Dispersion Measure

This document describes calculations related to the Dispersion Measure (DM) of FRBs.

Galactic ISM

The repository contains a HEALPix map of the Galactic ISM DM values based on the NE2001 model. This map is used to calculate the DM contribution from the Milky Way ISM. The map is in the frb/data/DM/ne2001_dm_healpix_map_128.fits file.

Here is an example usage:

from frb.dm import dm_ism_healpix_map

b, l = 5., 50.  # Galactic coordinates in degrees

dm_map = dm_ism_healpix_map.get_dm_map()
dm_ism = dm_ism_healpix_map.dm_ism_from_healpix_map(l, b, dm_map)

One can also input arrays of b,l coordinates.

Intervening Galaxies

A set of estimates for the DM from intervening galaxies is provided. These are based on the analysis given in PN17. All of the analysis is based on statistics of the damped Lya systems (DLAs).

Approximate

An approximate average DM to a given redshift may be calculated with the frb.dlas.approx_avgDM() method:

DM = approx_avgDM(1.)

This may be calculated for a single or an array of redshifts. The return value is an astropy Quantity with default units of pc/cm^3.

MonteCarlo

This algorithm generates realizations of the DM values by randomly placing intervening galaxies along a given sightline or sightlines. Here is an example call:

zeval = np.array([0.,1.,2.])
DMs = monte_DM(np.array(zeval), nrand=1000)  # Returned without units

The values returned are in units of pc/cm^3 but without astropy units attached. One can then perform stats on these outputs to the heart’s desire.