arXiv subjects

K. M. Rajwade

Publications and source records attributed to K. M. Rajwade.

At least 19 recordsLinked to original sources

arXiv2026

PIFFLE: Characterizing the Foreground Contributions from 4 Decades in Halo Mass to the FRB20230907D Dispersion Measure

We characterize the foreground environment of FRB20230907D, localized to a galaxy at $z=0.464$, which has an observed dispersion measure of ${\rm DM}_{\rm obs}=1031~{\rm pc~cm^{-3}}$. At its redshift, FRB20230907D lies above the Macquart relation, the expected relation between cosmological dispersion measure and the source redshift, indicating a substantial excess DM along this line of sight. We use Subaru/PFS and SDSS spectroscopy, published group catalogs, Rubin/LSST imaging, and eROSITA X-ray data to characterize the foreground structures that may account for this excess. A friends-of-friends search identifies a massive foreground system at $z\simeq0.09$ with $M_{200}\simeq5.2\times10^{14}~M_\odot$, while low redshift catalogs reveal an additional group at $z\simeq0.02565$. Assuming that the halo gas follows a modified-NFW halo density profile, we estimate observer frame contributions of $150^{+110}_{-70}~{\rm pc~cm^{-3}}$ and $80^{+60}_{-40}~{\rm pc~cm^{-3}}$ from these systems, respectively. Together with the Milky Way, diffuse intergalactic medium, Virgo cluster, M49 group, and host galaxy contributions, these foreground structures can account for the excess dispersion measure of FRB20230907D within uncertainties. This highlights the importance of dense foreground spectroscopy and multi-wavelength data.

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arXiv2026

Discovery of 27 new Rotating Radio Transients by MeerTRAP

We present the discovery of 27 new Galactic transients made by the commensal MeerTRAP single pulse search programme at the MeerKAT telescope. We determine the location of 14 of these discoveries with arcsecond accuracy by imaging the data captured in the dedicated transient buffer. A preliminary estimate of the period was made for 8 sources using the arrival times of several pulses detected by MeerTRAP. The periods of these transients range between 0.78 s and 4 s. For the previously published MeerTRAP source MTP0040 (PSR J1357-6507), we are able to provide the position and period using new pulses detected since it was reported. We also estimate the radio fluences and discuss the burst rates derived from the commensal search detections. The accurate image-domain localizations were used to conduct follow-up observations and timing analysis. Furthermore, some follow-up observations revealed ordinary pulsar-like pulsed behavior for four of the new sources, illustrating the blurred separation between rotating radio transients and canonical pulsars. We also present coherent timing solutions for 4 sources, providing insight into their rotational properties and their place within the Galactic neutron star population.

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arXiv2026

Rapid Response Triggering for Radio Transients with the SKA Observatory

Rapid-response triggering is when a telescope is able to automatically respond to an external or internal astronomical transient alert, causing it to rapidly repoint at that position in the sky to catch its earliest radio emission. Both SKA-Low and SKA-Mid will have the ability to perform rapid-response triggering observations on externally detected transients as well as those detected within the data streams. We first give a brief overview of those radio instruments with active rapid-response observing modes. We then describe the different science cases motivating the need for this observing capability on SKAO and how the additional sensitivity afforded by the SKAO will enable us to answer fundamental questions relating to particle acceleration, transient central engines, coherent emission models and outflow physics in astrophysical systems spanning the range from the Sun to the high redshift Universe. Several suggestions relating to existing technologies and necessary SKAO system requirements are described. Through this chapter, we aim to ensure this is an existing, common and useful capability for the SKA Observatory.

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arXiv2026

Fast Radio Bursts in the Era of the Vera C. Rubin Observatory's Legacy Survey of Space and Time

Identifying the host galaxies of fast radio bursts (FRBs), and comparing their redshifts and dispersion measures, has unlocked a new probe of the cosmological distribution of ionised gas. However the necessary optical observations to identify FRB hosts, and measure their redshifts, are becoming increasingly onerous as the detection rate of precisely localised FRBs increases. Here we analyse the ability of the Legacy Survey of Space and Time (LSST), being conducted by the Vera C. Rubin Observatory, to identify FRB host galaxies, and the utility of LSST photometric redshifts for FRB cosmology. By combining a model of FRB host galaxy r-band magnitudes, $m_r$, with predictions for the FRB z-DM distribution, we create a method to predict the $m_r(z)$ distribution for the host galaxies of FRBs detected by radio surveys. We then predict these distributions for the coherent modes of the Australian Square Kilometre Array Pathfinder (ASKAP) and MeerKAT. We find that even a single visit with Rubin will be able to identify 65% of FRB host galaxies detected by ASKAP's coherent upgrade, `CRACO'; while the final 10 year co-added images will identify 81% of those from MeerKAT's tied array beams. We also simulate the impact of using photometric redshifts for a simplified analysis to determine $H_0$, finding that estimated photo-z errors result in a decreased precision of only 7% on $H_0$ for ASKAP's CRACO system. The impact of missing faint FRB hosts, which are likely at higher redshifts, is more significant, and might degrade sensitivity to $H_0$ by 47%, or 62% when combined with photo-z errors. All told, Rubin's LSST will be an incredibly powerful survey for facilitating FRB cosmology, although supplemental observations may be useful for particularly faint and distant host galaxies.

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arXiv2026

A bright wideband radio burst from the isolated neutron star 2XMM J104608.7$-$594306

We present the discovery of a second coherent radio burst from the thermally emitting neutron star 2XMM J104608.7$-$594306 in our follow-up observations with the Murriyang Ultra-Wideband Low receiver. This burst shows complex morphology with multiple components and wideband emission spanning from 704 to 4032MHz. We measured a steep spectral index of $α=-2.18\pm0.16$. Our polarimetric analysis demonstrates that the burst is highly polarised with a linear and circular polarisation fraction of 54% and 22%, respectively. We identified an orthogonal jump in the polarisation position angles of the burst, resembling those seen in radio pulsars. We compared this burst with the first radio burst detected from the source with MeerKAT. These two bursts detected in a total of 40 hours on source with MeerKAT and Murriyang, combined, show that 2XMM J104608.7$-$594306 can emit sporadic radio emission with luminosity jumps comparable to those seen in the bright bursts from SGR 1935+2154. This suggests that previously thought radio-quiet neutron stars such as X-ray dim isolated neutron stars and central compact objects could exhibit rare radio bursting activity.

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arXiv2026

Twenty-four thousand hours of GREENBURST observations with the GBT

In addition to fast radio burst (FRB) searches carried out using dedicated surveys, a number of radio observatories take advantage of commensal opportunities with large facilities in which observations for other projects can be searched for FRBs and other transient sources. We present the results from one such effort, the first 24,186 hours of the GREENBURST search for dispersed radio pulses with the Green Bank Telescope (GBT). To date, GREENBURST has detected a total of 50 pulsars and three FRBs. One of the pulsars, PSR J0039+5407, has a period of 2.2 s and was previously unknown. Using follow-up observations with the Canadian Hydrogen Intensity Mapping Experiment, we found a timing solution for this pulsar which shows it to have a characteristic age of 2 Myr. Additional GBT observations show the pulsar has a very high nulling fraction ($\sim70-80\%$). All three of the FRBs are repeating sources that were previously known and were being monitored by the GBT as part of other projects. A major challenge for GREENBURST in the discovery of new FRBs is its single beam. This makes it hard to distinguish some of the pulses from sources of radio frequency interference. We highlight this problem with a case study of an FRB-like pulse that initially passed our interference filters. Upon closer inspection, the event appears to be part of a longer-duration narrow-band source of unknown origin. Further observations and monitoring are required to determine whether it is terrestrial or celestial.

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arXiv2026

Evaluating the effectiveness of radio frequency interference removal algorithms for single pulse searches

Radio Frequency Interference (RFI), the presence of artificial and/or terrestrial signals in astronomical data, poses a great challenge to the search for pulsars and radio transients, such as Rotating Radio Transients (RRATs) and Fast Radio Bursts (FRBs), by obscuring or distorting the signal of interest and resulting in large numbers of erroneous detections. RFI mitigation algorithms aim to remove this interference and improve the chance of detection of transients, but with the growing number of techniques, selecting the most appropriate method for a given survey can be problematic. The choice of method is particularly important in real-time searches planned for next-generation telescopes such as those of the SKAO, where there is no possibility to reprocess the data. In this paper, we explore the algorithm selection problem by injecting pulses into data which simulates several RFI environments. A set of these files is then cleaned using RFI mitigation algorithms and run through a single pulse search pipeline to analyse the recovery of the injected pulses. We examine the recovery of the injected single pulses with an emphasis on a number of cases spanning a range of pulse brightness, width and dispersion measure. The efficacy and side effects of a few popular RFI excision methods, namely IQRM, SKF, and ZDMF are evaluated.

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arXiv2025

Discovery of 30 Galactic radio transient pulsars with MeerTRAP

We present the discovery of 30 new Galactic sources from the MeerTRAP project, a commensal fast radio transient search programme using the MeerKAT telescope. These sources were all identified via a single pulse search. Most of them are likely to be rotating radio transients (RRATs) given their low pulse rates. Using data captured in our transient buffer we have localised nine sources in the image domain to arcsecond precision. This facilitates the timing of these sources and further follow-up with other telescopes. Using the arrival times of single pulses, we have constrained the periods of 14 sources, ranging from 121ms to 7.623s, and derived a phase-coherent timing solution for one of them. Follow-up observations of the MeerTRAP sources (including those published previously) performed with the Effelsberg telescope have detected regular but faint emission from three sources, confirming their long rotation period, including PSR J2218+2902 with a period of 17.5s, the fourth slowest in the radio pulsar population. A few of the sources exhibit interesting emission features, such as periodic microstructure in PSR J1243-0435 and possible nulling in PSR J1911-2020 and PSR J1243-0435. We find that the duty cycles of the three newly discovered pulsars are very low and follow the general trend for the duty cycle with period of known pulsars.

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arXiv2025

Sporadic radio pulses from a white dwarf binary at the orbital period

Recent observations have revealed rare, previously unknown flashes of cosmic radio waves lasting from milliseconds to minutes, and with periodicity of minutes to an hour. These transient radio signals must originate from sources in the Milky Way, and from coherent emission processes in astrophysical plasma. They are theorized to be produced in the extreme and highly magnetised environments around white dwarfs or neutron stars. However, the astrophysical origin of these signals remains contested, and multiple progenitor models may be needed to explain their diverse properties. Here we present the discovery of a transient radio source, ILT J1101+5521, whose roughly minute-long pulses arrive with a periodicity of 125.5 minutes. We find that ILT J1101+5521 is an M dwarf -- white dwarf binary system with an orbital period that matches the period of the radio pulses, which are observed when the two stars are in conjunction. The binary nature of ILT J1101+5521 establishes that some long-period radio transients originate from orbital motion modulating the observed emission, as opposed to an isolated rotating star. We conclude that ILT J1101+5521 is likely a polar system where magnetic interaction has synchronised the rotational and orbital periods of the white dwarf. Magnetic interaction and plasma exchange between two stars has been theorized to generate sporadic radio emission, making ILT J1101+5521 a potential low-mass analogue to such mechanisms.

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arXiv2025

MeerKAT discovery of a hyperactive repeating fast radio burst source

We present the discovery and localisation of a repeating fast radio burst (FRB) source from the MeerTRAP project, a commensal fast radio transient search programme using the MeerKAT telescope. FRB 20240619D was first discovered on 2024 June 19 with three bursts being detected within two minutes in the MeerKAT L-band (856 - 1712MHz). We conducted follow-up observations of FRB 20240619D with MeerKAT using the Ultra-High Frequency (UHF; 544 - 1088MHz), L-band and S-band (1968 - 2843MHz) receivers one week after its discovery, and recorded a total of 249 bursts. The MeerKAT-detected bursts exhibit band-limited emission with an average fractional bandwidth of 0.31, 0.34 and 0.48 in the UHF, L-band and S-band, respectively. We find our observations are complete down to a fluence limit of ~1Jy ms, above which the cumulative burst rate follows a power law $R (>F)\propto (F/1\,\text{Jy}\,\text{ms})^γ$ with $γ=-1.6\pm0.1$ and $-1.7\pm0.1$ in the UHF and L-band, respectively. The near-simultaneous L-band, UHF and S-band observations reveal a frequency dependent burst rate with $3\times$ more bursts being detected in the L-band than in the UHF and S-band, suggesting a spectral turnover in the burst energy distribution of FRB 20240619D. Our polarimetric analysis demonstrates that most of the bursts have $\sim100\%$ linear polarisation fractions and $\sim10\%\text{--}20\%$ circular polarisation fractions. We find no optical counterpart of FRB 20240619D in the MeerLICHT optical observations simultaneous to the radio observations and set a fluence upper limit in MeerLICHT's q-band of 0.76Jy ms and an optical-to-radio fluence ratio limit of 0.034 for a 15s exposure.

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arXiv2025

A coherent radio burst from an X-ray neutron star in the Carina Nebula

The neutron star zoo comprises several sub-populations that range from energetic magnetars and thermally emitting X-ray neutron stars to radio-emitting pulsars. Despite studies over the last five decades, it has been challenging to obtain a clear physical link between the various populations of neutron stars, vital to constrain their formation and evolutionary pathways. Here we report the detection of a burst of coherent radio emission from a known radio-quiet, thermally emitting neutron star 2XMM J104608.7$-$594306in the Carina Nebula. The burst has a distinctive sharp rise followed by a decay made up of multiple components, which is unlike anything seen from other radio-emitting neutron stars. It suggests an episodic event from the neutron star surface, akin to transient radio emission seen from magnetars. The radio burst confirms that the X-ray source is a neutron star and suggests a new link between these apparently radio-quiet X-ray emitting sources and other transient or persistent radio-emitting neutron stars. It also suggests that a common physical mechanism for emission might operate over a range of magnetic field strengths and neutron star ages. We propose that 2XMM J104608.7$-$594306 straddles the boundary between young, energetic neutron stars and their evolved radio-emitting cousins and may bridge these two populations. The detection of such a radio burst also shows that other radio-quiet neutron stars may also emit such sporadic radio emission that has been missed by previous radio surveys and highlights the need for regular monitoring of this unique sub-population of neutron stars.

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arXiv2025

An activity transition in FRB 20201124A: methodological rigor, detection of frequency-dependent cessation, and a geometric magnetar model

We report detections of fast radio bursts (FRBs) from the repeating source FRB 20201124A with Apertif/WSRT and GMRT, and measurements of basic burst properties, especially the dispersion measure (DM) and fluence. Based on comparisons of these properties with previously published larger samples, we argue that the excess DM reported earlier for pulses with integrated signal to noise ratio $\lesssim 1000$ is due to incompletely accounting for the so-called sad trombone effect, even when using structure-maximizing DM algorithms. Our investigations of fluence distributions next lead us to advise against formal power-law fitting, especially dissuading the use of the least-square method, and we demonstrate the large biases involved. A maximum likelihood estimator (MLE) provides a much more accurate estimate of the power law and we provide accessible code for direct inclusion in future research. Our GMRT observations were fortuitously scheduled around the end of the activity cycle as recorded by FAST. We detected several bursts (one of them very strong) at 400/600 MHz, a few hours after sensitive FAST non-detections already showed the 1.3 GHz FRB emission to have ceased. After FRB 20180916B, this is a second example of a frequency-dependent activity window identified in a repeating FRB source. Since numerous efforts have so-far failed to determine a spin period for FRB 20201124A, we conjecture it to be an ultra-long period magnetar, with a period on the scale of months, and with a very wide, highly irregular duty cycle. Assuming the emission comes from closed field lines, we use radius-to-frequency mapping and polarization information from other studies to constrain the magnetospheric geometry and location of the emission region. Our initial findings are consistent with a possible connection between FRBs and crustal motion events.

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arXiv2025

A long-period radio transient active for three decades

Recently several long-period radio transients have been discovered, with strongly polarised coherent radio pulses appearing on timescales between tens to thousands of seconds [1,2]. In some cases the radio pulses have been interpreted as coming from rotating neutron stars with extremely strong magnetic fields, known as magnetars; the origin of other, occasionally periodic and less well-sampled radio transients, is still debated [3]. Coherent periodic radio emission is usually explained by rotating dipolar magnetic fields and pair production mechanisms, but such models do not easily predict radio emission from such slowly-rotating neutron stars and maintain it for extended times. On the other hand, highly magnetic isolated white dwarfs would be expected to have long spin periodicities, but periodic coherent radio emission has not yet been directly detected from these sources. Here we report observations of a long-period (21 minutes) radio transient, which we have labeled GPMJ1839-10. The pulses vary in brightness by two orders of magnitude, last between 30 and 300 seconds, and have quasi-periodic substructure. The observations prompted a search of radio archives, and we found that the source has been repeating since at least 1988. The archival data enabled constraint of the period derivative to $<3.6\times10^{-13}$s s$^{-1}$, which is at the very limit of any classical theoretical model that predicts dipolar radio emission from an isolated neutron star.

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arXiv2025

Contemporaneous optical-radio observations of a fast radio burst in a close galaxy pair

We present the MeerKAT discovery and MeerLICHT contemporaneous optical observations of the Fast Radio Burst (FRB) 20230808F, which was found to have a dispersion measure of $\mathrm{DM}=653.2\pm0.4\mathrm{\,pc\,cm^{-3}}$. FRB 20230808F has a scattering timescale $τ_{s}=3.1\pm0.1\,\mathrm{ms}$ at $1563.6$ MHz, a rotation measure $\mathrm{RM}=169.4\pm0.2\,\mathrm{rad\,m^{-2}}$, and a radio fluence $F_{\mathrm{radio}}=1.72\pm0.01\,\mathrm{Jy\,ms}$. We find no optical counterpart in the time immediately after the FRB, nor in the three months after the FRB during which we continued to monitor the field of the FRB. We set an optical upper flux limit in MeerLICHT's $q$-band of $11.7\,\mathrm{μJy}$ for a 60 s exposure which started $\sim3.4$ s after the burst, which corresponds to an optical fluence, $F_{\mathrm{opt}}$, of $0.039\,\mathrm{Jy\,ms}$ on a timescale of $\sim3.4$ s. We obtain an estimate for the $q-$band luminosity limit of $vL_{v}\sim 1.3\times10^{43}\,\mathrm{erg\,s^{-1}}$. We localise the burst to a close galaxy pair at a redshift of $z_{\mathrm{spec}}=0.3472\pm0.0002$. Our time delay of $\sim3.4$ s between the FRB arrival time and the start of our optical exposure is the shortest ever for an as yet non-repeating FRB, and hence the closest to simultaneous optical follow-up that exists for such an FRB.

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arXiv2025

The emission of interpulses by a 6.45-hour period coherent radio transient

Long-period radio transients are a novel class of astronomical objects characterised by prolonged periods ranging from 18 minutes to 54 minutes. They exhibit highly polarised, coherent, beamed radio emission lasting only 10--100 seconds. The intrinsic nature of these objects is subject to speculation, with highly magnetised white dwarfs and neutron stars being the prevailing candidates. Here we present ASKAP J183950.5-075635.0 (hereafter, ASKAP J1839-0756), boasting the longest known period of this class at 6.45 hours. It exhibits emission characteristics of an ordered dipolar magnetic field, with pulsar-like bright main pulses and weaker interpulses offset by about half a period are indicative of an oblique or orthogonal rotator. This phenomenon, observed for the first time in a long-period radio transient, confirms that the radio emission originates from both magnetic poles and that the observed period corresponds to the rotation period. The spectroscopic and polarimetric properties of ASKAP J1839-0756 are consistent with a neutron star origin, and this object is a crucial piece of evidence in our understanding of long-period radio sources and their links to neutron stars.

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arXiv2024

A candidate coherent radio flash following a neutron star merger

In this paper, we present rapid follow-up observations of the short GRB 201006A, consistent with being a compact binary merger, using the LOw Frequency ARray (LOFAR). We have detected a candidate 5.6$σ$, short, coherent radio flash at 144 MHz at 76.6 mins post-GRB with a 3$σ$ duration of 38 seconds. This radio flash is 27 arcsec offset from the GRB location, which has a probability of occurring by chance of $\sim$0.05% (3.8$σ$) when accounting for measurement uncertainties. Despite the offset, we show that the probability of finding an unrelated transient within 40 arcsec of the GRB location is $<10^{-6}$ and conclude that this is a candidate radio counterpart to GRB 201006A. We performed image plane dedispersion and the radio flash is tentatively (2.4$σ$) shown to be highly dispersed, allowing a distance estimate, corresponding to a redshift of $0.58\pm0.06$. The corresponding luminosity of the event at this distance is $6.7^{+6.6}_{-4.4} \times 10^{32}$ erg s$^{-1}$ Hz$^{-1}$. If associated with GRB 201006A, this emission would indicate prolonged activity from the central engine that is consistent with being a newborn, supramassive, likely highly magnetised, millisecond spin neutron star (a magnetar).

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arXiv2024

Detection and localisation of the highly active FRB 20240114A with MeerKAT

We report observations of the highly active FRB 20240114A with MeerKAT using the Ultra-High Frequency (UHF; $544\text{--}1088$ MHz) and L-band ($856\text{--}1712$ MHz) receivers. A total of 62 bursts were detected in coherent tied-array beams using the MeerTRAP real-time transient detection pipeline. We measure a structure-optimising dispersion measure of $527.65\pm0.01\,\text{pc}\,\text{cm}^{-3}$ using the brightest burst in the sample. We find the bursts of FRB 20240114A are generally detected in part of the broad band of MeerKAT, $\sim40\%$ in the UHF and $\sim30\%$ in the L-band, indicating the band limited nature. We analyse the fluence distribution of the 44 bursts detected at UHF, constraining the fluence completeness limit to $\sim1\,$Jy ms, above which the cumulative burst rate follows a power law $R (>F)\propto (F/1\,\text{Jy}\,\text{ms})^γ$ with $γ=-1.8\pm0.2$. Using channelised telescope data captured in our transient buffer we localise FRB 20240114A in the image domain to RA = 21h27m39.86s, Dec = +04d19m45.01s with an uncertainty of 1.4 arcsec. This localisation allows us to confidently identify the host galaxy of FRB 20240114A. Also using the transient buffer data we perform a polarimetric study and demonstrate that most of the bursts have $\sim100\%$ linear polarisation fractions and up to $\sim20\%$ circular polarisation fractions. Finally, we predict the flux density of a potential persistent radio source (PRS) associated with FRB 20240114A is $\backsimeq[0.6\text{--}60]\,μ\text{Jy}$ based on the simple relation between the luminosity of the PRS and the rotation measure arising from the FRB local environment.

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arXiv2024

An emission state switching radio transient with a 54 minute period

Long-period radio transients are an emerging class of extreme astrophysical events of which only three are known. These objects emit highly polarised, coherent pulses of typically a few tens of seconds duration and minutes to hour-long periods. While magnetic white dwarfs and magnetars, either isolated or in binary systems, have been invoked to explain these objects, a consensus has not emerged. Here we report on the discovery of ASKAP J193505.1+214841.0 (henceforth ASKAPJ1935+2148) with a period of 53.8 minutes exhibiting three distinct emission states - a bright pulse state with highly linearly polarised pulses with widths of 10-50 seconds; a weak pulse state which is about 26 times fainter than the bright state with highly circularly polarised pulses of widths of approximately 370 milliseconds; and a quiescent or quenched state with no pulses. The first two states have been observed to progressively evolve over the course of 8 months with the quenched state interspersed between them suggesting physical changes in the region producing the emission. A constraint on the radius of the source for the observed period rules out a magnetic white dwarf origin. Unlike other long-period sources, ASKAPJ1935+2148 is the first to exhibit drastic variations in emission modes reminiscent of neutron stars. However, its radio properties challenge our current understanding of neutron star emission and evolution.

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