Low-energy Radio Bursts from Magnetar XTE J1810$-$197: Implications for Fast Radio Bursts

TL;DR

This work investigates whether the abundant low-energy bursts from the magnetar XTE J1810-197 can illuminate the magnetar-FRB connection.Using 4.5 years of multi-frequency radio monitoring with GMRT and GBT, the authors catalog over 97,000 bright pulses and analyze their fluence distributions, waiting times, and periodicity properties. They find that the magnetar exhibits both pulsar-like lognormal and giant-pulse-like power-law tails in its burst energetics, implying FRB-like giant pulses could occur on timescales shorter than 10^5 years, depending on the emission state; the underlying spin periodicity can be masked if bursts occupy a wide range of spin phases. These results support magnetars as viable FRB progenitors and provide a framework to estimate the Galactic FRB rate by linking burst energetics to emission states and spin-phase coverage.

Abstract

Magnetars are the leading candidate sources of fast radio bursts (FRBs). However, the observational probes of the connections between magnetars and FRBs are severely limited by the paucity of detection of highly energetic radio events from magnetars -- to date, only one radio burst as energetic as FRBs has been detected from a Galactic magnetar. Here, we present a detailed analysis of a large sample of low-energy bursts detected from the magnetar XTE J1810$-$197, and probe their implications for FRB emission from magnetars. We report detection of over 97000 bright radio pulses from 242 observations of the magnetar XTE J1810$-$197 over 4.5 years and two decades in frequency (300 MHz to 6.15 GHz), using the Giant Meterwave Radio Telescope and the Green Bank Telescope, after its recent outburst onset in December 2018. We present detailed analysis of the burst fluence distributions and their trends with time as well as frequency, and the waiting time distribution. We show that XTE J1810$-$197 rapidly switches between pulsar-like and giant-pulse-like emission states, and magnetars like XTE J1810$-$197 remain viable and likely emitters of FRBs, in the form of giant-pulses with energies comparable to FRBs. We also demonstrate that the lack of the detection of an underlying periodicity in the bursts from repeating FRBs might be caused by emission across a wide range of spin phases.

Figures (14)

• Figure 1: The frequency coverage for the individual observations is shown with the observation date (MJDs as well as decimal years). The GMRT observations are shown in blue and the GBT ones are shown in red color.
• Figure 2: One of the brightest detected pulses: the bottom panel shows the dynamic spectrum de-dispersed at a DM of 178.85 $\,$pc$\,$cm$^{-3}$, while the top panel shows the band-averaged pulse with flux density in arbitrary units.
• Figure 3: Left set of panels: The fluence histograms using all the pulses detected at different frequencies are shown in different panels. The different subplots, a, b, c, d, e and f correspond to 400 MHz, 650 MHz, 1360 MHz, 1500 MHz, 2000 MHz, and 5400 MHz, respectively. Right set of panels: The relative fluence histograms at different frequencies. The blue vertical line, at a relative fluence of 10, is to distinguish normal pulses from giant-pulses. Giant pulses are defined as pulses with a relative fluence of equal to or greater than 10.
• Figure 4: The bottom panel shows the relative fluence distributions for all the observations using box plots. The zoomed-in figure illustrates the box plot for a typical observation, where the top and bottom bars on the box plot are maximum and minimum relative fluences, the top and bottom of the box itself are the 75 and 25 percentile, and the line inside the box is the median of the relative fluence distribution. The black dashed line at 10 is for giant pulses. A log scale version of this figure is shown in Appendix \ref{['box_plot']}. Top: A few examples of cumulative fluence distributions at different epochs is shown. The blue points represent the observed number of pulses with Poissonian uncertainty and the red dashed line represents the fitted model. At the top of these plots, the epoch of observation is mentioned.
• Figure 5: Left: The best-fitted distribution is shown as a function of Epoch (in units of decimal year on the top and MJD in the bottom horizontal axis). The different symbols represent different models, as shown in the legends at the top. Right: The Fractions of the number of observations fitted with different models are shown, with the same color-symbol mapping for different models as in the left panel.