High precision measurements of interstellar dispersion measure with the upgraded GMRT^⋆

¹ Fakultät für Physik, Universität Bielefeld, Postfach 100131, 33501 Bielefeld, Germany
e-mail: kkma@physik.uni-bielefeld.de
² Arecibo Observatory, University of Central Florida, Arecibo, PR 00612, USA
³ National Centre for Radio Astrophysics, Tata Institute of Fundamental Research, Ganeshkhind, Pune, 411007 Maharashtra, India
⁴ Department of Physics, Indian Institute of Technology Hyderabad, Kandi, Telangana 502285, India
⁵ The Institute of Mathematical Sciences, CIT Campus, Tharamani, Chennai, 600113 Tamil Nadu, India
⁶ Homi Bhabha National Institute, Training School Complex, Anushakti Nagar, Mumbai, 400094 Maharashtra, India
⁷ Department of Physics, BITS Pilani Hyderabad Campus, Hyderabad, 500078 Telangana, India
⁸ Department of Astronomy and Astrophysics, Tata Institute of Fundamental Research, Dr. Homi Bhabha Road, Mumbai, 400005 Maharashtra, India
⁹ Indian Institute of Science Education and Research Thiruvananthapuram, Vithura, Kerala 695551, India
¹⁰ Jodrell Bank Centre for Astrophysics, University of Manchester, Oxford Road, Manchester M13 9PL, UK
¹¹ ASTRON, the Netherlands Institute for Radio Astronomy, Postbus 2, 7990 AA Dwingeloo, The Netherlands
¹² Department of Physics, Indian Institute of Technology Delhi, Hauz Khas, New Delhi 110016, India
¹³ Cahill Center for Astrophysics, California Institute of Technology, 1200 E. California Blvd., Pasadena, CA 91125, USA
¹⁴ University of Oxford, Sub-Department of Astrophysics, Denys Wilkinson Building, Keble Road, Oxford OX1 3RH, UK
¹⁵ Department of Physics, Indian Institute of Technology Roorkee, Roorkee, 247667 Uttarakhand, India
¹⁶ Raman Research Institute, Bengaluru, 560080 Karnataka, India

Received: 13 January 2021
Accepted: 26 May 2021

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.