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  • 1
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    The binary millisecond pulsar PSR J1023+0038 during its accretion state - I. Optical variability (2015)
    Oxford University Press
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    Publication Date: 2015-09-11
    Description: We present time-resolved optical photometry of the binary millisecond ‘redback’ pulsar PSR J1023+0038 (=AY Sex) during its low-mass X-ray binary phase. The light curves taken between 2014 January and April show an underlying sinusoidal modulation due to the irradiated secondary star and accretion disc. We also observe superimposed rapid flaring on time-scales as short as ~20 s with amplitudes of ~0.1–0.5 mag and additional large flare events on time-scales of ~5–60 min with amplitudes of ~0.5–1.0 mag. The power density spectrum of the optical flare light curves is dominated by a red-noise component, typical of aperiodic activity in X-ray binaries. Simultaneous X-ray and UV observations by the Swift satellite reveal strong correlations that are consistent with X-ray reprocessing of the UV light, most likely in the outer regions of the accretion disc. On some nights we also observe sharp-edged, rectangular, flat-bottomed dips randomly distributed in orbital phase, with a median duration of ~250 s and a median ingress/egress time of ~20 s. These rectangular dips are similar to the mode-switching behaviour between disc ‘active’ and ‘passive’ luminosity states, observed in the X-ray light curves of other redback millisecond pulsars. This is the first time that the optical analogue of the X-ray mode-switching has been observed. The properties of the passive- and active-state light curves can be explained in terms of clumpy accretion from a trapped inner accretion disc near the corotation radius, resulting in rectangular, flat-bottomed optical and X-ray light curves.
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    Electronic ISSN: 1365-2966
    Topics: Physics
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  • 2
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    Swift J1357.2-0933: a massive black hole in the Galactic thick disc (2015)
    Oxford University Press
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    Publication Date: 2015-10-15
    Description: Swift J1357.2–0933 is one of the shortest orbital period black hole X-ray transients. It exhibited deep optical dips together with an extremely broad H α line during outburst. We present 10.4-m Gran Telescopio Canarias (GTC) time-resolved spectroscopy during quiescence searching for donor star absorption features. The large contribution of the accretion flow to the total luminosity prevents the direct detection of the companion. Nevertheless, we constrain the non-stellar contribution to be larger than ~80 per cent of the total optical light, which sets new lower limits to the distance ( d 〉 2.29 kpc) and the height over the Galactic plane ( z 〉 1.75 kpc). This places the system in the Galactic thick disc. We measure a modulation in the centroid of the H α line with a period of P = 0.11 ± 0.04 d which, combined with the recently presented full width at half-maximum– K 2 correlation, results in a massive black hole ( M 1 〉 9.3 M ) and an ~M2V companion star ( M 2 ~ 0.4 M ). We also present further evidence supporting a very high orbital inclination ( i 80°).
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  • 3
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    Dynamical masses of a nova-like variable on the edge of the period gap (2015)
    Oxford University Press
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    Publication Date: 2015-07-03
    Description: We present the first dynamical determination of the binary parameters of an eclipsing SW Sextantis star in the 3–4 h orbital period range during a low state. We obtained time-resolved optical spectroscopy and photometry of HS 0220+0603 during its 2004–2005 low-brightness state, as revealed in the combined Small & Moderate Aperture Research Telescope System, IAC80 and M1 Group long-term optical light curve. The optical spectra taken during primary eclipse reveal a secondary star spectral type of M5.5 ± 0.5 as derived from molecular band-head indices. The spectra also provide the first detection of a DAB white dwarf in a cataclysmic variable. By modelling its optical spectrum we estimate a white dwarf temperature of 30 000 ± 5000 K. By combining the results of modelling the white dwarf eclipse from ULTRACAM light curves with those obtained by simultaneously fitting the emission- and absorption-line radial velocity curves and I -band ellipsoidal light curves, we measure the stellar masses to be M 1  = 0.87 ± 0.09 M and M 2  = 0.47 ± 0.05 M for the white dwarf and the M dwarf, respectively, and an inclination of the orbital plane of i   79°. A radius of 0.0103 ± 0.0007 R is obtained for the white dwarf. The secondary star in HS 0220+0603 is likely too cool and undersized for its mass.
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  • 4
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    The binary millisecond pulsar PSR J1023+0038 during its accretion state - I. Optical variability (2015)
    Oxford University Press
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    Publication Date: 2015-09-27
    Description: We present time-resolved optical photometry of the binary millisecond ‘redback’ pulsar PSR J1023+0038 (=AY Sex) during its low-mass X-ray binary phase. The light curves taken between 2014 January and April show an underlying sinusoidal modulation due to the irradiated secondary star and accretion disc. We also observe superimposed rapid flaring on time-scales as short as ~20 s with amplitudes of ~0.1–0.5 mag and additional large flare events on time-scales of ~5–60 min with amplitudes of ~0.5–1.0 mag. The power density spectrum of the optical flare light curves is dominated by a red-noise component, typical of aperiodic activity in X-ray binaries. Simultaneous X-ray and UV observations by the Swift satellite reveal strong correlations that are consistent with X-ray reprocessing of the UV light, most likely in the outer regions of the accretion disc. On some nights we also observe sharp-edged, rectangular, flat-bottomed dips randomly distributed in orbital phase, with a median duration of ~250 s and a median ingress/egress time of ~20 s. These rectangular dips are similar to the mode-switching behaviour between disc ‘active’ and ‘passive’ luminosity states, observed in the X-ray light curves of other redback millisecond pulsars. This is the first time that the optical analogue of the X-ray mode-switching has been observed. The properties of the passive- and active-state light curves can be explained in terms of clumpy accretion from a trapped inner accretion disc near the corotation radius, resulting in rectangular, flat-bottomed optical and X-ray light curves.
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  • 5
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    The spotty donor star in the X-ray transient Cen X-4 (2014)
    Oxford University Press
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    Publication Date: 2014-04-02
    Description: We accurately determine the fundamental system parameters of the neutron star X-ray transient Cen X–4 solely using phase-resolved high-resolution UV–Visual Echelle Spectrograph spectroscopy. We first determine the radial-velocity curve of the secondary star and then model the shape of the phase-resolved absorption line profiles using an X-ray binary model. The model computes the exact rotationally broadened, phase-resolved spectrum and does not depend on assumptions about the rotation profile, limb-darkening coefficients and the effects of contamination from an accretion disc. We determine the secondary star-to-neutron star binary mass ratio to be 0.1755 ± 0.0025, which is an order of magnitude more accurate than previous estimates. We also constrain the inclination angle to be 32 ${^{\circ }} ^{+8^{\circ }}_{-2^{\circ }}$ . Combining these values with the results of the radial-velocity study gives a neutron star mass of 1.94 $^{+0.37}_{-0.85}$ M consistent with previous estimates. Finally, we perform the first Roche tomography reconstruction of the secondary star in an X-ray binary. The tomogram reveals surface inhomogeneities that are due to the presence of cool starspots. A large cool polar spot, similar to that seen in Doppler images of rapidly rotating isolated stars, is present on the Northern hemisphere of the K7 secondary star and we estimate that ~4 per cent of the total surface area of the donor star is covered with spots. This evidence for starspots supports the idea that magnetic braking plays an important role in the evolution of low-mass X-ray binaries.
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  • 6
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    The multiwavelength polarization of Cygnus X-1 (2014)
    Oxford University Press
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    Publication Date: 2014-02-15
    Description: Polarization measurements of the microquasar Cygnus X–1 exist at -ray, X-ray, ultraviolet, optical and radio frequencies. The -ray emission has been shown to be highly linearly polarized. Here, we present new infrared polarimetric data of Cygnus X–1 taken with the 10.4 m Gran Telescopio Canarias and the 4.2 m William Herschel Telescope. We show that the broad-band, radio-to--ray flux spectrum and polarization spectrum in the hard state are largely consistent with a simple phenomenological model of a strongly polarized synchrotron jet, an unpolarized Comptonized corona and a moderately polarized interstellar dust component. In this model, the origin of the -ray, X-ray and some of the infrared polarization is the optically thin synchrotron power law from the inner regions of the jet. The model requires the magnetic field in this region to be highly ordered and perpendicular to the axis of the resolved radio jet. This differs from studies of some other X-ray binaries, in which the magnetic field is turbulent, variable and aligned with the jet axis. The model is able to explain the approximate polarization strength and position angle at all wavelengths including the detected X-ray (3–5 keV) polarization, except the observed position angle of the -ray polarization, which differs from the model by ~60°. Past numerical modelling has shown that a curved synchrotron spectrum can produce a shift in position angle by ~60°, which may account for this.
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  • 7
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    Roche tomography of cataclysmic variables - VI. Differential rotation of AE Aqr - not tidally locked! (2014)
    Oxford University Press
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    Publication Date: 2014-08-14
    Description: We present Roche tomograms of the K4V secondary star in the cataclysmic variable AE Aqr, reconstructed from two data sets taken 9 d apart, and measure the differential rotation of the stellar surface. The tomograms show many large, cool starspots, including a large high-latitude spot and a prominent appendage down the trailing hemisphere. We find two distinct bands of spots around 22° and 43° latitude, and estimate a spot coverage of 15.4–17 per cent on the Northern hemisphere. Assuming a solar-like differential rotation law, the differential rotation of AE Aqr was measured using two different techniques. The first method yields an equator–pole lap time of 269 d and the second yields a lap time of 262 d. This shows that the star is not fully tidally locked, as was previously assumed for CVs, but has a co-rotation latitude of ~40°. We discuss the implications that these observations have on stellar dynamo theory, as well as the impact that spot traversal across the L 1 point may have on accretion rates in CVs as well as some of their other observed properties. The entropy landscape technique was applied to determine the system parameters of AE Aqr. For the two independent data sets, we find M 1  = 1.20 and 1.17 M , M 2  = 0.81 and 0.78 M , and orbital inclinations of 50° to 51° at optimal systemic velocities of  = –64.7 and –62.9 km s –1 .
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  • 8
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    Roche tomography of cataclysmic variables - VII. The long-term magnetic activity of AE Aqr (2016)
    Oxford University Press
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    Publication Date: 2016-04-30
    Description: We present a long-term study of the secondary star in the cataclysmic variable AE Aqr, using Roche tomography to indirectly image starspots on the stellar surface spanning 8 years of observations. The seven maps show an abundance of spot features at both high and low latitudes. We find that all maps have at least one large high-latitude spot region, and we discuss its complex evolution between maps, as well as its compatibility with current dynamo theories. Furthermore, we see the apparent growth in fractional spot coverage, f s , around 45° latitude over the duration of observations, with a persistently high f s near latitudes of 20°. These bands of spots may form as part of a magnetic activity cycle, with magnetic flux tubes emerging at different latitudes, similar to the ‘butterfly’ diagram for the Sun. We discuss the nature of flux tube emergence in close binaries, as well as the activity of AE Aqr in the context of other stars.
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  • 9
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    Furiously fast and red: sub-second optical flaring in V404 Cyg during the 2015 outburst peak (2016)
    Oxford University Press
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    Publication Date: 2016-04-17
    Description: We present observations of rapid (sub-second) optical flux variability in V404 Cyg during its 2015 June outburst. Simultaneous three-band observations with the ULTRACAM fast imager on four nights show steep power spectra dominated by slow variations on ~100–1000 s time-scales. Near the peak of the outburst on June 26, a dramatic change occurs and additional, persistent sub-second optical flaring appears close in time to giant radio and X-ray flaring. The flares reach peak optical luminosities of ~ few  x  10 36  erg s –1 . Some are unresolved down to a time resolution of 24 ms. Whereas the fast flares are stronger in the red, the slow variations are bluer when brighter. The redder slopes, emitted power and characteristic time-scales of the fast flares can be explained as optically thin synchrotron emission from a compact jet arising on size scales ~140–500 Gravitational radii (with a possible additional contribution by a thermal particle distribution). The origin of the slower variations is unclear. The optical continuum spectral slopes are strongly affected by dereddening uncertainties and contamination by strong Hα emission, but the variations of these slopes follow relatively stable loci as a function of flux. Cross-correlating the slow variations between the different bands shows asymmetries on all nights consistent with a small red skew (i.e. red lag). X-ray reprocessing and non-thermal emission could both contribute to these. These data reveal a complex mix of components over five decades in time-scale during the outburst.
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  • 10
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    A millisecond pulsar candidate in a 21-h orbit: 3FGL J0212.1+5320 (2016)
    Oxford University Press
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    Publication Date: 2016-12-30
    Description: We present the discovery of a variable optical counterpart to the unidentified gamma-ray source 3FGL J0212.1+5320 and argue that this is a new compact binary millisecond pulsar (MSP) candidate. We show 3FGL J0212.1+5320 hosts a semidetached binary with a 0.869 55 ± 0.000 15 d orbital period and an F6-type companion star at an estimated distance of D = 1.1 ± 0.2 kpc, with a radial velocity curve semi-amplitude K 2 = 214.1 ± 5.0 km s –1 and a projected rotational velocity of V sin ( i ) = 73.2 ± 1.6 km s –1 . We find a hard X-ray source at the same location with a 0.5–10 keV luminosity L X = 2.6  x  10 32 ( D /1.1 kpc) 2  erg s –1 , which strengthens the MSP identification. Our results imply a mass ratio q  =  M 2 / M 1 = 0.26 $^{+0.02}_{-0.03}$ if the companion star fills its Roche lobe, and q  0.26 in any case. This classifies 3FGL J0212.1+5320 as a ‘redback’ binary MSP; if its MSP nature is confirmed, this will be the brightest compact binary MSP in the optical band ( r '' ~= 14.3 mag) and will have the longest orbital period among Galactic field systems (nearly 21 h). Based on the light curve peak-to-peak amplitude ( r = 0.19 mag), we further suggest that the orbital inclination is high and the putative pulsar mass is close to canonical ( M 1 ~= 1.3–1.6 M ). Finally, we discuss the lack of heating signatures and asymmetric optical light curves in the context of other redback MSPs.
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