arXiv subjects

K. Nimmo

Publications and source records attributed to K. Nimmo.

At least 19 recordsLinked to original sources

arXiv2026

Searching for links between energetic millisecond pulsars and repeating fast radio bursts

The unexpected localization of the repeating FRB 20200120E to a globular cluster challenges conventional FRB models based on magnetars formed via core collapse. One alternative model suggests that FRB 20200120E is a millisecond pulsar (MSP) producing giant pulses (GPs). To test this hypothesis, we compared the characteristics of FRB 20200120E bursts with the GPs of the most energetic Galactic MSP known, M28A (PSR B1821$-$24A), using observations with the Parkes (Murriyang) telescope's Ultra Wideband Low-frequency (UWL) receiver. Our analysis provides insight into the spectral structure and frequency extent of M28A's GPs, revealing broad-band spectra spanning $700-4000$ MHz (in some cases) with complex spectral peaks. We find that known M28A GP characteristics persist at this bandwidth, such as durations, luminosities, periodicity, wait-time, and energy distribution. A sub-band search for narrow-band GPs yielded no detection of genuinely narrow-band GPs. However, we do find narrow-band spectral peaks of $\sim100$ MHz bandwidth, a similar scale observed for FRB 20200120E's bursts. Compared to FRB 20200120E's bursts, M28A's GPs have $50\times$ shorter durations, $10^5\times$ lower spectral luminosities, clear periodicity (vs. no periodicity), a purely Poissonian wait-time distribution (vs. quasi-Poissonian), and generally broad-band spectra with narrow-band peaks (vs. only narrow-band bursts). Both sources show a steep energy distribution and minor dispersion measure variability. Our study finds no strong links between M28A and FRB 20200120E. However, we cannot rule out the possibility that FRB 20200120E is a rare and unique type of MSP with no Galactic analogue. Furthermore, higher-cadence monitoring of M28A, for hundreds to thousands of hours, might reveal rare but extremely luminous pulses.

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arXiv2025

Revisiting FRB 20121102A: milliarcsecond localisation and a decreasing dispersion measure

FRB 20121102A is the original repeating fast radio burst (FRB) source and also the first to be localised to milliarcsecond precision using very-long-baseline interferometry (VLBI). It has been active for over 13 years and resides in an extreme magneto-ionic environment in a dwarf host galaxy at a distance of ~1 Gpc. In this work, we use the European VLBI Network (EVN) to (re-)localise FRB 20121102A and its associated persistent radio source (PRS). We confirm that the two are co-located -- improving on previous results by a factor of ~4 and constraining the FRB and PRS co-location to ~12 pc transverse offset. Over a decade, the PRS luminosity on milliarcsecond scales remains consistent with measurements on larger angular scales, showing that the PRS is still compact. We also present the detection of 18 bursts with the Nancay Radio Telescope (NRT) as part of our \'ECLAT monitoring program. These bursts, together with previously published results, show that the observed dispersion measure (DM) of FRB 20121102A has dropped by ~25 pc/cc in the past five years, highlighting a fractional decrease in the local DM contribution of >15%. We discuss potential physical scenarios and highlight possible future observations that will help reveal the nature of FRB 20121102A, which is one of only a few known FRBs with a luminous PRS.

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arXiv2025

A HyperFlash and \'ECLAT view of the local environment and energetics of the repeating FRB 20240619D

Time-variable propagation effects provide a window into the local plasma environments of repeating fast radio burst (FRB) sources. Here we report high-cadence observations of FRB 20240619D, as part of the HyperFlash and \'ECLAT programs. We observed for $500$h and detected $217$ bursts, including $10$ bursts with high fluence ($>25$ Jy ms) and implied energy. We track burst-to-burst variations in dispersion measure (DM) and rotation measure (RM), from which we constrain the parallel magnetic field strength in the source's local environment: $0.27\pm0.13$ mG. Apparent DM variations between sub-bursts in a single bright event are interpreted as coming from plasma lensing or variable emission height. We also identify two distinct scintillation screens along the line of sight, one associated with the Milky Way and the other likely located in the FRB's host galaxy or local environment. Together, these (time-variable) propagation effects reveal that FRB 20240619D is embedded in a dense, turbulent and highly magnetised plasma. The source's environment is more dynamic than that measured for many other (repeating) FRB sources, but less extreme compared to several repeaters that are associated with a compact, persistent radio source. FRB 20240619D's cumulative burst fluence distribution shows a power-law break, with a flat tail at high energies. Along with previous studies, this emphasises a common feature in the burst energy distribution of hyperactive repeaters. Using the break in the burst fluence distribution, we estimate a source redshift of $z=0.042$-$0.240$. We discuss FRB 20240619D's nature in the context of similar studies of other repeating FRBs.

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arXiv2025

A Hyperactive FRB Pinpointed in an SMC-Like Satellite Host Galaxy

Precise localizations of fast radio bursts (FRBs) are essential for uncovering their host galaxies and immediate environments. We present the milliarcsecond-precision European VLBI Network localization of FRB 20240114A, a hyperactive repeating FRB, achieving <90x30 mas (1-sigma) accuracy. This precision places the burst 0.5 kpc from the nucleus of its low-metallicity star-forming dwarf host at a spectroscopic redshift of z = 0.130287. Our Gran Telescopio CANARIAS spectroscopic follow-up reveals that the dwarf FRB host is gravitationally bound to a more massive, star-forming spiral galaxy. This establishes the first known instance of an FRB residing in a satellite galaxy within a larger galactic system. This configuration, analogous to the Small Magellanic Cloud orbiting the Milky Way (but at a lower overall mass scale), expands the known diversity of FRB host environments and offers important insights for interpreting seemingly "hostless" or highly offset FRBs. Furthermore, our detailed dispersion measure (DM) budget analysis indicates that the dominant contribution to FRB 20240114A's DM likely originates from the foreground galaxy halos. This finding addresses the anomalously high DM observed for this FRB and underscores the significant role of intervening foreground structures in shaping observed FRB DMs, which is important for accurate FRB-based cosmological measurements. Our results highlight the importance of deep, high-resolution optical/infrared observations (e.g., with the Hubble or James Webb Space Telescopes) to fully leverage our precise radio localization and probe the immediate astrophysical birthplaces of FRB progenitors within these complex galactic systems.

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arXiv2025

Searching for Historical Extragalactic Optical Transients Associated with Fast Radio Bursts

We present a systematic search for past supernovae (SNe) and other historical optical transients at the positions of fast radio burst (FRB) sources to test FRB progenitor systems. Our sample comprises 83 FRBs detected by the Canadian Hydrogen Intensity Mapping Experiment (CHIME) and its KKO Outrigger, along with 93 literature FRBs representing all known well-localized FRBs. We search for optical transients coincident in position and redshift with FRBs and find no significant associations within the 5-sigma FRB localization uncertainties except for a previously identified potential optical counterpart to FRB 20180916B. By constraining the timescale for SN ejecta to become transparent to FRB emission, we predict that it takes at least 6-10 years before the FRB emission can escape. From this, we infer that approximately 7% of matched optical transients, up to 30% of currently known SNe, and up to 40% of core-collapse SNe could have an observable FRB based on timescales alone. We derive the number of new, well-localized FRBs required to produce one FRB-SN match by chance, and find it will take ~ 22,700 FRBs to yield one chance association at the projected CHIME/FRB Outrigger detection rate. Looking forward, we demonstrate redshift overlap between SNe detected by the upcoming Vera C. Rubin Observatory and CHIME/FRB Outrigger FRBs, indicating the prospect of an increase in potential associations at redshift z < 1. Our framework is publicly available, flexible to a wide range of transient timescales and FRB localization sizes, and can be applied to any optical transient populations in future searches.

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arXiv2024

Long term monitoring of FRB~20121102 with the Nan\c{c}ay Radio Telescope and multi-wavelength campaigns including INTEGRAL

The origin(s) of Fast Radio Bursts (FRBs), mysterious radio bursts coming from extragalactic distances, remains unknown. Multi-wavelength observations are arguably the only way to answer this question unambiguously. We attempt to detect hard X-ray/soft gamma-ray counterparts to one of the most active FRB sources, FRB20121102, as well as improve understanding of burst properties in radio through a long-term monitoring campaign using the Nan\c{c}ay Radio Telescope (NRT). Multi-wavelength campaigns involving the International Gamma-ray Astrophysics Laboratory (INTEGRAL) satellite, the Nan\c{c}ay Radio Observatory, the optical telescopes at the Observatoire de Haute Provence as well as Arecibo were conducted between 2017 and 2019. In 2017, the telescopes were scheduled to observe simultaneously between Sept 24-29. We specifically used the Fast Response Enhanced CCDs for the optical observations to ensure a high time resolution. In 2019, we changed the strategy to instead conduct ToO observations on INTEGRAL and other available facilities upon positive detection triggers from the NRT. In the 2017 campaign, FRB20121102 was not in its burst activity window. We obtain a 5-sigma optical flux limit of 12 mJy ms using the GASP and a 3-sigma limit from OHP T120cm R-band image of R=22.2 mag of any potential persistent emission not associated to radio bursts. In the 2019 campaign, we have simultaneous INTEGRAL data with 11 radio bursts from the NRT and Arecibo. We obtain a 5-sigma upper limit of 2.7e-7 erg/cm2 in the 25-400 keV energy range for contemporary radio and high energy bursts, and a 5-sigma upper limit of 3.8e-11 erg/cm2 for permanent emission in the 25-100 keV energy range. In addition, we report on the regular observations from NRT between 2016-2020, which accounts for 119 additional radio bursts from FRB20121102. We present an updated fit of the periodic active window of 154+/-2 days.

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arXiv2024

Polarization properties of 28 repeating fast radio burst sources with CHIME/FRB

As part of the Canadian Hydrogen Intensity Mapping Experiment Fast Radio Burst (CHIME/FRB) project, we report 41 new Rotation Measures (RMs) from 20 repeating Fast Radio Bursts (FRBs) obtained between 2019 and 2023 for which no previous RM was determined. We also report 22 additional RM measurements for eight further repeating FRBs. We observe temporal RM variations in practically all repeating FRBs. Repeaters appear to be separated into two categories: those with dynamic and those with stable RM environments, differentiated by the ratios of RM standard deviations over the averaged RM magnitudes. Sources from stable RM environments likely have little RM contributions from the interstellar medium (ISM) of their host galaxies, whereas sources from dynamic RM environments share some similarities with Galactic pulsars in eclipsing binaries but appear distinct from Galactic centre solitary pulsars. We observe a new stochastic, secular, and again stochastic trend in the temporal RM variation of FRB 20180916B, which does not support binary orbit modulation being the reason for this RM changes. We highlight two more repeaters that show RM sign change, namely FRBs 20290929C and 20190303A. We perform an updated comparison of polarization properties between repeating and non-repeating FRBs, which show a marginal dichotomy in their distribution of electron-density-weighted parallel-component line-of-sight magnetic fields.

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arXiv2024

The Massive and Quiescent Elliptical Host Galaxy of the Repeating Fast Radio Burst FRB20240209A

The discovery and localization of FRB20240209A by the Canadian Hydrogen Intensity Mapping Fast Radio Burst (CHIME/FRB) experiment marks the first repeating FRB localized with the CHIME/FRB Outriggers and adds to the small sample of repeating FRBs with associated host galaxies. Here we present Keck and Gemini observations of the host that reveal a redshift $z=0.1384\pm0.0004$. We perform stellar population modeling to jointly fit the optical through mid-infrared data of the host and infer a median stellar mass log$(M_*/{\rm M_{\odot}})=11.34\pm0.01$ and a mass-weighted stellar population age $\sim11$Gyr, corresponding to the most massive and oldest FRB host discovered to date. Coupled with a star formation rate $<0.36\,{\rm M_{\odot}\ yr^{-1}}$, the specific star formation rate $<10^{-11.8}\rm\ yr^{-1}$ classifies the host as quiescent. Through surface brightness profile modeling, we determine an elliptical galaxy morphology, marking the host as the first confirmed elliptical FRB host. The discovery of a quiescent early-type host galaxy within a transient class predominantly characterized by late-type star-forming hosts is reminiscent of short-duration gamma-ray bursts, Type Ia supernovae, and ultraluminous X-ray sources. Based on these shared host demographics, coupled with a large offset as demonstrated in our companion paper, we conclude that preferred progenitors for FRB20240209A include magnetars formed through merging binary neutron stars/white dwarfs or the accretion-induced collapse of a white dwarf, or a luminous X-ray binary. Together with FRB20200120E localized to a globular cluster in M81, our findings provide strong evidence that some fraction of FRBs may arise from a process distinct from the core collapse of massive stars.

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arXiv2024

A repeating fast radio burst source in the outskirts of a quiescent galaxy

We report the discovery of the repeating fast radio burst source FRB 20240209A using the CHIME/FRB telescope. We have detected 22 bursts from this repeater between February and July 2024, six of which were also recorded at the Outrigger station KKO. The 66-km long CHIME-KKO baseline can provide single-pulse FRB localizations along one dimension with $2^{\prime\prime}$ accuracy. The high declination of $\sim$86 degrees for this repeater allowed its detection with a rotating range of baseline vectors, enabling the combined localization region size to be constrained to $1^{\prime\prime}\times2^{\prime\prime}$. We present deep Gemini observations that, combined with the FRB localization, enabled a robust association of FRB 20240209A to the outskirts of a luminous galaxy (P(O|x) = 0.99; $L \approx 5.3 \times 10^{10}\,L_{\odot}$). FRB 20240209A has a projected physical offset of $40 \pm 5$ kpc from the center of its host galaxy, making it the FRB with the largest host galaxy offset to date. When normalized by the host galaxy size, the offset of FRB 20240209A is comparable to that of FRB 20200120E, the only FRB source known to originate in a globular cluster. We consider several explanations for the large offset, including a progenitor that was kicked from the host galaxy or in situ formation in a low-luminosity satellite galaxy of the putative host, but find the most plausible scenario to be a globular cluster origin. This, coupled with the quiescent, elliptical nature of the host as demonstrated in our companion paper, provide strong evidence for a delayed formation channel for the progenitor of the FRB source.

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arXiv2024

A probe of the maximum energetics of fast radio bursts through a prolific repeating source

Fast radio bursts (FRBs) are sufficiently energetic to be detectable from luminosity distances up to at least seven billion parsecs (redshift $z > 1$). Probing the maximum energies and luminosities of FRBs constrains their emission mechanism and cosmological population. Here we investigate the maximum energetics of a highly active repeater, FRB 20220912A, using 1,500 h of observations. We detect $130$ high-energy bursts and find a break in the burst energy distribution, with a flattening of the power-law slope at higher energy -- consistent with the behaviour of another highly active repeater, FRB 20201124A. There is a roughly equal split of integrated burst energy between the low- and high-energy regimes. Furthermore, we model the rate of the highest-energy bursts and find a turnover at a characteristic spectral energy density of $E^{\textrm{char}}_{\nu} = 2.09^{+3.78}_{-1.04}\times10^{32}$ erg/Hz. This characteristic maximum energy agrees well with observations of apparently one-off FRBs, suggesting a common physical mechanism for their emission. The extreme burst energies push radiation and source models to their limit: at this burst rate a typical magnetar ($B = 10^{15}$ G) would deplete the energy stored in its magnetosphere in $\sim$ 2150 h, assuming a radio efficiency $\epsilon_\mathrm{radio} = 10^{-5}$. We find that the high-energy bursts ($E_\nu > 3 \times 10^{30}$ erg Hz$^{-1}$) play an important role in exhausting the energy budget of the source.

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arXiv2024

A Radio Study of Persistent Radio Sources in Nearby Dwarf Galaxies: Implications for Fast Radio Bursts

We present 1 - 12 GHz Karl G. Jansky Very Large Array observations of 9 off-nuclear persistent radio sources (PRSs) in nearby (z < 0.055) dwarf galaxies, along with high-resolution European very-long baseline interferometry (VLBI) Network (EVN) observations for one of them at 1.7GHz. We explore the plausibility that these PRSs are associated with fast radio burst (FRB) sources by examining their properties, physical sizes, host-normalized offsets, spectral energy distributions (SEDs), radio luminosities, and light curves, and compare them to those of the PRSs associated with FRBs 20121102A and 20190520B, two known active galactic nuclei (AGN), and one likely AGN in our sample with comparable data, as well as other radio transients exhibiting characteristics analogous to FRB-PRSs. We identify a single source in our sample, J1136+2643, as the most promising FRB- PRS, based on its compact physical size and host-normalized offset. We further identify two sources, J0019+1507 and J0909+5955, with physical sizes comparable to FRB-PRSs, but which exhibit large offsets and flat spectral indices potentially indicative of a background AGN origin. We test the viability of neutron star wind nebulae and hypernebulae models for J1136+2643, and find that the physical size, luminosity, and SED of J1136+2643 are broadly consistent with these models. Finally, we discuss the alternative interpretation that the radio sources are instead powered by accreting massive black holes and outline future prospects and follow-up observations for differentiating between these scenarios.

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arXiv2023

Detection of ultra-fast radio bursts from FRB 20121102A

Fast radio bursts (FRBs) are extragalactic transients with typical durations of milliseconds. FRBs have been shown, however, to fluctuate on a wide range of timescales: some show sub-microsecond sub-bursts while others last up to a few seconds in total. Probing FRBs on a range of timescales is crucial for understanding their emission physics, how to detect them effectively, and how to maximize their utility as astrophysical probes. FRB 20121102A is the first-known repeating FRB source. Here we show that FRB 20121102A is able to produce isolated microsecond-duration bursts whose total durations are more than ten times shorter than all other known FRBs to date. The polarimetric properties of these micro-bursts resemble those of the longer-lasting bursts, suggesting a common emission mechanism producing FRBs spanning a factor of 1,000 in duration. Furthermore, this work shows that there exists a population of ultra-fast radio bursts that current wide-field FRB searches are missing due to insufficient time-resolution. These results indicate that FRBs occur more frequently and with greater diversity than initially thought. This could also influence our understanding of energy, wait time, and burst rate distributions.

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arXiv2023

Connecting repeating and non-repeating fast radio bursts via their energy distributions

Fast radio bursts (FRBs) are extremely energetic, millisecond-duration radio flashes that reach Earth from extragalactic distances. Broadly speaking, FRBs can be classified as repeating or (apparently) non-repeating. It is still unclear, however, whether the two types share a common physical origin, differing only in their activity rate. Here we report on an unprecedented observing campaign that targeted one hyperactive repeating source, FRB 20201124A, for more than $2000~\mathrm{hr}$ using four $25-32\mathrm{-m}$ class radio telescopes. In total, we detect $46$ high-energy bursts, many more than one would expect given previous observations of lower-energy bursts using larger radio telescopes. We find a high-energy burst distribution that resembles that of the non-repeating FRB population, suggesting that apparently non-repeating FRB sources may simply be the rarest bursts from repeating sources. We also discuss how FRB 20201124A contributes strongly to the all-sky FRB rate and how similar sources would be observable even at very high redshift.

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arXiv2023

Propagation effects at low frequencies seen in the LOFAR long-term monitoring of the periodically active FRB 20180916B

LOFAR (LOw Frequency ARray) has previously detected bursts from the periodically active, repeating fast radio burst (FRB) source FRB 20180916B down to unprecedentedly low radio frequencies of 110 MHz. Here we present 11 new bursts in 223 more hours of continued monitoring of FRB 20180916B in the 110-188 MHz band with LOFAR. We place new constraints on the source's activity window $w = 4.3^{+0.7}_{-0.2}$ day, and phase centre $\phi_{\mathrm{c}}^{\mathrm{LOFAR}} = 0.67^{+0.03}_{-0.02}$ in its 16.33-day activity cycle, strengthening the evidence for its frequency-dependent activity cycle. Propagation effects like Faraday rotation and scattering are especially pronounced at low frequencies and constrain properties of FRB 20180916B's local environment. We track variations in scattering and time-frequency drift rates, and find no evidence for trends in time or activity phase. Faraday rotation measure (RM) variations seen between June 2021 and August 2022 show a fractional change $>$50% with hints of flattening of the gradient of the previously reported secular trend seen at 600 MHz. The frequency-dependent window of activity at LOFAR appears stable despite the significant changes in RM, leading us to deduce that these two effects have different causes. Depolarization of and within individual bursts towards lower radio frequencies is quantified using LOFAR's large fractional bandwidth, with some bursts showing no detectable polarization. However, the degree of depolarization seems uncorrelated to the scattering timescales, allowing us to evaluate different depolarization models. We discuss these results in the context of models that invoke rotation, precession, or binary orbital motion to explain the periodic activity of FRB 20180916B.

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arXiv2022

A burst storm from the repeating FRB 20200120E in an M81 globular cluster

The repeating fast radio burst (FRB) source FRB 20200120E is exceptional because of its proximity and association with a globular cluster. Here we report $60$ bursts detected with the Effelsberg telescope at 1.4 GHz. We observe large variations in the burst rate, and report the first FRB 20200120E `burst storm', where the source suddenly became active and 53 bursts (fluence $\geq 0.04$ Jy ms) occurred within only 40 minutes. We find no strict periodicity in the burst arrival times, nor any evidence for periodicity in the source's activity between observations. The burst storm shows a steep energy distribution (power-law index $\alpha = 2.39\pm0.12$) and a bi-modal wait-time distribution, with log-normal means of 0.94$^{+0.07}_{-0.06}$ s and 23.61$^{+3.06}_{-2.71}$ s. We attribute these wait-time distribution peaks to a characteristic event timescale and pseudo-Poisson burst rate, respectively. The secondary wait-time peak at $\sim1$ s is $\sim50\times$ longer than the $\sim24$ ms timescale seen for both FRB 20121102A and FRB 20201124A -- potentially indicating a larger emission region, or slower burst propagation. FRB 20200120E shows order-of-magnitude lower burst durations and luminosities compared with FRB 20121102A and FRB 20201124A. Lastly, in contrast to FRB 20121102A, which has observed dispersion measure (DM) variations of $\Delta{\rm DM} >1$ pc cm$^{-3}$ on month-to-year timescales, we determine that FRB 20200120E's DM has remained stable ($\Delta{\rm DM} <0.15$ pc cm$^{-3}$) over $>10$ months. Overall, the observational characteristics of FRB 20200120E deviate quantitatively from other active repeaters, but it is unclear whether it is qualitatively a different type of source.

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arXiv2022

PRECISE localizations of repeating Fast Radio Bursts

Fast Radio Bursts (FRBs) are extremely luminous and brief signals (with duration of milliseconds or even shorter) of extragalactic origin. Despite the fact that hundreds of FRBs have been discovered to date, their nature still remains unclear. Precise localizations of FRBs can unveil their host galaxies and local environments -- and thus shed light on the physical processes that led to the burst production. However, this has only been achieved for a few FRBs to date. The European VLBI Network (EVN) is currently the only instrument capable of localizing FRBs down to the milliarcsecond level. This level of precision was critical to associate the first localized FRB, 20121102A, to a star-forming region in a low-metallicity dwarf galaxy and physically related it to a compact persistent radio source. Analogously, a second repeating FRB, 20180916B, was found to just outside the edge of a prominent star-forming region of a nearby spiral galaxy. The PRECISE project (Pinpointing REpeating ChIme Sources with EVN dishes), starting from 2019, has observed hundreds of hours per year with a subset of EVN telescopes with the goal of localizing repeating FRBs discovered by the CHIME/FRB Collaboration. The ultimate goal of PRECISE is to disentangling the environments where FRBs can be produced. Here we present the state of the art of the FRB field, the PRECISE project, and the localizations achieved until now, which have unveiled a variety of environments where FRBs can be found that challenges the current models.

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arXiv2022

FRB 121102: drastic changes in the burst polarization contrasts with the stability of the persistent emission

We study milliarcsecond-scale properties of the persistent radio counterpart to FRB 121102 and investigate the spectro-polarimetric properties of a bright burst. For the former, we use European VLBI Network (EVN) observations in 2017 at 1.7 and 4.8 GHz. For the latter, we re-analyse the 1.7-GHz data from the 100-m Effelseberg telescope taken in 2016. These observations predate other polarimetric studies of FRB 121102, and yield the highest burst Faraday rotation measure (RM) to date, RM = 1.27*10^5 rad m^-2, consistent with the decreasing RM trend. The fractional polarization of the burst emission is 15% at 1.7 GHz. This can be reconciled with the high fractional polarization at higher frequencies if the Faraday width of the burst environment is 150 rad m^-2 - a bare 0.1% of the total Faraday rotation. The width may originate from minor non-uniformities in the Faraday screen, or from effects in the emitting region itself. The upper limit on the persistent source size is 1 pc, barely consistent with a young supernova (SN) scenario. The flux variability limit of <10% is not in favor of the young SN scenario, and challenges other interpretations as well. The fractional polarization of the faint persistent source is constrained at <25% at 4.8 GHz ruling out a common origin with the highly polarized individual bursts.

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arXiv2022

The FRB 20121102A November rain in 2018 observed with the Arecibo Telescope

We present 849 new bursts from FRB 20121102A detected with the 305-m Arecibo Telescope. Observations were conducted as part of our regular campaign to monitor activity and evolution of burst properties. The 10 reported observations were carried out between 1150 and 1730 MHz and fall in the active period around November 2018. All bursts were dedispersed at the same dispersion measure and are consistent with a single value of 562.4(1) pc/cm$^3$. The rate varies between 0 bursts and 218(16) bursts per hour, the highest rate observed to date. The times between consecutive bursts show a bimodal distribution. We find that a Poisson process with varying rate best describes arrival times with separations >0.1 s. Clustering on timescales of 22 ms reflects a characteristic timescale of the source and possibly the emission mechanism. We analyse the spectro-temporal structure of the bursts by fitting 2D Gaussians with a temporal drift to each sub-burst in the dynamic spectra. We find a linear relationship between the sub-burst's drift and its duration. At the same time, the drifts are consistent with coming from the sad-trombone effect. This has not been predicted by current models. The energy distribution shows an excess of high energy bursts and is insufficiently modelled by a single power-law even within single observations. We find long-term changes in the energy distribution, the average spectrum, and the sad-trombone drift, compared to earlier and later published observations. Despite the large burst rate, we find no strict short-term periodicity.

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