66 research outputs found
Optical polarisation of the Crab pulsar: precision measurements and comparison to the radio emission
The linear polarisation of the Crab pulsar and its close environment was
derived from observations with the high-speed photo-polarimeter OPTIMA at the
2.56-m Nordic Optical Telescope in the optical spectral range (400 - 750 nm).
Time resolution as short as 11 microseconds, which corresponds to a phase
interval of 1/3000 of the pulsar rotation, and high statistics allow the
derivation of polarisation details never achieved before. The degree of optical
polarisation 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 require more elaborate theoretical models than those currently
available in the literature.Comment: 21 pages, 13 figures, uses AMS.sty, mn2e.cls, mn2e.bst and
natbib.sty, submitted to MNRA
Very fast optical flaring from a possible new Galactic magnetar
Highly luminous rapid flares are characteristic of processes around compact
objects like white dwarfs, neutron stars or black holes. In the high energy
regime of X- and gamma-rays, outbursts with variability time-scales of seconds
and faster are routinely observed, e.g. in gamma-ray bursts or Soft Gamma
Repeaters. In the optical, flaring activity on such time-scales has never been
observed outside the prompt phase of GRBs. This is mostly due to the fact that
outbursts with strong, fast flaring usually are discovered in the high-energy
regime. Most optical follow-up observations of such transients employ
instruments with integration times exceeding tens of seconds, which are
therefore unable to resolve fast variability. Here we show the observation of
extremely bright and rapid optical flaring in the galactic transient SWIFT
J195509.6+261406. Flaring of this kind has never previously been reported. Our
optical light-curves are phenomenologically similar to high energy light-curves
of Soft Gamma Repeaters and Anomalous X-ray Pulsars, which are thought to be
neutron stars with extremely high magnetic fields (magnetars). This suggests
similar emission processes may be at work, but in contrast to the other known
magnetars with strong emission in the optical.Comment: 8 pages, 3 figures. A substantially revised version of this
manuscript was published in Nature. Due to license issues, the accepted
manuscript will only be put on astro-ph as v2 6 months after this versio
Calibration of the Liverpool Telescope RINGO3 polarimeter
We present an analysis of polarimetric observations of standard stars performed over the period of more than three years with the RINGO3 polarimeter mounted on the Liverpool Telescope. The main objective was to determine the instrumental polarisation of the RINGO3 polarimeter in three spectral energy ranges: blue (350--640~nm), green (650--760~nm) and red (770--1000~nm). The observations were conducted between 2012 and 2016. The total time span of 1126 days was split into five epochs due to the hardware changes to the observing system. Our results should be applied to calibrate all polarimetric observations performed with the RINGO3 polarimeter
The study of neutron star magnetospheres with LOFT
This is a White Paper in support of the mission concept of the Large
Observatory for X-ray Timing (LOFT), proposed as a medium-sized ESA mission. We
discuss the potential of LOFT for the study of magnetospheres of isolated
neutron stars. For a summary, we refer to the paper.Comment: White Paper in Support of the Mission Concept of the Large
Observatory for X-ray Timin
The Optical Polarisation of the Vela Pulsar revisited
In this work we present a revised measurement of the phase-averaged optical
polarisation of the Vela pulsar (PSR B0833-45), for which only one value has
been published so far (Wagner & Seifert 2000). Our measurement has been
obtained through an accurate reanalysis of archival polarisation observations
obtained with the FORS instrument at the VLT. We have measured a phase-averaged
linear polarisation degree P=9.4% +/- 4% and a position angle 146 +/- 11 deg,
very close to the ones of the axis of symmetry of the X-ray arcs and jets
detected by Chandra and of the pulsar proper motion.We have compared the
measured phase-averaged optical polarisation with the expectations of different
pulsars' magnetosphere models. We have found that all models consistently
predict too large values of the phase-averaged linear polarization with respect
to the observed one. This is probably a consequence of present models'
limitations which neglect the contributions of various depolarisation effects.
Interestingly, for the outer gap model we have found that, assuming synchrotron
radiation for the optical emission, the observed polarisation position angle
also implies an alignment between the pulsar rotational axis and the axis of
symmetry of the X-ray arcs and jets.Comment: 8 pages, 4 figure
The near-infrared detection of PSR B0540-69 and its nebula
The ~1700 year old PSR B0540-69 in the LMC is considered the twin of the Crab
pulsar because of its similar spin parameters, magnetic field, and energetics.
Its optical spectrum is fit by a power-law, ascribed to synchrotron radiation,
like for the young Crab and Vela pulsars. 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 mIR, hinting at an even more complex particle
energy and density distribution in the pulsar magnetosphere. We observed PSR
B0540-69 in the J, H, and Ks bands with the 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 NACO. 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 magnitudes in
each band). The joint fit to the available optical and nIR photometry with a
power-law spectrum gives a spectral index alpha=0.70 +/-0.04. The comparison
between our NACO images and 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
alpha=0.56+/- 0.03, slightly flatter than the pulsar's. 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
JWST will make it possible to extend the study of the pulsar and PWN spectrum
towards the mid-IR.Comment: 11 pages, 10 figures, Accepted for publication on Astronomy and
Astrophysic
Stokes tomography of radio pulsar magnetospheres. I. Linear polarization
Polarimetric studies of pulsar radio emission traditionally concentrate on
how the Stokes vector (I, Q, U, V) varies with pulse longitude, with special
emphasis on the position angle (PA) swing of the linearly polarized component.
The interpretation of the PA swing in terms of the rotating vector model is
limited by the assumption of an axisymmetric magnetic field and the degeneracy
of the output with respect to the orientation and magnetic geometry of the
pulsar; different combinations of the latter two properties can produce similar
PA swings. This paper introduces Stokes phase portraits as a supplementary
diagnostic tool with which the orientation and magnetic geometry can be
inferred more accurately. The Stokes phase portraits feature unique patterns in
the I-Q, I-U, and Q-U planes, whose shapes depend sensitively on the magnetic
geometry, inclination angle, beam and polarization patterns, and emission
altitude. We construct look-up tables of Stokes phase portraits and PA swings
for pure and current-modified dipole fields, filled core and hollow cone beams,
and two empirical linear polarization models, L/I = \cos \theta_0 and L/I =
\sin \theta_0, where \theta_0 is the colatitude of the emission point. We
compare our look-up tables to the measured phase portraits of 24 pulsars in the
European Pulsar Network online database. We find evidence in 60% of the objects
that the radio emission region may depart significantly from low altitudes,
even when the PA swing is S-shaped and/or the pulse-width-period relation is
well satisfied. On the other hand, the data are explained adequately if the
emission altitude exceeds ~10% of the light cylinder radius. We conclude that
Stokes phase portraits should be analysed concurrently with the PA swing and
pulse profiles in future when interpreting radio pulsar polarization data.Comment: 60 pages, 58 figures, submitted to MNRAS, accepted 13 Oct 201
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