Lunch talk on May. 18, 2023

Dispersion measure variations predict lensing in pulsars

Speaker: Fan Xi Lin (University of Toronto)

Venue: Video Conference

Time: 09:30 AM, Thursday, May. 18, 2023

Abstract: 

As radio signals from a pulsar travel to the observer on Earth, they encounter electron plasma. This plasma delays the signals in a frequency-dependent manner, which can be characterized by the dispersion measure (DM), a fundamental quantity in pulsar astronomy. This quantity, which is a measure of the total electron content between the pulsar and source, is expected to change as the pulsar moves.  Since the refractive index of the medium is related to electron density, a changing DM is expected to be correlated with lensing effects such as a change in magnification or multiple imaging. 

In this talk, we show that such expectations are indeed valid, and show how they can be used to study structures in the interstellar medium (ISM) or the environment of a pulsar. We develop techniques developed based on standard lensing formalism and apply them to PSR J1713+0747, PSR J2051−0827, and PSR B1957+20. In PSR J1713+0747, we show that the multiple DM dip events, if they are indeed due to line of sight crossing a lower electron density region, must create a second, bright image. In the Black Widow pulsars PSR J2051−0827 and PSR B1957+20, we discover the existence of an apparently weak lensing regime in the two pulsars and show that this regime transitions to a strong lensing regime in PSR B1957+20. We show, for the first time, that the second derivative of variations in DM can predict observed flux density variations in the apparently weak regime. We use this correlation to directly measure the velocity of the lensing material, projected onto the plane of the sky. 

The techniques developed can be applied to improve pulsar timing by predicting and mitigating certain propagation effects.  The discovery of lensing regime transitions in PSR B1957+20 offers a valuable testing ground for theoretical developments in lensing.

Report PPT:  SWIFAR_Fang Xi Lin.pdf