A New Electron Density Model for Estimation of Pulsar and FRB Distances (1610.09448v2)

Abstract: We present a new model for the distribution of free electrons in the Galaxy, the Magellanic Clouds and the intergalactic medium (IGM) that can be used to estimate distances to real or simulated pulsars and fast radio bursts (FRBs) based on their dispersion measure (DM). The Galactic model has an extended thick disk representing the so-called warm interstellar medium, a thin disk representing the Galactic molecular ring, spiral arms based on a recent fit to Galactic HII regions, a Galactic Center disk and seven local features including the Gum Nebula, Galactic Loop I and the Local Bubble. An offset of the Sun from the Galactic plane and a warp of the outer Galactic disk are included in the model. Parameters of the Galactic model are determined by fitting to 189 pulsars with independently determined distances and DMs. Simple models are used for the Magellanic Clouds and the IGM. Galactic model distances are within the uncertainty range for 86 of the 189 independently determined distances and within 20\% of the nearest limit for a further 38 pulsars. We estimate that 95\% of predicted Galactic pulsar distances will have a relative error of less than a factor of 0.9. The predictions of YMW16 are compared to those of the TC93 and NE2001 models showing that YMW16 performs significantly better on all measures. Timescales for pulse broadening due to interstellar scattering are estimated for (real or simulated) Galactic and Magellanic Cloud pulsars and FRBs.

• The paper presents the YMW16 model that enhances distance estimates for pulsars and FRBs by accurately mapping complex electron distributions.
• It employs a multi-component framework, including thick and thin disks, spiral arms, and local features, to model the Galaxy’s free electron density.
• The model achieves 95% accuracy in DM-based distance predictions and uniquely incorporates extra-galactic structures such as the Magellanic Clouds and IGM.

A New Electron Density Model for Estimation of Pulsar and FRB Distances

The paper "A New Electron Density Model for Estimation of Pulsar and FRB Distances" by J. M. Yao, R. N. Manchester, and N. Wang introduces an advanced model designed to enhance the accuracy of pulsar and Fast Radio Burst (FRB) distance estimations through improved modeling of the free electron distribution in the Milky Way, the Magellanic Clouds, and the intergalactic medium (IGM). This model, denoted as YMW16, constitutes a significant enhancement over prior models like TC93 and NE2001.

Core Structure and Components of YMW16

The YMW16 model builds upon a structured framework that represents various components of the Galaxy, incorporating multiple scale heights and electron densities to encapsulate the diversity of electron distribution. The model is expressed through several components:

1. Thick Disk: Represents the warm interstellar medium with an extended scale height, accounting for a significant portion of the electron density distribution in high-latitude regions. The model optimizes the thick disk parameters to a mid-plane electron density of 0.01132 cm$^{-3}$ and a scale height of 1673 pc.
2. Thin Disk: Reflects the Galactic molecular ring, characterized by a high density concentrated around a Galactocentric radius of 4 kpc. This component is modeled with a sech$^2$ radial profile to capture its rapid density decline beyond the ring.
3. Spiral Arms: YMW16 employs a four-armed spiral model based on HII region data, incorporating explicit parameters for the Perseus, Norma-Outer, Carina-Sagittarius, and Crux-Scutum arms, each contributing localized spikes in electron density.
4. Galactic Center: A distinct component with its high density and unique contributions, largely determined by measurements towards the Galactic Center, PSR J1745-2900.
5. Local Features: These include notable local nebulae such as the Gum Nebula and enhancements surrounding the Local Bubble, as well as features like Loop I, which all contribute to significant local deviations in DM.
6. Magellanic Clouds and IGM: Unlike earlier models, YMW16 includes representations of the ionized structures in nearby galaxies and a simplified model for the IGM to facilitate the estimation of extragalactic pulsar and FRB distances.

Results and Implications

By fitting the model parameters against 189 pulsars with independently determined distances, the YMW16 model shows a considerable improvement in predicting Galactic distances from DM, with 95% of predicted distances exhibiting relative errors below 0.9. This advance is crucial for accurately determining pulsar positioning, which is fundamental for understanding pulsar luminosity, Galactic distribution, and other astrophysical phenomena. Moreover, the model's ability to account for extra-galactic DMs enhances our capability to estimate distances and redshifts for FRBs, a rapidly expanding area of research.

The YMW16 model's implementation addresses one of the main limitations of previous models by avoiding individual clump and void adjustments, thus providing a more generalized and potentially more accurate electron distribution map. It also sets a new standard by modeling the Magellanic Clouds and the IGM, which are pivotal for FRB analyses.

Future Directions

As additional data becomes available through advancing observation technologies and methodologies, the YMW16 model may undergo iterative enhancements, integrating more precise measurements of the Galactic structure and resolving small-scale discrepancies such as those related to individual HII regions or under-dense zones. The expansion of pulsar catalogs and improved distance measurements will further refine the model, bolstering our understanding of cosmic electron distributions and enhancing the accuracy of extragalactic object modeling. The intersections of this work with observational and theoretical advancements continue to be fertile ground for astrophysical exploration and discovery.