MY RESEARCH
My research mostly revolves around understanding the processes of mass transfer and accretion within binary systems, its effects and consequences to the binaries, and their evolution. I am particulary fond of accreting white dwarfs (and more specifically so-called Cataclysmic Variables), but all accretion processes intrigue me, no matter the accretor! I have extensivley used data from space-based optical timing missions such as Kepler and TESS to study accretion-driven variability, as well as discover new transient variability phenomena in these systems. I like to test the notion that accretion physics is scale-invariant, and to do this I compare phenomenological results in accreting white dwarfs to accreting young stars, neutron stars and black holes (both stellar-mass and supermassive).
Below are abstracts and links to all my first-author papers. This ADS link lists all of my publicatons.
Cataclysmic variables are a key population of gravitational wave sources for LISA
The gravitational wave (GW) signals from the Galactic population of cataclysmic variables (CVs) have yet to be carefully assessed. Here we estimate these signals and evaluate their significance for LISA. First, we find that at least three known systems are expected to produce strong enough signals to be individually resolved within the first four years of LISA's operation. Second, CVs will contribute significantly to the LISA Galactic binary background, limiting the mission's sensitivity in the relevant frequency band. Third, we predict a spike in the unresolved GW background at a frequency corresponding to the CV minimum orbital period. This excess noise may impact the detection of other systems near this characteristic frequency. Fourth, we note that the excess noise spike amplitude and location associated with P_min of about 80 minutes can be used to measure the CV space density and period bounce location with complementary and simple GW biases compared to the biases and selection effects plaguing samples selected from electromagnetic signals. Our results highlight the need to explicitly include the Galactic CV population in the LISA mission planning, both as individual GW sources and generators of background noise, as well as the exciting prospect of characterising the CV population through their GW emission.
Monthly Notices of the Royal Astronomical Society: Letters
ArXiv
XGAPS: a sub-arcsec cross-match of galactic plane surveys
We present a sub-arcsec cross-match of Gaia Data Release 3 (DR3) against the INT Galactic Plane Surveys (IGAPS) and the United Kingdom Infrared Deep Sky Survey (UKIDSS). The resulting cross-match of Galactic Plane Surveys (XGAPS) provides additional precise photometry (U_RGO, g, r, i, H-alpha, J, H, and K) to the Gaia photometry. In building the catalogue, proper motions given in Gaia DR3 are wound back to match the epochs of the IGAPS constituent surveys (INT Photometric H-alpha Survey of the Northern Galactic Plane, IPHAS, and the UV-Excess Survey of the Northern Galactic Plane, UVEX) and UKIDSS, ensuring high-proper motion objects are appropriately cross-matched. The catalogue contains 33 987 180 sources. The requirement of >3 sigma parallax detection for every included source means that distances out to 1-1.5 kpc are well covered. In producing XGAPS, we have also trained a Random Forest classifier to discern targets with problematic astrometric solutions. Selection cuts based on the classifier results can be used to clean colour-magnitude and colour-colour diagrams in a controlled and justified manner, as well as producing subsets of astrometrically reliable targets. We provide XGAPS as a 111 column table. Uses of the catalogue include the selection of Galactic targets for multi-object spectroscopic surveys as well as identification of specific Galactic populations.
XGAPS catalogue
ArXiv
Triggering micronovae through magnetically confined accretion flows in accreting white dwarfs
Rapid bursts at optical wavelengths have been reported for several accreting white dwarfs. In these bursts, the optical luminosity can increase by up to a factor of 30 in less than an hour, before fading on time-scales of several hours, and the energy release can reach ~1e39 erg ('micronovae'). Several systems have also shown these bursts to be semirecurrent on time-scales of days to months, and the temporal profiles of these bursts strongly resemble those observed in Type-I X-ray bursts in accreting neutron stars. It has been suggested that the observed micronovae may be the result of localized thermonuclear runaways in the surface layers of accreting white dwarfs. Here we propose a model in which the magnetic confinement of accretion streams on to the accreting magnetic white dwarf may trigger localized thermonuclear runaways. The model proposed to trigger micronovae appears to favour magnetic systems with both a high white dwarf mass and a high mass-transfer rate.
Monthly Notices of the Royal Astronomical Society: Letters, Volume 514, Issue 1, pp.L11-L15
ArXiv
Localized thermonuclear bursts from accreting magnetic white dwarfs
Nova explosions are caused by global thermonuclear runaways triggered in the surface layers of accreting white dwarfs. It has been predicted that localized thermonuclear bursts on white dwarfs can also take place, similar to type-I X-ray bursts observed in accreting neutron stars. Unexplained rapid bursts from the binary system TV Columbae, in which mass is accreted onto a moderately strong magnetized white dwarf from a low-mass companion, have been observed on several occasions in the past 40 years. During these bursts, the optical/ultraviolet luminosity increases by a factor of more than three in less than an hour and fades in around ten hours. Fast outflows have been observed in ultraviolet spectral lines, with velocities of more than 3,500 kilometres per second, comparable to the escape velocity from the white dwarf surface. Here we report on optical bursts observed in TV Columbae and in two additional accreting systems, EI Ursae Majoris and ASASSN-19bh. The bursts have a total energy of approximately 1e−6 times than those of classical nova explosions (micronovae) and bear a strong resemblance to type-I X-ray bursts. We exclude accretion or stellar magnetic reconnection events as their origin and suggest thermonuclear runaway events in magnetically confined accretion columns as a viable explanation.
Nature, Volume 604, Issue 7906, p.447-450
ArXiv
An accreting white dwarf displaying fast transitional mode switching
Accreting white dwarfs are often found in close binary systems with orbital periods ranging from tens of minutes to several hours. In most cases, the accretion process is relatively steady, with significant modulations only occurring on timescales of ~days or longer. Here we report the discovery of abrupt drops in the optical luminosity of the accreting white dwarf binary system TW Pictoris by factors up to 3.5 on timescales as short as 30 minutes. The optical light curve of this binary system obtained by the Transiting Exoplanet Survey Satellite (TESS) clearly displays fast switches between two distinct intensity modes that probably track the changing mass accretion rate onto the white dwarf. In the low mode, the system also displays magnetically gated accretion bursts, which implies that a weak magnetic field of the white dwarf truncates the inner disc at the co-rotation radius in this mode. The properties of the mode switching observed in TW Pictoris appear analogous to those observed in transitional millisecond pulsars, where similar transitions occur, although on timescales of ~tens of seconds. Our discovery establishes a previously unrecognized phenomenon in accreting white dwarfs and suggests a tight link to the physics governing magnetic accretion onto neutron stars.
Nature Astronomy, Volume 6, p. 98-102
ArXiv
The Gaia/IPHAS and Gaia/KIS value-added catalogues
We present a sub-arcsecond crossmatch of Gaia DR2 against the INT Photometric H α Survey of the Northern Galactic Plane Data Release 2 (IPHAS DR2) and the Kepler-INT Survey (KIS). The resulting value-added catalogues (VACs) provide additional precise photometry to the Gaia photometry (r, i, and H α for IPHAS, with additional U and g for KIS). In building the catalogue, proper motions given in Gaia DR2 are wound back to match the epochs of IPHAS DR2, thus ensuring high proper motion objects are appropriately crossmatched. The catalogues contain 7927 224 and 791 071 sources for IPHAS and KIS, respectively. The requirement of >5σ parallax detection for every included source means that distances out to 1-1.5 kpc are well covered. We define two additional parameters for each catalogued object: (i) fc, a magnitude-dependent tracer of the quality of the Gaia astrometric fit; (ii) fFP, the false-positive rate for parallax measurements determined from astrometric fits of a given quality at a given magnitude. Selection cuts based on these parameters can be used to clean colour-magnitude and colour-colour diagrams in a controlled and justified manner. We provide both full and light versions of the VAC, with VAC-light containing only objects that represent our recommended trade-off between purity and completeness. Uses of the catalogues include the identification of new variable stars in the matched data sets, and more complete identification of H α-excess emission objects due to separation of high-luminosity stars from the main sequence.
Monthly Notices of the Royal Astronomical Society, Volume 481, Issue 3, p.3357-3369
ArXiv
Magnetically gated accretion in an accreting `non-magnetic' white dwarf
White dwarfs are often found in binary systems with orbital periods ranging from tens of minutes to hours in which they can accrete gas from their companion stars. In about 15% of these binaries, the magnetic field of the white dwarf is strong enough (>106 Gauss) to channel the accreted matter along field lines onto the magnetic poles. The remaining systems are referred to as 'non-magnetic', since to date there has been no evidence that they have a dynamically significant magnetic field. Here we report an analysis of archival optical observations of the 'non-magnetic' accreting white dwarf in the binary system MV Lyrae (hereafter MV Lyr), whose lightcurve displayed quasi-periodic bursts of ~30 minutes duration every ~2 hours. The observations indicate the presence of an unstable magnetically-regulated accretion mode, revealing the existence of magnetically gated accretion, where disk material builds up around the magnetospheric boundary (at the co-rotation radius) and then accretes onto the white dwarf, producing bursts powered by the release of gravitational potential energy. We infer a surface magnetic field strength for the white dwarf in MV Lyr between 2 x 104 < B < 105 Gauss, too low to be detectable by other current methods. Our discovery provides a new way of studying the strength and evolution of magnetic fields in accreting white dwarfs and extends the connections between accretion onto white dwarfs, young stellar objects and neutron stars, for which similar magnetically gated accretion cysles have been identified.
Nature, 552, 210, 2017
ArXiv
The peculiar dipping events in the disk-bearing young-stellar object EPIC 204278916
EPIC 204278916 has been serendipitously discovered from its K2 light curve which displays irregular dimmings of up to 65% for ~25 consecutive days out of 78.8 days of observations. For the remaining duration of the observations, the variability is highly periodic and attributed to stellar rotation. The star is a young, low-mass (M-type) pre-main-sequence star with clear evidence of a resolved tilted disk from ALMA observations. We examine the K2 light curve in detail and hypothesise that the irregular dimmings are caused by either a warped inner-disk edge or transiting cometary-like objects in either circular or eccentric orbits. The explanations discussed here are particularly relevant for other recently discovered young objects with similar absorption dips.
ArXiv link
See here for a nice article from IFLScience!
Accretion-induced variability links young stellar objects, white dwarfs and black holes
The central engines of disc-accreting stellar-mass black holes appear to be scaled down versions of the supermassive black holes that power active galactic nuclei. However, if the physics of accretion is universal, it should also be possible to extend this scaling to other types of accreting systems, irrespective of accretor mass, size, or type. We examine new observations, obtained with Kepler/K2 and ULTRACAM, regarding accreting white dwarfs and young stellar objects. Every object in the sample displays the same linear correlation between the brightness of the source and its amplitude of variability (rms-flux relation) and obeys the same quantitative scaling relation as stellar-mass black holes and active galactic nuclei. We also show that the most important parameter in this scaling relation is the physical size of the accreting object. This establishes the universality of accretion physics from proto-stars still in the star-forming process to the supermassive black holes at the centers of galaxies.
Science Advances link
Interested in listening to the sound of accretion? Go to my Outreach section and experience the sound of feasting black holes, and other stuff!
Sco X-1 revisited with Kepler, MAXI and HERMES: outflows, time-lags and echoes unveiled
Sco X-1 has been the subject of many multi-wavelength studies in the past, being the brightest
persistent extra-solar X-ray source ever observed. Here we revisit Sco X-1 with simultaneous short cadence
Kepler optical photometry and MAXI X-ray photometry over a 78 day period, as well as optical spectroscopy
obtained with HERMES. We find Sco X-1 to be highly variable in all our datasets. The optical fluxes are clearly
bimodal, implying the system can be found in two distinct optical states. These states are generally associated
with the known flaring/normal branch X-ray states, although the flux distributions associated with these states
overlap. Furthermore, we find that the optical power spectrum of Sco X-1 differs substantially between optical
luminosity states. Additionally we find rms-flux relations in both optical states, but only find a linear
relation during periods of low optical luminosity. The full optical/X-ray discrete correlation function displays
a broad ~12.5 hour optical lag. However during the normal branch phase the X-ray and optical fluxes are
anti-correlated, whilst being correlated during the flaring branch. We also performed a Cepstrum analysis on the
full Kepler light curve to determine the presence of any echoes within the optical light curve alone. We find
significant echo signals, consistent with the optical lags found using the discrete cross-correlation. We
speculate that whilst some of the driving X-ray emission is reflected by the disk, some is absorbed and
re-processed on the thermal timescale, giving rise to both the observed optical lags and optical echoes.
ArXiv link
Reversibility of time series: revealing the hidden messages in X-ray binaries and cataclysmic variables
We explore the non-linear, high-frequency, aperiodic variability properties in the three cataclysmic
variables MV Lyr, KIC 8751494 and V1504 Cyg observed with Kepler, as well as the X-ray binary Cyg X-1 observed
with RXTE. This is done through the use of a high-order Fourier statistic called the bispectrum and its related
biphase and bicoherence, as well as the time-skewness statistic. We show how all objects display qualitatively
similar biphase trends. In particular all biphase amplitudes are found to be smaller than pi/2, suggesting that
the flux distributions for all sources are positively skewed on all observed timescales, consistent with the
log-normal distributions expected from the fluctuating accretion disk model. We also find that for all objects
the biphases are positive at frequencies where the corresponding power spectral densities display their high
frequency break. This suggests that the noise-like flaring observed is rising more slowly than it is falling,
and thus not time-reversible. This observation is also consistent with the fluctuating accretion disk model.
Furthermore, we observe the same qualitative biphase trends in all four objects, where the biphases display a
distinct decrease at frequencies below the high-frequency break in their respective power spectral densities.
This behaviour can also be observed in the time-skewness of all four objects. As far as we are aware, there is
no immediate explanation for the observed biphase decreases. The biphase decreases may thus suggest that the
fluctuating accretion disk model begins to break down at frequencies below the high frequency break.
ArXiv link
A physical model for the flickering variability in cataclysmic variables
Aperiodic broad-band variability (also known as flickering) is observed throughout all types of
accreting compact objects. Many statistical properties of this variability can be naturally explained with the
fluctuating accretion disk model, where variations in the mass-transfer rate through the disk are modulated on
the local viscous timescale and propagate towards the central compact object. Here, a recently developed
implementation of the model is applied for the first time to the time-averaged, high-frequency variability of a
cataclysmic variable star (MV Lyrae) observed with the Kepler satellite. A qualitatively good fit to the data is
achieved, suggesting the presence of geometrically thick inner flow with large viscosity parameter, extending
from ∼0.12 solar radii all the way to the white dwarf surface. A simple spectral model of the system suggests
that the geometrically thick component would not contribute much to the observed optical flux originating from
the geometrically thin outer disk. Instead, X-ray reprocessing from the geometrically thick disk onto the thin
disk is proposed as a mechanism to explain the observed variability. Similar flows are also deduced in accreting
neutron stars/black holes (X-ray binaries) and Active Galactic Nuclei. Additionally, eclipse mapping studies of
cataclysmic variables also seem to suggest the presence of a geometrically extended flow towards the inner-edges
of the accretion disk. The fluctuating accretion disk model applied here is encouraging in understanding the
origin of flickering in cataclysmic variables, as well as in X-ray binaries and Active Galactic Nuclei, by
providing a unifying scheme by which to explain the observed broad-band variability features observed throughout
all compact accreting systems.
ArXiv link
Kepler observations of the eclipsing cataclysmic variable
KIS J192748.53+444724.5
We present results from long cadence Kepler observations covering 97.6 days of the newly discovered
eclipsing cataclysmic variable KIS J192748.53+444724.5/KIC 8625249. We detect deep eclipses of the accretion
disk by the donor star every 3.97 hours. Additionally, the Kepler observations also cover a full outburst for
this cataclysmic variable, making KIS J192748.53+444724.5 the second known eclipsing cataclysmic variable system
in the Kepler field of view. We show how in quiescence a significant component associated to the hot-spot is
visible preceding the eclipse, and that this component is swamped by the brightness increase during the
outburst, potentially associated with the accretion disk. Furthermore we present evidence for accretion disk
radius changes during the outburst by analysing the out-of-eclipse light levels and eclipse depth through each
orbital cycle. We show how these parameters are linearly correlated in quiescence, and discuss how their
evolution during the outburst is suggesting disk radius changes and/or radial temperature gradient variations in
the disk.
ArXiv link
Discovery of Fourier-dependent time lags in cataclysmic variables
We report the first study of Fourier-frequency-dependent coherence and phase/time lags at optical
wavelengths of cataclysmic variables (MV Lyr and LU Cam) displaying typical flickering variability in white
light. Observations were performed on the William Herschel Telescope using ULTRACAM. Lightcurves for both
systems have been obtained with the SDSS filters u′, g′ and r′ simultaneously with cadences between ~0.5−2
seconds, and allow us to probe temporal frequencies between ~10^{-3} Hz and ~1 Hz. We find high levels of
coherence between the u', g' and r' lightcurves up to at least ~10^{-2} Hz. Furthermore we detect red/negative
lags where the redder bands lag the bluer ones at the lowest observed frequencies. For MV Lyr time lags up to ~3
seconds are observed, whilst LU Cam displays larger time lags of ~10 seconds. Mechanisms which seek to explain
red/negative lags observed in X-ray binaries and Active Galactic Nuclei involve reflection of photons generated
close to the compact object onto the surface layers of the accretion disk, where the lag delay is simply the
light-travel time from the emitting source to the reflecting accretion disk area. Although this could be a
viable explanation for the lags observed in MV Lyr, the lags observed in LU Cam are too large to be explained by
reflection from the disk and/or the donor star. We suggest reprocessing on the thermal timescale of boundary
layer photons onto the accretion disk as a possible mechanism to explain the lags observed in accreting white
dwarfs, or reverse (inside-out) shocks within the disk travelling through cooler disk regions as they move
outwards.
ArXiv link
Spectroscopic identifications of blue H-alpha excess sources in the Kepler field-of-view
We present the first results of an ongoing spectroscopic follow-up program of blue H-alpha excess
sources within the Kepler field-of-view, in order to identify new cataclysmic variables. Kepler observations of
the identified targets in this work will then provide detailed, time-resolved, studies of accretion. Candidates
selected from the Kepler-INT Survey were observed with the 4.2 meter William Herschel Telescope. Out of 38
observed candidates, we found 11 new cataclysmic variables reported here for the first time, as well as 13 new
quasars. Our target selection has a success rate of 29% when searching for cataclysmic variables, and we show
how this can be improved by including photometry obtained with the Wide-field Infrared Survey Explorer.
ArXiv link
Broad-band timing properties of the accreting white dwarf MV Lyrae
We present a broad-band timing analysis of the accreting white dwarf system MV Lyrae based on data
obtained with the Kepler satellite. The observations span 633 days at a cadence of 58.8 seconds and allow us to
probe 4 orders of magnitude in temporal frequency. The modelling of the observed broad-band noise components is
based on the superposition of multiple Lorentzian components, similar to the empirical modelling adopted for
X-ray binary systems. We also present the detection of a frequency varying Lorentzian component in the
lightcurve of MV Lyrae, where the Lorentzian characteristic frequency is inversely correlated with the mean
source flux. Because in the literature similar broad-band noise components have been associated to either the
viscous or dynamical timescale for different source types (accreting black holes or neutron stars), we here
systematically explore both scenarios and place constraints on the accretion disk structure. In the viscous case
we employ the fluctuating accretion disk model to infer parameters for the viscosity and disk scale height, and
infer uncomfortably high parameters to be accommodated by the standard thin disk, whilst in the dynamical case
we infer a large accretion disk truncation radius of ~10 white dwarf radii. More importantly however, the
phenomenological properties between the broad-band variability observed here and in X-ray binaries and Active
Galactic Nuclei are very similar, potentially suggesting a common origin for the broad-band variability.
ArXiv link
The Universal Nature of Accretion-induced Variability: The RMS-Flux Relation in an Accreting White Dwarf
We report the discovery of a linear relationship between the root-mean-square (rms) variability
amplitude and the mean flux in the accreting white dwarf binary system MV Lyrae. Our lightcurve, obtained with
the Kepler satellite, spans 633 days with quasi-continuous 58.8 second cadence resolution. We show, for the
first time, how this cataclysmic variable displays linear rms-flux relations similar to those observed in many
other black hole binaries, neutron star binaries and Active Galactic Nuclei. The phenomenological similarity
between the rms-flux relation observed here and in other X-ray binaries suggests a common physical origin for
the broad-band variability, independent of source type, mass or size of the compact accretor. Furthermore, we
infer the viscosity parameter, alpha, and disk scale height, H/R, using two independent methods. In both cases,
both values are found to be uncomfortably high to be accommodated by the disk instability model.
ArXiv link
Confirmation of 1RXS J165443.5-191620 as an intermediate polar and its orbital and spin periods
Aims: We investigate the physical nature of the X-ray emitting source 1RXS J165443.5-191620 through
optical photometry and time-resolved spectroscopy. Methods: Optical photometry is obtained from a variety of
telescopes all over the world spanning about 27 days. Additionally, time-resolved spectroscopy is obtained from
the MDM observatory. Results: The optical photometry clearly displays modulations consistent with those observed
in magnetic cataclysmic variables: a low-frequency signal interpreted as the orbital period, a high-frequency
signal interpreted as the white dwarf spin period, and an orbital sideband modulation. Our findings and
interpretations are further confirmed through optical, time-resolved, spectroscopy that displays H-alpha radial
velocity shifts modulated on the binary orbital period. Conclusion: We confirm the true nature of 1RXS
J165443.5-191620 as an intermediate polar with a spin period of 546 seconds and an orbital period of 3.7 hours.
In particular, 1RXS J165443.5-191620 is part of a growing subset of intermediate polars, all displaying hard
X-ray emission above 15keV, white dwarf spin periods below 30 minutes, and spin-to-orbital ratios below 0.1.
ArXiv link
A new determination of the INTEGRAL/IBIS point source location accuracy
AIMS: To determine the Point Source Location Accuracy (PSLA) for the INTEGRAL/IBIS telescope based
on analysis of archival in-flight data. METHODS: Over 40000 individual pointings (science windows) of
INTEGRAL/IBIS data were analysed using the latest Off-line Science Analysis software, version 7.0. Reconstructed
source positions were then compared against the most accurate positions available, determined from focusing
X-ray telescopes. Since the PSLA is a strong function of source detection significance, the offsets from true
position were histogrammed against significance, so that the 90% confidence limits could be determined. This has
been done for both sources in the fully coded field of view (FCFOV) and partially coded field of view (PCFOV).
RESULTS: The PSLA is found to have improved significantly since values derived from early mission data and
software for both FCFOV and PCFOV. CONCLUSIONS: This result has implications for observers executing follow-up
programs on IBIS sources since the sky area to be searched is reduced by over 50% in some cases.
ArXiv link
Hard X-ray properties of magnetic cataclysmic variables
Hard X-ray surveys have proven remarkably efficient in detecting intermediate polars and
asynchronous polars, two of the rarest type of cataclysmic variable (CV). Here we present a global study of hard
X-ray selected intermediate polars and asynchronous polars, focusing particularly on the link between hard X-ray
properties and spin/orbital periods. To this end, we first construct a new sample of these objects by
cross-correlating candidate sources detected in INTEGRAL/IBIS observations against catalogues of known CVs. We
find 23 cataclysmic variable matches, and also present an additional 9 (of which 3 are definite) likely magnetic
cataclysmic variables (mCVs) identified by others through optical follow-ups of IBIS detections. We also include
in our analysis hard X-ray observations from Swift/BAT and SUZAKU/HXD in order to make our study more complete.
We find that most hard X-ray detected mCVs have P_{spin}/P_{orb}<0.1 above the period gap. In this respect we
also point out the very low number of detected systems in any band between P_{spin}/P_{orb}=0.3 and
P_{spin}/P_{orb}=1 and the apparent peak of the P_{spin}/P_{orb} distribution at about 0.1. The observational
features of the P_{spin} - P_{orb} plane are discussed in the context of mCV evolution scenarios. We also
present for the first time evidence for correlations between hard X-ray spectral hardness and P_{spin}, P_{orb}
and P_{spin}/P_{orb}. An attempt to explain the observed correlations is made in the context of mCV evolution
and accretion footprint geometries on the white dwarf surface.
ArXiv link
Classifying Broad Absorption Line Quasars: Metrics, Issues and a New Catalogue Constructed from SDSS DR5
We apply a recently developed method for classifying broad absorption line quasars (BALQSOs) to the
latest QSO catalogue constructed from Data Release 5 of the Sloan Digital Sky Survey. Our new hybrid
classification scheme combines the power of simple metrics, supervised neural networks and visual inspection. In
our view the resulting BALQSO catalogue is both more complete and more robust than all previous BALQSO
catalogues, containing 3552 sources selected from a parent sample of 28,421 QSOs in the redshift range
1.7$lt;z$lt;4.2. This equates to a raw BALQSO fraction of 12.5%. In the process of constructing a robust
catalogue, we shed light on the main problems encountered when dealing with BALQSO classification, many of which
arise due to the lack of a proper physical definition of what constitutes a BAL. This introduces some
subjectivity in what is meant by the term BALQSO, and because of this, we also provide all of the meta-data used
in constructing our catalogue, for every object in the parent QSO sample. This makes it easy to quickly isolate
and explore sub-samples constructed with different metrics and techniques. By constructing composite QSO spectra
from sub-samples classified according to the meta-data, we show that no single existing metric produces clean
and robust BALQSO classifications. Rather, we demonstrate that a variety of complementary metrics are required
at the moment to accomplish this task. Along the way, we confirm the finding that BALQSOs are redder than
non-BALQSOs and that the raw BALQSO fraction displays an apparent trend with signal-to-noise, steadily
increasing from 9% in low signal-to-noise data, up to 15%.
ArXiv link
ISINA: INTEGRAL Source Identification Network Algorithm
We give an overview of ISINA: INTEGRAL Source Identification Network Algorithm. This machine
learning algorithm, using Random Forests, is applied to the IBIS/ISGRI dataset in order to ease the production
of unbiased future soft gamma-ray source catalogues. First we introduce the dataset and the problems encountered
when dealing with images obtained using the coded mask technique. The initial step of source candidate searching
is introduced and an initial candidate list is created. A description of the feature extraction on the initial
candidate list is then performed together with feature merging for these candidates. Three training and testing
sets are created in order to deal with the diverse timescales encountered when dealing with the gamma-ray sky.
Three independent Random Forest are built: one dealing with faint persistent source recognition, one dealing
with strong persistent sources and a final one dealing with transients. For the latter, a new transient
detection technique is introduced and described: the Transient Matrix. Finally the performance of the network is
assessed and discussed using the testing set and some illustrative source examples.
ArXiv link