Strongly magnetized accretion in ultracompact binary systems

AM CVn systems are binary star systems with orbital periods less than 70 minutes in which a white dwarf accretes matter from a companion star, which must be either a stripped helium burning star, or a white dwarf of lower mass than the accretor. Here, we present the discoveries of two of these systems in which there is mass transfer from the lighter white dwarf or helium star onto a strongly magnetized heavier white dwarf. These represent the first clear example of magnetized accretion in ultracompact binaries. These systems, along with similar systems that are slightly more widely separated, and that have not started to transfer mass yet, are expected to be the primary source of gravitational waves to be detected by space-based gravitational wave observatories. The presence of strong magnetic fields can substantially affect both the evolution of the binaries, and also the particular wave forms of the gravitational waves themselves, and understanding these magnetic effects is vital for understanding what to expect from the Laser Interferometer Space Antenna

Strongly magnetized accretion in two ultracompact binary systems

We present the discoveries of two of AM CVn systems, Gaia14aae and SDSS~J080449.49+161624.8, which show X-ray pulsations at their orbital periods, indicative of magnetically collimated accretion. Both also show indications of higher rates of mass transfer relative to the expectations from binary evolution driven purely by gravitational radiation, based on existing optical data for Gaia14aae, which show a hotter white dwarf temperature than expected from standard evolutionary models, and X-ray data for SDSS~J080449.49+161624.8 which show a luminosity 10-100 times higher than those for other AM~CVn at similar orbital periods. The higher mass transfer rates could be driven by magnetic braking from the disk wind interacting with the magnetosphere of the tidally locked accretor. We discuss implications of this additional angular momentum transport mechanism for evolution and gravitational wave detectability of AM CVn objects.Comment: 5 pages, 3 figures, accepted to MNRAS Letter

Accretion spin-up and a strong magnetic field in the slow-spinning Be X-ray binary MAXI J0655-013

We present MAXI and NuSTAR observations of the Be X-ray binary, MAXI J0655-013, in outburst. NuSTAR observed the source once early in the outburst, when spectral analysis yields a bolometric (0.1--100 keV), unabsorbed source luminosity of $L_{\mathrm{bol}}=5.6\times10^{36}\mathrm{erg\,s^{-1}}$, and a second time 54 days later, by which time the luminosity dropped to $L_{\mathrm{bol}}=4\times10^{34}\,\mathrm{erg\,s^{-1}}$ after first undergoing a dramatic increase. Timing analysis of the NuSTAR data reveals a neutron star spin period of $1129.09\pm0.04$ s during the first observation, which decreased to $1085\pm1$ s by the time of the second observation, indicating spin-up due to accretion throughout the outburst. Furthermore, during the first NuSTAR observation, we observed quasiperiodic oscillations with centroid frequency $\nu_0=89\pm1$ mHz, which exhibited a second harmonic feature. By combining the MAXI and NuSTAR data with pulse period measurements reported by Fermi/GBM, we are able to show that apparent flaring behavior in the MAXI light-curve is an artifact introduced by uneven sampling of the pulse profile, which has a large pulsed fraction. Finally, we estimate the magnetic field strength at the neutron star surface via three independent methods, invoking a tentative cyclotron resonance scattering feature at $44$ keV, QPO production at the inner edge of the accretion disk, and spin-up via interaction of the neutron star magnetic field with accreting material. Each of these result in a significantly different value. We discuss the strengths and weaknesses of each method and infer that MAXI J0655-013 is likely to have a high surface magnetic field strength, $B_{s}>10^{13}$ G.Comment: 19 pages, 10 figure, 4 tables; submitted to ApJ on May 24, 202

Spectral and Timing Analysis of NuSTAR and Swift/XRT Observations of the X-Ray Transient MAXI J0637-430

We present results for the first observed outburst from the transient X-ray binary source MAXI J0637-430. This study is based on eight observations from the Nuclear Spectroscopic Telescope Array (NuSTAR) and six observations from the Neil Gehrels Swift Observatory X-Ray Telescope (Swift/XRT) collected from 2019 November 19 to 2020 April 26 as the 3-79 keV source flux declined from 8.2 × 10-10 to 1.4 × 10-12 erg cm-2 s-1. We see the source transition from a soft state with a strong disk-blackbody component to a hard state dominated by a power-law or thermal Comptonization component. NuSTAR provides the first reported coverage of MAXI J0637-430 above 10 keV, and these broadband spectra show that a two-component model does not provide an adequate description of the soft-state spectrum. As such, we test whether blackbody emission from the plunging region could explain the excess emission. As an alternative, we test a reflection model that includes a physical Comptonization continuum. Finally, we also test a spectral component based on reflection of a blackbody illumination spectrum, which can be interpreted as a simple approximation to the reflection produced by returning disk radiation due to the bending of light by the strong gravity of the black hole. We discuss the physical implications of each scenario and demonstrate the value of constraining the source distance

Multi-wavelength observations of Galactic black hole X-ray transients

Galactic black hole X-ray transients (BHXRTs) represent the ideal opportunity to study accretion physics in extreme environments. Questions relating to accretion geometry, the mass of the compact object, how transient outbursts start and how jets form and evolve can only be answered by observing BHXRTs across multiple wavelengths. In this thesis I take a multi-wavelength approach to studying BHXRTs, and attempt to reconcile many aspects of accretion physics using a number of techniques.I have used timing analysis techniques to discover a super-orbital periodicity of 420 days in the X-ray and optical light curves of the BHXRT Swift J1753.5-0127, whilst also uncovering the likely orbital period of the candidate BHXRT MAXI J1305-704. X-ray spectral observations of Swift J1753.5-0127 revealed its first ever transition to a soft accretion state. This was found to be one of the lowest luminosity soft states ever recorded in such a system, at < 1% the Eddington luminosity, which proved crucial in the subsequent radio observations. Collaborators and I found that the compact jet had been quenched by a factor > 25, indicating that jet quenching was not dependent on accretion rate.In the optical regime, spectroscopy can reveal details about the mass of the compact object, which is important for population studies of black holes (BHs). I place a lower limit on the mass of Swift J1753.5-0127at M1 > 7:41:2M, confirming it as a BH and removing it from the so-called 'mass gap.' However, optical observations can also reveal details about the accretion geometry of a source, as in the case of the BHXRT V404 Cyg, in which collaborators and I combined optical spectroscopy and photometry and found evidence for the 2015 outburst initiating 1 week before the X-ray outburst was detected. Our results were found to be consistent with the current disc instability model for transient outbursts. I summarise these findings and suggest avenues for future work in the closing pages of this thesis. Overall, this work shows that we can only truly understand BHXRTs once we have studied them across multiple wavelengths, and only in doing so can we discuss the underlying physics behind some of the most extreme regions of the Universe

Soft excess in the quiescent Be/X-ray pulsar RX J0812.4-3114

We report a 72 ks XMM-Newton observation of the Be/X-ray pulsar (BeXRP) RX J0812.4-3114 in quiescence (Lx=1.6x10^33 erg/s). Intriguingly, we find a two-component spectrum, with a hard power-law (Gamma=1.5) and a soft blackbody-like excess below =1 keV. The blackbody component is consistent in kT with a prior quiescent Chandra observation reported by Tsygankov et al. and has an inferred blackbody radius of =10 km, consistent with emission from the entire neutron star (NS) surface. There is also mild evidence for an absorption line at =1 keV and/or =1.4 keV. The hard component shows pulsations at P=31.908 s (pulsed fraction 0.84+/-0.10), agreeing with the pulse period seen previously in outbursts, but no pulsations were found in the soft excess (pulsed fraction <~ 31%). We conclude that the pulsed hard component suggests low-level accretion onto the neutron star poles, while the soft excess seems to originate from the entire NS surface. We speculate that, in quiescence, the source switches between a soft thermal-dominated state (when the propeller effect is at work) and a relatively hard state with low-level accretion, and use the propeller cutoff to estimate the magnetic field of the system to be <~8.4x10^11 G. We compare the quiescent thermal Lx predicted by the standard deep crustal heating model to our observations and find that RX J0812.4-3114 has a high thermal Lx, at or above the prediction for minimum cooling mechanisms. This suggests that RX J0812.4-3114 either contains a relatively low-mass NS with minimum cooling, or that the system may be young enough that the NS has not fully cooled from the supernova explosion

Classifying IGR J18007-4146 as an intermediate polar using XMM and NuSTAR

International audienceMany new and unidentified Galactic sources have recently been revealed by ongoing hard X-ray surveys. A significant fraction of these have been shown to be the type of accreting white dwarfs known as cataclysmic variables (CVs). Follow-up observations are often required to categorize and classify these sources, and may also identify potentially unique or interesting cases. One such case is IGR J18007-4146, which is likely a CV based on follow-up Chandra observations and constraints from optical/IR catalogues. Utilizing simultaneous XMM-Newton and NuSTAR observations, as well as the available optical/IR data, we confirm the nature of IGR J18007-4146 as an intermediate polar type CV. Timing analysis of the XMM data reveals a periodic signal at 424.4 ± 0.7 s that we interpret as the spin period of the white dwarf. Modelling the 0.3-78 keV spectrum, we use a thermal bremsstrahlung continuum but require intrinsic absorption as well as a soft component and strong Fe lines between 6 and 7 keV. We model the soft component using a single-temperature blackbody with $kT = 73^{+8}_{-6}$ eV. From the X-ray spectrum, we are able to measure the mass of the white dwarf to be $1.06^{+0.19}_{-0.10}$ $\mathrm{ M}_{\mathord \odot }$, which means IGR J18007-4146 is more massive than the average for magnetic CVs

Hitting a New Low: The Unique 28 hr Cessation of Accretion in the TESS Light Curve of YY Dra (DO Dra)

We present the Transiting Exoplanet Surveying Satellite light curve of the intermediate polar YY Draconis (YY Dra, also known as DO Dra). The power spectrum indicates that while there is stream-fed accretion for most of the observational period, there is a day-long, flat-bottomed low state at the beginning of 2020 during which the only periodic signal is ellipsoidal variation and there is no appreciable flickering. We interpret this low state to be a complete cessation of accretion, a phenomenon that has been observed only once before in an intermediate polar. Simultaneous ground-based observations of this faint state establish that when accretion is negligible, YY Dra fades to g = 17.37 ± 0.12, which we infer to be the magnitude of the combined photospheric contributions of the white dwarf and its red dwarf companion. Using survey photometry, we identify additional low states in 2018–2019 during which YY Dra repeatedly fades to—but never below—this threshold. This implies relatively frequent cessations in accretion. Spectroscopic observations during future episodes of negligible accretion can be used to directly measure the field strength of the white dwarf by Zeeman splitting. Separately, we search newly available catalogs of variable stars in an attempt to resolve the long-standing dispute over the proper identifier of this system