UBVRI photopolarimetry of the long period eclipsing AM Herculis binary V1309

We report simultaneous UBVRI photo-polarimetric observations of the long period (7.98 h) AM Her binary V1309 Ori. The length and shape of the eclipse ingress and egress varies from night to night. We suggest this is due to the variation in the brightness of the accretion stream. By comparing the phases of circular polarization zero-crossovers with previous observations, we confirm that V1309 Ori is well synchronized, and find an upper limit of 0.002 percent for the difference between the spin and orbital periods. We model the polarimetry data using a model consisting of two cyclotron emission regions at almost diametrically opposite locations, and centered at colatitude 35 (deg) and 145 (deg) on the surface of the white dwarf. We also present archive X-ray observations which show that the negatively polarised accretion region is X-ray bright.Comment: 11 pages, 12 figures (2 colour), Fig1 and Fig 4 are in lower resolution than in original paper, accepted for publication in Monthly Notices of the Royal Astronomical Societ

Identification of the soft X-ray source WGA J1802.1+1804 with a new magnetic cataclysmic variable

We have discovered a bright (V~14.5) cataclysmic variable during observations of the soft X-ray sources in the list of Singh et al. The optical source, which is coincident with the X-ray position of WGA J1802.1+1804, shows all the characteristics of a magnetic AM Herculis-type system: circular polarization, He II strength greater than Hβ, multiple line components, and a consistent photometric, polarimetric, spectroscopic, and X-ray period of 113 minutes. The X-ray spectrum shows a dominant soft blackbody (kT=20-45 eV) and a weaker bremsstrahlung component (kT>1 keV), while the circular polarization is relatively low (4% in the red)

Multiband optical polarimetry of BL Lac objects with the Nordic Optical Telescope

Optical polarization of seven selected BL Lac objects in UBVRI bands was studied with the Nordic Optical Telescope from December 10-14, 1999. Two of them, 3C 66A and PKS 0735+178, were monitored for 4 nights for a total integration time of 4.75 and 5.5 hours, respectively. Other objects (1Jy 0138-097, H 0414+009, PKS 0823-223, OJ287 and BL Lac) were observed sparsely during the run. Apart from PKS 0823-223 (more polarized than observed in the past), the sources show levels of flux and polarization consistent with results at previous epochs. 3C 66A and PKS 0735+178 were intensively observed during December 11 and 12 and exhibited variability of polarization, both on internight and intranight time scales. Wavelength dependence of polarization has been investigated, as well as circular polarization. The results are discussed within the standard model for BL Lacs.Comment: 11 pages, 7 figures, accepted to Astronomy & Astrophysic

Evolving optical polarisation of the black hole X-ray binary MAXI J1820+070

Aims. The optical emission of black hole transients increases by several magnitudes during the X-ray outbursts. Whether the extra light arises from the X-ray heated outer disc, from the inner hot accretion flow, or from the jet is currently debated. Optical polarisation measurements are able to distinguish the relative contributions of these components. Methods. We present the results of BVR polarisation measurements of the black hole X-ray binary MAXI J1820+070 during the period of March-April 2018. Results. We detect small, $\sim$0.7%, but statistically significant polarisation, part of which is of interstellar origin. Depending on the interstellar polarisation estimate, the intrinsic polarisation degree of the source is between $\sim$0.3% and 0.7%, and the polarisation position angle is between $\sim10\deg-30\deg$. We show that the polarisation increases after MJD 58222 (2018 April 14). The change is of the order of 0.1% and is most pronounced in the R band. The change of the source Stokes parameters occurs simultaneously with the drop of the observed V-band flux and a slow softening of the X-ray spectrum. The Stokes vectors of intrinsic polarisation before and after the drop are parallel, at least in the V and R filters. Conclusions. We suggest that the increased polarisation is due to the decreasing contribution of the non-polarized component, which we associate with the the hot flow or jet emission. The low polarisation can result from the tangled geometry of the magnetic field or from the Faraday rotation in the dense, ionised, and magnetised medium close to the black hole. The polarized optical emission is likely produced by the irradiated disc or by scattering of its radiation in the optically thin outflow.Comment: 11 pages, 12 figures, A&A in pres

On the collisional disalignment of dust grains in illuminated and shaded regions of IC 63

Interstellar dust grain alignment causes polarization from UV to mm wavelengths, allowing the study of the geometry and strength of the magnetic field. Over the last couple of decades, observations and theory have led to the establishment of the radiative alignment torque mechanism as a leading candidate to explain the effect. With a quantitatively well constrained theory, polarization can be used not only to study the interstellar magnetic field, but also the dust and other environmental parameters. Photodissociation regions, with their intense, anisotropic radiation fields, consequent rapid H2 formation, and high spatial density-contrast provide a rich environment for such studies. Here we discuss an expanded optical, NIR, and mm-wave study of the IC 63 nebula, showing strong H2 formation-enhanced alignment and the first direct empirical evidence for disalignment due to gas-grain collisions using high-resolution HCO+(J = 1-0) observations. We find that a relative amount of polarization is marginally anticorrelated with column density of HCO+. However, separating the lines of sight of optical polarimetry into those behind, or in front of, a dense clump as seen from γ Cas, the distribution separates into two well defined sets, with data corresponding to "shaded" gas having a shallower slope. This is expected if the decrease in polarization is caused by collisions since collisional disalignment rate is proportional to RC∝nT" role="presentation">RC∝nT−−√ . Ratios of the best-fit slopes for the "illuminated" and "shaded" samples of lines of sight agrees, within the uncertainties, with the square root of the two-temperature H2 excitation in the nebula seen by Thi et al.Fil: Soam, Archana. Sofia Science Center; Estados UnidosFil: Anderson, B. G. Sofia Science Center; Estados UnidosFil: Acosta Pulido, Jose. Instituto de Astrofisica de Canarias; EspañaFil: Fernandez Lopez, Manuel. Provincia de Buenos Aires. Gobernación. Comisión de Investigaciones Científicas. Instituto Argentino de Radioastronomía. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata. Instituto Argentino de Radioastronomía; ArgentinaFil: Vaillancourt, J. E.. Lincoln Laboratory; Estados UnidosFil: Widicus Weaver, S. L.. Department Of Chemistry; Estados UnidosFil: Piirola, V.. University Of Turku; FinlandiaFil: Gordon, M. S.. Sofia Science Center; Estados Unido

Cyclotron modeling phase-resolved infrared spectroscopy of polars I: EF Eridani

We present phase-resolved low resolution infrared spectra of the polar EF Eridani obtained over a period of 2 years with SPEX on the IRTF. The spectra, covering the wavelength range 0.8 to 2.4 microns, are dominated by cyclotron emission at all phases. We use a ``Constant Lambda'' prescription to attempt to model the changing cyclotron features seen in the spectra. A single cyclotron emission component with B = 12.6 MG, and a plasma temperature of kT = 5.0 keV, does a reasonable job in matching the features seen in the H and K bands, but fails to completely reproduce the morphology shortward of 1.6 microns. We find that a two component model, where both components have similar properties, but whose contributions differ with viewing geometry, provides an excellent fit to the data. We discuss the implications of our models and compare them with previously published results. In addition, we show that a cyclotron model with similar properties to those used for modeling the infrared spectra, but with a field strength of B = 115 MG, can explain the GALEX observations of EF Eri.Comment: 25 pages, 5 figures, to appear in Ap

X-Ray Emission and Optical Polarization of V1432 Aquilae: An Asynchronous Polar

A detailed analysis of X-ray data from ROSAT, ASCA, XMM and RXTE for the asynchronous polar V1432 Aql along with Stokes polarimetry data from SAAO, is presented. Power spectra from long-baseline ROSAT data show a spin period of 12150s along with several system related frequency components. However, the second harmonic of the spin period dominates power spectrum in the XMM data. For the optical circular polarization, the dominant period corresponds to half the spin period. The ROSAT data can be explained as due to accretion onto two hot spots that are not anti-podal. The variations seen in the optical polarization and the ASCA and XMM data suggest the presence of at least three accretion foot prints on the white dwarf surface. Two spectral models, a multi-temperature plasma and a photo-ionized plasma model, are used for spectral study. The RXTE PCA data are used to constrain the white dwarf mass to 1.2$\pm$0.1 M_odot using the multi-temperature plasma model. A strong soft X-ray excess (<0.8 keV) in the XMM MOS data is well modeled by a blackbody component having a temperature of 80-90 eV. The plasma emission lines seen at 6.7 and 7.0 keV are well fitted using the multi-temperature plasma model, however an additional Gaussian is needed for the 6.4 keV line. The multi-temperature plasma model requires a homogeneous absorber fully covering the source and a partial absorber covering 65% of the source. The photo-ionized plasma model, with a range of Fe column densities, gives a slightly better overall fit and fits all emission lines. The presence of a strong blackbody component, a spin period of 12150s, modulation of the 6.4 keV line flux with spin period, and a very hard X-ray component suggest that V1432 Aql is a polar with X-ray spectral properties similar to that of a soft intermediate polar.Comment: 46 pages, including 13 figures and 4 tables, To appear in The Astrophysical Journal, 20 May 2005 issue, vol. 625, Added Report-no and Journal-ref, no change in the text of the pape

An Extreme Case of a Misaligned Highly Flattened Wind in the Wolf-Rayet Binary CX Cephei

CX Cep (WR 151) is the WR+O binary (WN5+O5V) with the second shortest period known in our Galaxy. To examine the circumstellar matter distribution and to better constraint the orbital parameters and mass-loss rate of the WR star, we obtained broadband and multi-band (i.e. UBVRI) linear polarization observations of the system. Our analysis of the phase-locked polarimetric modulation confirms the high orbital inclination of the system (i.e. $i=65^o$). Using the orbital solution of Lewis et al. (1993) we obtain masses of $33.9 M_{\odot}$ and $23.9 M_{\odot}$ for the O and WR stars respectively, which agree with their spectral types. A simple polarimetric model accounting for finite stellar size effects allowed us to derive a mass-loss rate for the WR star of $0.3-0.5\times10^{-5} M_{\odot}/yr$. This result was remarkably independent of the model's input parameters and favors an earlier spectral type for the WR component (i.e. WN4). Finally, using our multi-band observations, we fitted and subtracted from our data the interstellar polarization. The resulting constant intrinsic polarization of $3-4$ is misaligned in relation to the orbital plane (i.e. $\theta_{CIP}=26^o$ vs. $\Omega=75^o$) and is the highest intrinsic polarization ever observed for a WR star. This misalignment points towards a rotational (or magnetic) origin for the asymmetry and contradicts the most recent evolutionary models for massive stars (Meynet & Maeder 2003) which predict spherically symmetric winds during the WR phase (i.e. $CIP=0$).Comment: 26 pages, 4 figures. Astrophysical Journal (submited

Multiband optical polarimetry of the BL Lac object PKS 2155--304. Intranight and long term variability

The polarized and total flux of the BL Lac object PKS 2155-304 were monitored intensively and simultaneously in the optical UBVRI bands with the Turin photopolarimeter at the CASLEO 2.15 m telescope during 4 campaigns in June, August, November 1998 and August 1999. The effective observation time amounted to ~47 hours. PKS 2155-304 showed a linear polarization percentage (P) usually ranging between 3% and 7% and a polarization position angle (PA) mainly between 70 and 120 deg. The highest temporal resolution of our observations, 15 minutes, is unprecedented for polarimetric monitoring of this source, and has allowed us to detect amplitude variations of the linear polarization percentage from 6 to 7.5% in time scales of hours. In some nights the polarization percentage seems to increase toward shorter wavelengths, however the polarized spectrum does not vary significantly with time. The most remarkable variability event occurred on 1998 June 18, when the degree of linear polarization decreased by more than a factor 2 in one day in all bands, while the PA rotated by 90 deg. This is consistent with the presence of two emission components, of different polarization degree and position angle. Intranight variability of P and PA can be interpreted with small amplitude physical or geometrical changes within the jet. Measurements of the circular polarization over time intervals of days set upper limits of 0.2%. Simultaneous photometry taken with the Turin Photopolarimeter and with a CCD camera at Cordoba Astronomical Observatory did not show light variations correlated with those of the linearly polarized flux.Comment: 21 pages, 6 eps figures. Accepted for The Astrophysical Journal Supplement Serie