50 research outputs found
Spectropolarimetric observations of cool DQ white dwarfs
Following our recent discovery of a new magnetic DQ white dwarf (WD) with CH
molecular features, we report the results for the rest of the DQ WDs from our
survey. We use high signal-to-noise spectropolarimetric data to search for
magnetic fields in a sample of 11 objects. One object in our sample,
WD1235+422, shows the signs of continuum circular polarization that is similar
to some peculiar DQs with unidentified molecular absorption bands, but the low
S/N and spectral resolution of these data make more observations necessary to
reveal the true nature of this object.Comment: 4 pages, 4 figures. Accepted for publication in Astronomy &
Astrophysic
High-precision broadband linear polarimetry of early-type binaries IV. Binary system of DH Cephei in the open cluster of NGC 7380
DH~Cephei is a well known massive O+O-type binary system on the northern sky,
residing in the young open cluster NGC~7380. Our high-precision multi-band
polarimetry has clearly revealed that variations of linear polarizations in
this system are synchronous with the phase of the orbital period. We have used
the observed variations of Stokes parameters and to derive the orbital
inclination , orientation , and the direction of rotation. In order
to determine the contribution from interstellar polarization, we have carried
out new observations of polarization of field stars with precisely measured
parallaxes.
The variations of Stokes parameters in all three , , and passbands
clearly exhibit an unambiguous periodic signal at 1.055 d with the amplitude of
variations which corresponds to half of known orbital period of
2.11 d. This type of polarization variability is expected for a binary system
with light scattering material distributed symmetrically with respect to the
orbital plane. Even though most of the observed polarization (2) is
of interstellar origin, about one third of it is due to the intrinsic
component. In addition to the regular polarization variability, there is a
non-periodic component, strongest in the passband. We obtained in the
passband our most reliable values for the orbital inclination and the orientation of the orbit on the sky
, with 1 confidence intervals. The
direction of the binary system rotation on the plane of the sky is clockwise
An optical spectroscopic and polarimetric study of the microquasar binary system SS 433
Aims. Our aim is to study the mass transfer, accretion environment, and wind outflows in the SS 433 system, concentrating on the so-called stationary lines.Methods. We used archival high-resolution (X-shooter) and low-resolution (EMMI) optical spectra, new optical multi-filter polarimetry, and low-resolution optical spectra (Liverpool Telescope), spanning an interval of a decade and a broad range of precessional and orbital phases, to derive the dynamical properties of the system.Results. Using optical interstellar absorption lines and H I 21 cm profiles, we derive E(B-V) = 0.86 +/- 0.10, with an upper limit of E(B-V) = 1.8 +/- 0.1 based on optical Diffuse Interstellar Bands. We obtain revised values for the ultraviolet and U band polarizations and polarization angles (PA), based on a new calibrator star at nearly the same distance as SS 433 that corrects the published measurement and yields the same PA as the optical. The polarization wavelength dependence is consistent with optical-dominating electron scattering with a Rayleigh component in U and the UV filters. No significant phase modulation was found for PA while there is significant variability in the polarization level. We fortuitously caught a flare event; no polarization changes were observed but we confirm the previously reported associated emission line variations. Studying profile modulation of multiple lines of H I, He I, O I, Na I, Si II, Ca II, Fe II with precessional and orbital phase, we derive properties for the accretion disk and present evidence for a strong disk wind, extending published results. Using transition-dependent systemic velocities, we probe the velocity gradient of the wind, and demonstrate that it is also variable on timescales unrelated to the orbit. Using the rotational velocity, around 140 +/- 20 km s(-1), a redetermined mass ratio q=0.37 +/- 0.04, and masses M-X=4.2 +/- 0.4 M-circle dot, M-A=11.3 +/- 0.6 M-circle dot, the radius of the A star fills - or slightly overfills - its Roche surface. We devote particular attention to the O I 7772 angstrom and 8446 angstrom lines, finding that they show different but related orbital and precessional modulation and there is no evidence for a circumbinary component. The spectral line profile variability can, in general, be understood with an ionization stratified outflow predicted by thermal wind modeling, modulated by different lines of sight through the disk produced by its precession. The wind can also account for an extended equatorial structure detected at long wavelength.</div
High-precision broadband linear polarimetry of early-type binaries
Aims. The fact that the O-type close binary star system AO Cassiopeiae exhibits variable phase-locked linear polarization has been known since the mid-1970s. In this work, we re-observe the polarization arising from this system more than 50 yr later to better estimate the interstellar polarization and to independently derive the orbital parameters, such as inclination, i, orientation, Omega, and the direction of the rotation for the inner orbit from the phase-folded polarization curves of the Stokes q and u parameters.Methods. The Dipol-2 polarimeter was used to obtain linear polarization measurements of AO Cassiopeiae in the B, V, and R passbands with the T60 remotely controlled telescope at an unprecedented accuracy level of similar to 0.003%. We have obtained the first proper quantification of the interstellar polarization in the direction heading towards AO Cas by observing the polarization of three neighboring field stars. We employed a Lomb-Scargle algorithm and detected a clear periodic signal for the orbital period of AO Cas. The standard analytical method based on a two-harmonics Fourier fit was used to obtain the inclination and orientation of the binary orbit.Results. Our polarimetric data exhibited an unambiguous periodic signal at 1.76 days, thus confirming the orbital period of the binary system of 3.52 days. Most of the observed polarization is of interstellar origin. The de-biased values of the orbital inclination are i = 63. + 2. /-3. and orientation of Omega = 29.(209.) +/- 8.. The direction of the binary system rotation on the plane of the sky is clockwise.</p
Connection Between Optical and VHE Gamma-ray Emission in Blazar Jets
MAGIC has been performing optically triggered Target of Opportunity
observations of flaring blazars since the beginning of its scientific
operations. The alerts of flaring blazars originate from Tuorla Blazar
Monitoring Programme, which started the optical monitoring of candidtate TeV
blazars in 2002 and has now collected up to eight years of data on more than 60
blazars. These ToO observations have resulted in the discovery of five new VHE
gamma-ray emitting blazars (S5 0716+714, 1ES 1011+496, Mrk 180, ON 325 and B3
2247+381). In addition part of the discovery of BL Lac and the discovery of 3C
279 was made during a high optical state. In this contribution we present a
detailed analysis of the optical light curves which are then compared to MAGIC
observations of the same sources. We aim to answer the question: "Is there a
connection between optical and VHE -ray high states in blazars or have we just
been lucky?"Comment: 6 pages, 1 figure, 2 tables, proceedings for the Beamed and Unbeamed
Gamma-Rays from Galaxies workshop, Olos, April 11-15 201
Highly sensitive search for magnetic fields in white dwarfs using broad-band circular polarimetry
Circular polarisation measurements of white dwarfs of various ages and spectral types are useful to understand the origin and evolution of the magnetic field in degenerate stars. In the latest stages of white dwarf evolution, when stars are so cool that spectral lines are no longer formed in the normal H- or He-dominated atmospheres, magnetic fields can be probed only by means of circular polarimetry of the continuum. The study of the fields of featureless DC white dwarfs may reveal whether Ohmic decay acts on magnetic white dwarfs, or if magnetic fields continue to be generated even several billion years after white dwarf formation. Compared to spectropolarimetry, broad-band circular polarisation measurements have the advantage of reaching a higher accuracy in the continuum, with the potential of detecting magnetic fields as weak as a fraction of a MG in DC stars, if the telescope size is adequate for the star's magnitude. Here we present the results of a first (short) observing campaign with the DIPol-UF polarimeter, which we have used to measure broad-band circular polarisation of white dwarfs. Our observing run was in part aimed to fully characterise the instrument, and in part to study the relationship between magnetic field strength (when known from spectropolarimetry) and circular polarisation of the continuum. We also observed a small number of previously unexplored DC white dwarfs, and we present the discovery of two new magnetic white dwarfs of spectral class DC, probably the first discovery of this kind made with broad-band circular polarimetric techniques since the late 1970s. We also discuss the characteristics of our instrument, and predict the level of polarimetric accuracy that may be reached as a function of stellar magnitude, exposure time, and telescope size