129 research outputs found
Discovery and characterisation of fast radio transients
Many types of astronomical objects are detectable through the radio waves that they produce. The observed properties of sources in the 'radio sky' can vary on a wide range of timescales, both for intrinsic and extrinsic reasons. Transients are those sources whose observed properties change drastically within a timescale that a human astronomer can measure. Traditionally, the qualifier 'fast' refers to transients whose emission properties change significantly over less than a second, and most commonly on millisecond timescales. The two classes of fast radio transients studied in this thesis are pulsars and fast radio bursts. We are motivated to study these sources because they probe extreme astrophysical environments as well as the intervening magnetised and ionised medium between observer and source. Furthermore, the origin of the relatively recently discovered fast radio bursts remains enigmatic, though the properties of these signals share traits of pulsar emission. Thus, we aim to better understand the physical nature of fast radio bursts and their possible link to pulsar emission. We discuss the characteristics of these source classes and outline some of the outstanding scientific questions we can address through observations with a radio telescope and other complementary, multi-wavelength information
A sample of low energy bursts from FRB 121102
We present 41 bursts from the first repeating fast radio burst discovered
(FRB 121102). A deep search has allowed us to probe unprecedentedly low burst
energies during two consecutive observations (separated by one day) using the
Arecibo telescope at 1.4 GHz. The bursts are generally detected in less than a
third of the 580-MHz observing bandwidth, demonstrating that narrow-band FRB
signals may be more common than previously thought. We show that the bursts are
likely faint versions of previously reported multi-component bursts. There is a
striking lack of bursts detected below 1.35 GHz and simultaneous VLA
observations at 3 GHz did not detect any of the 41 bursts, but did detect one
that was not seen with Arecibo, suggesting preferred radio emission frequencies
that vary with epoch. A power law approximation of the cumulative distribution
of burst energies yields an index that is much steeper than the
previously reported value of . The discrepancy may be evidence for a
more complex energy distribution. We place constraints on the possibility that
the associated persistent radio source is generated by the emission of many
faint bursts ( ms). We do not see a connection between burst
fluence and wait time. The distribution of wait times follows a log-normal
distribution centered around s; however, some bursts have wait times
below 1 s and as short as 26 ms, which is consistent with previous reports of a
bimodal distribution. We caution against exclusively integrating over the full
observing band during FRB searches, because this can lower signal-to-noise.Comment: Accepted version. 16 pages, 7 figures, 1 tabl
Scattering analysis of LOFAR pulsar observations
We measure the effects of interstellar scattering on average pulse profiles
from 13 radio pulsars with simple pulse shapes. We use data from the LOFAR High
Band Antennas, at frequencies between 110 and 190~MHz. We apply a forward
fitting technique, and simultaneously determine the intrinsic pulse shape,
assuming single Gaussian component profiles. We find that the constant ,
associated with scattering by a single thin screen, has a power-law dependence
on frequency , with indices ranging from to , despite simplest theoretical models predicting
or . Modelling the screen as an isotropic or extremely anisotropic
scatterer, we find anisotropic scattering fits lead to larger power-law
indices, often in better agreement with theoretically expected values. We
compare the scattering models based on the inferred, frequency dependent
parameters of the intrinsic pulse, and the resulting correction to the
dispersion measure (DM). We highlight the cases in which fits of extreme
anisotropic scattering are appealing, while stressing that the data do not
strictly favour either model for any of the 13 pulsars. The pulsars show
anomalous scattering properties that are consistent with finite scattering
screens and/or anisotropy, but these data alone do not provide the means for an
unambiguous characterization of the screens. We revisit the empirical
versus DM relation and consider how our results support a frequency dependence
of . Very long baseline interferometry, and observations of the
scattering and scintillation properties of these sources at higher frequencies,
will provide further evidence.Comment: 24 pages, 23 figures, supplementary appendi
Simultaneous X-ray, gamma-ray, and Radio Observations of the repeating Fast Radio Burst FRB 121102
We undertook coordinated campaigns with the Green Bank, Effelsberg, and
Arecibo radio telescopes during Chandra X-ray Observatory and XMM-Newton
observations of the repeating fast radio burst FRB 121102 to search for
simultaneous radio and X-ray bursts. We find 12 radio bursts from FRB 121102
during 70 ks total of X-ray observations. We detect no X-ray photons at the
times of radio bursts from FRB 121102 and further detect no X-ray bursts above
the measured background at any time. We place a 5 upper limit of
erg cm on the 0.5--10 keV fluence for X-ray bursts at
the time of radio bursts for durations ms, which corresponds to a burst
energy of erg at the measured distance of FRB 121102. We also
place limits on the 0.5--10 keV fluence of erg cm and
erg cm for bursts emitted at any time during the
XMM-Newton and Chandra observations, respectively, assuming a typical X-ray
burst duration of 5 ms. We analyze data from the Fermi Gamma-ray Space
Telescope Gamma-ray Burst Monitor and place a 5 upper limit on the
10--100 keV fluence of erg cm ( erg at
the distance of FRB 121102) for gamma-ray bursts at the time of radio bursts.
We also present a deep search for a persistent X-ray source using all of the
X-ray observations taken to date and place a 5 upper limit on the
0.5--10 keV flux of erg s cm (
erg~s at the distance of FRB 121102). We discuss these non-detections in
the context of the host environment of FRB 121102 and of possible sources of
fast radio bursts in general.Comment: 13 pages, 5 figures, published in Ap
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