arXiv subjects

Ryan M. Shannon

Publications and source records attributed to Ryan M. Shannon.

At least 91 records · Page 5Linked to original sources

arXiv2022

Circularly polarized radio emission from the repeating fast radio burst source FRB 20201124A

The mechanism that produces fast radio burst (FRB) emission is poorly understood. Targeted monitoring of repeating FRB sources provides the opportunity to fully characterize the emission properties in a manner impossible with one-off bursts. Here, we report observations of the source of FRB 20201124A, with the Australian Square Kilometre Array Pathfinder (ASKAP) and the ultra-wideband low (UWL) receiver at the Parkes 64-m radio telescope (Murriyang). The source entered a period of emitting bright bursts during early 2021 April. We have detected 16 bursts from this source. One of the bursts detected with ASKAP is the brightest burst ever observed from a repeating FRB source with an inferred fluence of $640\pm70$ Jy ms. Of the five bursts detected with the Parkes UWL, none display any emission in the range 1.1--4 GHz. All UWL bursts are highly polarized, with their Faraday rotation measures (RMs) showing apparent variations. We obtain an average RM of $-614$ rad m$^{-2}$ for this FRB source with a standard deviation of $16$ rad m$^{-2}$ in the UWL bursts. In one of the UWL bursts, we see evidence of significant circularly polarized emission with a fractional extent of $47\pm1$ per cent. Such a high degree of circular polarization has never been seen before in bursts from repeating FRB sources. We also see evidence for significant variation in the linear polarization position angle in the pulse profile of this UWL repeat burst. Models for repeat burst emission will need to account for the increasing diversity in the burst polarization properties.

astro-ph.HE↗

arXiv2021

High-precision search for dark photon dark matter with the Parkes Pulsar Timing Array

The nature of dark matter remains obscure in spite of decades of experimental efforts. The mass of dark matter candidates can span a wide range, and its coupling with the Standard Model sector remains uncertain. All these unknowns make the etection of dark matter extremely challenging. Ultralight dark matter, with $m \sim10^{-22}$ eV, is proposed to reconcile the disagreements between observations and predictions from simulations of small-scale structures in the cold dark matter paradigm, while remaining consistent with other observations. Because of its large de Broglie wavelength and large local occupation number within galaxies, ultralight dark matter behaves like a coherently oscillating background field with an oscillating frequency dependent on its mass. If the dark matter particle is a spin-1 dark photon, such as the $U(1)_B$ or $U(1)_{B-L}$ gauge boson, it can induce an external oscillating force and lead to displacements of test masses. Such an effect would be observable in the form of periodic variations in the arrival times of radio pulses from highly stable millisecond pulsars. In this study, we search for evidence of ultralight dark photon dark matter (DPDM) using 14-year high-precision observations of 26 pulsars collected with the Parkes Pulsar Timing Array. While no statistically significant signal is found, we place constraints on coupling constants for the $U(1)_B$ and $U(1)_{B-L}$ DPDM. Compared with other experiments, the limits on the dimensionless coupling constant $ε$ achieved in our study are improved by up to two orders of magnitude when the dark photon mass is smaller than $3\times10^{-22}$~eV ($10^{-22}$~eV) for the $U(1)_{B}$ ($U(1)_{B-L}$) scenario.

hep-ph↗

arXiv2021

Dissecting the Local Environment of FRB 190608 in the Spiral Arm of its Host Galaxy

We present a high-resolution analysis of the host galaxy of fast radio burst (FRB)~190608, an SB(r)c galaxy at $z=0.11778$ (hereafter HG 190608), to dissect its local environment and its contributions to the FRB properties. Our Hubble Space Telescope Wide Field Camera 3 ultraviolet and visible light image reveals that the subarcsecond localization of FRB~190608 is coincident with a knot of star-formation ($Σ_{\rm SFR} = 1.5 \times 10^{-2}~ M_{\odot} \, \rm \, yr^{-1} \, kpc^{-2}$) in the northwest spiral arm of HG~190608. Using H$β$ emission present in our Keck Cosmic Web Imager integral field spectrum of the galaxy with a surface brightness of $μ_{\rm Hβ}= \mathrm{(3.36\pm0.21)\times10^{-17}\;erg\;s^{-1}\;cm^{-2}\;arcsec^{-2}}$, we infer an extinction-corrected H$α$ surface brightness and compute a dispersion measure (DM) from the interstellar medium of HG 190608 of $\rm DM_{\rm Host,ISM} = 94 \pm 38~ \;pc\;cm^{-3}$. The galaxy rotates with a circular velocity $v_{\rm circ} = \rm 141 \pm 8~ km\;s^{-1}$ at an inclination $i_{\mathrm{gas}} = 37 \pm 3^\circ$, giving a dynamical mass $M_{\rm halo}^{\rm dyn} \approx 10^{11.96 \pm 0.08}~M_{\odot}$. This implies a halo contribution to the DM of $\rm DM_{\rm Host,Halo}= \rm 55\pm25 \;pc\;cm^{-3}$ subject to assumptions on the density profile and fraction of baryons retained. From the galaxy rotation curve, we infer a bar-induced pattern speed of $Ω_p=34\pm 6\;\mathrm{km\;s^{-1}\;kpc^{-1}}$ using linear resonance theory. We then calculate the maximum time since star-formation for a progenitor using the furthest distance to the arm's leading edge within the localization, and find $t_{\mathrm{enc}} = 21_{-6}^{+25}$ Myr. Unlike previous high-resolution studies of FRB environments, we find no evidence of disturbed morphology, emission, or kinematics for FRB 190608.

astro-ph.GA↗

arXiv2021

Mode changing in J1909$-$3744: the most precisely timed pulsar

We present baseband radio observations of the millisecond pulsar J1909$-$3744, the most precisely timed pulsar, using the MeerKAT telescope as part of the MeerTime pulsar timing array campaign. During a particularly bright scintillation event the pulsar showed strong evidence of pulse mode changing, among the first millisecond pulsars and the shortest duty cycle millisecond pulsar to do so. Two modes appear to be present, with the weak (lower signal-to-noise ratio) mode arriving $9.26$ $\pm 3.94$ $μ$s earlier than the strong counterpart. Further, we present a new value of the jitter noise for this pulsar of $8.20 \pm 0.14$ ns in one hour, finding it to be consistent with previous measurements taken with the MeerKAT ($9 \pm 3$ ns) and Parkes ($8.6 \pm 0.8$ ns) telescopes, but inconsistent with the previously most precise measurement taken with the Green Bank telescope ($14 \pm 0.5$ ns). Timing analysis on the individual modes is carried out for this pulsar, and we find an approximate $10\%$ improvement in the timing precision is achievable through timing the strong mode only as opposed to the full sample of pulses. By forming a model of the average pulse from templates of the two modes, we time them simultaneously and demonstrate that this timing improvement can also be achieved in regular timing observations. We discuss the impact an improvement of this degree on this pulsar would have on searches for the stochastic gravitational wave background, as well as the impact of a similar improvement on all MeerTime PTA pulsars.

astro-ph.HE↗

arXiv2021

Characterizing the FRB host galaxy population and its connection to transients in the local and extragalactic Universe

We present the localization and host galaxies of one repeating and two apparently non-repeating Fast Radio Bursts. FRB20180301A was detected and localized with the Karl G. Jansky Very Large Array to a star-forming galaxy at $z=0.3304$. FRB20191228A, and FRB20200906A were detected and localized by the Australian Square Kilometre Array Pathfinder to host galaxies at $z=0.2430$ and $z=0.3688$, respectively. We combine these with 13 other well-localized FRBs in the literature, and analyze the host galaxy properties. We find no significant differences in the host properties of repeating and apparently non-repeating FRBs. FRB hosts are moderately star-forming, with masses slightly offset from the star-forming main-sequence. Star formation and low-ionization nuclear emission-line region (LINER) emission are major sources of ionization in FRB host galaxies, with the former dominant in repeating FRB hosts. FRB hosts do not track stellar mass and star formation as seen in field galaxies (more than 95% confidence). FRBs are rare in massive red galaxies, suggesting that progenitor formation channels are not solely dominated by delayed channels which lag star formation by Gigayears. The global properties of FRB hosts are indistinguishable from core-collapse supernovae (CCSNe) and short gamma-ray bursts (SGRBs) hosts, and the spatial offset (from galaxy centers) of FRBs is mostly inconsistent with that of the Galactic neutron star population (95% confidence). The spatial offsets of FRBs (normalized to the galaxy effective radius) also differ from those of globular clusters (GCs) in late- and early-type galaxies with 95% confidence.

astro-ph.HE↗

arXiv2021

The impact of glitches on young pulsar rotational evolution

We report on a timing programme of 74 young pulsars that have been observed by the Parkes 64-m radio telescope over the past decade. Using modern Bayesian timing techniques, we have measured the properties of 124 glitches in 52 of these pulsars, of which 74 are new. We demonstrate that the glitch sample is complete to fractional increases in spin-frequency greater than $Δν^{90\%}_{g}/ν\approx 8.1 \times 10^{-9}$. We measure values of the braking index, $n$, in 33 pulsars. In most of these pulsars, their rotational evolution is dominated by episodes of spin-down with $n > 10$, punctuated by step changes in the spin-down rate at the time of a large glitch. The step changes are such that, averaged over the glitches, the long-term $n$ is small. We find a near one-to-one relationship between the inter-glitch value of $n$ and the change in spin-down of the previous glitch divided by the inter-glitch time interval. We discuss the results in the context of a range of physical models.

astro-ph.HE↗

arXiv2021

Chronicling the Host Galaxy Properties of the Remarkable Repeating FRB 20201124A

We present the Australian Square Kilometre Array Pathfinder (ASKAP) localization and follow-up observations of the host galaxy of the repeating fast radio burst (FRB) source, FRB20201124A, the fifth such extragalactic repeating FRB with an identified host. From spectroscopic observations using the 6.5-m MMT Observatory, we derive a redshift of $z=0.0979 \pm 0.0001$, a star formation rate inferred from H$α$ emission of SFR(H$α$) $\approx 2.1 M_{\odot}$ yr$^{-1}$, and a gas-phase metallicity of 12+log(O/H)$\approx 9.0$. By jointly modeling the 12-filter optical-mid-infrared (MIR) photometry and spectroscopy of the host, we infer a median stellar mass of $\approx 2 \times 10^{10} M_{\odot}$, internal dust extinction of $A_V\approx 1-1.5$ mag, and a mass-weighted stellar population age of $\approx 5-6$ Gyr. Connecting these data to the radio and X-ray observations, we cannot reconcile the broad-band behavior with strong AGN activity and instead attribute the dominant source of persistent radio emission to star formation, likely originating from the circumnuclear region of the host. The modeling also indicates a hot dust component contributing to the MIR luminosity at a level of $\approx 10-30\%$. We model the host galaxy's star formation and mass assembly histories, finding that the host assembled $>90\%$ of its mass by 1 Gyr ago and exhibited a fairly constant SFR for most of its existence, with no clear evidence of past star-burst activity.

astro-ph.GA↗

arXiv2021

Estimating the contribution of foreground halos to the FRB 180924 dispersion measure

Fast Radio Burst (FRB) dispersion measures (DMs) record the presence of ionized baryons that are otherwise invisible to other techniques enabling resolution of the matter distribution in the cosmic web. In this work, we aim to estimate the contribution to FRB 180924 DM from foreground galactic halos. Localized by ASKAP to a massive galaxy, this sightline is notable for an estimated cosmic web contribution to the DM ($\rm DM_{cosmic} = 220~pc~cm^{-3}$), which is less than the average value at the host redshift ($\rm z = 0.3216$) estimated from the Macquart relation ($280~\rm pc~cm^{-3}$). In the favored models of the cosmic web, this suggests few intersections with foreground halos at small impact parameters ($\lesssim 100$ kpc). To test this hypothesis, we carried out spectroscopic observations of the field galaxies within $\sim$1' of the sightline with VLT/MUSE and Keck/LRIS. Furthermore, we developed a probabilistic methodology that leverages photometric redshifts derived from wide-field DES and WISE imaging. We conclude that there is no galactic halo that closely intersects the sightline and also that the net DM contribution from halos, $\rm DM_{halos}< 45~pc~cm^{-3}$ (95 % c.l.). This value is lower than the $\rm DM_{halos}$ estimated from an "average" sightline ($121~\rm pc~cm^{-3}$) using the Planck $ΛCDM$ model and the Aemulus halo mass function and reasonably explains its low $\rm DM_{cosmic}$ value. We conclude that FRB 180924 represents the predicted majority of sightlines in the universe with no proximate foreground galactic halos. Our framework lays the foundation for a comprehensive analysis of FRB fields in the near future.

astro-ph.CO↗

arXiv2021

A High-Resolution View of Fast Radio Burst Host Environments

We present Hubble Space Telescope (HST/WFC3) ultraviolet and infrared observations of eight fast radio burst (FRB) host galaxies with sub-arcsecond localizations, including the hosts of three known repeating FRBs. We quantify their spatial distributions and locations with respect to their host galaxy light distributions, finding that they occur at moderate host normalized-offsets of 1.4 $r_e$ ([0.6,2.1] $r_e$; 68% interval), occur on fainter regions of their hosts in terms of IR light, but overall trace the radial distribution of IR light in their galaxies. The FRBs in our tested distribution do not clearly trace the distributions of any other transient population with known progenitors, and are statistically distinct from the locations of LGRBs, H-poor SLSNe, SGRBs, and Ca-rich transients. We further find that most FRBs are not in regions of elevated local star formation rate and stellar mass surface densities in comparison to the mean global values of their hosts. We also place upper limits to the IR flux at the FRB positions of $m_{\rm IR}\gtrsim\!24.8-27.6$~AB~mag, constraining both satellite and background galaxies to luminosities well below the host luminosity of FRB121102. We find that 5/8 FRB hosts exhibit clear spiral arm features in IR light, and that the positions of all well-localized FRBs located in such hosts are consistent with their spiral arms, although not on their brightest regions. Our results do not strongly support the primary progenitor channel of FRBs being connected either with the most massive (stripped-envelope) stars, or with events which require kicks and long delay times (neutron star mergers).

astro-ph.GA↗

arXiv2020

Disentangling the Cosmic Web Towards FRB 190608

FRB 190608 was detected by ASKAP and localized to a spiral galaxy at $z_{host}=0.11778$ in the SDSS footprint. The burst has a large dispersion measure ($DM_{FRB}=339.8$ $pc/cm^3$) compared to the expected cosmic average at its redshift. It also has a large rotation measure ($RM_{FRB}=353$ $rad/m^2$) and scattering timescale ($τ=3.3$ $ms$ at $1.28$ $GHz$). Chittidi et al (2020) perform a detailed analysis of the ultraviolet and optical emission of the host galaxy and estimate the host DM contribution to be $110\pm 37$ $pc/cm^3$. This work complements theirs and reports the analysis of the optical data of galaxies in the foreground of FRB 190608 to explore their contributions to the FRB signal. Together, the two manuscripts delineate an observationally driven, end-to-end study of matter distribution along an FRB sightline; the first study of its kind. Combining KCWI observations and public SDSS data, we estimate the expected cosmic dispersion measure $DM_{cosmic}$ along the sightline to FRB 190608. We first estimate the contribution of hot, ionized gas in intervening virialized halos ($DM_{halos} \approx 7-28$ $pc/cm^3$). Then, using the Monte Carlo Physarum Machine (MCPM) methodology, we produce a 3D map of ionized gas in cosmic web filaments and compute the DM contribution from matter outside halos ($DM_{IGM} \approx 91-126$ $pc/cm^3$). This implies a greater fraction of ionized gas along this sightline is extant outside virialized halos. We also investigate whether the intervening halos can account for the large FRB rotation measure and pulse width and conclude that it is implausible. Both the pulse broadening and the large Faraday rotation likely arise from the progenitor environment or the host galaxy.

astro-ph.GA↗

arXiv2020

Extremely band-limited repetition from a fast radio burst source

The fast radio burst (FRB) population is observationally divided into sources that have been observed to repeat and those that have not. There is tentative evidence that the bursts from repeating sources have different properties than the non-repeating ones. In order to determine the occurrence rate of repeating sources and characterize the nature of repeat emission, we have been conducting sensitive searches for repetitions from bursts detected with the Australian Square Kilometre Array Pathfinder (ASKAP) with the 64-m Parkes radio telescope, using the recently commissioned Ultra-wideband Low (UWL) receiver system, over a band spanning 0.7$-$4.0 GHz. We report the detection of a repeat burst from the source of FRB 20190711A. The detected burst is 1 ms wide and has a bandwidth of just 65 MHz. We find no evidence of any emission in the remaining part of the 3.3 GHz UWL band. While the emission bandwidths of the ASKAP and UWL bursts show $ν^{-4}$ scaling consistent with a propagation effect, the spectral occupancy is inconsistent with diffractive scintillation. This detection rules out models predicting broad-band emission from the FRB 20190711A source and puts stringent constraints on the emission mechanism. The low spectral occupancy highlights the importance of sub-banded search methods in detecting FRBs.

astro-ph.HE↗

arXiv2020

The dynamic magnetosphere of Swift J1818.0$-$1607

Radio-loud magnetars display a wide variety of radio-pulse phenomenology seldom seen among the population of rotation-powered pulsars. Spectropolarimetry of the radio pulses from these objects has the potential to place constraints on their magnetic topology and unveil clues about the magnetar radio emission mechanism. Here we report on eight observations of the magnetar Swift J1818.0$-$1607 taken with the Parkes Ultra-Wideband Low receiver covering a wide frequency range from 0.7 to 4 GHz over a period of 5 months. The magnetar exhibits significant temporal profile evolution over this period, including the emergence of a new profile component with an inverted spectrum, two distinct types of radio emission mode switching, detected during two separate observations, and the appearance and disappearance of multiple polarization modes. These various phenomena are likely a result of ongoing reconfiguration of the plasma content and electric currents within the magnetosphere. Geometric fits to the linearly polarized position angle indicate we are viewing the magnetar at an angle of $\sim$99$^{\circ}$ from the spin axis, and its magnetic and rotation axes are misaligned by $\sim$112$^{\circ}$. While conducting these fits, we found the position angle swing had reversed direction on MJD 59062 compared to observations taken 15 days earlier and 12 days later. We speculate this phenomena may be evidence the radio emission from this magnetar originates from magnetic field lines associated with two co-located magnetic poles that are connected by a coronal loop.

astro-ph.HE↗

arXiv2020

Host Galaxy Properties and Offset Distributions of Fast Radio Bursts: Implications for their Progenitors

We present observations and detailed characterizations of five new host galaxies of fast radio bursts (FRBs) discovered with the Australian Square Kilometre Array Pathfinder (ASKAP) and localized to $\lesssim 1''$. Combining these galaxies with FRB hosts from the literature, we introduce criteria based on the probability of chance coincidence to define a sub-sample of 10 highly-confident associations (at $z=0.03-0.52$), three of which correspond to known repeating FRBs. Overall, the FRB host galaxies exhibit a broad, continuous range of color ($M_u-M_r = 0.9 - 2.0$), stellar mass ($M_\star = 10^{8} - 6\times 10^{10}\,M_{\odot}$), and star-formation rate (${\rm SFR} = 0.05 - 10\,M_{\odot}\,{\rm yr}^{-1}$) spanning the full parameter space occupied by $z<0.5$ galaxies. However, they do not track the color-magnitude, SFR-$M_\star$, nor BPT diagrams of field galaxies surveyed at similar redshifts. There is an excess of "green valley" galaxies and an excess of emission-line ratios indicative of a harder radiation field than that generated by star-formation alone. From the observed stellar mass distribution, we rule out the hypothesis that FRBs strictly track stellar mass in galaxies ($>99\%$ c.l.). We measure a median offset of 3.3 kpc from the FRB to the estimated center of the host galaxies and compare the host-burst offset distribution and other properties with the distributions of long- and short-duration gamma-ray bursts (LGRBs and SGRBs), core-collapse supernovae (CC-SNe), and Type Ia SNe. This analysis rules out galaxies hosting LGRBs (faint, star-forming galaxies) as common hosts for FRBs ($>95\%$ c.l.). Other transient channels (SGRBs, CC- and Type Ia SNe) have host galaxy properties and offsets consistent with the FRB distributions. All of the data and derived quantities are made publicly available on a dedicated website and repository.

astro-ph.GA↗

arXiv2020

Limits on precursor and afterglow radio emission from a fast radio burst in a star-forming galaxy

We present a new fast radio burst (FRB) at 920 MHz discovered during commensal observations conducted with the Australian Square Kilometre Array Pathfinder (ASKAP) as part of the Commensal Real-time ASKAP Fast Transients (CRAFT) survey. FRB 191001 was detected at a dispersion measure (DM) of 506.92(4) pc cm$^{-3}$ and its measured fluence of 143(15) Jy ms is the highest of the bursts localized to host galaxies by ASKAP to date. The subarcsecond localization of the FRB provided by ASKAP reveals that the burst originated in the outskirts of a highly star-forming spiral in a galaxy pair at redshift $z=0.2340(1)$. Radio observations show no evidence for a compact persistent radio source associated with the FRB 191001 above a flux density of $15 μ$Jy. However, we detect diffuse synchrotron radio emission from the disk of the host galaxy that we ascribe to ongoing star formation. FRB 191001 was also detected as an image-plane transient in a single 10 s snapshot with a flux density of 19.3 mJy in the low-time-resolution visibilities obtained simultaneously with CRAFT data. The commensal observation facilitated a search for repeating and slowly varying radio emissions 8 hr before and 1 hr after the burst. We found no variable radio emission on timescales ranging from 1 ms to 1.4 hr. We report our upper limits and briefly review FRB progenitor theories in the literature that predict radio afterglows. Our data are still only weakly constraining of any afterglows at the redshift of the FRB. Future commensal observations of more nearby and bright FRBs will potentially provide stronger constraints.

astro-ph.HE↗

arXiv2020

First Constraints on Compact Dark Matter from Fast Radio Burst Microstructure

Despite existing constraints, it remains possible that up to $35\%$ of all dark matter is comprised of compact objects, such as the black holes in the 10-100\,M$_\odot$ range whose existence has been confirmed by LIGO. The strong gravitational lensing of transients such as FRBs and GRBs has been suggested as a more sensitive probe for compact dark matter than intensity fluctuations observed in microlensing experiments. Recently ASKAP has reported burst substructure down to $15μ$s timescales in FRBs in the redshift range $0.3-0.5$. We investigate here the implications of this for the detectability of compact dark matter by FRBs. We find that a sample size of $\sim130$ FRBs would be required to constrain compact dark matter to less than the existing 35$\%$ limit with 95$\%$ confidence, if it were distributed along $\gtrsim 1\,$Gpc-long FRB sightlines through the cosmic web. Conversely, existing constraints on the fraction of compact dark matter permit as many as 1 in $\approx 40$ of all $z \lesssim 0.4$ FRBs to exhibit micro-lensed burst structure. Approximately $170$ FRBs intercepting halos within $\sim 50\,$kpc would be required to exclude the fraction of compact dark matter in each intercepted halo to a similar level. Furthermore, we consider the cumulative effects of lensing of the FRB signal by a macroscopic dark matter distribution. We conclude that lensing from a uniform distribution of compact objects is likely not observable, but suggest that FRBs may set meaningful limits on power-law distributions of dark matter.

astro-ph.CO↗

arXiv2020

A population analysis of pulse broadening in ASKAP Fast Radio Bursts

The pulse morphology of fast radio bursts (FRBs) provides key information in both understanding progenitor physics and the plasma medium through which the burst propagates. We present a study of the profiles of 33 bright FRBs detected by the Australian Square Kilometre Array Pathfinder. We identify seven FRBs with measureable intrinsic pulse widths, including two FRBs that have been seen to repeat. In our modest sample we see no evidence for bimodality in the pulse width distribution. We also identify five FRBs with evidence of millisecond timescale pulse broadening caused by scattering in inhomogeneous plasma. We find no evidence for a relationship between pulse broadening and extragalactic dispersion measure. The scattering could be either caused by extreme turbulence in the host galaxy or chance propagation through foreground galaxies. With future high time resolution observations and detailed study of host galaxy properties we may be able to probe line-of-sight turbulence on gigaparsec scales.

astro-ph.HE↗

arXiv2020

A search for supernova-like optical counterparts to ASKAP-localised Fast Radio Bursts

Fast radio bursts (FRBs) are millisecond-scale radio pulses, which originate in distant galaxies and are produced by unknown sources. The mystery remains partially because of the typical difficulty in localising FRBs to host galaxies. Accurate localisations delivered by the Commensal Real-time ASKAP Fast Transients (CRAFT) survey now provide an opportunity to study the host galaxies and potential transient counterparts of FRBs at a large range of wavelengths. In this work, we investigate whether the first three FRBs accurately localised by CRAFT have supernova-like transient counterparts. We obtained two sets of imaging epochs with the Very Large Telescope for three host galaxies, one soon after the burst detection and one several months later. After subtracting these images no optical counterparts were identified in the associated FRB host galaxies, so we instead place limits on the brightness of any potential optical transients. A Monte Carlo approach, in which supernova light curves were modelled and their base properties randomised, was used to estimate the probability of a supernova associated with each FRB going undetected. We conclude that Type Ia and IIn supernovae are unlikely to accompany every apparently non-repeating FRB.

astro-ph.HE↗

arXiv2020

Spectropolarimetric properties of Swift J1818.0$-$1607: a 1.4 s radio magnetar

The soft-gamma repeater Swift J1818.0$-$1607 is only the fifth magnetar found to exhibit pulsed radio emission. Using the Ultra-Wideband Low receiver system of the Parkes radio telescope, we conducted a 3 h observation of Swift J1818.0$-$1607. Folding the data at a rotation period of $P = 1.363$ s, we obtained wideband polarization profiles and flux density measurements covering radio frequencies between 704-4032 MHz. After measuring, and then correcting for the pulsar's rotation measure of $1442.0 \pm 0.2$ rad m$^{-2}$, we find the radio profile is between 80-100 per cent linearly polarized across the wide observing band, with a small amount of depolarization at low frequencies that we ascribe to scatter broadening. We also measure a steep spectral index of $α= -2.26^{+0.02}_{-0.03}$ across our large frequency range, a significant deviation from the flat or inverted spectra often associated with radio-loud magnetars. The steep spectrum and temporal rise in flux density bears some resemblance to the behaviour of the magnetar-like, rotation-powered pulsar PSR J1119$-$6127. This leads us to speculate that Swift J1818.0$-$1607 may represent an additional link between rotation-powered pulsars and magnetars.

astro-ph.HE↗