Free-free absorption in hot relativistic flows: application to fast radio bursts

Abstract

Magnetic flares create hot relativistic shocks outside the light cylinder radius of a magnetised star. Radio emission produced in such a shock or at a radius smaller than the shock undergoes free-free absorption while passing through the shocked medium. In this work, we demonstrate that this free-free absorption can lead to a negative drift in the frequency-time spectra. Whether it is related to the downward drift pattern observed in fast radio bursts (FRBs) is unclear. However, if the FRB down drifting is due to this mechanism then it will be pronounced in those shocks that have isotropic kinetic energies $\gtrsim 10^{44}$ erg. In this model, for an internal shock with a Lorentz factor $\sim 100$, the normalised drift rate $|\mathrm{D}{\mathrm{R}}_{\mathrm{obs}}|/{\nu }_{\mathrm{mean}}$ is $\sim 10^{-2}$ per ms, where ${\nu }_{\mathrm{mean}}$ is the central frequency of the radio pulses. The corresponding radius of the shocked shell is, therefore, in the range of $10^{10}$ cm and $10^{11}$ cm. This implies that, for an outflow consisting of hydrogen ion, the upper limit on the mass of the relativistic shocks is a few $\times 10^{-10}{M}_{\odot }$, which is considerably low compared to that ejected from SGR 1806-20 during the 2004 outburst.

5 pages, 4 figures, Accepted in MNRAS Letters

References in corpus (29)

• A bright millisecond radio burst of extragalactic origin
• A fast radio burst associated with a Galactic magnetar
• A bright millisecond-duration radio burst from a Galactic magnetar
• The Host Galaxy and Redshift of the Repeating Fast Radio Burst FRB 121102
• Fast Radio Bursts
• Fast radio bursts as synchrotron maser emission from decelerating relativistic blast waves
• A repeating fast radio burst source localised to a nearby spiral galaxy
• A single fast radio burst localized to a massive galaxy at cosmological distance
• INTEGRAL discovery of a burst with associated radio emission from the magnetar SGR 1935+2154
• A fast radio burst localised to a massive galaxy
• The Physical Mechanisms of Fast Radio Bursts
• A Second Source of Repeating Fast Radio Bursts
• HXMT Identification of a non-thermal X-ray burst from SGR J1935+2154 and with FRB 200428
• Nine New Repeating Fast Radio Burst Sources from CHIME/FRB
• A peculiar hard X-ray counterpart of a Galactic fast radio burst
• Diverse polarization angle swings from a repeating fast radio burst source
• The host galaxies and progenitors of Fast Radio Bursts localized with the Australian Square Kilometre Array Pathfinder
• Implications of a "Fast Radio Burst" from a Galactic Magnetar
• On the FRB luminosity function -- II. Event rate density
• The prevalence of repeating fast radio bursts
• The synchrotron maser emission from relativistic shocks in Fast Radio Bursts: 1D PIC simulations of cold pair plasmas
• A bright gamma-ray flare interpreted as a giant magnetar flare in NGC 253
• A Distant Fast Radio Burst Associated to its Host Galaxy with the Very Large Array
• On the Time-Frequency Downward Drifting of Repeating Fast Radio Bursts
• Dispersion Measure Variation of Repeating Fast Radio Burst Sources
• Limits on precursor and afterglow radio emission from a fast radio burst in a star-forming galaxy
• Induced scattering of short radio pulses
• The confrontation of the shock-powered synchrotron maser model with the Galactic FRB 200428
• The Impact of the Environment of White Dwarf Mergers on Fast Radio Bursts

Cited by in corpus (3)

• The Physics of Fast Radio Bursts
• Coherent inverse Compton scattering by bunches in fast radio bursts
• Optical Transients from Fast Radio Bursts Heating Companion Stars in Close Binary Systems