Time-resolved polarization measurements of pulsars offer a unique insight into the geometry of their emission regions. Such measurements provide observational constraints on the different models ...proposed for the pulsar emission mechanisms. Optical polarization data of the Crab nebula were obtained from the Hubble Space Telescope (HST) archive. The data set consists of a series of observations of the nebula taken with the HST/Advanced Camera for Surveys (ACS). We produced polarization vector maps of the inner nebula and measured, for the first time, the degree of linear polarization (P.D.) and the position angle (P.A.) of the pulsar's integrated pulse beam, and of its nearby synchrotron knot. This yielded P.D. = 5.2 ± 0.3 per cent and P.A. = 105
1 ± 1
6 for the pulsar, and P.D. = 59.0 ± 1.9 per cent and P.A. = 124
7 ± 1
0 for the synchrotron knot. This is the first high-spatial resolution multi-epoch study of the polarization of the inner nebula and pulsar. None of the main features in the nebula shows evidence of significant polarization evolution in the period covered by these observations. The results for the pulsar are consistent with those obtained by S owikowska et al. using the high-time resolution photo-polarimeter - Optical Pulsar Timing Analyzer (OPTIMA), once the constant component (DC) component has been subtracted. Our results clearly prove that the knot is the main source of the DC component.
The HU Aqr planetary system hypothesis revisited Goździewski, K; Słowikowska, A; Dimitrov, D ...
Monthly notices of the Royal Astronomical Society,
04/2015, Letnik:
448, Številka:
2
Journal Article
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We study the mid-egress eclipse timing data gathered for the cataclysmic binary HU Aquarii during the years 1993–2014. The (O−C) residuals were previously attributed to a single ∼7 Jupiter mass ...companion in ∼5 au orbit or to a stable two-planet system with an unconstrained outermost orbit. We present 22 new observations gathered between 2011 June and 2014 July with four instruments around the world. They reveal a systematic deviation of ∼60–120 s from the older ephemeris. We re-analyse the whole set of the timing data available. Our results provide an erratum to the previous HU Aqr planetary models, indicating that the hypothesis for a third and fourth body in this system is uncertain. The dynamical stability criterion and a particular geometry of orbits rule out coplanar two-planet configurations. A putative HU Aqr planetary system may be more complex, e.g. highly non-coplanar. Indeed, we found examples of three-planet configurations with the middle planet in a retrograde orbit, which are stable for at least 1 Gyr, and consistent with the observations. The (O−C) may be also driven by oscillations of the gravitational quadrupole moment of the secondary, as predicted by the Lanza et al. modification of the Applegate mechanism. Further systematic, long-term monitoring of HU Aqr is required to interpret the (O−C) residuals.
ABSTRACT
At optical wavelengths, blazar Electric Vector Position Angle (EVPA) rotations linked with gamma-ray activity have been the subject of intense interest and systematic investigation for over ...a decade. One difficulty in the interpretation of EVPA rotations is the inherent 180° ambiguity in the measurements. It is therefore essential, when studying EVPA rotations, to ensure that the typical time-interval between successive observations – i.e. the cadence – is short enough to ensure that the correct modulo 180° value is selected. This optimal cadence depends on the maximum intrinsic EVPA rotation speed in blazars, which is currently not known. In this paper, we address the following questions for the RoboPol sample: What range of rotation speeds for rotations greater than 90° can we expect? What observation cadence is required to detect such rotations? Have rapid rotations been missed in EVPA rotation studies thus far? What fraction of data is affected by the ambiguity? And how likely are detected rotations affected by the ambiguity? We answer these questions with three seasons of optical polarimetric observations of a statistical sample of blazars sampled weekly with the RoboPol instrument and an additional season with daily observations. We model the distribution of EVPA changes on time-scales from 1–30 d and estimate the fraction of changes exceeding 90°. We show that at least daily observations are necessary to measure ${\gt}96{{\ \rm per\ cent}}$ of optical EVPA variability in the RoboPol sample of blazars correctly and that intraday observations are needed to measure the fastest rotations that have been seen thus far.
Context. Optical polarisation measurements are key tests for different models of the pulsar magnetosphere. Furthermore, comparing the relative orientation of the phase-averaged linear polarisation ...direction and the pulsar proper motion vector may unveil a peculiar alignment, clearly seen in the Crab pulsar. Aims. Our goal is to obtain the first measurement of the phase-averaged optical linear polarisation of the fifth brightest optical pulsar, PSR B0656+14, which also has a precisely measured proper motion, and to verify a possible alignment between the polarisation direction and the proper motion vector. Methods. We carried out observations with the Very Large Telescope (VLT) to measure the phase-averaged optical polarisation degree (PD) and position angle (PA) of PSR B0656+14. Results. We measured a PD of 11.9% ± 5.5% and a PA of 125.8° ± 13.2°, measured east of north. Albeit of marginal significance, this is the first measurement of the phase-averaged optical PD for this pulsar. Moreover, we found that the PA of the phase-averaged polarisation vector is close to that of the pulsar proper motion (93.12° ± 0.38°). Conclusions. Deeper observations are needed to confirm our polarisation measurement of PSR B0656+14, whereas polarisation measurements for more pulsars will better assess possible correlations of the polarisation degree with the pulsar parameters.
Context.
Optical polarimeters are typically calibrated using measurements of stars with known and stable polarization parameters. However, there is a lack of such stars available across the sky. Many ...of the currently available standards are not suitable for medium and large telescopes due to their high brightness. Moreover, as we find, some of the polarimetric standards used are in fact variable or have polarization parameters that differ from their cataloged values.
Aims.
Our goal is to establish a sample of stable standards suitable for calibrating linear optical polarimeters with an accuracy down to 10
−3
in fractional polarization.
Methods.
For 4 yr, we have been running a monitoring campaign of a sample of standard candidates comprised of 107 stars distributed across the northern sky. We analyzed the variability of the linear polarization of these stars, taking into account the non-Gaussian nature of fractional polarization measurements. For a subsample of nine stars, we also performed multiband polarization measurements.
Results.
We created a new catalog of 65 stars (see Table 2) that are stable, have small uncertainties of measured polarimetric parameters, and can be used as calibrators of polarimeters at medium and large telescopes.
The linear polarization of the Crab pulsar and its close environment was derived from observations with the high-speed photopolarimeter Optical Pulsar TIMing Analyser at the 2.56-m Nordic Optical ...Telescope in the optical spectral range (400–750 nm). Time resolution as short as 11 μs, which corresponds to a phase interval of 1/3000 of the pulsar rotation, and high statistics allow the derivation of polarization details never achieved before. The degree of optical polarization and the position angle correlate in surprising details with the light curves at optical wavelengths and at radio frequencies of 610 and 1400 MHz. Our observations show that there exists a subtle connection between presumed non-coherent (optical) and coherent (radio) emissions. This finding supports previously detected correlations between the optical intensity of the Crab and the occurrence of giant radio pulses. Interpretation of our observations requires more elaborate theoretical models than those currently available in the literature.
RoboPol: AGN polarimetric monitoring data Blinov, D; Kiehlmann, S; Pavlidou, V ...
Monthly notices of the Royal Astronomical Society,
03/2021, Letnik:
501, Številka:
3
Journal Article
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ABSTRACT
We present uniformly reprocessed and re-calibrated data from the RoboPol programme of optopolarimetric monitoring of active galactic nuclei (AGNs), covering observations between 2013, when ...the instrument was commissioned, and 2017. In total, the data set presented in this paper includes 5068 observations of 222 AGN with Dec. > −25○. We describe the current version of the RoboPol pipeline that was used to process and calibrate the entire data set, and we make the data publicly available for use by the astronomical community. Average quantities summarizing optopolarimetric behaviour (average degree of polarization, polarization variability index) are also provided for each source we have observed and for the time interval we have followed it.
ABSTRACT
We present the design and performance of RoboPol, a four-channel optical polarimeter operating at the Skinakas Observatory in Crete, Greece. RoboPol is capable of measuring both relative ...linear Stokes parameters q and u (and the total intensity I) in one sky exposure. Though primarily used to measure the polarization of point sources in the R band, the instrument features additional filters (B, V, and I), enabling multiwavelength imaging polarimetry over a large field of view (13.6′ × 13.6′). We demonstrate the accuracy and stability of the instrument throughout its 5 yr of operation. Best performance is achieved within the central region of the field of view and in the R band. For such measurements the systematic uncertainty is below 0.1 per cent in fractional linear polarization, p (0.05 per cent maximum likelihood). Throughout all observing seasons the instrumental polarization varies within 0.1 per cent in p and within ∼1° in polarization angle.
Cardiac myxomas are the most common primary cardiac tumours in adults. They usually present as a solitary, solid mass in the left atrium. Their most common radiographic appearance is that of a ...hypodense lesion on computed tomography (CT) and inhomogeneous lesion (hypo to isointense on T1 sequences and hyperintense on T2 sequences) on magnetic resonance (MR) with some contrast enhancement. However, different patterns are recognized due to secondary changes within the tumour. We present a case of a 60-year-old man with a hypervascular myxoma. The lesion was a sessile mass located in the left atrium and rigidly attached to the interatrial septum. On CT and MR, it showed vivid contrast enhancement due to intratumoural flush of arterial blood form branches of dominant left circumflex artery and a possible fistula to the left atrium. Furthermore, we review the literature for different atypical radiographic appearances of myxomas.
Context. The ~1700 year old PSR B0540−69 in the Large Magellanic Clouds (LMC) is considered the twin of the Crab pulsar because of its similar spin parameters, magnetic field, and energetics. ...PSR B0540−69 (V ~ 22.5) is also one of the very few pulsars for which both optical pulsations and polarised emission have been measured. Its optical spectrum is fit by a power-law, ascribed to synchrotron radiation, like for the young Crab and Vela pulsars. At variance with them, however, a double break is required to join the X-ray and optical power-law spectra, with the first one possibly occurring in the near ultraviolet (nUV). Aims. Near-infrared (nIR) observations, never performed for PSR B0540−69, are crucial to determine whether the optical power-law spectrum extends to longer wavelengths or a new break occurs, like it happens for both the Crab and Vela pulsars in the mid-infrared (mIR), hinting at an even more complex particle energy and density distribution in the pulsar magnetosphere. Methods. We observed PSR B0540−69 in the J, H, and KS bands with the Very Large Telescope (VLT) to detect it, for the first time, in the nIR and characterise its optical-to-nIR spectrum. To disentangle the pulsar emission from that of its pulsar wind nebula (PWN), we obtained high-spatial resolution adaptive optics images with the NAOS-CONICA instrument (NACO). Results. We could clearly identify PSR B0540−69 in our J, H, and KS-band images and measure its flux (J = 20.14, H = 19.33, KS = 18.55, with an overall error of ± 0.1 mag in each band). The joint fit to the available optical and nIR photometry with a power-law spectrum Fν ∝ ν−α gives a spectral index α = 0.70 ± 0.04, slightly more precise than measured in the optical only. This clearly implies that there is no spectral break between the optical and the nIR. We also detected, for the first time, the PSR B0540−69 PWN in the nIR. The comparison between our NACO images and Hubble Space Telescope (HST) optical ones does not reveal any apparent difference in the PWN morphology as a function of wavelength. The PWN optical-to-nIR spectrum is also fit by a single power-law, with spectral index α = 0.56 ± 0.03, slightly flatter than the pulsar’s. Conclusions. Using NACO at the VLT, we obtained the first detection of PSR B0540−69 and its PWN in the nIR. Due to the small angular scale of the PWN (~4″) only the spatial resolution of the James Webb Space Telescope (JWST) will make it possible to extend the study of the pulsar and PWN spectrum towards the mid-IR.
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