arXiv subjects

Stuart P. Littlefair

Publications and source records attributed to Stuart P. Littlefair.

At least 19 recordsLinked to original sources

arXiv2026

Optical observations of candidate host galaxies of eight fast X-ray transients

Fast X-ray transients (FXTs) are extragalactic flashes of X-rays with a typical duration of minutes to hours for which a variety of origins has been proposed and observed. To decipher the origin of FXTs, particularly those lacking multi-wavelength counterparts, we aim to understand their energetics and environments. We present deep optical ground-based observations of the positions of eight FXTs in order to try and identify and characterize candidate host galaxies. We use their properties to discriminate between possible progenitor scenarios. We identify candidate host galaxies for Swift~J050400.2+673405, XRT140507, XRT040610, XRT151121, XRT191127 and EP240708a. For each candidate, we infer the spectroscopic or photometric redshift, stellar mass, star formation rate, metallicity and stellar population age by fitting our data with the spectral energy distribution fitting code BAGPIPES. We re-identify XRT191223 as a Galactic stellar flare. For several FXTs, there are multiple candidate host galaxies, which complicates deriving constraints on the origin of the FXT. Assuming association with (one of) those candidates, all are consistent with a (non-)relativistic white dwarf - intermediate mass black hole tidal disruption event (WD-IMBH TDE) and a binary neutron star (BNS) merger. Two are consistent with a supernova shock breakout and only EP240708a with cocoon emission from a long gamma-ray burst. We also discuss the possibility that the host galaxies remain undetected in our observations. We conclude that FXTs detected by Chandra and XMM-Newton are likely to arise from a variety of origins, and we discuss that part of this population differs from FXTs detected by Einstein Probe many of which appear consistent with a collapsar scenario.

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arXiv2026

An eclipsing 8.56 minute orbital period mass-transferring binary

We report the discovery of ATLAS J101342.5-451656.8 (hereafter ATLAS J1013-4516), an 8.56 minute orbital period mass transferring AM Canum Venaticorum binary with mean Gaia magnitude G=19.51. The system was identified via periodic variability in Asteroid Terrestrial-impact Last Alert System light curves of Gaia white dwarf candidates. Follow-up spectroscopy with the Large Lenslet Array Magellan Spectrograph reveals a helium dominated accretion disk, while high speed ULTRACAM photometry shows pronounced primary and secondary eclipses. We construct a decade long orbital timing baseline using ATLAS and Gaia survey photometry together with high speed observations from ULTRACAM on the NTT and proto Lightspeed on the Magellan Clay telescope. From this baseline we measure an orbital period derivative Pdot = -1.60 +/- 0.07 x 10^-12 seconds per second. Interpreted in the context of stable mass transfer, the magnitude and sign of Pdot indicate orbital evolution governed by the interplay between gravitational wave driven angular momentum losses and mass transfer, directly probing the donor star structural response to mass loss. Assuming angular momentum loss dominated by gravitational radiation, we constrain the component masses and infer the characteristic gravitational wave strain. We predict a four year Laser Interferometer Space Antenna signal to noise ratio greater than 10, establishing ATLAS J1013-4516 as a strong prospective space based gravitational wave source that probes long term orbital evolution in the mass transferring regime.

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arXiv2025

EP240801a/XRF 240801B: An X-ray Flash Detected by the Einstein Probe and Implications of its Multiband Afterglow

We present multiband observations and analysis of EP240801a, a low-energy, extremely soft gamma-ray burst (GRB) discovered on August 1, 2024 by the Einstein Probe (EP) satellite, with a weak contemporaneous signal also detected by Fermi/GBM. Optical spectroscopy of the afterglow, obtained by GTC and Keck, identified the redshift of $z = 1.6734$. EP240801a exhibits a burst duration of 148 s in X-rays and 22.3 s in gamma-rays, with X-rays leading by 80.61 s. Spectral lag analysis indicates the gamma-ray signal arrived 8.3 s earlier than the X-rays. Joint spectral fitting of EP/WXT and Fermi/GBM data yields an isotropic energy $E_{\gamma,\rm{iso}} = (5.57^{+0.54}_{-0.50})\times 10^{51}\,\rm{erg}$, a peak energy $E_{\rm{peak}} = 14.90^{+7.08}_{-4.71}\,\rm{keV}$, a fluence ratio $\rm S(25-50\,\rm{keV})/S(50-100\,\rm{keV}) = 1.67^{+0.74}_{-0.46}$, classifying EP240801a as an X-ray flash (XRF). The host-galaxy continuum spectrum, inferred using Prospector, was used to correct its contribution for the observed outburst optical data. Unusual early $R$-band behavior and EP/FXT observations suggest multiple components in the afterglow. Three models are considered: two-component jet model, forward-reverse shock model and forward-shock model with energy injection. Both three provide reasonable explanations. The two-component jet model and the energy injection model imply a relatively small initial energy and velocity of the jet in the line of sight, while the forward-reverse shock model remains typical. Under the two-component jet model, EP240801a may resemble GRB 221009A (BOAT) if the bright narrow beam is viewed on-axis. Therefore, EP240801a can be interpreted as an off-beam (narrow) jet or an intrinsically weak GRB jet. Our findings provide crucial clues for uncovering the origin of XRFs.

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arXiv2024

Expanding the ultracompacts: gravitational wave-driven mass transfer in the shortest-period binaries with accretion disks

We report the discovery of three ultracompact binary white dwarf systems hosting accretion disks, with orbital periods of 7.95, 8.68, and 13.15 minutes. This significantly augments the population of mass-transferring binaries at the shortest periods, and provides the first evidence that accretors in ultracompacts can be dense enough to host accretion disks even below 10 minutes (where previously only direct-impact accretors were known). In the two shortest-period systems, we measured changes in the orbital periods driven by the combined effect of gravitational wave emission and mass transfer; we find $\dot{P}$ is negative in one case, and positive in the other. This is only the second system measured with a positive $\dot{P}$, and it the most compact binary known that has survived a period minimum. Using these systems as examples, we show how the measurement of $\dot{P}$ is a powerful tool in constraining the physical properties of binaries, e.g. the mass and mass-radius relation of the donor stars. We find that the chirp masses of ultracompact binaries at these periods seem to cluster around $\mathcal{M}_c \sim 0.3 M_\odot$, perhaps suggesting a common origin for these systems or a selection bias in electromagnetic discoveries. Our new systems are among the highest-amplitude known gravitational wave sources in the millihertz regime, providing exquisite opportunity for multi-messenger study with future space-based observatories such as \textit{LISA} and TianQin; we discuss how such systems provide fascinating laboratories to study the unique regime where the accretion process is mediated by gravitational waves.

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arXiv2024

The Einstein Probe transient EP240414a: Linking Fast X-ray Transients, Gamma-ray Bursts and Luminous Fast Blue Optical Transients

Detections of fast X-ray transients (FXTs) have been accrued over the last few decades. However, their origin has remained mysterious. There is now rapid progress thanks to timely discoveries and localisations with the Einstein Probe mission. Early results indicate that FXTs may frequently, but not always, be associated with gamma-ray bursts (GRBs). Here, we report on the multi-wavelength counterpart of FXT EP240414a, which has no reported gamma-ray counterpart. The transient is located 25.7~kpc in projection from a massive galaxy at $z=0.40$. We perform comprehensive photometric and spectroscopic follow-up. The optical light curve shows at least three distinct emission episodes with timescales of $\sim 1, 4$ and 15 days and peak absolute magnitudes of $M_R \sim -20$, $-21$, and $-19.5$, respectively. The optical spectrum at early times is extremely blue, inconsistent with afterglow emission. It may arise from the interaction of both jet and supernova shock waves with the stellar envelope and a dense circumstellar medium, as has been suggested for some Fast Blue Optical Transients (LFBOTs). At late times, the spectrum evolves to a broad-lined~Type~Ic supernova, similar to those seen in collapsar long-GRBs. This implies that the progenitor of EP240414a is a massive star creating a jet-forming supernova inside a dense envelope, resulting in an X-ray outburst with a luminosity of $\sim 10^{48}$ erg s$^{-1}$, and the complex observed optical/IR light curves. If correct, this argues for a causal link between the progenitors of long-GRBs, FXTs and LFBOTs.

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arXiv2024

The only inflated brown dwarf in an eclipsing white dwarf-brown dwarf binary: WD1032+011B

Due to their short orbital periods and relatively high flux ratios, irradiated brown dwarfs in binaries with white dwarfs offer better opportunities to study irradiated atmospheres than hot Jupiters, which have lower planet-to-star flux ratios. WD1032+011 is an eclipsing, tidally locked white dwarf-brown dwarf binary with a 9950 K white dwarf orbited by a 69.7 M$_{Jup}$ brown dwarf in a 0.09 day orbit. We present time-resolved Hubble Space Telescope Wide Field Camera 3 spectrophotometric data of WD1032+011. We isolate the phase-dependent spectra of WD1032+011B, finding a 210 K difference in brightness temperature between the dayside and nightside. The spectral type of the brown dwarf is identified as L1 peculiar, with atmospheric retrievals and comparison to field brown dwarfs showing evidence for a cloud-free atmosphere. The retrieved temperature of the dayside is $1748^{+66}_{-67}$ K, with a nightside temperature of $1555^{+76}_{-62}$ K, showing an irradiation-driven temperature contrast coupled with inefficient heat redistribution from the dayside to the nightside. The brown dwarf radius is inflated, likely due to the constant irradiation from the white dwarf, making it the only known inflated brown dwarf in an eclipsing white dwarf-brown dwarf binary.

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arXiv2024

The fast X-ray transient EP240315a: a z ~ 5 gamma-ray burst in a Lyman continuum leaking galaxy

The nature of the minute-to-hour long Fast X-ray Transients (FXTs) localised by telescopes such as Chandra, Swift, and XMM-Newton remains mysterious, with numerous models suggested for the events. Here, we report multi-wavelength observations of EP240315a, a 1600 s long transient detected by the Einstein Probe, showing it to have a redshift of z=4.859. We measure a low column density of neutral hydrogen, indicating that the event is embedded in a low-density environment, further supported by direct detection of leaking ionising Lyman-continuum. The observed properties are consistent with EP240315a being a long-duration gamma-ray burst, and these observations support an interpretation in which a significant fraction of the FXT population are lower-luminosity examples of similar events. Such transients are detectable at high redshifts by the Einstein Probe and, in the (near) future, out to even larger distances by SVOM, THESEUS, and Athena, providing samples of events into the epoch of reionisation.

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arXiv2024

Using Gaussian Processes to detect AGN flares

A key feature of active galactic nuclei (AGN) is their variability across all wavelengths. Typically, AGN vary by a few tenths of a magnitude or more over periods lasting from hours to years. By contrast, extreme variability of AGN -- large luminosity changes that are a significant departure from the baseline variability -- are known as AGN flares. These events are rare and their timescales poorly constrained, with most of the literature focusing on individual events. It has been suggested that extreme AGN variability including flares can provide insights into the accretion processes in the disk. With surveys such as the Legacy Survey of Space and Time (LSST) promising millions of transient detections per night in the coming decade, there is a need for fast and efficient classification of AGN flares. The problem with the systematic detection of AGN flares is the requirement to detect them against a stochastically variable baseline; the ability to define a signal as a significant departure from the ever-present variability is a statistical challenge. Recently, Gaussian Processes (GPs) have revolutionised the analysis of time-series data in many areas of astronomical research. They have, however, seen limited uptake within the field of transient detection and classification. Here we investigate the efficacy of Gaussian Processes to detect AGN flares in both simulated and real optical light curves. We show that GP analysis can successfully detect AGN flares with a false-positive rate of less than seven per cent, and we present examples of AGN light curves that show extreme variability.

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arXiv2023

Measuring the Initial-Final Mass-Relation using wide double white dwarf binaries from Gaia DR3

The Initial-Final Mass-Relation (IFMR) maps the masses of main sequence stars to their white dwarf descendants. The most common approach to measure the IFMR has been to use white dwarfs in clusters. However, it has been shown that wide double white dwarfs can also be used to measure the IFMR using a Bayesian approach. We have observed a large sample of 90 Gaia double white dwarfs using FORS2 on the VLT. Considering 52 DA+DA, DA+DC, and DC+DC pairs, we applied our extended Bayesian framework to probe the IFMR in exquisite detail. Our monotonic IFMR is well constrained by our observations for initial masses of 1-5 Msun, with the range 1-4 Msun mostly constrained to a precision of 0.03 Msun or better. We add an important extension to the framework, using a Bayesian mixture-model to determine the IFMR robustly in the presence of systems departing from single star evolution. We find a large but uncertain outlier fraction of 59$\pm$21 percent, with outlier systems requiring an additional $0.70^{+0.40}_{-0.22}$ Gyr uncertainty in their cooling age differences. However, we find that this fraction is dominated by a few systems with massive components near 0.9 Msun, where we are most sensitive to outliers, but are also able to establish four systems as merger candidates

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arXiv2023

A rotating white dwarf shows different compositions on its opposite faces

White dwarfs, the extremely dense remnants left behind by most stars after their death, are characterised by a mass comparable to that of the Sun compressed into the size of an Earth-like planet. In the resulting strong gravity, heavy elements sink toward the centre and the upper layer of the atmosphere contains only the lightest element present, usually hydrogen or helium. Several mechanisms compete with gravitational settling to change a white dwarf's surface composition as it cools, and the fraction of white dwarfs with helium atmospheres is known to increase by a factor ~2.5 below a temperature of about 30,000 K; therefore, some white dwarfs that appear to have hydrogen-dominated atmospheres above 30,000 K are bound to transition to be helium-dominated as they cool below it. Here we report observations of ZTF J203349.8+322901.1, a transitioning white dwarf with two faces: one side of its atmosphere is dominated by hydrogen and the other one by helium. This peculiar nature is likely caused by the presence of a small magnetic field, which creates an inhomogeneity in temperature, pressure or mixing strength over the surface. ZTF J203349.8+322901.1 might be the most extreme member of a class of magnetic, transitioning white dwarfs -- together with GD 323, a white dwarf that shows similar but much more subtle variations. This new class could help shed light on the physical mechanisms behind white dwarf spectral evolution.

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arXiv2023

Photometric follow-up of 43 new eclipsing white dwarf plus main-sequence binaries from the ZTF survey

Wide-field time-domain photometric sky surveys are now finding hundreds of eclipsing white dwarf plus M dwarf binaries, a population encompassing a wealth of information and potential insight into white dwarf and close binary astrophysics. Precise follow-up observations are essential in order to fully constrain these systems and capitalise on the power of this sample. We present the first results from our program of high-speed, multi-band photometric follow-up. We develop a method to measure temperatures, (model-dependent) masses, and radii for both components from the eclipse photometry alone and characterize 34 white dwarf binaries, finding general agreement with independent estimates using an alternative approach while achieving around a factor of two increase in parameter precision. In addition to these parameter estimates, we discover a number of interesting systems -- finding four with sub-stellar secondaries, doubling the number of eclipsing examples, and at least six where we find the white dwarf to be strongly magnetic, making these the first eclipsing examples of such systems and key to investigating the mechanism of magnetic field generation in white dwarfs. We also discover the first two pulsating white dwarfs in detached and eclipsing post-common-envelope binaries -- one with a low-mass, likely helium core, and one with a relatively high mass, towards the upper end of the known sample of ZZ Cetis. Our results demonstrate the power of eclipse photometry, not only as a method of characterising the population, but as a way of discovering important systems that would have otherwise been missed by spectroscopic follow-up.

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arXiv2022

Characterising eclipsing white dwarf M dwarf binaries from multi-band eclipse photometry

With the prevalence of wide-field, time-domain photometric sky surveys, the number of eclipsing white dwarf systems being discovered is increasing dramatically. An efficient method to follow these up will be key to determining any population trends and finding any particularly interesting examples. We demonstrate that multi-band eclipse photometry of binaries containing a white dwarf and an M~dwarf can be used to determine the masses and temperatures of the white dwarfs to better than 5 per cent. For the M~dwarfs we measure their parameters to a precision of better than 6 per cent with the uncertainty dominated by the intrinsic scatter of the M~dwarf mass-radius relationship. This precision is better than what can typically be achieved with low-resolution spectroscopy. The nature of this method means that it will be applicable to LSST data in the future, enabling direct characterisation without follow-up spectroscopy. Additionally, we characterise three new post-common-envelope binaries from their eclipse photometry, finding two systems containing hot helium-core white dwarfs with low-mass companions (one near the brown dwarf transition regime) and a possible detached cataclysmic variable at the lower edge of the period gap.

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arXiv2021

The SNIa Runaway LP 398-9: Detection of Circumstellar Material and Surface Rotation

A promising progenitor scenario for Type Ia supernovae (SNeIa) is the thermonuclear detonation of a white dwarf in a close binary system with another white dwarf. After the primary star explodes, the surviving donor can be spontaneously released as a hypervelocity runaway. One such runaway donor candidate is LP 398-9, whose orbital trajectory traces back $\approx 10^5$ years to a known supernova remnant. Here we report the discovery of carbon-rich circumstellar material around LP 398-9, revealed by a strong infrared excess and analyzed with follow-up spectroscopy. The circumstellar material is most plausibly composed of inflated layers from the star itself, mechanically and radioactively heated by the past companion's supernova. We also detect a 15.4 hr periodic signal in the UV and optical light curves of LP 398-9, which we interpret as surface rotation. The rotation rate is consistent with theoretical predictions from this supernova mechanism, and the brightness variations could originate from surface inhomogeneity deposited by the supernova itself. Our observations strengthen the case for this double-degenerate SNIa progenitor channel, and motivate the search for more runaway SNIa donors.

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arXiv2020

A pulsating white dwarf in an eclipsing binary

White dwarfs are the burnt out cores of Sun-like stars and are the final fate of 97% of all stars in our Galaxy. The internal structure and composition of white dwarfs are hidden by their high gravities, which causes all elements, apart from the lightest ones, to settle out of their atmospheres. The most direct method to probe the inner structure of stars and white dwarfs in detail is via asteroseismology. Here we present the first known pulsating white dwarf in an eclipsing binary system, enabling us to place extremely precise constraints on the mass and radius of the white dwarf from the light curve, independent of the pulsations. This 0.325M$_\odot$ white dwarf --- one member of SDSS J115219.99+024814.4 --- will serve as a powerful benchmark to constrain empirically the core composition of low-mass stellar remnants and investigate the effects of close binary evolution on the internal structure of white dwarfs.

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arXiv2019

The Type II-Plateau Supernova 2017eaw in NGC 6946 and Its Red Supergiant Progenitor

We present extensive optical photometric and spectroscopic observations, from 4 to 482 days after explosion, of the Type II-plateau (II-P) supernova (SN) 2017eaw in NGC 6946. SN 2017eaw is a normal SN II-P intermediate in properties between, for example, SN 1999em and SN 2012aw and the more luminous SN 2004et, also in NGC 6946. We have determined that the extinction to SN 2017eaw is primarily due to the Galactic foreground and that the SN site metallicity is likely subsolar. We have also independently confirmed a tip-of-the-red-giant-branch (TRGB) distance to NGC 6946 of 7.73+/-0.78 Mpc. The distances to the SN that we have also estimated via both the standardized candle method and expanding photosphere method corroborate the TRGB distance. We confirm the SN progenitor identity in pre-explosion archival Hubble Space Telescope (HST) and Spitzer Space Telescope images, via imaging of the SN through our HST Target of Opportunity program. Detailed modeling of the progenitor's spectral energy distribution indicates that the star was a dusty, luminous red supergiant consistent with an initial mass of ~15 Msuns.

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arXiv2019

Accurate mass and radius determinations of a cool subdwarf in an eclipsing binary

Cool subdwarfs are metal-poor low-mass stars that formed during the early stages of the evolution of our Galaxy. Because they are relatively rare in the vicinity of the Sun, we know of few cool subdwarfs in the solar neighbourhood, and none with both the mass and the radius accurately determined. This hampers our understanding of stars at the low-mass end of the main-sequence. Here we report the discovery of SDSSJ235524.29+044855.7 as an eclipsing binary containing a cool subdwarf star, with a white dwarf companion. From the light-curve and the radial-velocity curve of the binary we determine the mass and the radius of the cool subdwarf and we derive its effective temperature and luminosity by analysing its spectral energy distribution. Our results validate the theoretical mass-radius-effective temperature-luminosity relations for low-mass low-metallicity stars.

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arXiv2019

Weighing Melnick 34: the most massive binary system known

Here we confirm Melnick 34, an X-ray bright star in the 30 Doradus region of the Large Magellanic Cloud, as an SB2 binary comprising WN5h+WN5h components. We present orbital solutions using 26 epochs of VLT/UVES spectra and 22 epochs of archival Gemini/GMOS spectra. Radial-velocity monitoring and automated template fitting methods both reveal a similar high eccentricity system with a mass ratio close to unity, and an orbital period in agreement with the 155.1 +/- 1 day X-ray light curve period previously derived by Pollock et al. Our favoured solution derived an eccentricity of 0.68 +/- 0.02 and mass ratio of 0.92 +/- 0.07, giving minimum masses of Ma_sin^{3}(i) = 65 +/- 7 Msun and Mb_sin^{3}(i) = 60 +/- 7 Msun. Spectral modelling using WN5h templates with CMFGEN reveals temperatures of T ~53 kK for each component and luminosities of log(La/Lsun) = 6.43 +/- 0.08 and log(Lb/Lsun) = 6.37 +/- 0.08, from which BONNSAI evolutionary modelling gives masses of Ma = 139 (+21,-18) Msun and Mb = 127 (+17,-17) Msun and ages of ~0.6 Myrs. Spectroscopic and dynamic masses would agree if Mk34 has an inclination of i ~50°, making Mk34 the most massive binary known and an excellent candidate for investigating the properties of colliding wind binaries. Within 2-3 Myrs, both components of Mk34 are expected to evolve to stellar mass black holes which, assuming the binary system survives, would make Mk34 a potential binary black hole merger progenitor and gravitational wave source.

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arXiv2017

An elevation of 0.1 light-seconds for the optical jet base in an accreting Galactic black hole system

Relativistic plasma jets are observed in many accreting black holes. According to theory, coiled magnetic fields close to the black hole accelerate and collimate the plasma, leading to a jet being launched. Isolating emission from this acceleration and collimation zone is key to measuring its size and understanding jet formation physics. But this is challenging because emission from the jet base cannot be easily disentangled from other accreting components. Here, we show that rapid optical flux variations from a Galactic black-hole binary are delayed with respect to X-rays radiated from close to the black hole by ~0.1 seconds, and that this delayed signal appears together with a brightening radio jet. The origin of these sub-second optical variations has hitherto been controversial. Not only does our work strongly support a jet origin for the optical variations, it also sets a characteristic elevation of <~10$^3$ Schwarzschild radii for the main inner optical emission zone above the black hole, constraining both internal shock and magnetohydrodynamic models. Similarities with blazars suggest that jet structure and launching physics could potentially be unified under mass-invariant models. Two of the best-studied jetted black hole binaries show very similar optical lags, so this size scale may be a defining feature of such systems.

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