Exploring the Effects of Solar Wind on Pulsars using the Low Frequency Array (LOFAR) Telescope*

Pulsars are rapidly rotating neutron stars mainly visible as pulsating radio sources. Their rotation is so reliable that it can be used as a highly-precise clock-like signal. By studying this clock signal, via their emitted radio pulses, pulsars can be used to probe a number of effects, such as interstellar weather like the Solar Wind. The Solar Wind (SW) is a highly magnetized stream of plasma out of the Sun due to the pressure of the hot solar corona. The free electrons in the SW induce noise in the pulsar signals that can be measured to gain a deeper understanding of the heliospheric magnetic field and the Solar corona. The ecliptic pulsar whose line-of-sight (LoS) comes close to the sun are best used to track the variations in the electron content of the SW. This is quantified by a parameter known as the Dispersion Measure (DM). Variations in DMs have strong, inverse dependencies on the observing frequency which makes LOFAR, that covers frequencies below 240 MHz, one of the best-suited instruments to measure the effects of the SW. We have obtained data from 7 LOFAR stations which have been observed in the frequency range 110-190 MHz band at a weekly cadence for the past 10 years.

The effect of SW is starkly observed in the DM variations once every year when the pulsar’s LoS is closest to the sun. We measure the effect of SW by calculating the electron density at 1 AU (ne_sw). In this talk, I will describe my solar wind modeling using the Enterprise pulsar timing software which employs a Bayesian MCMC approach to additionally estimate the value of various other noise parameters. The value of these SW noise parameters can be highly covariant with those of the pulsar (Red Noise) and of the interstellar medium DM noise. I will describe the difficulties in disentangling these intermingled effects using the regular-cadence of these ecliptic pulsars and the low frequency observations. I will show how these problems can be solved and how it is theoretically possible to get the true value of ne_sw that is several times more precise than previous measurements. These will, in turn, help us to model and understand this ever changing noise process and get us closer to the discovery of nano-hertz gravitational waves.