28 research outputs found
Can magnetar spin-down power extended emission in some short GRBs?
Extended emission gamma-ray bursts are a subset of the `short' class of burst
which exhibit an early time rebrightening of gamma emission in their light
curves. This extended emission arises just after the initial emission spike,
and can persist for up to hundreds of seconds after trigger. When their light
curves are overlaid, our sample of 14 extended emission bursts show a
remarkable uniformity in their evolution, strongly suggesting a common central
engine powering the emission. One potential central engine capable of this is a
highly magnetized, rapidly rotating neutron star, known as a magnetar.
Magnetars can be formed by two compact objects coallescing, a scenario which is
one of the leading progenitor models for short bursts in general. Assuming a
magnetar is formed, we gain a value for the magnetic field and late time spin
period for 9 of the extended emission bursts by fitting the magnetic dipole
spin-down model of Zhang & Meszaros (2001). Assuming the magnetic field is
constant, and the observed energy release during extended emission is entirely
due to the spin-down of this magnetar, we then derive the spin period at birth
for the sample. We find all birth spin periods are in good agreement with those
predicted for a newly born magnetar.Comment: 7 pages, 4 figures, 3 tables. Accepted for publication in MNRA
The braking index of millisecond magnetars
We make the first measurement of the braking index of two putative
millisecond magnetars born in short gamma-ray bursts. We measure
and for millisecond magnetars born in GRB 130603B and GRB 140903A
respectively. The neutron star born in GRB 130603B has the only known braking
index consistent with the fiducial value. This value is ruled out with
99.95\% confidence for GRB 140903A. We discuss possible causes of braking
indices in millisecond magnetars, showing that several models can account for
the measurement of the braking index in GRB 140903A, while it is more difficult
to account for a braking index consistent with .Comment: ApJ
LOFAR observations of gravitational wave merger events: O3 results and O4 strategy
The electromagnetic counterparts to gravitational wave (GW) merger events
hold immense scientific value, but are difficult to detect due to the typically
large localisation errors associated with GW events. The Low-Frequency Array
(LOFAR) is an attractive GW follow-up instrument owing to its high sensitivity,
large instantaneous field of view, and ability to automatically trigger on
events to probe potential prompt emission within minutes. Here, we report on
144-MHz LOFAR radio observations of three GW merger events containing at least
one neutron star that were detected during the third GW observing run.
Specifically, we probe 9 and 16 per cent of the location probability density
maps of S190426c and S200213t, respectively, and place limits at the location
of an interesting optical transient (PS19hgw/AT2019wxt) found within the
localisation map of S191213g. While these GW events are not particularly
significant, we use multi-epoch LOFAR data to devise a sensitive wide-field GW
follow-up strategy to be used in future GW observing runs. In particular, we
improve on our previously published strategy by implementing direction
dependent calibration and mosaicing, resulting in nearly an order of magnitude
increase in sensitivity and more uniform coverage. We achieve a uniform
sensitivity of Jy across a single instantaneous LOFAR
pointing's 21 deg core, and a median sensitivity of 1.1 mJy when
including the full 89 deg hexagonal beam pattern. We also place the
deepest transient surface density limits yet on of order month timescales for
surveys between 60--340 MHz (0.017 deg above mJy and 0.073
deg above mJy).Comment: Replaced with accepted version for publication in MNRA
Constraining a neutron star merger origin for localized fast radio bursts
What the progenitors of fast radio bursts (FRBs) are, and whether there are
multiple types of progenitors are open questions. The advent of localized FRBs
with host galaxy redshifts allows the various emission models to be directly
tested for the first time. Given the recent localizations of two non-repeating
FRBs (FRB 180924 and FRB 190523), we discuss a selection of FRB emission models
and demonstrate how we can place constraints on key model parameters like the
magnetic field strength and age of the putative FRB-emitting neutron star. In
particular, we focus on models related to compact binary merger events
involving at least one neutron star, motivated by commonalities between the
host galaxies of the FRBs and the hosts of such merger events/short gamma-ray
bursts (SGRBs). We rule out the possibility that either FRB was produced during
the final inspiral stage of a merging binary system. Where possible, we predict
the light curve of electromagnetic emission associated with a given model and
use it to recommend multi-wavelength follow-up strategies that may help confirm
or rule out models for future FRBs. In addition, we conduct a targeted
sub-threshold search in Fermi Gamma-ray Burst Monitor data for potential SGRB
candidates associated with either FRB, and show what a non-detection means for
relevant models. The methodology presented in this study may be easily applied
to future localized FRBs, and adapted to sources with possibly core-collapse
supernova progenitors, to help constrain potential models for the FRB
population at large.Comment: MNRAS accepted version. Published in MNRAS, 12 pages, 6 figure
Long-term study of extreme giant pulses from PSR B0950+08 with AARTFAAC
We report on the detection of extreme giant pulses (GPs) from one of the
oldest-known pulsars, the highly variable PSR B0950+08, with the
Amsterdam-ASTRON Radio Transient Facility And Analysis Centre (AARTFAAC), a
parallel transient detection instrument operating as a subsystem of the LOw
Frequency ARray (LOFAR). During processing of our Northern Hemisphere survey
for low-frequency radio transients, a sample of 275 pulses with fluences
ranging from 42 to 177 kJy ms were detected in one-second snapshot images. The
brightest pulses are an order of magnitude brighter than those previously
reported at 42 and 74 MHz, on par with the levels observed in a previous
long-term study at 103 MHz. Both their rate and fluence distribution differ
between and within the various studies done to date. The GP rate is highly
variable, from 0 to 30 per hour, with only two three-hour observations
accounting for nearly half of the pulses detected in the 96 h surveyed. It does
not vary significantly within a few-hour observation, but can vary strongly one
from day to the next. The spectra appear strongly and variably structured, with
emission sometimes confined to a single 195.3 kHz subband, and the pulse
spectra changing on a timescale of order 10 min.Comment: 10 pages, 9 figures, 1 table. Submitted to MNRA
Transient study using LoTSS -- framework development and preliminary results
We present a search for transient radio sources on time-scales of seconds to
hours at 144 MHz using the LOFAR Two-metre Sky Survey (LoTSS). This search is
conducted by examining short time-scale images derived from the LoTSS data. To
allow imaging of LoTSS on short time-scales, a novel imaging and filtering
strategy is introduced. This includes sky model source subtraction, no cleaning
or primary beam correction, a simple source finder, fast filtering schemes and
source catalogue matching. This new strategy is first tested by injecting
simulated transients, with a range of flux densities and durations, into the
data. We find the limiting sensitivity to be 113 and 6 mJy for 8 second and 1
hour transients respectively. The new imaging and filtering strategies are
applied to 58 fields of the LoTSS survey, corresponding to LoTSS-DR1 (2% of the
survey). One transient source is identified in the 8 second and 2 minute
snapshot images. The source shows one minute duration flare in the 8 hour
observation. Our method puts the most sensitive constraints on/estimates of the
transient surface density at low frequencies at time-scales of seconds to
hours; at 1 hour at a sensitivity of 6.3
mJy; at 2 minutes at a sensitivity of 30
mJy; and at 8 seconds at a sensitivity of
113 mJy. In the future, we plan to apply the strategies presented in this paper
to all LoTSS data.Comment: submitted to MNRA
Limits on long-time-scale radio transients at 150 MHz using the TGSS ADR1 and LoTSS DR2 catalogues
We present a search for transient radio sources on timescales of 2 to 9 yr at
150 MHz. This search is conducted by comparing the first Alternative Data
Release of the TIFR GMRT Sky Survey (TGSS ADR1) and the second data release of
the LOFAR Two-metre Sky Survey (LoTSS DR2). The overlapping survey area covers
5570 on the sky, or 14 per cent of the total sky. We introduce a
method to compare the source catalogues that involves a pair match of sources,
a flux density cutoff to meet the survey completeness limit and a newly
developed compactness criterion. This method is used to identify both transient
candidates in the TGSS source catalogue that have no counterpart in the LoTSS
catalogue and transient candidates in LoTSS without a counterpart in TGSS. We
find that imaging artefacts and uncertainties and variations in the flux
density scales complicate the transient search. Our method to search for
transients by comparing two different surveys, while taking into account
imaging artefacts around bright sources and misaligned flux scales between
surveys, is universally applicable to future radio transient searches. No
transient sources were identified, but we are able to place an upper limit on
the transient surface density of at 150
MHz for compact sources with an integrated flux density over 100 mJy. Here we
define a transient as a compact source with flux density greater than 100 mJy
that appears in the catalogue of one survey without a counterpart in the other
survey.Comment: 14 pages, 11 figure
Identifying transient and variable sources in radio images
With the arrival of a number of wide-field snapshot image-plane radio transient surveys, there will be a huge influx of images in the coming years making it impossible to manually analyse the datasets. Automated pipelines to process the information stored in the images are being developed, such as the LOFAR Transients Pipeline, outputting light curves and various transient parameters. These pipelines have a number of tuneable parameters that require training to meet the survey requirements. This paper utilises both observed and simulated datasets to demonstrate different machine learning strategies that can be used to train these parameters. We use a simple anomaly detection algorithm and a penalised logistic regression algorithm. The datasets used are from LOFAR observations and we process the data using the LOFAR Transients Pipeline; however the strategies developed are applicable to any light curve datasets at different frequencies and can be adapted to different automated pipelines. These machine learning strategies are publicly available as PYTHON tools that can be downloaded and adapted to different datasets (https://github.com/AntoniaR/TraP_ML_tools)