arXiv subjects

David A. H. Buckley

Publications and source records attributed to David A. H. Buckley.

At least 55 records · Page 3Linked to original sources

arXiv2023

Unveiling the white dwarf in J191213.72-441045.1 through ultraviolet observations

J191213.72-441045.1 is a binary system composed of a white dwarf and an M-dwarf in a 4.03-hour orbit. It shows emission in radio, optical, and X-ray, all modulated at the white dwarf spin period of 5.3 min, as well as various orbital sideband frequencies. Like in the prototype of the class of radio-pulsing white dwarfs, AR Scorpii, the observed pulsed emission seems to be driven by the binary interaction. In this work, we present an analysis of far-ultraviolet spectra obtained with the Cosmic Origins Spectrograph at the Hubble Space Telescope, in which we directly detect the white dwarf in J191213.72-441045.1. We find that the white dwarf has an effective temperature of 11485+/-90 K and mass of 0.59+/-0.05 solar masses. We place a tentative upper limit on the magnetic field of ~50 MG. If the white dwarf is in thermal equilibrium, its physical parameters would imply that crystallisation has not started in the core of the white dwarf. Alternatively, the effective temperature could have been affected by compressional heating, indicating a past phase of accretion. The relatively low upper limit to the magnetic field and potential lack of crystallisation that could generate a strong field pose challenges to pulsar-like models for the system and give preference to propeller models with a low magnetic field. We also develop a geometric model of the binary interaction which explains many salient features of the system.

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arXiv2023

Rapid Evolution of the White Dwarf Pulsar AR Scorpii

Analysis of AR Sco optical light curves spanning nine years show a secular change in the relative amplitudes of the beat pulse pairs generated by the two magnetic poles of its rotating white dwarf. Recent photometry now shows that the primary and secondary beat pulses have similar amplitudes, while in 2015 the primary pulse was approximately twice that of the secondary peak. The equalization in the beat pulse amplitudes is also seen in the linearly polarized flux. This rapid evolution is consistent with precession of the white dwarf spin axis. The observations imply that the pulse amplitudes cycle over a period of $\gtrsim 40$ yrs, but that the upper limit is currently poorly constrained. If precession is the mechanism driving the evolution, then over the next 10 years the ratio of the beat pulse amplitudes will reach a maximum followed by a return to asymmetric beat pulses.

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arXiv2023

A two-component jet model for the optical plateau in the afterglow of GRB 191221B

The long gamma-ray burst GRB 191221B has abundant observations in X-ray, optical and radio bands. In the literature, the observed optical light curve of GRB 191221B displays a plateau around 0.1-day, which is rather peculiar in gamma-ray bursts. Here we performed detailed analysis of the observational data from Swift/UVOT, VLT and LCO, obtained the light curve of the multi-band afterglow of GRB 191221B. By examining optical, ultraviolet, X-ray, and radio data for this event, we demonstrate that an on-axis two-component jet model can explain the observations. Our analysis suggests that the narrow component has an initial Lorentz factor of 400 and a jet opening half-angle of $1.4^{\circ}$, while the wide component has an initial Lorentz factor of 25 and a jet opening half-angle of $2.8^{\circ}$. The narrow jet dominates the early decay, whereas the wider jet causes the optical plateau and dominates late decay. According to this model, the reason for the absence of the X-ray plateau is due to the steeper spectral index of the wide component, resulting in a less significant flux contribution from the wide jet in the X-ray bands than in the optical bands. Moreover, we have explained the inconsistency in the decay indices of the UVOT and Rc-band data around 2000 seconds using reverse shock emission.

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arXiv2023

The Early Light Curve of SN 2023bee: Constraining Type Ia Supernova Progenitors the Apian Way

We present very early photometric and spectroscopic observations of the Type Ia supernova (SN Ia) 2023bee, starting about 8 hr after the explosion, which reveal a strong excess in the optical and nearest UV (U and UVW1) bands during the first several days of explosion. This data set allows us to probe the nature of the binary companion of the exploding white dwarf and the conditions leading to its ignition. We find a good match to the Kasen model in which a main-sequence companion star stings the ejecta with a shock as they buzz past. Models of double detonations, shells of radioactive nickel near the surface, interaction with circumstellar material, and pulsational delayed detonations do not provide good matches to our light curves. We also observe signatures of unburned material, in the form of carbon absorption, in our earliest spectra. Our radio nondetections place a limit on the mass-loss rate from the putative companion that rules out a red giant but allows a main-sequence star. We discuss our results in the context of other similar SNe Ia in the literature.

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arXiv2023

A 5.3-minute-period pulsing white dwarf in a binary detected from radio to X-rays

White dwarf stars are the most common stellar fossils. When in binaries, they make up the dominant form of compact object binary within the Galaxy and can offer insight into different aspects of binary formation and evolution. One of the most remarkable white dwarf binary systems identified to date is AR Scorpii (henceforth AR Sco). AR Sco is composed of an M-dwarf star and a rapidly-spinning white dwarf in a 3.56-hour orbit. It shows pulsed emission with a period of 1.97 minutes over a broad range of wavelengths, which led to it being known as a white dwarf pulsar. Both the pulse mechanism and the evolutionary origin of AR Sco provide challenges to theoretical models. Here we report the discovery of the first sibling of AR Sco, J191213.72-441045.1 (henceforth J1912-4410), which harbours a white dwarf in a 4.03-hour orbit with an M-dwarf and exhibits pulsed emission with a period of 5.30 minutes. This discovery establishes binary white dwarf pulsars as a class and provides support for proposed formation models for white dwarf pulsars.

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arXiv2023

Serendipitous discovery of the magnetic cataclysmic variable SRGE J075818-612027

We report the discovery of SRGE J075818-612027, a deep stream-eclipsing magnetic cataclysmic variable found serendipitously in SRG/eROSITA CalPV observations of the open cluster NGC~2516 as an unrelated X-ray source. An X-ray timing and spectral analysis of the eROSITA data is presented and supplemented by an analysis of TESS photometry and SALT spectroscopy. X-ray photometry reveals two pronounced dips repeating with a period of $106.144(1)$ min. The 14-month TESS data reveal the same unique period. A low-resolution identification spectrum obtained with SALT displays hydrogen Balmer emission lines on a fairly blue continuum. The spectrum and the stability of the photometric signal led to the classification of the new object as a polar-type cataclysmic variable. In this picture, the dips in the X-ray light curve are explained by absorption in the intervening accretion stream and by a self-eclipse of the main accretion region. The object displays large magnitude differences on long (months) timescales both at optical and X-ray wavelengths, being interpreted as high and low states and thus supporting the identification as a polar. The bright phase X-ray spectrum can be reflected with single temperature thermal emission with 9.7 keV and bolometric X-ray luminosity $L_{\rm X} \simeq 8\times 10^{32}$erg s$^{-1}$ at a distance of about 2.7 kpc. It lacks the pronounced soft X-ray emission component prominently found in ROSAT-discovered polars.

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arXiv2022

The First Data Release of CNIa0.02 -- A Complete Nearby (Redshift <0.02) Sample of Type Ia Supernova Light Curves

The CNIa0.02 project aims to collect a complete, nearby sample of Type Ia supernovae (SNe Ia) light curves, and the SNe are volume-limited with host-galaxy redshifts z_host < 0.02. The main scientific goal is to infer the distributions of key properties (e.g., the luminosity function) of local SNe Ia in a complete and unbiased fashion in order to study SN explosion physics. We spectroscopically classify any SN candidate detected by the All-Sky Automated Survey for Supernovae (ASAS-SN) that reaches peak brightness < 16.5 mag. Since ASAS-SN scans the full sky and does not target specific galaxies, our target selection is effectively unbiased by host-galaxy properties. We perform multi-band photometric observations starting from the time of discovery. In the first data release (DR1), we present the optical light curves obtained for 247 SNe from our project (including 148 SNe in the complete sample), and we derive parameters such as the peak fluxes, dm15 and s_BV.

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arXiv2022

TUVO-21acq: a new cataclysmic variable discovered through a UV outburst

Outbursts from cataclysmic variables, such as dwarf novae (DNe), are prevalent throughout the galaxy and are known to emit strongly in the ultraviolet (UV). However, the UV emission of DNe has not been studied extensively compared with the optical. Characterising in detail the physical processes responsible for DN outburst behaviour requires further UV data. Here we report, as part of our recently launched Transient UV Objects Project (TUVO), the discovery of TUVO-21acq, a new transient which we detected in the UV using data from the Ultraviolet Optical Telescope aboard Swift. We detected two separate outbursts and used the UV data to constrain source properties, focusing on the amplitudes and timescales of the outbursts. During the first outburst the source increased in brightness by >4.1, >2.4, and >3.5 magnitudes and during the second outburst by >4.4, >3.4, and >3.6 magnitudes in the UVW1, UVM2, and UVW2 bands, respectively. The outburst durations were in the range 6-21 days and 11-46 days, and the recurrence time is <316 days. We additionally obtained an optical spectrum during quiescence with SALT. The spectrum exhibited hydrogen Balmer series and helium emission lines, and a flat overall spectral shape. We confirm the nature of the source as an accreting white dwarf which underwent DN outburst based on photometric and spectroscopic properties. This study serves as a proof-of-concept for the TUVO project strategy, demonstrating its capability of discovering and classifying new, interesting UV transients. We discuss the implications of our findings for our understanding of the physics underlying DN outbursts, in particular with respect to the UV emission. We examine the need for simultaneous UV and optical observations during the onset of DN outbursts in order to help answer remaining questions, for example in the characteristics and implications of the UV delay.

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arXiv2022

Disentangling the neighbouring pulsars SXP 15.3 and SXP 305

SXP 15.3 and SXP 305 are two Be X-ray binaries in the Small Magellanic Cloud that are spatially separated by ~7 arcsec. The small separation between these sources has, in the past, resulted in confusion about the origin of the emission from the combined region. We present long-term optical and X-ray monitoring results of both sources, where we study the historic and recent behaviour. In particular, from data collected as part of the S-CUBED project we see repeating X-ray outbursts from the combined region of the two sources in the recent lightcurve from the Neil Gehrels Swift Observatory, and we investigate the origin of this emission. Using the H-alpha emission line from the Southern African Large Telescope (SALT) and photometric flux from the Optical Gravitational Lensing Experiment (OGLE) to study the changes in the size and structure of the Be disc, we demonstrate that the X-ray emission likely originates from SXP 15.3. Timing analysis reveals unusual behaviour, where the optical outburst profile shows modulation at twice the frequency of the X-ray outbursts. We consider either of these periodicities being the true orbital period in SXP 15.3 and propose models based on the geometric orientations of the Be disc and neutron star to explain the physical origin of the outbursts.

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arXiv2021

A Linear Relation Between the Color Stretch $s_{BV}$ and the Rising Color Slope $s_0^*(B-V)$ of Type Ia Supernovae

Using data from the Complete Nearby ($z_{host}<0.02$) sample of Type Ia Supernovae (CNIa0.02), we discover a linear relation between two parameters derived from the $B-V$ color curves of Type Ia supernovae: the "color stretch" $s_{BV}$ and the rising color slope $s_0^*(B-V)$ after the peak, and this relation applies to the full range of $s_{BV}$. The $s_{BV}$ parameter is known to be tightly correlated with the peak luminosity, and especially for "fast decliners" (dim Type Ia supernovae), and the luminosity correlation with $s_{BV}$ is markedly better than with the classic light-curve width parameters such as $Δ{m_{15}(B)}$. Thus our new linear relation can be used to infer peak luminosity from $s_0^*$. Unlike $s_{BV}$ (or $Δ{m_{15}}$), the measurement of $s_0^*(B-V)$ does not rely on the well-determined time of light-curve peak or color maximum, making it less demanding on the light-curve coverage than past approaches.

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arXiv2021

Probing the Non-thermal Emission Geometry of AR Sco via Optical Phase-Resolved Polarimetry

AR Sco is a binary system that contains a white and red dwarf. The rotation rate of the white dwarf has been observed to slow down, analogous to rotation-powered radio pulsars; it has thus been dubbed a "white dwarf pulsar". We previously fit the traditional radio pulsar rotating vector model to the linearly polarised optical data from this source, constraining the system geometry as well as the white dwarf mass. Using a much more extensive dataset, we now explore the application of the same model to binary phase-resolved optical polarimetric data, thought to be the result of non-thermal synchrotron radiation, and derive the magnetic inclination angle $α$ and the observer angle $ζ$ at different orbital phases. We obtain a $\sim 10^{\circ}$ variation in $α$ and $\sim 30^{\circ}$ variation in $ζ$ over the orbital period. The variation patterns in these two parameters is robust, regardless of the binning and epoch of data used. We speculate that the observer is detecting radiation from an asymmetric emission region that is a stable structure over several orbital periods. The success of this simple model lastly implies that the pitch angles of the particles are small and the pulsed, non-thermal emission originates relatively close to the white dwarf surface.

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arXiv2021

Modeling the Spectral Energy Distributions and Spectropolarimetry of Blazars -- Application to 4C+01.02 in 2016-2017

The optical radiation emitted by blazars contains contributions from synchrotron radiation by relativistic electrons in the jets, as well as thermal radiation emitted mainly by the Accretion Disk (AD), the Broad Line Region (BLR) and the host galaxy. The unpolarized radiation components from the AD, BLR and host galaxy present themselves by decreasing the total polarization in the optical/ultraviolet(UV) spectrum. A combined model for the Spectral Energy Distribution (SED) and degree of optical/UV polarization is constructed, enabling the disentanglement of the synchrotron and AD components. Our model is applied to the multi-wavelength SED and spectropolarimetry observations of the Flat Spectrum Radio Quasar 4C+01.02 ($z = 2.1$) in its 2016 July-August flaring state and July-August 2017 quiescent state, using data from the Fermi Large Area Telescope, the Southern African Large Telescope and the Las Cumbres Observatory network of telescopes. By constraining the AD component, the mass of the super massive black hole is obtained as $3 \times 10^9 \rm M_{\odot}$. Furthermore, the model retrieves the characteristics of the relativistic electron distribution in the jet and the degree of ordering of the magnetic field. Our results highlight the potential of spectropolarimetry observations for disentangling thermal from non-thermal (jet) emission components and thus revealing the physics of particle acceleration and high-energy emission in active galactic nuclei jets.

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arXiv2021

Synchronous X-ray/Optical QPOs from the Black Hole LMXB MAXI J1820+070

We present high-speed optical photometry from SAAO and SALT on the black hole LMXB \MAXI (ASSASN-18ey), some of it simultaneous with NICER, Swift and Insight-HXMT X-ray coverage. We detect optical Quasi-Periodic Oscillations (QPOs) that move to higher frequencies as the outburst progresses, tracking both the frequency and evolution of similar X-ray QPOs previously reported. Correlated X-ray/optical data reveal a complex pattern of lags, including an anti-correlation and a sub-second lag that evolve over the first few weeks of outburst. They also show correlated components separated by a lag equal to the QPO period roughly centered on zero lag, implying that the inter-band variability is strongly and consistently affected by these QPOs at a constant phase lag of roughly +/- pi. The synchronisation of X-ray and optical QPOs indicates that they must be produced in regions physically very close to each other; we thus propose that they can be explained by a precessing jet model, based on analogies with V404 Cyg and MAXI J1348-630.

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arXiv2021

On the nature of the X-ray pulsar XTE J1859+083 and its broadband properties

This work is devoted to the study of the broadband 0.8-79 keV spectral and timing properties of the poorly studied X-ray pulsar XTE J1859+083 during its 2015 outburst based on the data from the NuSTAR and Swift observatories. We show that the source pulse profile has complex shape that depends on the energy band. Pulse fraction of XTE J1859+083 has constant value around 35% in the broad energy band, this behaviour is atypical for X-ray pulsars. At the same time its energy spectrum is typical of this class of objects and has a power-law shape with an exponential cutoff at high energies. No cyclotron absorption line was discovered in the source spectrum. On the basis of indirect method and the absence of a cyclotron line, an estimation was made for the magnetic field strength as less than $5\times10^{11}$ G or belonging to the interval from $5\times10^{12}$ to $2.0^{+0.9}_{-1.2}\times10^{13}$ G. Data from the NOT and SALT telescopes as well as optical and IR sky surveys allowed us also to study the nature of its optical companion. We have proposed and studied new possible candidates for the optical companion of XTE J1859+083 and the most likely candidate was identified. The results of the optical and IR photometry and spectroscopy of these possible companions showed that the system is a Be X-ray binary, showing Br$γ$, He I and strong H$α$ spectral lines.

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arXiv2021

Large optical modulations during 2018 outburst of MAXI J1820+070 reveal evolution of warped accretion disc through X-ray state change

The black-hole X-ray transient MAXI J1820+07 (=ASSASN-18ey) discovered in March 2018 was one of the optically brightest ever seen, which has resulted in very detailed optical outburst light-curves being obtained. We combine them here with X-ray and radio light-curves to show the major geometric changes the source undergoes. We present a detailed temporal analysis that reveals the presence of remarkably high amplitude (>0.5 mag) modulations, which evolve from the superhump (16.87 h) period towards the presumed orbital (16.45 h) period. These modulations appear ~87d after the outburst began, and follow the Swift/BAT hard X-ray light-curve, which peaks 4 days before the radio flare and jet ejection, when the source undergoes a rapid hard to soft state transition. The optical modulation then moves closer to the orbital period, with a light curve peak that drifts slowly in orbital phase from ~0.8 to ~0.3 during the soft state. We propose that the unprecedentedly large amplitude modulation requires a warp in the disc in order to provide a large enough radiating area, and for the warp to be irradiation-driven. Its sudden turn-on implies a change in the inner disc geometry that raises the hard X-ray emitting component to a height where it can illuminate the warped outer disc regions.

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arXiv2021

Identification of SRGt 062340.2-265715 as a bright, strongly variable, novalike cataclysmic variable

We report the identification and follow-up of the transient SRG 062340.2-265715 detected with both instruments on board the Spektrum-Roentgen-Gamma mission. Optical spectroscopy of the G=12.5 counterpart firmly classifies the object as a novalike cataclysmic variable (CV) at a distance of 495 pc. A highly significant TESS period of 3.941 hours, tentatively identified with the orbital period of the binary, could not be found when the object was reobserved with TESS two years later. The newer high-cadence TESS data revealed quasi-periodic oscillations around 25 min, while ground-based photometry indicated periodic variability at 32 min. Located in very sparsely populated regions of color-magnitude diagrams involving X-ray and optical magnitudes and colors, the new object could be an X-ray underluminous magnetic CV, an intermediate polar, or an overluminous nonmagnetic CV. The lack of uniquely identified spin and orbital periods prevents a final classification. The site of X-ray production in the system, L(X, bol) = 4.8 x 10^{32} erg/s, remains to be understood given its high variability on long and short timescales.

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arXiv2021

Discovery of eRASSt J192932.9-560346: a bright, two-pole accreting, eclipsing polar

We report the discovery of a bright (V ~ 15), eclipsing, two-pole accreting magnetic cataclysmic variable (CV), a polar, as counterpart of the eROSITA and Gaia transients eRASSt 192932.9-560346 and Gaia21bxo. Frequent large amplitude changes of its brightness at X-ray and optical wavelengths by more than 4 magnitudes was indicative of a CV nature of the source. Identification spectra obtained with the 10m SALT telescope revealed the typical features of a magnetic CV, strong, broad HeI, HeII and hydrogen Balmer emission lines superposed on a blue continuum. Time-resolved photoelectric polarimetry revealed circular polarization to vary from -20% to +20%, and linear polarization from 0% to 10% confirming the system to be magnetic CV of the polar subclass. High cadence photometry revealed deep, structured eclipses, indicating that the system is a two pole accretor. The orbital period determined from the eclipse times is 92.5094 +- 0.0002 min. The X-ray spectrum is thermal only and the implied luminosity is L_X=2.2 x 10^(31) erg/s at the Gaia-determined distance of 376 pc.

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arXiv2021

Progenitor mass constraints for the type Ib intermediate-luminosity SN 2015ap and the highly extinguished SN 2016bau

Photometric and spectroscopic analyses of the intermediate-luminosity Type Ib supernova (SN) 2015ap and of the heavily reddened Type Ib SN~2016bau are discussed. Photometric properties of the two SNe, such as colour evolution, bolometric luminosity, photospheric radius, temperature, and velocity evolution, are also constrained. The ejecta mass, synthesised nickel mass, and kinetic energy of the ejecta are calculated from their light-curve analysis. We also model and compare the spectra of SN~2015ap and SN~2016bau at various stages of their evolution. The P~Cygni profiles of various lines present in the spectra are used to determine the velocity evolution of the ejecta. To account for the observed photometric and spectroscopic properties of the two SNe, we have computed 12\,$M_\odot$ zero-age main sequence (ZAMS) star models and evolved them until the onset of core collapse using the publicly available stellar-evolution code {\tt MESA}. Synthetic explosions were produced using the public version of {\tt STELLA} and another publicly available code, {\tt SNEC}, utilising the {\tt MESA} models. {\tt SNEC} and {\tt STELLA} provide various observable properties such as the bolometric luminosity and velocity evolution. The parameters produced by {\tt SNEC}/{\tt STELLA} and our observations show close agreement with each other, thus supporting a 12\,$M_\odot$ ZAMS star as the possible progenitor for SN~2015ap, while the progenitor of SN~2016bau is slightly less massive, being close to the boundary between SN and non-SN as the final product.

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