Timing and spectral studies of magnetic cataclysmic variables.

This thesis describes observations of a number of the magnetic cataclysmic variables known as AM Herculis stars (or polars) made in the X-ray and optical wavebands. Chapter 1 provides an introduction to cataclysmic variables (CVs), outlines their different classifications and evolution, and then describes the properties of the AM Her stars in detail. Chapter 2 describes the Ginga and ROSAT X-ray satellites from which much of the data discussed in the remaining chapters were obtained. In chapter 3 the phase resolved Ginga hard X-ray spectrum of AM Her is discussed. We find that simple thermal models do not describe the data well, with the observed iron line emission at lower energies and much stronger than expected from the inferred continuum temperatures. However, if we allow for an extra spectral component due to reflection of X-rays from the white dwarf surface, a consistent physical description of the data can be obtained. The inclusion of the hard reflection spectrum allows a reduction in the required incident continuum temperature, which provides a larger amount of thermal iron line emission at slightly lower energies. Together with the fluorescent line from reflection, this correctly accounts for the observed line energy and strength. The inclusion of the reflection component can also explain the variability of the spectrum and the iron line strength as a function of the white dwarf spin phase. Chapter 4 describes variability studies of AM Her and related systems. To begin with, the results of a search for the expected X-ray counterpart to the optical 1 - 3s quasi-periodic oscillations in the Ginga data of the three systems AM Her, EF Eri, and V834 Cen is presented. No QPO is detected, and the upper limits suggest we are not sensitive enough to the oscillating phenomenon (which is thought to be caused by an oscillating shock). The latter half of this chapter examines the longer timescale variability of AM Her as observed with Ginga, and also reports the results of some simultaneous UBVRI optical coverage. The variability observed in the Ginga band (2 - l0keV) is best described by a shot noise model, which consists of random bursts of emission with an e-folding decay timescale of 70s, allowing the characteristic lengthscale of inhomogeneities in the accreting material to be estimated. The optical red variability is found to correlate well with that observed by Ginga, whereas the optical blue does not. It is argued that multiple emitting blobs, of varying density, can account for the observed correlations. In chapter 5, the results of quasi-simultaneous ROSAT and Ginga observations of the polar QQ Vul are presented. The soft X-ray light curve is complex, at times dominated by a ~ 7.5 min timescale modulation. The detection of a soft X-ray spectral variation, together with soft and hard X-ray light curves which peak at different orbital phases, provides further evidence for the presence of two pole accretion. The data also confirms the soft X-ray excess observed previously in this object. Finally, chapter 6 describes the optical follow up observations of a new source, RE1307+535, which was discovered by the ROSAT WFC all sky survey. The optical photometry of this faint CV show it to be a short period system (Porb = 79.7min), at a distance d ? 750pc. The source is identified as a new polar on the basis of the optical discovery spectrum, and the EUV and optical light curves. The optical spectrum also reveals the presence of a strongly magnetic white dwarf, with B ~ 30-40 MG

Neil Gehrels Swift Observatory studies of supersoft novae

The rapid response capabilities of the Neil Gehrels Swift Observatory, together with the daily planning of its observing schedule, make it an ideal mission for following novae in the X-ray and UV bands, particularly during their early phases of rapid evolution and throughout the supersoft source interval. Many novae, both classical and recurrent, have been extensively monitored by Swift throughout their supersoft phase and later decline. We collect here results from observations of novae with outbursts which occurred between the start of 2006 and the end of 2017

The unusual 2006 dwarf nova outburst of GK Persei

The 2006 outburst of GK Persei differed significantly at optical and ultraviolet (UV) wavelengths from typical outbursts of this object. We present multiwavelength (X-ray, UV and optical) Swift and AAVSO data, giving unprecedented broad-band coverage of the outburst, allowing us to follow the evolution of the longer-than-normal 2006 outburst across these wavelengths. In the optical and UV we see a triple-peaked morphology with maximum brightness ∼1.5 mag lower than in previous years. In contrast, the peak hard X-ray flux is the same as in previous outbursts. We resolve this dichotomy by demonstrating that the hard X-ray flux only accounts for a small fraction of the total energy liberated during accretion, and interpret the optical/UV outburst profile as arising from a series of heating and cooling waves traversing the disc, caused by its variable density profile

The discovery of RE 1307 + 535: the shortest period AM Her system

We report on a ROSAT Wide Field Camera EUV survey observation of RE 1307 + 535 which, together with optical spectroscopic and photometric observations, shows that this object is an AM Her system. These observations reveal that RE 1307 + 535 has an orbital period of 79.69 min, the shortest orbital period known for such a system. RE 1307 + 535 was observed by us in both high (V = 17–18) and low (V = 20–21) states, with evidence of cyclotron emission being seen in both cases. We use the high-state spectroscopy to show that the optical emission comes from a region with a magnetic field, B ≈ 30–40 MG. We derive EUV temperature and luminosity limits. IRCAM infrared photometry is used to put a lower limit on the distance to RE 1307 + 535 of d > 705 pc. RE 1307 + 535 is thus one of the most distant AM Her systems known, and is the first that is at a measured distance of z > 630 pc above the Galactic plane

Lord of the Rings - Return of the King: Swift-XRT observations of dust scattering rings around V404 Cygni

On 2015 June 15, the black hole X-ray binary V404 Cygni went into outburst, exhibiting extreme X-ray variability which culminated in a final flare on June 26. Over the following days, the Swift-X-ray Telescope detected a series of bright rings, comprising five main components that expanded and faded with time, caused by X-rays scattered from the otherwise unobservable dust layers in the interstellar medium in the direction of the source. Simple geometrical modelling of the rings' angular evolution reveals that they have a common temporal origin, coincident with the final, brightest flare seen by INTEGRAL's JEM X-1, which reached a 3-10 keV flux of ~25 Crab. The high quality of the data allows the dust properties and density distribution along the line of sight to the source to be estimated. Using the Rayleigh-Gans approximation for the dust scattering cross-section and a power-law distribution of grain sizes a, ∝a-q, the average dust emission is well modelled by q = 3.90-0.08+0.09 and maximum grain size of a+ = 0.147-0.004+0.024 μm, though significant variations in q are seen between the rings. The recovered dust density distribution shows five peaks associated with the dense sheets responsible for the rings at distances ranging from 1.19 to 2.13 kpc, with thicknesses of ~40-80 pc and a maximum density occurring at the location of the nearest sheet. We find a dust column density of Ndust ≈ (2.0-2.5) × 1011 cm-2, consistent with the optical extinction to the source. Comparison of the inner rings' azimuthal X-ray evolution with archival Wide-field Infrared Survey Explorer mid-IR data suggests that the second most distant ring follows the general IR emission trend, which increases in brightness towards the Galactic north side of the source

Cross-calibration of the X-ray instruments onboard the Chandra, INTEGRAL, RXTE, Suzaku, Swift, and XMM-Newton observatories using G21.5-0.9

Context. For many years, X-ray astronomy missions have used the Crab nebula as a celestial calibration source for the X-ray flux and spectral shape. However, the object is often too bright for current and future missions equipped with instruments with improved sensitivity. Aims. We use G21.5–0.9, a pulsar-wind nebula with a time-constant power-law spectrum and a flux of a few milli-Crab in the X-ray band, as a viable, fainter substitute to the Crab. Using this source, we conduct a cross-calibration study of the instruments onboard currently active observatories: Chandra ACIS, Suzaku XIS, Swift XRT, and XMM-Newton EPIC (MOS and pn) for the soft-band, and INTEGRAL IBIS-ISGRI, RXTE PCA, and Suzaku HXD-PIN for the hard band. Methods. We extract spectra from all instruments and fit under the same astrophysical assumptions. We compare the spectral parameters of the G21.5–0.9 model: power-law photon index, H-equivalent column density of the interstellar photoelectric absorption, and flux in the soft (2–8 keV) or hard (15–50 keV) energy band. Results. We identify systematic differences in the best-fit parameter values unattributable to statistical scatter of the data alone. We interpret these differences as due to residual cross-calibration problems. The differences can be as large as 20% and 9% for the soft-band flux and power-law index, respectively, and 46% for the hard-band flux. The results are plotted and tabulated as a useful reference for future calibration and scientific studies using multiple missions

The spectroscopic evolution of the symbiotic-like recurrent nova V407 Cygni during its 2010 outburst

Context. V407 Cyg was, before 2010 Mar., known only as a D-type symbiotic binary system in which the Mira variable has a pulsation period of approximately 750 days, one of the longest known. On 2010 Mar. 10, it was discovered in outburst, eventually reaching V < 8. This is the first recorded nova event for this system, but it closely resembles the spectroscopic development of RS Oph, the prototypical symbiotic-like recurrent nova. It was also detected by Fermi above 100 MeV and displayed strong, likely nonthermal centimeter wavelength radio emission. Aims. Unlike classical novae occurring in compact cataclysmic binary systems, for which the ejecta undergo free ballistic expansion, this explosion occurred within the dense, complex wind of a Mira variable companion. This paper concentrates on the development of the shock and its passage through the Mira wind. We also present some constraints on the binary system properties. Methods. Using medium and high resolution ground-based optical spectra, visual and Swift UV photometry, and Swift X-ray spectrophotometry, we describe the behavior of the high-velocity profile evolution for this nova during its first three months. Results. Using the diffuse interstellar bands visible in the high-resolution optical spectra, we obtain an extinction E(B − V) ≈ 0.45 ± 0.05. The spectral type of the red giant during this period, when the star was at R minimum, was no earlier than M7 III. The peak of the X-ray emission occurred at about day 40 with a broad maximum and decline after day 50. The main changes in the optical spectrum began at around that time. The He II 4686 Å line first appeared between days 7 and 14 and initially displayed a broad, symmetric profile that is characteristic of all species before day 60. The profile development thereafter depended on ionization state. Low-excitation lines remained comparatively narrow, with vrad,max of order 200–400 km s-1. They were systematically more symmetric than lines such as [Ca V], [Fe VII], [Fe X], and He II, all of which showed a sequence of profile changes going from symmetric to a blue wing similar to that of the low ionization species but with a red wing extended to as high as 600 km s-1. The [O I] 6300, 6364 doublet showed a narrow wind-emission component near the rest velocity of the system and a broad component, 200–300 km s-1, whose relative intensity increased in time. Forbidden lines of N II and O III had two separate contributors to the profiles, a broad line that increased in strength and velocity width, exceeding 200 km s-1, and narrow components from a surrounding ionized region at higher velocity than the Mira wind. The Na I D doublet developed a broad component with similar velocity width to the other low-ionization species. The O VI Raman features observed in recent outbursts of RS Oph were not detected. We interpret these variations as aspherical expansion of the ejecta within the Mira wind. The blue side is from the shock penetrating into the wind while the red wing is from the low-density periphery. The maximum radial velocities obey power laws, vmax ~ t − n with n ≈ 1/3 for red wing and ≈ 0.8 for the blue

GRB 090618: Detection of thermal X-ray emission from a bright gamma-ray burst

GRB 090618 was an extremely bright burst, detected across the electromagnetic spectrum. It has a redshift of 0.54 and a supernova (SN) was identified in ground-based photometry. We present a thorough analysis of the prompt and early afterglow emission using data from Swift, Fermi Gamma-ray Burst Monitor and ROTSE, in which we track the evolution of the synchrotron spectral peak during the prompt emission and through the steep decay phase. We find evidence of a thermal X-ray component alongside the expected non-thermal power-law continuum. Such a component is rare among gamma-ray bursts (GRBs), with firm data for only GRBs 060218 and 100316D so far, and could potentially originate from an SN shock breakout, although there remains doubt regarding this explanation for any of the bursts. However, in contrast to these other Swift GRB–SNe with similar thermal signatures, GRB 090618 is a much more ‘typical’ burst: GRB–SNe 060218 and 100316D were both low-luminosity events, with long durations and low peak energies, while GRB 090618 was more representative of the wider population of long GRBs in all of these areas. It has been argued, based both on theory and observations, that most long GRBs should be accompanied by SNe. If this thermal X-ray component is related to the SN, its detection in GRB 090618, a fairly typical burst in many ways, may prove an important development in the study of the GRB–SN connection

Multiple flaring activity in the supergiant fast X-ray transient IGR J08408-4503 observed with Swift

IGR J08408−4503 is a supergiant fast X–ray transient discovered in 2006 with a confirmed association with a O8.5Ib(f) supergiant star, HD 74194. We report on the analysis of two outbursts caught by Swift/Burst Alert Telescope (BAT) on 2006 October 4 and 2008 July 5, and followed up at softer energies with Swift/X-ray Telescope (XRT). The 2008 XRT light curve shows a multiple-peaked structure with an initial bright flare that reached a flux of ∼10[superscript: −9] erg cm[superscript: -2] s[superscript: −1] (2–10 keV), followed by two equally bright flares within 75 ks. The spectral characteristics of the flares differ dramatically, with most of the difference, as derived via time-resolved spectroscopy, being due to absorbing column variations. We observe a gradual decrease in the N[subscript: H], derived with a fit using absorbed power-law model, as time passes. We interpret these N[subscript: H] variations as due to an ionization effect produced by the first flare, resulting in a significant decrease in the measured column density towards the source. The durations of the flares as well as the times of the outbursts suggest that the orbital period is ∼35 d, if the flaring activity is interpreted within the framework of the Sidoli et al. model with the outbursts triggered by the neutron star passage inside an equatorial wind inclined with respect to the orbital plane

The Swift Burst Analyser I. BAT and XRT spectral and flux evolution of gamma ray bursts

Context: Gamma ray burst models predict the broadband spectral evolution and the temporal evolution of the energy flux. In contrast, standard data analysis tools and data repositories provide count-rate data, or use single flux conversion factors for all of the data, neglecting spectral evolution. Aims: We produce Swift BAT and XRT light curves in flux units, where the spectral evolution is accounted for. Methods: We have developed software to use the hardness ratio information to track spectral evolution of GRBs, and thus to convert the count-rate light curves from the BAT and XRT instruments on Swift into accurate, evolution-aware flux light curves. Results: The Swift Burst Analyser website (http://www.swift.ac.uk/burst_analyser) contains BAT, XRT and combined BAT-XRT flux light curves in three energy regimes for all GRBs observed by the Swift satellite. These light curves are automatically built and updated when data become available, are presented in graphical and plain-text format, and are available for download and use in research