Low-frequency pulse profile variation in PSR B2217+47: evidence for echoes from the interstellar medium

Abstract

We have observed a complex and continuous change in the integrated pulse profile of PSR B2217+47, manifested as additional components trailing the main peak. These transient components are detected over 6 years at 150 MHz using the LOw Frequency ARray (LOFAR), but they are not seen in contemporaneous Lovell observations at 1.5 GHz. We argue that propagation effects in the ionized interstellar medium (IISM) are the most likely cause. The putative structures in the IISM causing the profile variation are roughly half-way between the pulsar and the Earth and have transverse radii R ∼ 30 AU. We consider different models for the structures. Under the assumption of spherical symmetry, their implied average electron density is n e ∼ 100 cm −3 . Since PSR B2217+47 is more than an order of magnitude brighter than the average pulsar population visible to LOFAR, similar profile variations would not have been identified in most pulsars, suggesting that subtle profile variations in low-frequency profiles might be more common than we have observed to date. Systematic studies of these variations at low frequencies can provide a new tool to investigate the proprieties of the IISM and the limits to the precision of pulsar timing.

Detection of Echoes in PSR B1508+55 at Frequencies below 100 MHz Using the LWA1

“…We calculate the third side (X1) of the triangle using the extra path length measurement, traveled by light, using X1 + X2 − D = τ d × c. We estimate the deflection angle to be ∼ 320 mas.Using both observing frequencies (49.8 and 64.5 MHz), we obtain the electron density to be ∼ 40 − 100 cm −3 . This value is of the same order of magnitude as reported in Michilli et al (2018) and consistent with the extreme scattering event electron density obtained in Hill et al (2005).…”

“…Given the similarity of the profile evolution to others previously attributed to propagation effects (Backer et al 2000;Lyne et al 2001;Michilli et al 2018) and the lack of evidence favoring a different interpretation, we favor a model based on a lens in the ISM to explain our observations. Considering this interpretation, we calculate the physical characteristics of this structure below.…”

“…This variation was discovered serendipitously while studying the scattering for a sample of eight pulsars using the LWA1. We compare our results from Michilli et al (2018), where they have observed a similar phenomena for PSR B2217+47. Our analysis of component separation and DM evolution suggests that this observation is the result of echoes due to the presence of structures in the ISM.…”

“…For more details see Section 3.1. 6with time we believe it is intrinsic to the pulsar Michilli et al (2018). report a similar weak precursor for PSR B2217+47 at 150 MHz.…”

“…PSR B1508+55, similar to PSR B2217+47, shows a change in DM of ∼ 6 × 10 −3 pc cm −3 over a period of three years ( Figure 5). In the case of PSR B2217+47, Michilli et al (2018) associate it with the pulse profile evolution. Moreover, in PSR B2217+47, the minimum phase delay between the main component and the transient postcursor is 1 ms at 150 MHz while in the case of PSR B1508+55 it is 17 ms at 49.8 MHz.…”