34 research outputs found
One thousand days of SN 2015bn: HST imaging shows a light curve flattening consistent with magnetar predictions
We present the first observations of a Type I superluminous supernova (SLSN)
at days after maximum light. We observed SN 2015bn using the
Hubble Space Telescope Advanced Camera for Surveys in the F475W, F625W and
F775W filters at 721 days and 1068 days. SN 2015bn is clearly detected and
resolved from its compact host, allowing reliable photometry. A galaxy template
constructed from these data further enables us to isolate the SLSN flux in deep
ground-based imaging. We measure a light curve decline rate at days of
mag (100 d), much shallower than the earlier evolution,
and slower than previous SLSNe (at any phase) or the decay rate of Co.
Neither additional radioactive isotopes nor a light echo can consistently
account for the slow decline. A spectrum at 1083 days shows the same [O I] and
[Ca II] lines as seen at days, with no new features to indicate
strong circumstellar interaction. Radio limits with the Very Large Array rule
out an extended wind for mass-loss rates M yr (where is the wind velocity in
units of 10 km s). The optical light curve is consistent with , which we show is expected for magnetar spin-down with inefficient
trapping; furthermore, the evolution matches predictions from earlier magnetar
model fits. The opacity to magnetar radiation is constrained at
cm g, consistent with photon-matter pair-production over a broad
GeV-TeV range. This suggests the magnetar spectral energy distribution,
and hence the 'missing energy' leaking from the ejecta, may peak in this range.Comment: Accepted for publication in ApJL, updated to match accepted versio
The Type I Superluminous Supernova PS16aqv: Lightcurve Complexity and Deep Limits on Radioactive Ejecta in a Fast Event
[Abridged] We present UV/optical observations of PS16aqv (SN 2016ard), a Type
I superluminous supernova (SLSN-I) classified as part of our search for low-
SLSNe. PS16aqv is a fast evolving SLSNe-I that reached a peak absolute
magnitude of . The lightcurves exhibit a significant
undulation at 30 rest-frame days after peak, with a behavior similar to
undulations seen in the slowly fading SLSN-I SN 2015bn. This similarity
strengthens the case that fast and slow SLSNe-I form a continuum with a common
origin. At days after peak, the lightcurves exhibit a transition
to a slow decline, followed by significant subsequent steepening, indicative of
a plateau phase or a second significant undulation. Deep limits at
days after peak imply a tight constraint on the nickel mass, M (lower than for previous SLSNe-I), and indicate that
some SLSNe-I do not produce significantly more nickel than normal Type Ic SNe.
Using MOSFiT, we model the lightcurve with a magnetar central engine model and
find ms, G, and
M. The implied rapid spin-down time and large
reservoir of available energy coupled with the high ejecta mass may account for
the fast evolving lightcurve and slow spectroscopic evolution. We also study
the location of PS16aqv in its host galaxy and find that it occurred at an
offset of kpc from the central star-forming region. We find the
host galaxy exhibits low metallicity and spatially varying extinction and star
formation rate, with the explosion site exhibiting lower values than the
central region. The complexity seen in the lightcurves of PS16aqv and other
events highlights the importance of obtaining well-sampled lightcurves for
exploring deviations from a uniform decline.Comment: 16 pages, 10 figures, submitted to Ap
The first JWST spectrum of a GRB afterglow: No bright supernova in observations of the brightest GRB of all time, GRB 221009A
We present JWST and Hubble Space Telescope (HST) observations of the
afterglow of GRB\,221009A, the brightest gamma-ray burst (GRB) ever observed.
Observations obtained with NIRSPEC (0.6-5.5 micron) and MIRI (5-12 micron) 12
days after the burst are the first mid-IR spectroscopy performed for a GRB.
Assuming the underlying slope is that of a single power-law, we obtain and , in excess of the notional Galactic value. This
is suggestive of extinction above the notional Galactic value, possibly due to
patchy extinction within the Milky Way or dust in the GRB host galaxy. It
further implies that the X-ray and optical/IR regimes are not on the same
branch of the synchrotron spectrum of the afterglow. If the cooling break lies
between the X-ray and optical/IR, then the temporal declines would only match
for a post jet break, ISM medium and electron index with . The shape of
the JWST spectrum is near-identical in the optical/nIR to X-shooter
spectroscopy obtained at 0.5 days and to later time observations with HST. The
lack of spectral evolution suggests the SNe is either substantially fainter or
bluer than SN~1998bw. Our {\em HST} observations also reveal a disc-like host
galaxy, viewed close to edge-on that further complicates the isolation of any
supernova component. The host galaxy appears rather typical amongst long-GRB
hosts and suggests that the extreme properties of GRB 221009A are not directly
tied to its galaxy-scale environment.Comment: Submitted to the Astrophysical Journal Letters for the GRB 221009A
Special Issue. The results of this paper are under press embargo. 18 pages, 8
figures, 2 table
Panning for gold, but finding helium: discovery of the ultra-stripped supernova SN2019wxt from gravitational-wave follow-up observations
We present the results from multi-wavelength observations of a transient
discovered during the follow-up of S191213g, a gravitational wave (GW) event
reported by the LIGO-Virgo Collaboration as a possible binary neutron star
merger in a low latency search. This search yielded SN2019wxt, a young
transient in a galaxy whose sky position (in the 80\% GW contour) and distance
(150\,Mpc) were plausibly compatible with the localisation uncertainty of
the GW event. Initially, the transient's tightly constrained age, its
relatively faint peak magnitude (\,mag) and the band
decline rate of \,mag per 5\,days appeared suggestive of a compact
binary merger. However, SN2019wxt spectroscopically resembled a type Ib
supernova, and analysis of the optical-near-infrared evolution rapidly led to
the conclusion that while it could not be associated with S191213g, it
nevertheless represented an extreme outcome of stellar evolution. By modelling
the light curve, we estimated an ejecta mass of , with
Ni comprising of this. We were broadly able to reproduce its
spectral evolution with a composition dominated by helium and oxygen, with
trace amounts of calcium. We considered various progenitors that could give
rise to the observed properties of SN2019wxt, and concluded that an
ultra-stripped origin in a binary system is the most likely explanation.
Disentangling electromagnetic counterparts to GW events from transients such as
SN2019wxt is challenging: in a bid to characterise the level of contamination,
we estimated the rate of events with properties comparable to those of
SN2019wxt and found that such event per week can occur within the
typical GW localisation area of O4 alerts out to a luminosity distance of
500\,Mpc, beyond which it would become fainter than the typical depth of
current electromagnetic follow-up campaigns.Comment: By the ENGRAVE collaboration (engrave-eso.org). 35 pages, 20 figures,
final version accepted by A&
A long-duration gamma-ray burst of dynamical origin from the nucleus of an ancient galaxy
The majority of long-duration (>2 s) gamma-ray bursts (GRBs) arise from the collapse of massive stars, with a small proportion created from the merger of compact objects. Most of these systems form via standard stellar evolution pathways. However, a fraction of GRBs may result from dynamical interactions in dense environments. These channels could also contribute substantially to the samples of compact object mergers detected as gravitational wave sources. Here we report the case of GRB 191019A, a long GRB (a duration of T 90 = 64.4 ± 4.5 s), which we pinpoint close (âȘ
100 pc projected) to the nucleus of an ancient (>1 Gyr old) host galaxy at z = 0.248. The lack of evidence for star formation and deep limits on any supernova emission disfavour a massive star origin. The most likely route for progenitor formation is via dynamical interactions in the dense nucleus of the host. The progenitor, in this case, could be a compact object merger. These may form in dense nuclear clusters or originate in a gaseous disc around the supermassive black hole. Identifying, to the best of our knowledge, a first example of a dynamically produced GRB demonstrates the role that such bursts may have in probing dense environments and constraining dynamical fractions in gravitational wave populations
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Heavy element production in a compact object merger observed by JWST
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)1, sources of high-frequency gravitational waves (GW)2 and likely production sites for heavy element nucleosynthesis via rapid neutron capture (the r-process)3. 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 mergers4â6, and contains a kilonova similar to AT2017gfo, associated with the gravitational-wave merger GW1708177â12. 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