arXiv subjects

D. Buckley

Publications and source records attributed to D. Buckley.

At least 19 recordsLinked to original sources

arXiv2025

Accretion and Ejection Physics at High Time Resolution

Accretion onto compact objects is one of the most fundamental phenomena in the astrophysics, powering some of the most luminous objects in the sky. Along with this, accretion has also a key impact on the evolution of the Universe, through the launch of powerful outflows that affect the surrounding medium. In the last years sub-second optical-infrared observations of accreting X-ray binaries have opened a new window in the study of inflow-outflow connection, discovering a wide range of previously unkown phenomena. Here we review the key open questions in accretion and ejection physics and discuss how a dedicated facility, equipped with photon-counting detectors and high spectral resolution from the UV to the mid-infrared, can enable transformative advances in our understanding of accretion processes.

astro-ph.IM↗

arXiv2025

A low mass, binary-stripped envelope for the Type IIb SN 2024abfo

Type IIb supernovae (SNe) are a transitional subclass of stripped-envelope SNe showing hydrogen lines in their spectra that gradually weaken and give way to helium lines reminiscent of SNe Ib, which is indicative of stripping through stellar winds or binary interaction. SN 2024abfo is the seventh SN IIb with a direct progenitor detection. We find that the position of the supernova in our ERIS adaptive optics imaging agrees with the progenitor position in archival HST imaging to within 19 mas. The progenitor SED is consistent with an A5 giant. Single star models predict an initial mass in the range 11 to 15 solar masses, while the most probable binary model is a 12+1.2 solar mass system with an initial period of 1.73 years. We also find significant evidence for variability of the progenitor candidate in the years prior to core-collapse. SN 2024abfo is the least luminous SN IIb with a direct progenitor detections. At late times the r-band light curve decays more slowly than the comparison SNe, which may be due to increased gamma-ray trapping. Similar to SN 2008ax, SN 2024abfo does not show a prominent double-peaked light curve. Our semi-analytic light curve modelling shows that this may be due to a very low mass of hydrogen in the outer envelope. Spectrally, SN 2024abfo is most similar to SN 2008ax at early times while at later times (80 days) it appears to show persistent Halpha absorption compared to the comparison sample. We prefer a binary system to explain the supernova and its progenitor, although we are unable to rule out single-star models. We recommend late-time observations to search for a binary companion and signatures of CSM-interaction. The absence of these features would support the hypothesis that SN 2024abfo resulted from a system which underwent a period of binary mass transfer well before (1000 yr) the explosion, resulting in a low-mass hydrogen-rich envelope.

astro-ph.HE↗

arXiv2025

XMM-Newton follow-up of two eROSITA X-ray binary candidates

We report on the follow-up observations with XMM-Newton of two X-ray binary candidates identified in the first eROSITA all-sky survey data (eRASS1), 1eRASS J061330.8+160440 and 1eRASS J161201.9-464622. Based on the obtained results, in particular, the observed X-ray spectra and lack of pulsations, as well as properties of the identified optical counterparts, we conclude that both candidates are unlikely to be XRBs. Based on LAMOST optical spectroscopy and SED fit results for 1eRASS J061330.8+160440 we classify it as an M0 chromospherically active subgiant star. ZTF and TESS photometry reveal highly significant period for this object of 7.189 days, which likely attributed to starspot(s). On the other hand, SALT follow-up spectroscopy of 1eRASS J161201.9-464622 solidly classifies this source as a bright novalike cataclysmic variable (CV), the second discovered with eROSITA. A persistent 4.802 h signal is found across all three available TESS observations, and is tentatively identified as the orbital period of the binary. Follow-up high-speed photometry and time-resolved spectroscopy are required to confirm the derived orbital modulation.

astro-ph.HE↗

arXiv2025

SRG/eROSITA No. 5: Discovery of quasi-periodic eruptions every ~3.7 days from a galaxy at z>0.1

Quasi-periodic eruptions (QPEs) are repeating soft X-ray bursts from the nuclei of galaxies, tantalizingly proposed to be extreme mass ratio inspirals. Here, we report the discovery of a new galaxy showing X-ray QPEs, the fifth found through a dedicated blind search in the \emph{SRG}/eROSITA all-sky survey data, hereafter named eRO-QPE5. Its QPE duration ($t_{\rm dur}\sim0.6$\,d), recurrence time ($t_{\rm recur}\sim3.7\,$d), integrated energy per eruption ($\sim3.4 \times 10^{47}\,$erg), and black hole mass ($M_{\rm BH}=2.9^{+5.4}_{-2.2}\times10^7\,M_{\astrosun}$) sit at the high end of the known population. Like other eROSITA or X-ray-discovered QPEs, no previous or concurrent optical-IR transient is found in archival photometric datasets, and the optical spectrum looks almost featureless. With a spectroscopic redshift of $0.1155$, eRO-QPE5 is the most distant QPE source discovered to date. Given the number of recent discoveries, we test for possible correlations and confirm a connection between $t_{\rm dur}$ and $t_{\rm recur}$, while we do not find any significant correlation involving either $M_{\rm BH}$ or the QPE temperature. The slope of the $t_{\rm dur}-t_{\rm recur}$ relation ($1.14\pm0.16$) is roughly consistent with predictions from star-disk collision models, with a preference for those that suggest that QPEs are powered by stellar debris streams around the orbiter. Considering this and previous discoveries, eROSITA has proved extremely successful in finding many QPE candidates given its grasp, namely its sensitivity and large field of view, and scanning capabilities over the full sky. We advocate the need of sensitive wide-area and time-domain oriented surveys from future-generation soft X-ray missions.

astro-ph.HE↗

arXiv2024

Characterization of a peculiar Einstein Probe transient EP240408a: an exotic gamma-ray burst or an abnormal jetted tidal disruption event?

We present the results of our multi-wavelength (X-ray to radio) follow-up campaign of the Einstein Probe transient EP240408a. The initial 10 s trigger displayed bright soft X-ray (0.5-4 keV) radiation with peak luminosity $L_\textrm{X} \gtrsim 10^{49}$ ($10^{50}$) erg s$^{-1}$ for an assumed redshift z>0.5 (2.0). The Neil Gehrels Swift Observatory and Neutron star Interior Composition ExploreR discovered a fading X-ray counterpart lasting for $\sim$5 d (observer frame), which showed a long-lived (~4 d) plateau-like emission ($t^{-0.5}$) before a sharp powerlaw decline ($t^{-7}$). The plateau emission was in excess of $L_\textrm{X} \gtrsim 10^{46}$ ($10^{47}$) erg s$^{-1}$ at z>0.5 (2.0). Deep optical and radio observations resulted in non-detections of the transient. Our observations with Gemini South revealed a faint potential host galaxy ($r \approx 24$ AB mag) near the edge of the X-ray localization. The faint candidate host, and lack of other potential hosts ($r \gtrsim 26$ AB mag; $J \gtrsim 23$ AB mag), implies a higher redshift origin (z>0.5), which produces extreme X-ray properties that are inconsistent with many known extragalactic transient classes. In particular, the lack of a bright gamma-ray counterpart, with the isotropic-equivalent energy ($10 - 10,000$ keV) constrained by GECam and Konus-Wind to $E_{\gamma,\textrm{iso}} \lesssim 4\times10^{51}$ ($6\times10^{52}$) erg at z>0.5 (2.0), conflicts with known gamma-ray bursts (GRBs) of similar X-ray luminosities. We therefore favor a jetted tidal disruption event (TDE) as the progenitor of EP240408a at z>1.0, possibly caused by the disruption of a white dwarf by an intermediate mass black hole. The alternative is that EP240408a may represent a new, previously unknown class of transient.

astro-ph.HE↗

arXiv2024

Massive stars exploding in a He-rich circumstellar medium $-$ X. Flash spectral features in the Type Ibn SN 2019cj and observations of SN 2018jmt

We present optical and near-infrared observations of two Type Ibn supernovae (SNe), SN 2018jmt and SN 2019cj. Their light curves have rise times of about 10 days, reaching an absolute peak magnitude of $M_g$(SN 2018jmt) = $-$19.07 $\pm$ 0.37 and $M_V$(SN 2019cj) = $-$18.94 $\pm$ 0.19 mag, respectively. The early-time spectra of SN 2018jmt are dominated by a blue continuum, accompanied by narrow (600$-$1000 km~s$^{-1}$) He I lines with P-Cygni profile. At later epochs, the spectra become more similar to those of the prototypical SN Ibn 2006jc. At early phases, the spectra of SN 2019cj show flash ionisation emission lines of C III, N III and He II superposed on a blue continuum. These features disappear after a few days, and then the spectra of SN 2019cj evolve similarly to those of SN 2018jmt. The spectra indicate that the two SNe exploded within a He-rich circumstellar medium (CSM) lost by the progenitors a short time before the explosion. We model the light curves of the two SNe Ibn to constrain the progenitor and the explosion parameters. The ejecta masses are consistent with either that expected for a canonical SN Ib ($\sim$ 2 M$_{\odot}$) or those from a massive WR star ($>$ $\sim$ 4 M$_{\odot}$), with the kinetic energy on the order of $10^{51}$ erg. The lower limit on the ejecta mass ($>$ $\sim$ 2 M$_{\odot}$) argues against a scenario involving a relatively low-mass progenitor (e.g., $M_{ZAMS}$ $\sim$ 10 M$_{\odot}$). We set a conservative upper limit of $\sim$0.1 M$_{\odot}$ for the $^{56}$Ni masses in both SNe. From the light curve modelling, we determine a two-zone CSM distribution, with an inner, flat CSM component, and an outer CSM with a steeper density profile. The physical properties of SN 2018jmt and SN 2019cj are consistent with those expected from the core collapse of relatively massive, stripped-envelope (SE) stars.

astro-ph.HE↗

arXiv2024

The more the merrier: SRG/eROSITA discovers two further galaxies showing X-ray quasi-periodic eruptions

X-ray quasi-periodic eruptions (QPEs) are a novel addition to the group of extragalactic transients. In this work, we report the discovery of two further galaxies showing QPEs, eRO-QPE3 and eRO-QPE4, with the eROSITA X-ray telescope on board the Spectrum Roentgen Gamma observatory. Among the properties in common with those of known QPEs are: the thermal-like spectral shape in eruption (up to $kT\sim110-120$ eV) and quiescence ($kT\sim50-90$ eV) and its evolution during the eruptions (with a harder rise than decay); the lack of strong canonical signatures of active nuclei (from current optical, UV, infrared and radio data); and the low-mass nature of the host galaxies ($\log M_*\approx 9-10$) and their massive central black holes ($\log M_{\rm BH}\approx 5-7$). These discoveries also bring several new insights into the QPE population: i) eRO-QPE3 shows eruptions on top of a decaying quiescence flux, providing further evidence for a connection between QPEs and a preceding tidal disruption event; ii) eRO-QPE3 exhibits the longest recurrence times and faintest peak luminosity of QPEs, compared to the known QPE population, excluding a correlation between the two; iii) we find evidence, for the first time, of a transient component that is harder, albeit much fainter, than the thermal QPE spectrum in eRO-QPE4; and iv) eRO-QPE4 displays the appearance (or significant brightening) of the quiescence disk component after the detection of QPEs, supporting its short-lived nature against a preexisting active galactic nucleus. Overall, the newly discovered properties (e.g., recent origin and/or transient nature of the quiescent accretion disk; lack of correlation between eruption recurrence timescales and luminosity) are qualitatively consistent with recent models that identify QPEs as extreme mass-ratio inspirals.

astro-ph.HE↗

arXiv2023

The radio to GeV picture of PSR B1259-63 during the 2021 periastron passage

PSR B1259-63 is a gamma-ray binary system with a radio pulsar orbiting an O9.5Ve star, LS 2883, with a period of ~3.4 yr. Close to the periastron the system is detected at all wavelengths, from radio to the TeV energies. The emission in this time period is believed to originate from the interaction of LS 2883 and pulsar's outflows. The observations of 4 periastra passages taken in 2010-2021 show strong correlation of the radio and X-ray lightcurves with two peaks just before and after the periastron. The observations of the latest 2021 periastron passage reveal the presence of the 3rd X-ray peak and subsequent disappearance of radio/X-ray flux correlation. In this paper we present the results of our optical, radio and X-ray observational campaigns on PSR B1259-63 performed in 2021 accompanied with the analysis of the publicly available GeV FERMI/LAT data. We compare the properties of different periastron passages, discuss the obtained results and show that they can be explained in terms of the 2-zone emission cone model proposed by us previously.

astro-ph.HE↗

arXiv2017

ThunderKAT: The MeerKAT Large Survey Project for Image-Plane Radio Transients

ThunderKAT is the image-plane transients programme for MeerKAT. The goal as outlined in 2010, and still today, is to find, identify and understand high-energy astrophysical processes via their radio emission (often in concert with observations at other wavelengths). Through a comprehensive and complementary programme of surveying and monitoring Galactic synchrotron transients (across a range of compact accretors and a range of other explosive phenomena) and exploring distinct populations of extragalactic synchrotron transients (microquasars, supernovae and possibly yet unknown transient phenomena) - both from direct surveys and commensal observations - we will revolutionise our understanding of the dynamic and explosive transient radio sky. As well as performing targeted programmes of our own, we have made agreements with the other MeerKAT large survey projects (LSPs) that we will also search their data for transients. This commensal use of the other surveys, which remains one of our key programme goals in 2016, means that the combined MeerKAT LSPs will produce by far the largest GHz-frequency radio transient programme to date.

astro-ph.HE↗

arXiv2017

Follow up of GW170817 and its electromagnetic counterpart by Australian-led observing programs

The discovery of the first electromagnetic counterpart to a gravitational wave signal has generated follow-up observations by over 50 facilities world-wide, ushering in the new era of multi-messenger astronomy. In this paper, we present follow-up observations of the gravitational wave event GW170817 and its electromagnetic counterpart SSS17a/DLT17ck (IAU label AT2017gfo) by 14 Australian telescopes and partner observatories as part of Australian-based and Australian-led research programs. We report early- to late-time multi-wavelength observations, including optical imaging and spectroscopy, mid-infrared imaging, radio imaging, and searches for fast radio bursts. Our optical spectra reveal that the transient source afterglow cooled from approximately 6400K to 2100K over a 7-day period and produced no significant optical emission lines. The spectral profiles, cooling rate, and photometric light curves are consistent with the expected outburst and subsequent processes of a binary neutron star merger. Star formation in the host galaxy probably ceased at least a Gyr ago, although there is evidence for a galaxy merger. Binary pulsars with short (100 Myr) decay times are therefore unlikely progenitors, but pulsars like PSR B1534+12 with its 2.7 Gyr coalescence time could produce such a merger. The displacement (about 2.2 kpc) of the binary star system from the centre of the main galaxy is not unusual for stars in the host galaxy or stars originating in the merging galaxy, and therefore any constraints on the kick velocity imparted to the progenitor are poor.

astro-ph.HE↗

arXiv2017

Multiwavelength observations of nova SMCN 2016-10a --- one of the brightest novae ever observed

We report on multiwavelength observations of nova SMCN 2016-10a. The present observational set is one of the most comprehensive for any nova in the Small Magellanic Cloud, including: low, medium, and high resolution optical spectroscopy and spectropolarimetry from SALT, FLOYDS, and SOAR; long-term OGLE $V$- and $I$- bands photometry dating back to six years before eruption; SMARTS optical and near-IR photometry from $\sim$ 11 days until over 280 days post-eruption; $Swift$ satellite X-ray and ultraviolet observations from $\sim$ 6 days until 319 days post-eruption. The progenitor system contains a bright disk and a main sequence or a sub-giant secondary. The nova is very fast with $t_2 \simeq$ 4.0 $\pm$ 1.0 d and $t_3 \simeq$ 7.8 $\pm$ 2.0 d in the $V$-band. If the nova is in the SMC, at a distance of $\sim$ 61 $\pm$ 10 kpc, we derive $M_{V,\mathrm{max}} \simeq - 10.5$ $\pm$ 0.5, making it the brightest nova ever discovered in the SMC and one of the brightest on record. At day 5 post-eruption the spectral lines show a He/N spectroscopic class and a FWHM of $\sim$ 3500 kms$^{-1}$ indicating moderately high ejection velocities. The nova entered the nebular phase $\sim$ 20 days post-eruption, predicting the imminent super-soft source turn-on in the X-rays, which started $\sim$ 28 days post-eruption. The super-soft source properties indicate a white dwarf mass between 1.2 M$_{\odot}$ and 1.3 M$_{\odot}$ in good agreement with the optical conclusions.

astro-ph.SR↗

arXiv2015

Early polarization observations of the optical emission of gamma-ray bursts: GRB150301B and GRB150413A

We report early optical linear polarization observations of two gamma-ray bursts made with the MASTER robotic telescope network. We found the minimum polar- ization for GRB150301B to be 8% at the beginning of the initial stage, whereas we detected no polarization for GRB150413A either at the rising branch or after the burst reached the power-law afterglow stage. This is the earliest measurement of the polarization (in cosmological rest frame) of gamma-ray bursts. The primary intent of the paper is to discover optical emission and publish extremely rare (unique) high- quality light curves of the prompt optical emission of gamma-ray bursts during the non-monotonic stage of their evolution. We report that our team has discovered the optical counterpart of one of the bursts, GRB150413A.

astro-ph.HE↗

arXiv2015

Multi-wavelength Observations of the Binary System PSR B1259-63/LS~2883 around the 2014 Periastron Passage

We report on the results of the extensive multi-wavelength campaign from optical to GeV gamma-rays of the 2014 periastron passage of PSR B1259-63, which is a unique high-mass gamma-ray emitting binary system with a young pulsar companion. Observations demonstrate the stable nature of the post-periastron GeV flare and prove the coincidence of the flare with the start of rapid decay of the H$α$ equivalent width, usually interpreted as a disruption of the Be stellar disk. Intensive X-ray observations reveal changes in the X-ray spectral behaviour happening at the moment of the GeV flare. We demonstrate that these changes can be naturally explained as a result of synchrotron cooling of monoenergetic relativistic electrons injected into the system during the GeV flare.

astro-ph.HE↗

arXiv2013

Multi-Wavelength Observations of Supernova 2011ei: Time-Dependent Classification of Type IIb and Ib Supernovae and Implications for their Progenitors

We present X-ray, UV/optical, and radio observations of the stripped-envelope, core-collapse supernova (SN) 2011ei, one of the least luminous SNe IIb or Ib observed to date. Our observations begin with a discovery within 1 day of explosion and span several months afterward. Early optical spectra exhibit broad, Type II-like hydrogen Balmer profiles that subside rapidly and are replaced by Type Ib-like He-rich features on the timescale of one week. High-cadence monitoring of this transition suggests that absorption attributable to a high velocity (> 12,000 km/s) H-rich shell is not rare in Type Ib events. Radio observations imply a shock velocity of v = 0.13c and a progenitor star mass-loss rate of 1.4 x 10^{-5} Msun yr^{-1} (assuming wind velocity v_w=10^3 km/s). This is consistent with independent constraints from deep X-ray observations with Swift-XRT and Chandra. Overall, the multi-wavelength properties of SN 2011ei are consistent with the explosion of a lower-mass (3-4 Msun), compact (R* <= 1x10^{11} cm), He core star. The star retained a thin hydrogen envelope at the time of explosion, and was embedded in an inhomogeneous circumstellar wind suggestive of modest episodic mass-loss. We conclude that SN 2011ei's rapid spectral metamorphosis is indicative of time-dependent classifications that bias estimates of explosion rates for Type IIb and Ib objects, and that important information about a progenitor star's evolutionary state and mass-loss immediately prior to SN explosion can be inferred from timely multi-wavelength observations.

astro-ph.HE↗

arXiv2013

Abell 48 - a rare WN-type central star of a planetary nebula

A considerable fraction of the central stars of planetary nebulae (CSPNe) are hydrogen-deficient. Almost all of these H-deficient central stars (CSs) display spectra with strong carbon and helium lines. Most of them exhibit emission line spectra resembling those of massive WC stars. Therefore these stars are classed as CSPNe of spectral type [WC]. Recently, quantitative spectral analysis of two emission-line CSs, PB 8 and IC 4663, revealed that these stars do not belong to the [WC] class. Instead PB 8 has been classified as [WN/WC] type and IC 4663 as [WN] type. In this work we report the spectroscopic identification of another rare [WN] star, the CS of Abell 48. We performed a spectral analysis of Abell 48 with the Potsdam Wolf-Rayet (PoWR) models for expanding atmospheres. We find that the expanding atmosphere of Abell 48 is mainly composed of helium (85 per cent by mass), hydrogen (10 per cent), and nitrogen (5 per cent). The residual hydrogen and the enhanced nitrogen abundance make this object different from the other [WN] star IC 4663. We discuss the possible origin of this atmospheric composition.

astro-ph.SR↗

arXiv2012

Empirical Determination of Convection Parameters in White Dwarfs I : Whole Earth Telescope Observations of EC14012-1446

We report on analysis of 308.3 hrs of high speed photometry targeting the pulsating DA white dwarf EC14012-1446. The data were acquired with the Whole Earth Telescope (WET) during the 2008 international observing run XCOV26. The Fourier transform of the light curve contains 19 independent frequencies and numerous combination frequencies. The dominant peaks are 1633.907, 1887.404, and 2504.897 microHz. Our analysis of the combination amplitudes reveals that the parent frequencies are consistent with modes of spherical degree l=1. The combination amplitudes also provide m identifications for the largest amplitude parent frequencies. Our seismology analysis, which includes 2004--2007 archival data, confirms these identifications, provides constraints on additional frequencies, and finds an average period spacing of 41 s. Building on this foundation, we present nonlinear fits to high signal-to-noise light curves from the SOAR 4.1m, McDonald 2.1m, and KPNO 2m telescopes. The fits indicate a time-averaged convective response timescale of 99.4 +/- 17 s, a temperature exponent 85 +/- 6.2 and an inclination angle of 32.9 +/- 3.2 degrees. We present our current empirical map of the convective response timescale across the DA instability strip.

astro-ph.SR↗

arXiv2012

OGLE-2008-BLG-510: first automated real-time detection of a weak microlensing anomaly - brown dwarf or stellar binary?

The microlensing event OGLE-2008-BLG-510 is characterised by an evident asymmetric shape of the peak, promptly detected by the ARTEMiS system in real time. The skewness of the light curve appears to be compatible both with binary-lens and binary-source models, including the possibility that the lens system consists of an M dwarf orbited by a brown dwarf. The detection of this microlensing anomaly and our analysis demonstrates that: 1) automated real-time detection of weak microlensing anomalies with immediate feedback is feasible, efficient, and sensitive, 2) rather common weak features intrinsically come with ambiguities that are not easily resolved from photometric light curves, 3) a modelling approach that finds all features of parameter space rather than just the `favourite model' is required, and 4) the data quality is most crucial, where systematics can be confused with real features, in particular small higher-order effects such as orbital motion signatures. It moreover becomes apparent that events with weak signatures are a silver mine for statistical studies, although not easy to exploit. Clues about the apparent paucity of both brown-dwarf companions and binary-source microlensing events might hide here.

astro-ph.EP↗

arXiv2012

Discovery of two new Galactic candidate luminous blue variables with WISE

We report the discovery of two new Galactic candidate luminous blue variable (cLBV) stars via detection of circular shells (typical of known confirmed and cLBVs) and follow-up spectroscopy of their central stars. The shells were detected at 22 um in the archival data of the Mid-Infrared All Sky Survey carried out with the Wide-field Infrared Survey Explorer (WISE). Follow-up optical spectroscopy of the central stars of the shells conducted with the renewed Southern African Large Telescope (SALT) showed that their spectra are very similar to those of the well-known LBVs P Cygni and AG Car, and the recently discovered cLBV MN112, which implies the LBV classification for these stars as well. The LBV classification of both stars is supported by detection of their significant photometric variability: one of them brightened in the R- and I-bands by 0.68\pm0.10 mag and 0.61\pm0.04 mag, respectively, during the last 13-18 years, while the second one (known as Hen 3-1383) varies its B,V,R,I and K_s brightnesses by \simeq 0.5-0.9 mag on time-scales from 10 days to decades. We also found significant changes in the spectrum of Hen 3-1383 on a timescale of \simeq 3 months, which provides additional support for the LBV classification of this star. Further spectrophotometric monitoring of both stars is required to firmly prove their LBV status. We discuss a connection between the location of massive stars in the field and their fast rotation, and suggest that the LBV activity of the newly discovered cLBVs might be directly related to their possible runaway status.

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