arXiv subjects

M. J. Darnley

Publications and source records attributed to M. J. Darnley.

At least 19 recordsLinked to original sources

arXiv2024

The Accelerating Decline of the Mass Transfer Rate in the Recurrent Nova T Pyxidis

The recurrent nova T Pyxidis has erupted six times since 1890, with its last outburst in 2011, and the relatively short recurrence time between classical nova explosions indicates that T Pyx must have a massive white dwarf accreting at a high rate. It is believed that, since its outburst in 1890, the mass transfer rate in T Pyx was very large due to a feedback loop where the secondary is heated by the hot white dwarf. The feedback loop has been slowly shutting off, reducing the mass transfer rate, and thereby explaining the magnitude decline of T Pyx from $\sim13.8$ (before 1890) to 15.7 just before the 2011 eruption. We present an analysis of the latest $Hubble~Space~Telescope$ (HST) far ultraviolet and optical spectra, obtained 12 years after the 2011 outburst, showing that the mass transfer rate has been steadily declining and is now below its pre-outburst level by about 40%: $\dot{M} \sim 1-3\times 10^{-7}M_\odot$/yr for a WD mass of $\sim 1.0-1.4 M_\odot$, an inclination of $50^\circ - 60^\circ$, reddening $E(B-V)=0.30 \pm 0.05$ and a Gaia DR3 distance of $2860^{+816}_{-471}$~pc. This steady decrease in the mass transfer rate in the $\sim$decade after the 2011 ourbutst is in sharp contrast with the more constant pre-outburst UV continuum flux level from archival international ultraviolet explorer (IUE) spectra. The flux (i.e. $\dot{M}$) decline rate is 29 times faster now in the last $\sim$decade than observed since 1890 to $\sim$2010. The feedback loop shut off seems to be accelerating, at least in the decade following its 2011 outburst. In all eventualities, our analysis confirms that T Pyx is going through an unusually peculiar short-lived phase.

astro-ph.SR↗

arXiv2024

Discovery of a nova super-remnant cavity surrounding RS Ophiuchi

The prototypical nova super-remnant (NSR) was uncovered around the most rapidly recurring nova (RN), M31N 2008-12a. Simulations of the growth of NSRs revealed that these large structures should exist around all novae, whether classical or recurrent. NSRs consist of large shell-like structures surrounding excavated cavities. Predictions, informed by these simulations, led to the discovery of an extended cavity coincident with the Galactic RN, RS Ophiuchi, in far-infrared archival IRAS images. We propose that this cavity is associated with RS Oph and is therefore evidence of another NSR to be uncovered.

astro-ph.HE↗

arXiv2024

On an apparent dearth of recurrent nova super-remnants in the Local Group

The Andromeda Galaxy is home to the annually erupting recurrent nova (RN) M31N 2008-12a (12a); the first nova found to host a nova super-remnant (NSR). A NSR is an immense structure surrounding a RN, created from many millions of eruptions sweeping up material in the local environment to form a shell tens of parsecs across. Theory has demonstrated that NSRs should be found around all RNe, even those systems with long periods between eruptions. Befittingly, the second NSR was found around the Galactic classical (and long suspected recurrent) nova, KT Eridani. In this Paper, we aim to find more of these phenomena through conducting the first ever survey for NSRs in M31 and the Large Magellanic Cloud (LMC). We find that the surroundings of fourteen RNe in M31 as well as the surroundings of the four RNe in the LMC do not show any evidence of vast parsec-scale structures in narrowband (H$α$ and [S II]) images, unlike the one clearly seen around 12a, and therefore conclude that observable NSRs are either rare structures, or they are too faint (or small) to be detected in our existing datasets. Yet, the NSR surrounding 12a would also likely to have been overlooked in our study if it were approximately one magnitude fainter. Searches for NSRs around other RNe 'masquerading' as classical novae may prove to be fruitful as would whole surveys of other Local Group galaxies.

astro-ph.HE↗

arXiv2023

Machine Learning applications for Cataclysmic Variable discovery in the ZTF alert stream

Cataclysmic variables (CV) encompass a diverse array of accreting white dwarf binary systems. Each class of CV represents a snapshot along an evolutionary journey, one with the potential to trigger a type Ia supernova event. The study of CVs offers valuable insights into binary evolution and accretion physics, with the rarest examples potentially providing the deepest insights. However, the escalating number of detected transients, coupled with our limited capacity to investigate them all, poses challenges in identifying such rarities. Machine Learning (ML) plays a pivotal role in addressing this issue by facilitating the categorisation of each detected transient into its respective transient class. Leveraging these techniques, we have developed a two-stage pipeline tailored to the ZTF transient alert stream. The first stage is an alerts filter aimed at removing non-CVs, while the latter is an ML classifier produced using XGBoost, achieving a macro average AUC score of 0.92 for distinguishing between CV classes. By utilising the Generative Topographic Mapping algorithm with classifier posterior probabilities as input, we obtain representations indicating that CV evolutionary factors play a role in classifier performance, while the associated feature maps present a potent tool for identifying the features deemed most relevant for distinguishing between classes. Implementation of the pipeline in June 2023 yielded 51 intriguing candidates that are yet to be reported as CVs or classified with further granularity. Our classifier represents a significant step in the discovery and classification of different CV classes, a domain of research still in its infancy.

astro-ph.SR↗

arXiv2023

Hydrodynamic simulations of the KT Eridani nova super-remnant

A nova super-remnant (NSR) is an immense structure associated with a nova that forms when frequent and recurrent nova eruptions sweep up surrounding interstellar material (ISM) into a high density and distant shell. The prototypical NSR, measuring over 100 pc across, was discovered in 2014 around the annually erupting nova M31N 2008-12a. Hydrodynamical simulations demonstrated that the creation of a dynamic NSR by repeated eruptions transporting large quantities of ISM is not only feasible but that these structures should exist around all novae, whether the white dwarf (WD) is increasing or decreasing in mass. But it is only the recurrent nova (RNe) with the highest WD masses and accretion rates that should host observable NSRs. KT Eridani is, potentially, the eleventh RNe recorded in the Galaxy and is also surrounded by a recently unveiled Hα shell tens of parsecs across, consistent with a NSR. Through modelling the nova ejecta from KT Eri, we demonstrate that such an observable NSR could form in approximately 50,000 years, which fits with the proper motion history of the nova. We compute the expected Hα emission from the KT Eri NSR and predict that the structure might be accessible to wide-field X-ray facilities.

astro-ph.HE↗

arXiv2023

Shock shaping? Nebular Spectroscopy of Nova V906 Carinae

V906 Carinae was one of the best observed novae of recent times. It was a prolific dust producer and harboured shocks in the early evolving ejecta outflow. Here, we take a close look at the consequences of these early interactions through study of high-resolution UVES spectroscopy of the nebular stage and extrapolate backwards to investigate how the final structure may have formed. A study of ejecta geometry and shaping history of the structure of the shell is undertaken following a spectral line SHAPE model fit. A search for spectral tracers of shocks in the nova ejecta is undertaken and an analysis of the ionised environment. Temperature, density and abundance analyses of the evolving nova shell are presented.

astro-ph.SR↗

arXiv2023

On the Observability of Recurrent Nova Super-Remnants

The nova super-remnant (NSR) surrounding M31N 2008-12a (12a), the annually erupting recurrent nova (RN), is the only known example of this phenomenon. As this structure has grown as a result of frequent eruptions from 12a, we might expect to see NSRs around other RNe; this would confirm the RN--NSR association and strengthen the connection between novae and type Ia supernovae (SN Ia) as NSRs centered on SN Ia provide a lasting, unequivocal signpost to the single degenerate progenitor type of that explosion. The only previous NSR simulation used identical eruptions from a static white dwarf (WD). In this Paper, we simulate the growth of NSRs alongside the natural growth/erosion of the central WD, within a range of environments, accretion rates, WD temperatures, and initial WD masses. The subsequent evolving eruptions create dynamic NSRs tens of parsecs in radius comprising a low-density cavity, bordered by a hot ejecta pile-up region, and surrounded by a cool high-density, thin, shell. Higher density environments restrict NSR size, as do higher accretion rates, whereas the WD temperature and initial mass have less impact. NSRs form around growing or eroding WDs, indicating that NSRs also exist around old novae with low-mass WDs. Observables such as X-ray and H$α$ emission from the modelled NSRs are derived to aid searches for more examples; only NSRs around high accretion rate novae will currently be observable. The observed properties of the 12a NSR can be reproduced when considering both the dynamically grown NSR and photoionisation by the nova system.

astro-ph.HE↗

arXiv2022

High Resolution X-ray Spectra of RS Ophiuchi (2006 and 2021): Revealing the cause of SSS variability

Swift observed the SSS phase in RS Oph much fainter in 2021 than in 2006, and we compare an XMM-Newton grating spectrum on day 55.6 in 2021 (2021d55.6) to SSS Chandra and XMM-Newton grating spectra from days 2006d39.7, 2006d54, and 2006d66.9. We present a novel approach to down-scale the observed (brighter) 2006 SSS spectra to match the 2021d55.6 spectrum by parameter optimisation of: (1) A constant factor, (2) a multi-ionisation photoelectric absorption model, and (3) scaling with a ratio of two blackbody models with different effective temperatures. This approach avoids defining a source model and is more sensitive to incremental changes than modeling source plus absorption simultaneously. The 2021d55.6 spectrum can be reproduced remarkably well by multiplying the brighter 2006 spectra with an absorption model. Only for the 2006d66.9 spectrum, an additional temperature change is needed. We further find the 2021d55.6 spectrum to resemble much more the 2006d39.7 spectrum in shape and structure than the same-epoch 2006d54 spectrum with more absorption lines with a deeper OI absorption edge, and higher blue shifts (1200km/s) than on day 2006d54 (700km/s). On days 2006d39.7, 2006d54 and 2021d55.6, brightness and hardness variations are correlated indicating variations of the OI column density. The 35s period was detected on day 2021d55.6 with lower significance compared to 2006d54. We conclude absorption to be the principal reason for observing lower soft X-ray emission in 2021 compared to 2006. We explain the reduction in line blue shift, depth in OI edge, and number of absorption lines from day 2006d39.7 to 2006d54 by deceleration and heating of the ejecta within the stellar wind of the companion. Less such deceleration and heating in 2021 indicates viewing at different angles through an inhomogeneous stellar wind.

astro-ph.HE↗

arXiv2022

Machine Learning based search for Cataclysmic Variables within Gaia Science Alerts

Wide-field time domain facilities detect transient events in large numbers through difference imaging. For example, Zwicky Transient Facility produces alerts for hundreds of thousands of transient events per night, a rate set to be dwarfed by the upcoming Vera Rubin Observatory. The automation provided by Machine Learning (ML) is, therefore, necessary to classify these events and select the most interesting sources for follow-up observations. Cataclysmic Variables (CVs) are a transient class that are numerous, bright, and nearby, providing excellent laboratories for the study of accretion and binary evolution. Here we focus on our use of ML to identify CVs from photometric data of transient sources published by the Gaia Science Alerts program (GSA) - a large, easily accessible resource, not fully explored with ML. The use of light curve feature extraction techniques and source metadata from the Gaia survey resulted in a Random Forest model capable of distinguishing CVs from supernovae, Active Galactic Nuclei, and Young Stellar Objects with a 92\% precision score and an 85\% hit rate. Of 13,280 sources within GSA without an assigned transient classification our model predicts the CV class for $\sim$2800. Spectroscopic observations are underway to classify a statistically significant sample of these targets to validate the performance of the model. This work puts us on a path towards the classification of rare CV subtypes from future wide-field surveys such as the Legacy Survey of Space and Time.

astro-ph.SR↗

arXiv2022

V392 Persei: a γ-ray bright nova eruption from a known dwarf nova

V392 Persei is a known dwarf nova (DN) that underwent a classical nova eruption in 2018. Here we report ground-based optical, Swift UV and X-ray, and Fermi-LAT γ-ray observations following the eruption for almost three years. V392 Per is one of the fastest evolving novae yet observed, with a $t_2$ decline time of 2 days. Early spectra present evidence for multiple and interacting mass ejections, with the associated shocks driving both the γ-ray and early optical luminosity. V392 Per entered Sun-constraint within days of eruption. Upon exit, the nova had evolved to the nebular phase, and we saw the tail of the super-soft X-ray phase. Subsequent optical emission captured the fading ejecta alongside a persistent narrow line emission spectrum from the accretion disk. Ongoing hard X-ray emission is characteristic of a standing accretion shock in an intermediate polar. Analysis of the optical data reveals an orbital period of 3.230 \pm 0.003 days, but we see no evidence for a white dwarf (WD) spin period. The optical and X-ray data suggest a high mass WD, the pre-nova spectral energy distribution (SED) indicates an evolved donor, and the post-nova SED points to a high mass accretion rate. Following eruption, the system has remained in a nova-like high mass transfer state, rather than returning to the pre-nova DN low mass transfer configuration. We suggest that this high state is driven by irradiation of the donor by the nova eruption. In many ways, V392 Per shows similarity to the well-studied nova and DN GK Persei.

astro-ph.HE↗

arXiv2022

The 2021 outburst of the recurrent nova RS Ophiuchi observed in X-rays by the Neil Gehrels Swift Observatory: a comparative study

On 2021 August 8, the recurrent nova RS Ophiuchi erupted again, after an interval of 15.5 yr. Regular monitoring by the Neil Gehrels Swift Observatory began promptly, on August 9.9 (0.37 day after the optical peak), and continued until the source passed behind the Sun at the start of November, 86 days later. Observations then restarted on day 197, once RS Oph emerged from the Sun constraint. This makes RS Oph the first Galactic recurrent nova to have been monitored by Swift throughout two eruptions. Here we investigate the extensive X-ray datasets from 2006 and 2021, as well as the more limited data collected by EXOSAT in 1985. The hard X-rays arising from shock interactions between the nova ejecta and red giant wind are similar following the last two eruptions. In contrast, the early super-soft source (SSS) in 2021 was both less variable and significantly fainter than in 2006. However, 0.3-1 keV light-curves from 2021 reveal a 35 s quasi-periodic oscillation consistent in frequency with the 2006 data. The Swift X-ray spectra from 2021 are featureless, with the soft emission typically being well parametrized by a simple blackbody, while the 2006 spectra showed much stronger evidence for superimposed ionized absorption edges. Considering the data after day 60 following each eruption, during the supersoft phase the 2021 spectra are hotter, with smaller effective radii and lower wind absorption, leading to an apparently reduced bolometric luminosity. We explore possible explanations for the gross differences in observed SSS behaviour between the 2006 and 2021 outbursts.

astro-ph.HE↗

arXiv2022

An H-alpha survey of the host environments of 77 type IIn supernovae within z<0.02

Type IIn supernovae (SNe\,IIn) are an uncommon and highly heterogeneous class of SN where the SN ejecta interact with pre-existing circumstellar media (CSM). Previous studies have found a mass ladder in terms of the association of the SN location with H$α$ emission and the progenitor masses of SN classes. In this paper, we present the largest environmental study of SNe\,IIn. We analyse the H$α$ environments of 77 type IIn supernovae using continuum subtracted H$α$ images. We use the pixel statistics technique, normalised cumulative ranking (NCR), to associate SN pixels with H$α$ emission. We find that our 77 SNe\,IIn do not follow the H$α$ emission. This is not consistent with the proposed progenitors of SNe\,IIn, luminous blue variables (LBVs) as LBVs are high mass stars that undergo dramatic episodic mass loss. However, a subset of the NCR values follow the H$α$ emission, suggesting a population of high mass progenitors. This suggests there may be multiple progenitor paths with $\sim$60\% having non-zero NCR values with a distribution consistent with high mass progenitors such as LBVs and $\sim$40\% of these SNe not being associated with H$α$ emission. We discuss the possible progenitor routes of SNe\,IIn, especially for the zero NCR value population. We also investigate the radial distribution of the SNe in their hosts in terms of H$α$ and $r'$-band flux.

astro-ph.HE↗

arXiv2021

The Remarkable Spin-down and Ultra-fast Outflows of the Highly-Pulsed Supersoft Source of Nova Hercules 2021

Nova Her 2021 (V1674 Her), which erupted on 2021 June 12, reached naked-eye brightness and has been detected from radio to $γ$-rays. An extremely fast optical decline of 2 magnitudes in 1.2 days and strong Ne lines imply a high-mass white dwarf. The optical pre-outburst detection of a 501.42s oscillation suggests a magnetic white dwarf. This is the first time that an oscillation of this magnitude has been detected in a classical nova prior to outburst. We report X-ray outburst observations from {\it Swift} and {\it Chandra} which uniquely show: (1) a very strong modulation of super-soft X-rays at a different period from reported optical periods; (2) strong pulse profile variations and the possible presence of period variations of the order of 0.1-0.3s; and (3) rich grating spectra that vary with modulation phase and show P Cygni-type emission lines with two dominant blue-shifted absorption components at $\sim 3000$ and 9000 km s$^{-1}$ indicating expansion velocities up to 11000 km s$^{-1}$. X-ray oscillations most likely arise from inhomogeneous photospheric emission related to the magnetic field. Period differences between reported pre- and post-outburst optical observations, if not due to other period drift mechanisms, suggest a large ejected mass for such a fast nova, in the range $2\times 10^{-5}$-$2\times 10^{-4} M_\odot$. A difference between the period found in the {\it Chandra} data and a reported contemporaneous post-outburst optical period, as well as the presence of period drifts, could be due to weakly non-rigid photospheric rotation.

astro-ph.HE↗

arXiv2021

A systematic reclassification of type IIn supernovae

Type IIn supernovae (SNe IIn) are a relatively infrequently observed subclass of SNe whose photometric and spectroscopic properties are varied. A common thread among SNe IIn are the complex multiple-component hydrogen Balmer lines. Owing to the heterogeneity of SNe IIn, online databases contain some outdated, erroneous, or even contradictory classifications. SN IIn classification is further complicated by SN impostors and contamination from underlying HII regions. We have compiled a catalogue of systematically classified nearby (redshift z < 0.02) SNe IIn using the Open Supernova Catalogue (OSC). We present spectral classifications for 115 objects previously classified as SNe IIn. Our classification is based upon results obtained by fitting multiple Gaussians to the H-alpha profiles. We compare classifications reported by the OSC and Transient Name Server (TNS) along with the best matched templates from SNID. We find that 28 objects have been misclassified as SNe IIn. TNS and OSC can be unreliable; they disagree on the classifications of 51 of the objects and contain a number of erroneous classifications. Furthermore, OSC and TNS hold misclassifications for 34 and twelve (respectively) of the transients we classify as SNe IIn. In total, we classify 87 SNe IIn. We highlight the importance of ensuring that online databases remain up to date when new or even contemporaneous data become available. Our work shows the great range of spectral properties and features that SNe IIn exhibit, which may be linked to multiple progenitor channels and environment diversity. We set out a classification sche me for SNe IIn based on the H-alpha profile which is not greatly affected by the inhomogeneity of SNe IIn.

astro-ph.SR↗

arXiv2020

Two New Nova Shells associated with V4362 Sagittarii and DO Aquilae

A classical nova is an eruption on the surface of a white dwarf in an accreting binary system. The material ejected from the white dwarf surface generally forms an axisymmetric shell. The shaping mechanisms of nova shells are probes of the processes that take place at energy scales between planetary nebulae and supernova remnants. We report on the discovery of nova shells surrounding the post-nova systems V4362 Sagittarii (1994) and more limited observations of DO Aquilae (1925). Distance measurements of 0.5p/m1.4 kpc for V4362 Sgr and 6.7 p/m 3.5 kpc -0.2 for DO Aql are found based on the expansion parallax method. The growth rates are measured to be 0.07``/year for DO Aql and 0.32``/year for V4362 Sgr. A preliminary investigation into the ionisation structure of the nova shell associated with V4362 Sgr is presented. The observed ionisation structure of nova shells depends strongly on their morphology and the orientation of the central component towards the observer. X-ray, IR and UV observations as well as optical integral field unit spectroscopy are required to better understand these interesting objects.

astro-ph.SR↗

arXiv2020

AT 2019abn: multi-wavelength observations over the first 200 days

AT 2019abn was discovered in the nearby M51 galaxy by the Zwicky Transient Facility at more than two magnitudes and around three weeks prior to its optical peak. We aim to conduct a detailed photometric and spectroscopic follow-up campaign for AT 2019abn, with early discovery allowing for significant pre-maximum observations of an intermediate luminosity red transient (ILRT) for the first time. This work is based on the analysis of u'BVi'z'H photometry and low-resolution spectroscopy using the Liverpool Telescope, medium-resolution spectroscopy with the Gran Telescopio Canarias (GTC), and near-infrared imaging with the GTC and the Nordic Optical Telescope. We present the most detailed optical light curve of an ILRT to date, with multi-band photometry starting around three weeks before peak brightness. The transient peaked at an observed absolute magnitude of M_r' = -13.1, although it is subject to significant reddening from dust in M51, implying an intrinsic M_r' ~ -15.2. The initial light curve showed a linear, achromatic rise in magnitude before becoming bluer at peak. After peak brightness, the transient gradually cooled. This is reflected in our spectra, which at later times show absorption from such species as Fe I, Ni I and Li I. A spectrum taken around peak brightness shows narrow, low-velocity absorption lines, which we interpret as likely to originate from pre-existing circumstellar material. We conclude that while there are some peculiarities, such as the radius evolution, AT 2019abn fits in well overall with the ILRT class of objects and is the most luminous member of the class seen to date.

astro-ph.SR↗

arXiv2020

AT 2016dah and AT 2017fyp: the first classical novae discovered within a tidal stream

AT2016dah and AT2017fyp are fairly typical Andromeda Galaxy (M31) classical novae. AT2016dah is an almost text book example of a 'very fast' declining, yet uncommon, Fe II'b' (broad-lined) nova, discovered during the rise to peak optical luminosity, and decaying with a smooth broken power-law light curve. AT2017fyp is classed as a 'fast' nova, unusually for M31, its early decline spectrum simultaneously shows properties of both Fe II and He/N spectral types - a 'hybrid'. Similarly, the light curve of AT2017fyp has a broken power-law decline but exhibits an extended flat-topped maximum. Both novae were followed in the UV and X-ray by the Neil Gehrels Swift Observatory, but no X-ray source was detected for either nova. The pair were followed photometrically and spectroscopically into their nebular phases. The progenitor systems were not visible in archival optical data, implying that the mass donors are main sequence stars. What makes AT2016dah and AT2017fyp particularly interesting is their position with respect to M31. The pair are close on the sky but are located far from the centre of M31, lying almost along the semi-minor axis of their host. Radial velocity measurements and simulations of the M31 nova population leads to the conclusion that both novae are members of the Andromeda Giant Stellar Stream (GSS). We find the probability of at least two M31 novae appearing coincident with the GSS by chance is ~1%. Therefore, we claim that these novae arose from the GSS progenitor, not M31 - the first confirmed novae discovered in a tidal steam.

astro-ph.SR↗

arXiv2019

Regulation of accretion by its outflow in a symbiotic star: the 2016 outflow fast state of MWC 560

How are accretion discs affected by their outflows? To address this question for white dwarfs accreting from cool giants, we performed optical, radio, X-ray, and ultraviolet observations of the outflow-driving symbiotic star MWC 560 (=V694 Mon) during its 2016 optical high state. We tracked multi-wavelength changes that signalled an abrupt increase in outflow power at the initiation of a months-long outflow fast state, just as the optical flux peaked: (1) an abrupt doubling of Balmer absorption velocities; (2) the onset of a $20$ $μ$Jy/month increase in radio flux; and (3) an order-of-magnitude increase in soft X-ray flux. Juxtaposing to prior X-ray observations and their coeval optical spectra, we infer that both high-velocity and low-velocity optical outflow components must be simultaneously present to yield a large soft X-ray flux, which may originate in shocks where these fast and slow absorbers collide. Our optical and ultraviolet spectra indicate that the broad absorption-line gas was fast, stable, and dense ($\gtrsim10^{6.5}$ cm$^{-3}$) throughout the 2016 outflow fast state, steadily feeding a lower-density ($\lesssim10^{5.5}$ cm$^{-3}$) region of radio-emitting gas. Persistent optical and ultraviolet flickering indicate that the accretion disc remained intact. The stability of these properties in 2016 contrasts to their instability during MWC 560's 1990 outburst, even though the disc reached a similar accretion rate. We propose that the self-regulatory effect of a steady fast outflow from the disc in 2016 prevented a catastrophic ejection of the inner disc. This behaviour in a symbiotic binary resembles disc/outflow relationships governing accretion state changes in X-ray binaries.

astro-ph.HE↗