Recent Publications
10 most recent research publications
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ADS (authors="STEEGHS, D")
The Gravitational-wave Optical Transient Observer (GOTO) is a project
dedicated to identifying optical counter-parts to gravitational-wave
detections using a network of dedicated, wide-field telescopes. After
almost a decade of design, construction, and commissioning work, the
GOTO network is now fully operational with two antipodal sites: La Palma
in the Canary Islands and Siding Spring in Australia. Both sites host
two independent robotic mounts, each with a field-of-view of 44 square
degrees formed by an array of eight 40 cm telescopes, resulting in an
instantaneous 88 square degree field-of-view per site. All four
telescopes operate as a single integrated network, with the ultimate aim
of surveying the entire sky every 2-3 days and allowing near-24-hour
response to transient events within a minute of their detection. In the
modern era of transient astronomy, automated telescopes like GOTO form a
vital link between multi-messenger discovery facilities and in-depth
follow-up by larger telescopes. GOTO is already producing a wide range
of scientific results, assisted by an efficient discovery pipeline and a
successful citizen science project: Kilonova Seekers.
We present mid-infrared (MIR) spectral-timing measurements of the
prototypical Galactic microquasar GRS 1915+105. The source was observed
with the Mid-Infrared Instrument (MIRI) onboard JWST in June 2023 at a
MIR luminosity L(MIR)~10^{36} erg/s exceeding past IR levels by about a
factor of 10. By contrast, the X-ray flux is much fainter than the
historical average, in the source's now-persistent 'obscured' state. The
MIRI low-resolution spectrum shows a plethora of emission lines, the
strongest of which are consistent with recombination in the hydrogen
Pfund (Pf) series and higher. Low amplitude (~1%) but highly significant
peak-to-peak photometric variability is found on timescales of ~1,000 s.
The brightest Pf(6-5) emission line lags the continuum. Though difficult
to constrain accurately, this lag is commensurate with light-travel
timescales across the outer accretion disc or with expected
recombination timescales inferred from emission line diagnostics. Using
the emission line as a bolometric indicator suggests a moderate (~5-30%
Eddington) intrinsic accretion rate. Multiwavelength monitoring shows
that JWST caught the source close in-time to unprecedentedly bright MIR
and radio long-term flaring. Assuming a thermal bremsstrahlung origin
for the MIRI continuum suggests an unsustainably high mass-loss rate
during this time unless the wind remains bound, though other possible
origins cannot be ruled out. PAH features previously detected with
Spitzer are now less clear in the MIRI data, arguing for possible
destruction of dust in the interim. These results provide a preview of
new parameter space for exploring MIR spectral-timing in XRBs and other
variable cosmic sources on rapid timescales.
The ESO public survey VISTA Variables in the Vía Láctea
(VVV) surveyed the inner Galactic bulge and the adjacent southern
Galactic disk from $2009-2015$. Upon its conclusion, the complementary
VVV eXtended (VVVX) survey has expanded both the temporal as well as
spatial coverage of the original VVV area, widening it from $562$ to
$1700$ sq. deg., as well as providing additional epochs in $JHK_{\rm s}$
filters from $2016-2023$. With the completion of VVVX observations
during the first semester of 2023, we present here the observing
strategy, a description of data quality and access, and the legacy of
VVVX. VVVX took $\sim 2000$ hours, covering about 4% of the sky in the
bulge and southern disk. VVVX covered most of the gaps left between the
VVV and the VISTA Hemisphere Survey (VHS) areas and extended the VVV
time baseline in the obscured regions affected by high extinction and
hence hidden from optical observations. VVVX provides a deep $JHK_{\rm
s}$ catalogue of $\gtrsim 1.5\times10^9$ point sources, as well as a
$K_{\rm s}$ band catalogue of $\sim 10^7$ variable sources. Within the
existing VVV area, we produced a $5D$ map of the surveyed region by
combining positions, distances, and proper motions of well-understood
distance indicators such as red clump stars, RR Lyrae, and Cepheid
variables. In March 2023 we successfully finished the VVVX survey
observations that started in 2016, an accomplishment for ESO Paranal
Observatory upon 4200 hours of observations for VVV+VVVX. The VVV+VVVX
catalogues complement those from the Gaia mission at low Galactic
latitudes and provide spectroscopic targets for the forthcoming ESO
high-multiplex spectrographs MOONS and 4MOST.
Time-domain astrophysics continues to grow rapidly, with the inception
of new surveys drastically increasing data volumes. Democratised,
distributed approaches to training sets for machine learning classifiers
are crucial to make the most of this torrent of discovery -- with
citizen science approaches proving effective at meeting these
requirements. In this paper, we describe the creation of and the initial
results from the $\textit{Kilonova Seekers}$ citizen science project,
built to find transient phenomena from the GOTO telescopes in near
real-time. $\textit{Kilonova Seekers}$ launched in July 2023 and
received over 600,000 classifications from approximately 2,000
volunteers over the course of the LIGO-Virgo-KAGRA O4a observing run.
During this time, the project has yielded 20 discoveries, generated a
`gold-standard' training set of 17,682 detections for augmenting
deep-learned classifiers, and measured the performance and biases of
Zooniverse volunteers on real-bogus classification. This project will
continue throughout the lifetime of GOTO, pushing candidates at
ever-greater cadence, and directly facilitate the next-generation
classification algorithms currently in development.
The mergers of binary compact objects such as neutron stars and black
holes are of central interest to several areas of astrophysics,
including as the progenitors of gamma-ray bursts (GRBs), sources of
high-frequency gravitational waves and likely production sites for heavy
element nucleosynthesis via rapid neutron capture (the r-process). These
heavy elements include some of great geophysical, biological and
cultural importance, such as thorium, iodine and gold. Here we present
observations of the exceptionally bright gamma-ray burst GRB 230307A. We
show that GRB 230307A belongs to the class of long-duration gamma-ray
bursts associated with compact object mergers, and contains a kilonova
similar to AT2017gfo, associated with the gravitational-wave merger
GW170817. We obtained James Webb Space Telescope mid-infrared (mid-IR)
imaging and spectroscopy 29 and 61 days after the burst. The
spectroscopy shows an emission line at 2.15 microns which we interpret
as tellurium (atomic mass A=130), and a very red source, emitting most
of its light in the mid-IR due to the production of lanthanides. These
observations demonstrate that nucleosynthesis in GRBs can create
r-process elements across a broad atomic mass range and play a central
role in heavy element nucleosynthesis across the Universe.