143 research outputs found
Are all fast radio bursts repeating sources?
We present Monte-Carlo simulations of a cosmological population of repeating
fast radio burst (FRB) sources whose comoving density follows the cosmic star
formation rate history. We assume a power-law model for the intrinsic energy
distribution for each repeating FRB source located at a randomly chosen
position in the sky and simulate their dispersion measures (DMs) and
propagation effects along the chosen lines-of-sight to various telescopes. In
one scenario, an exponential distribution for the intrinsic wait times between
pulses is chosen, and in a second scenario we model the observed pulse arrival
times to follow a Weibull distribution. For both models we determine whether
the FRB source would be deemed a repeater based on the telescope sensitivity
and time spent on follow-up observations. We are unable to rule out the
existence of a single FRB population based on comparisons of our simulations
with the longest FRB follow-up observations performed. We however rule out the
possibility of FRBs 171020 and 010724 repeating with the same rate statistics
as FRB 121102 and also constrain the slope of a power-law fit to the FRB energy
distribution to be . All-sky simulations of repeating FRB
sources imply that the detection of singular events correspond to the bright
tail-end of the adopted energy distribution due to the combination of the
increase in volume probed with distance, and the position of the burst in the
telescope beam.Comment: 10 pages, 4 figures, accepted for publication in MNRA
Multi-frequency observations and spectral analysis of two gigahertz-peaked spectra pulsars
We report the multi-frequency observations of two pulsars: J1740+1000 and
B1800-21, using the Giant Metrewave Radio Telescope and the Green Bank
Telescope. The main aim of these observations was to estimate the flux density
spectrum of these pulsars, as both of them were previously reported to exhibit
gigahertz-peaked spectra. J1740+1000 is a young pulsar far from the Galactic
plane and the interpretation of its spectrum was inconclusive in the light of
the recent flux density measurements. Our result supports the gigahertz-peaked
interpretation of the PSR J1740+1000 spectrum. B1800-21 is a Vela-like pulsar
near the W30 complex, whose spectrum exhibit a significant change between 2012
and 2014 year. Our analysis shows that the current shape of the spectrum is
similar to that observed before 2009 and confirms that the observed spectral
change happen in a time-scale of a few years.Comment: 9 pages, 7 figure
Detecting fast radio bursts at decametric wavelengths
Fast radio bursts (FRBs) are highly dispersed, sporadic radio pulses that are likely extragalactic in nature. Here we investigate the constraints on the source population from surveys carried out at frequencies ~GHz. All but one FRB has so far been discovered in the 1--2~GHz band, but new and emerging instruments look set to become valuable probes of the FRB population at sub-GHz frequencies in the near future. In this paper, we consider the impacts of free-free absorption and multi-path scattering in our analysis via a number of different assumptions about the intervening medium. We consider previous low frequency surveys alongwith an ongoing survey with the University of Technology digital backend for the Molonglo Observatory Synthesis Telescope (UTMOST) as well as future observations with the Canadian Hydrogen Intensity Mapping Experiment (CHIME) and the Hydrogen Intensity and Real-Time Analysis Experiment (HIRAX). We predict that CHIME and HIRAX will be able to observe 30 or more FRBs per day, even in the most extreme scenarios where free-free absorption and scattering can significantly impact the fluxes below 1~GHz. We also show that UTMOST will detect 1--2 FRBs per month of observations. For CHIME and HIRAX, the detection rates also depend greatly on the assumed FRB distance scale. Some of the models we investigated predict an increase in the FRB flux as a function of redshift at low frequencies. If FRBs are truly cosmological sources, this effect may impact future surveys in this band, particularly if the FRB population traces the cosmic star formation rate. Rajwade, Kaustubh; Lorimer, Dunca
A search for rotating radio transients and fast radio bursts in the Parkes high-latitude pulsar survey
Discoveries of rotating radio transients and fast radio bursts (FRBs) in
pulsar surveys suggest that more of such transient sources await discovery in
archival data sets. Here we report on a single-pulse search for dispersed radio
bursts over a wide range of Galactic latitudes (|b| < ) in data
previously searched for periodic sources by Burgay et al. We re-detected 20 of
the 42 pulsars reported by Burgay et al. and one rotating radio transient
reported by Burke-Spolaor. No FRBs were discovered in this survey. Taking into
account this result, and other recent surveys at Parkes, we corrected for
detection sensitivities based on the search software used in the analyses and
the different backends used in these surveys and find that the all-sky FRB
event rate for sources with a fluence above 4.0 Jy ms at 1.4 GHz to be FRBs day sky, where the
uncertainties represent a confidence interval. While this rate is lower
than inferred from previous studies, as we demonstrate, this combined event
rate is consistent with the results of all systematic FRB searches at Parkes to
date and does not require the need to postulate a dearth of FRBs at
intermediate latitudes.Comment: Accepted, 10 pages, 6 figure
IQRM: real-time adaptive RFI masking for radio transient and pulsar searches
In a search for short timescale astrophysical transients in time-domain data,
radio-frequency interference (RFI) causes both large quantities of false
positive candidates and a significant reduction in sensitivity if not correctly
mitigated. Here we propose an algorithm that infers a time-variable frequency
channel mask directly from short-duration (1 s) data blocks: the method
consists of computing a spectral statistic that correlates well with the
presence of RFI, and then finding high outliers among the resulting values. For
the latter task, we propose an outlier detection algorithm called
Inter-Quartile Range Mitigation (IQRM), that is both non-parametric and robust
to the presence of a trend in sequential data. The method requires no training
and can in principle adapt to any telescope and RFI environment; its efficiency
is shown on data from both the MeerKAT and Lovell 76-m radio telescopes. IQRM
is fast enough to be used in a streaming search and has been integrated into
the MeerTRAP real-time transient search pipeline. Open-source Python and C++
implementations are also provided.Comment: Accepted for publication in MNRAS, 11 pages, 9 figures. Minor
corrections, improved clarity, additional analysis of impact on single pulse
searches. IQRM implementations can be found at
https://github.com/v-morello/iqrm (Python) and
https://gitlab.com/kmrajwade/iqrm_apollo (C++
Detecting pulsars in the Galactic Centre
Although high-sensitivity surveys have revealed a number of highly dispersed pulsars in the inner Galaxy, none have so far been found in the Galactic Centre (GC) region, which we define to be within a projected distance of 1 pc from Sgr A*. This null result is surprising given that several independent lines of evidence predict a sizable population of neutron stars in the region. Here, we present a detailed analysis of both the canonical and millisecond pulsar populations in the GC and consider free–free absorption and multipath scattering to be the two main sources of flux density mitigation. We demonstrate that the sensitivity limits of previous surveys are not sufficient to detect GC pulsar population, and investigate the optimum observing frequency for future surveys. Depending on the degree of scattering and free–free absorption in the GC, current surveys constrain the size of the potentially observable population (i.e. those beaming towards us) to be up to 52 canonical pulsars and 10 000 millisecond pulsars. We find that the optimum frequency for future surveys is in the range of 9–13 GHz. We also predict that future deeper surveys with the Square Kilometre array will probe a significant portion of the existing radio pulsar population in the GC
Detecting pulsars in the Galactic Centre
Although high-sensitivity surveys have revealed a number of highly dispersed pulsars in the inner Galaxy, none have so far been found in the Galactic Centre (GC) region, which we define to be within a projected distance of 1 pc from Sgr A*. This null result is surprising given that several independent lines of evidence predict a sizable population of neutron stars in the region. Here, we present a detailed analysis of both the canonical and millisecond pulsar populations in the GC and consider free–free absorption and multipath scattering to be the two main sources of flux density mitigation. We demonstrate that the sensitivity limits of previous surveys are not sufficient to detect GC pulsar population, and investigate the optimum observing frequency for future surveys. Depending on the degree of scattering and free–free absorption in the GC, current surveys constrain the size of the potentially observable population (i.e. those beaming towards us) to be up to 52 canonical pulsars and 10 000 millisecond pulsars. We find that the optimum frequency for future surveys is in the range of 9–13 GHz. We also predict that future deeper surveys with the Square Kilometre array will probe a significant portion of the existing radio pulsar population in the GC
Evidence for an abundant old population of Galactic ultra long period magnetars and implications for fast radio bursts
Two recent discoveries, namely PSR J0901-4046 and GLEAM-X J162759.5-523504.3
(hereafter GLEAM-X J1627), have corroborated an extant population of radio-loud
periodic sources with long periods (76 s and 1091 s respectively) whose
emission can hardly be explained by rotation losses. We argue that GLEAM-X
J1627 is a highly-magnetized object consistent with a magnetar (an ultra long
period magnetar - ULPM), and demonstrate it is unlikely to be either a
magnetically or a rotationally-powered white dwarf. By studying these sources
together with previously detected objects, we find there are at least a handful
of promising candidates for Galactic ULPMs. The detections of these objects
imply a substantial number, and for PSR
J0901--4046 like and GLEAM-X J1627 like objects, respectively, within our
Galaxy. These source densities, as well as cooling age limits from
non-detection of thermal X-rays, Galactic offsets, timing stability and dipole
spindown limits, all imply the ULPM candidates are substantially older than
confirmed Galactic magnetars and that their formation channel is a common one.
Their existence implies widespread survival of magnetar-like fields for several
Myr, distinct from the inferred behaviour in confirmed Galactic magnetars.
ULPMs may also constitute a second class of FRB progenitors which could
naturally exhibit very long periodic activity windows. Finally, we show that
existing radio campaigns are biased against detecting objects like these and
discuss strategies for future radio and X-ray surveys to identify more such
objects. We estimate that more such objects should be detected
with SKA-MID and DSA-2000.Comment: 22 pages, 10 figures. Published in MNRA
Point counting on reductions of CM elliptic curves
We give explicit formulas for the number of points on reductions of elliptic
curves with complex multiplication by any imaginary quadratic field. We also
find models for CM -curves in certain cases. This generalizes
earlier results of Gross, Stark, and others.Comment: Minor corrections. To appear in Journal of Number Theor
- …