Abstract
Context. Pulsar radio emission undergoes dispersion due to the presence of free electrons in the interstellar medium (ISM). The dispersive delay in the arrival time of the pulsar signal changes over time due to the varying ISM electron column density along the line of sight. Accurately correcting for this delay is crucial for the detection of nanohertz gravitational waves using pulsar timing arrays. Aims. We aim to demonstrate the precision in the measurement of the dispersion delay achieved by combining 400−500 MHz (BAND3) wide-band data with those at 1360−1460 MHz (BAND5) observed using the upgraded GMRT, employing two different template alignment methods. Methods. To estimate the high precision dispersion measure (DM), we measure high precision times-of-arrival (ToAs) of pulses using carefully generated templates and the currently available pulsar timing techniques. We use two different methods for aligning the templates across frequency to obtain ToAs over multiple sub-bands and therefrom measure the DMs. We study the effects of these two different methods on the measured DM values in detail. Results. We present in-band and inter-band DM estimates of four pulsars over the timescale of a year using two different template alignment methods. The DMs obtained using both these methods show only subtle differences for PSRs J1713+0747 and J1909−3744. A considerable offset is seen in the DM of PSRs J1939+2134 and J2145−0750 between the two methods. This could be due to the presence of scattering in the former and profile evolution in the latter. We find that both methods are useful but could have a systematic offset between the DMs obtained. Irrespective of the template alignment methods followed, the precision on the DMs obtained is about 10−3 pc cm−3 using only BAND3 and 10−4 pc cm−3 after combining data from BAND3 and BAND5 of the uGMRT. In a particular result, we detected a DM excess of about 5 × 10−3 pc cm−3 on 24 February 2019 for PSR J2145−0750. This excess appears to be due to the interaction region created by fast solar wind from a coronal hole and a coronal mass ejection observed from the Sun on that epoch. A detailed analysis of this interesting event is presented.
Highlights
Pulsars are rotating neutron stars that emit broadband radiation received as pulsed signals by the observers
This dispersion delay is directly proportional to the integrated column density of free electrons in the ionised interstellar medium (IISM), usually referred to as the dispersion measure (DM), and inversely proportional to the square of the observing frequency (ν)
The DM of a pulsar can vary with time due to a number of factors, which include the relative motion of the pulsar with respect to the observer, solar wind, the terrestrial ionosphere, and the dynamical nature of the IISM
Summary
Pulsars are rotating neutron stars that emit broadband radiation received as pulsed signals by the observers. Typical DM variations observed in pulsars range from 10−3−10−5 pc cm−3 (Kumar et al 2013; Alam et al 2021; Donner et al 2020) If these variations are not accounted for, systematic errors of the order of 1 μs or more can arise while correcting for the DM delay to generate infinite-frequency ToAs in the Solar System barycentre (SSB) frame (Hobbs et al 2006; Edwards et al 2006). The technique of pulsar timing that creates such celestial clocks requires us to model and correctly characterise the pulse propagation effects (Edwards et al 2006) This technique is crucial for the rapidly maturing pulsar timing array (PTA) efforts to detect nanohertz gravitational waves (GWs; Foster & Backer 1990; Arzoumanian et al 2020). Pulsar timing arrays pursue the timing of tens of MSPs to detect mainly a stochastic nanohertz GW background due to an ensemble of merging supermassive black hole binaries (Burke-Spolaor et al 2019)
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