 pages, 3 figure
"Inter-Longitude Astronomy" (ILA) project : current highlights and perspectives : I. Magnetic vs. non-magneticinteracting binary stars
We present a review of highlights of our photometric and photo-polarimetric monitoring and mathematical modeling of interacting binary stars of different types classical, asynchronous, intermedi ate polars with 25 timescales corresponding to differ ent physical mechanisms and their combinations (part "Polar"); negative and positive superhumpers in nova-like and dwarf novae stars ("Superhumper"); symbiotic ("Symbiosis"); eclipsing variables with and without ev idence for a current mass transfer ("Eclipser") with a special emphasis on systems with a direct impact of the stream into the gainor star's atmosphere, which we propose to call "Impactors", or V361 Lyr-type stars. Other parts of the ILA project are "Stellar Bell" (pul sating variables of different types and periods - M, SR, RV Tau, RR Lyr, Delta Sct) and "New Variable".
The complex variability of blazars: time-scales and periodicity analysis in S4 0954+65
Among active galactic nuclei, blazars show extreme variability properties. We here investigate the case of the BL Lac object S4 0954+65 with data acquired in 2019–2020 by the Transiting Exoplanet Survey Satellite (TESS) and by the Whole Earth Blazar Telescope (WEBT) Collaboration. The 2-min cadence optical light curves provided by TESS during three observing sectors of nearly 1 month each allow us to study the fast variability in great detail. We identify several characteristic short-term time-scales, ranging from a few hours to a few days. However, these are not persistent, as they differ in the various TESS sectors. The long-term photometric and polarimetric optical and radio monitoring undertaken by the WEBT brings significant additional information, revealing that (i) in the optical, long-term flux changes are almost achromatic, while the short-term ones are strongly chromatic; (ii) the radio flux variations at 37 GHz follow those in the optical with a delay of about 3 weeks; (iii) the range of variation of the polarization degree and angle is much larger in the optical than in the radio band, but the mean polarization angles are similar; (iv) the optical long-term variability is characterized by a quasi-periodicity of about 1 month. We explain the source behaviour in terms of a rotating inhomogeneous helical jet, whose pitch angle can change in time.Accepted manuscrip
The complex variability of blazars: time-scales and periodicity analysis in S4 0954+65
Among active galactic nuclei, blazars show extreme variability properties. We here investigate the case of the BL Lac object S4 0954+65 with data acquired in 2019-2020 by the Transiting Exoplanet Survey Satellite (TESS) and by the Whole Earth Blazar Telescope (WEBT) Collaboration. The 2-min cadence optical light curves provided by TESS during three observing sectors of nearly 1 month each allow us to study the fast variability in great detail. We identify several characteristic short-term time-scales, ranging from a few hours to a few days. However, these are not persistent, as they differ in the various TESS sectors. The long-term photometric and polarimetric optical and radio monitoring undertaken by the WEBT brings significant additional information, revealing that (i) in the optical, long-term flux changes are almost achromatic, while the short-term ones are strongly chromatic; (ii) the radio flux variations at 37 GHz follow those in the optical with a delay of about 3 weeks; (iii) the range of variation of the polarization degree and angle is much larger in the optical than in the radio band, but the mean polarization angles are similar; (iv) the optical long-term variability is characterized by a quasi-periodicity of about 1 month. We explain the source behaviour in terms of a rotating inhomogeneous helical jet, whose pitch angle can change in time