Andrew Shearer

Enhanced optical emission during Crab giant radio pulses

We detected a correlation between optical and giant radio pulse emission from the Crab pulsar. Optical pulses coincident with the giant radio pulses were on average 3% brighter than those coincident with normal radio pulses. Combined with the lack of any other pulse profile changes, this result indicates that both the giant radio pulses and the increased optical emission are linked to an increase in the electron-positron plasma density.

Optical Polarimetry of the Inner Crab Nebula and Pulsar

Time-resolved polarisation measurements of pulsars oer an unique insight into the geometry of their emission regions. Such measurements provide observational constraints on the dierent models proposed for the pulsar emission mechanisms. Optical polarisation data of the Crab Nebula was obtained from the HST archive. The dataset consists of a series of observations of the nebula taken with the HST/ACS. We produced polarisation vector maps of the inner nebula and measured, for the rst time, the degree of linear polarisation (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% and P:A: = 105:1 1:6 for the pulsar, and P:D: = 59:0 1:9% and P:A: = 124:7 1:0 for the synchrotron knot. This is the rst high-spatial resolution multi-epoch study of the polarisation of the inner nebula and pulsar. None of the main features in the nebula show evidence of signicant polarisation evolution in the period covered by these observations. The results for the pulsar are consistent with those obtained by S lowikowska et al. (2009) using the high-time resolution photo-polarimeter OPTIMA, once the DC component has been subtracted. Our results clearly prove that the knot is the main source of the DC component.

Pulsed Optical Emission from PSR 0656+14

Using data collected with the Special Astrophysical Observatory (SAO) 6 m telescope and the University College Galway (UCG) Transputer Instrument for Fast Image Deconvolution (TRIFFID) imaging photometer, we show that the radio, X-ray, and γ-ray pulsar PSR 0656+14 exhibits pulsed optical emission. We observed that the pulsed fraction was consistent with 100% and that the flux was higher than that expected from a thermal source. The magnitude of PSR 0656+14 in the B band was observed to be 25.1 ± 0.3, consistent with previous CCD observations. The peak of the optical signal is at phase 0.2 compared to the radio and in phase with the weak γ-ray pulse.

High-speed, multicolour optical photometry of the anomalous X-ray pulsar 4U 0142+61 with ULTRACAM

We present high-speed, multicolour optical photometry of the anomalous X-ray pulsar 4U 0142+61, obtained with ULTRACAM on the 4.2-m William Herschel Telescope. We detect 4U 0142+61 at magnitudes of i� = 23.7 ± 0.1, g� = 27.2 ± 0.2 and u� > 25.8, consistent with the magnitudes found recently by Hulleman et al. and hence confirming their discovery of both a spectral break in the optical and a lack of long-term optical variability. We also confirm the earlier discovery of Kern & Martin that 4U 0142+61 shows optical pulsations with an identical period (∼8.7 s) to the X-ray pulsations. The rms pulsed fraction in our data is 29 ± 8 per cent, 5–7 times greater than the 0.2–8 keV X-ray rms pulsed fraction. The optical and X-ray pulse profiles show similar morphologies and appear to be approximately in phase with each other, the former lagging the latter by only 0.04 ± 0.02 cycles. In conjunction with the constraints imposed by X-ray observations, the results presented here favour a magnetar interpretation for the anomalous X-ray pulsars.

A recent change in the optical and γ-ray polarization of the Crab nebula and pulsar

We report on observations of the polarization of optical and γ-ray photons from the Crab nebula and pulsar system using the Galway Astronomical Stokes Polarimeter (GASP), the Hubble Space Telescope, Advanced Camera for Surveys and the International Gamma-Ray Astrophysics Laboratory satellite (INTEGRAL). These, when combined with other optical polarization observations, suggest that the polarized optical emission and γ-ray polarization changes in a similar manner. A change in the optical polarization angle has been observed by this work, from 109.5 ± 0∘.7 in 2005 to 85.3 ± 1∘.4 in 2012. On the other hand, the γ-ray polarization angle changed from 115 ± 11° in 2003–2007 to 80 ± 12° in 2012–2014. Strong flaring activities have been detected in the Crab nebula over the past few years by the high-energy γ-ray missions Agile and Fermi, and magnetic reconnection processes have been suggested to explain these observations. The change in the polarized optical and γ-ray emission of the Crab nebula/pulsar as observed, for the first time, by GASP and INTEGRAL may indicate that reconnection is possibly at work in the Crab nebula. We also report, for the first time, a non-zero measure of the optical circular polarization from the Crab pulsar+knot system.

Implications of the Optical Observations of Isolated Neutron Stars

We show that observations of pulsars with pulsed optical emission indicate that the peak flux scales according to the magnetic field strength at the light cylinder. The derived relationships indicate that the emission mechanism is common across all of the observed pulsars with periods ranging from 33 to 385 ms and ages of 1000-300,000 yr. It is noted that similar trends exist for γ-ray pulsars. Furthermore, the model proposed by Pacini and developed by Pacini and Salvati still has validity and gives an adequate explanation of the optical phenomena.

Dark spot, spiral waves and the SW Sextantis behaviour: it is all about UX Ursae Majoris

We present an analysis of time-resolved, medium resolution optical spectroscopic observations of UX UMa in the blue (3920‐5250 ˚ A) and red (6100‐7200 ˚ A) wavelength ranges, that were obtained in April 1999 and March 2008 respectively. The observed characteristics of our spectra indicate that UX UMa has been in different states during those observations. The blue spectra are very complex. They are dominated by strong and broad single-peaked emission lines of hydrogen. The high-excitation lines of He II �4686 and the Bowen blend are quite strong as well. All the lines consist of a mixture of absorption and emission components. Using Doppler tomography we have identified four dist inct components of the system: the accretion disc, the secondary star, the bright spot from the gas stream/disc impact region, and the unique compact area of absorption in the accretion disc seen as a dark spot in the lower-left quadrant of the tomograms. In the red wavelength range, both the hydrogen (H� ) and neutral helium (He I �6678 and He I �7065) lines were observed in emission and both exhibited double-peaked profiles. Doppler tomography of th ese lines reveals spiral structure in the accretion disc, but in contrast to the blue wavelength range, there is no evidence for either the dark spot or the gas stream/disc impact region emission, while the emission from the secondary star is weak. During the observations in 1999, UX UMa showed many of the defining properties of the SW Sex stars. However, all these fe atures almost completely disappeared in 2008. We have also estimated the radial velocity semi-amplitudes K1 and K2 and evaluated the system parameters of UX UMa. These estimates are inconsistent with previous values derived by means of analysis of WD eclipse features in the light curve in the different wavelength ranges.

Unpulsed Optical Emission from the Crab Pulsar

Based on observations of the Crab pulsar using the TRIFFID high-speed imaging photometer in the UBV bands using the Special Astrophysical Observatorys 6 m telescope in the Russian Caucasus, we report the detection of pronounced emission during the so-called off phase of emission. Following de-extinction, this unpulsed component of emission is shown to be consistent with a power law with an exponent of α = -0.60 ± 0.37, the uncertainty being dominated by the error associated with the independent CCD photometry used to reference the TRIFFID data. This suggests a steeper power-law form than that reported elsewhere in the literature for the total integrated spectrum, which is essentially flat with α ~ 0.1, although the difference in this case is only significant at the ≤2 σ level. Deeper reference integrated and TRIFFID phase-resolved photometry, in these bands in conjunction with further observations in the UV and R region, would constrain this fit further.

HST optical polarimetry of the Vela pulsar and nebula

Polarisation measurements of pulsars offer an unique insight into the geometry of the emission regions in the neutron star magnetosphere. Therefore, they provide observational constraints on the different models proposed for the pulsar emission mechanisms. Optical polarisation data of the Vela pulsar was obtained from the {\em Hubble Space Telescope} ({\em HST}) archive. The data, obtained in two filters (F606W; central wavelength = 590.70 nm, and F550M; central wavelength = 558.15 nm), consists of a series of observations of the pulsar taken with the {\em HST}/Advanced Camera for Surveys (ACS) and covers a time span of 5 days. This data have been used to carry out the first high-spatial resolution and multi-epoch study of the polarisation of the pulsar. We produced polarisation vector maps of the region surrounding the pulsar and measured the degree of linear polarisation (P.D.) and the position angle (P.A.) of the pulsars integrated pulse beam. %This yielded We obtained

From TORTORA to MegaTORTORA—Results and Prospects of Search for Fast Optical Transients

\rm P.D.=8.1\%\pm0.7\%