Fast Radio Bursts

Fast Radio Bursts (FRBs) are highly dispersed, millisecond duration bursts of unknown origin. Even more than a decade since their discovery, they are one of the most exciting challenges for present-day astrophysics. My work focused on finding and characterizing FRBs across a wide range of frequencies. I have developed a Graphical Processing Unit (GPU)-enabled FRB searching pipeline named SPANDAK. In Gajjar et al. (2018), I reported the discovery of one of the very first detection of FRBs at the highest radio frequencies of 8 GHz. Moreover, my discovery led us to the findings that the first known repeating FRB is embedded in an extreme and dynamic magneto-ionic environment (Michilli et al. 2018, Nature); paving a way to unravel the underlying progenitor. Working with one of my students, we developed a Convolution Neural Network (CNN) classifier which further identified around 100 bursts (Zhang et al. 2018). Moreover, we also used the SPANDAK pipeline to detect FRBs at the lowest radio frequencies of 300 MHz (Pilia et al. 2020). Two of my students, also led one of the very first discoveries of FRB with the Giant Meterwave Radio Telescope (uGMRT) and Robert C. Byrd Green Bank Telescope (GBT) at 300 MHz and 800 MHz, respectively, using the SPANDAK pipeline (Sand et al. 2022). I have also led the design and commissioning of a fully commensal real-time FRB detection backend with the Five hundred meter Aperture Spherical Telescope (FAST), in China. In 2019, my pipeline, in parallel, detected around 1700 FRB bursts exhibiting bi-modal energy distribution (Li et al. 2021, Nature); similar to the magnetospheric emission switching we reported in radio pulsars (Gajjar et al. 2014a).

Among the plethora of emission models, the two main competing scenarios include emission from GRB-like shock driven emission mechanism or pulsar-like magnetospheric emission. Polarization angle (PA) and Rotation Measure (RM) provide a unique look into the geometry and inferred magnetic field at the source to scrutinize these emission models. In the case of shock driven emission models, the emission is generated due to various proposed variants of maser synchrotron processes from the forward shocks where B-field is expected to be uniform predicting a relatively constant PA as seen for FRB121102. In the pulsar-like curvature radiation models, the emission can show significant swing and variations in the PA as seen for a few other FRBs. It should be noted that scattering at lower frequencies has been shown to significantly dampen any PA swings in radio pulsars. Thus, higher-frequencies are the best possible ways to get accurate, scatter independent, PA measurements to truly scrutinize between GRB-like and pulsar-like emission models. Similarly, the measurement of RM depends on the changes in the PA as a function of frequency where scattering also plays an important role in determining accurate RM measurements. FRB 121102 is so far the only FRB that was detected across 400 MHz to 8 GHz (Gajjar et al. 2018), giving insight into the spectral energy distributions. Over the next few years, the Candadian Hydrogen Intensity Mapping Experiment (CHIME) is likely localized several FRBs. I have signed a Memorandum-of-Understanding to exclusively obtain pre-published precise positions to trigger high-frequency follow-up. Thus, I aim to carry out a high waveband study of these FRBs to discriminate between GRB-like and Pulsar-like emission models.

Top three of my research highlights on this topic are below.  

Mobirise
First highest frequency detection of FRB

We reported the first detections of the repeating fast radio burst source FRB 121102 across 4 -- 8 GHz receiver of the GBT with the Breakthrough Listen digital backend. We found 21 bursts within the first 60 minutes of a total of 6 hr of observations. At the time of these detection, these observations were the highest burst density reported in the literature, with 18 bursts being detected in the first 30 minutes. A few bursts clearly show temporal sub-structure with distinct spectral properties. These sub-structures superimpose to provide an enhanced peak signal-to-noise ratio at higher trial DM. The most important finding was that the bursts exhibit nearly 100% linear polarization, and a large average rotation measure of around 10^5 rad m^-2 (in the observer’s frame). No circular polarization was found for any burst. These detections were combined with another set of detection made with the Arecibo telescope which was published in Michilli  et al. (2018) and created quite a flurry of news.  

Mobirise
First low-frequency detection of periodic repeating FRB from GMRT

I have deployed a GPU-accelerated FRB searching pipeline at the GMRT and we have been searching for FRBs from various pulsar surveys. We carried out dedicated DDT observations of 
periodic repeating FRB, named FRB 20180916B, which repeats every 16.3 days. We reported the detection of four and seven bursts from observations spanning 3 days using the upgraded GMRT (300-500 MHz) and the GBT (600-1000 MHz), respectively. These detections were the first reported detection of any FRB with the GMRT and first detection of any FRB at the frequency range of 800 MHz to 1 GHz. We identify 30 μs wide structures in one of the bursts at 800 MHz, making it the lowest frequency detection of such structures for this FRB thus far. There is also a clear indication of high activity of the source at a higher frequency during earlier phases of the activity cycle. We identify a gradual decrease in the rotation measure over two years and no significant variations in the dispersion measure. 

Mobirise
First Direct detection of FRBs using Machine Learning

In Zhang et al. (2018), we reported the detection of 72 new pulses from the repeating fast radio burst FRB 121102 from the GBT using the Breakthrough Listen backend. The new pulses were found with a convolutional neural network in data taken on 2017 August 26, where 21 bursts have been previously detected. Our technique combines neural network detection with dedispersion verification. For the current application, we demonstrate its advantage over a traditional brute-force dedispersion algorithm in terms of higher sensitivity, lower false-positive rates, and faster computational speed. Together with the 21 previously reported pulses, this observation marks the highest number of FRB 121102 pulses from a single observation, totaling 93 pulses in five hours, including 45 pulses within the first 30 minutes. The number of data points reveals trends in pulse fluence, pulse detection rate, and pulse frequency structure. We introduce a new periodicity search technique, based on the Rayleigh test, to analyze the time of arrivals (TOAs), with which we exclude with 99% confidence periodicity in TOAs with periods larger than 5.1 times the model-dependent timestamp uncertainty. In particular, we rule out constant periods ≳10 ms in the barycentric arrival times, though intrinsic periodicity in the time of emission remains plausible.

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