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 $-2.0 < \gamma <-1.0$. 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 $\u3c1$~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 $\sim$ 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| < $60^{\circ}$) 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 ${\cal R} = 4.4^{+5.2}_{-3.1} \times 10^3$ FRBs day$^{-1}$ sky$^{-1}$, where the uncertainties represent a $99\%$ 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 ($\sim$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

Simultaneous Radio And X-Ray Observations Of Psr B0611+22

We report results from simultaneous radio and X-ray observations of PSR B0611+22 which is known to exhibit bursting in its single-pulse emission. The pulse phase of the bursts vary with radio frequency. The bursts are correlated in 327/150 MHz data sets while they are anti-correlated, with bursts at one frequency associated with normal emission at the other, in 820/150 MHz data sets. Also, the flux density of this pulsar is lower than expected at 327 MHz assuming a power law. We attribute this unusual behaviour to the pulsar itself rather than absorption by external astrophysical sources. Using this data set over an extensive frequency range, we show that the bursting phenomenon in this pulsar exhibits temporal variance over a span of few hours. We also show that the bursting is quasi-periodic over the observed band. The anti-correlation in the phase offset of the burst mode at different frequencies suggests that the mechanisms responsible for phase offset and flux enhancement have different dependencies on the frequency. We did not detect the pulsar with XMM-Newton and place a 99 per cent confidence upper limit on the X-ray efficiency of 10-5

Multi-frequency study of the peculiar pulsars PSR B0919+06 and PSR B1859+07

Since their discovery more than 50 years ago, broadband radio studies of pulsars have generated a wealth of information about the underlying physics of radio emission. In order to gain some further insights into this elusive emission mechanism, we performed a multi-frequency study of two very well-known pulsars, PSR~B0919+06 and PSR~B1859+07. These pulsars show peculiar radio emission properties whereby the emission shifts to an earlier rotation phase before returning to the nominal emission phase in a few tens of pulsar rotations (also known as `swooshes'). We confirm the previous claim that the emission during the swoosh is not necessarily absent at low frequencies and the single pulses during a swoosh show varied behaviour at 220~MHz. We also confirm that in PSR~B0919+06, the pulses during the swoosh show a chromatic dependence of the maximum offset from the normal emission phase with the offset following a consistent relationship with observing frequency. We also observe that the flux density spectrum of the radio profile during the swoosh is inverted compared to the normal emission. For PSR~B1859+07, we have discovered a new mode of emission in the pulsar that is potentially quasi-periodic with a different periodicity than is seen in its swooshes. We invoke an emission model previously proposed in the literature and show that this simple model can explain the macroscopic observed characteristics in both pulsars. We also argue that pulsars that exhibit similar variability on short timescales may have the same underlying emission mechanism.Comment: 13 pages, 13 figures, 1 table, accepted for publication in MNRA