Time variation in the low-frequency spectrum of Vela-like pulsar B1800−21

Abstract

We report the flux measurement of the Vela like pulsar B1800−21 at the low radio frequency regime over multiple epochs spanning several years. The spectrum shows a turnover around the GHz frequency range and represents a typical example of gigahertz-peaked spectrum (GPS) pulsar. Our observations revealed that the pulsar spectrum show a significant evolution during the observing period with the low frequency part of the spectrum becoming steeper, with a higher turnover frequency, for a period of several years before reverting back to the initial shape during the latest measurements. The spectral change over times spanning several years requires dense structures, with free electron densities around 1000-20000 cm −3 and physical dimensions ∼220 AU, in the interstellar medium (ISM) traversing across the pulsar line of sight. We look into the possible sites of such structures in the ISM and likely mechanisms particularly the thermal free-free absorption as possible explanations for the change.

Gigahertz-peaked spectra pulsars in Pulsar Wind Nebulae

“…The detailed low frequency spectra, around GHz frequencies, using the GMRT have been established in ten pulsars. In this work we have studied six pulsars, additionally the pulsar J1740+1000 is reported in Kijak et al (2011a); Dembska et al (2014), B1800−21 in Basu et al (2016), B1823−13 and J1856+0245 in Kijak et al (2017). PSR J1856+0245 was not detected at 610 MHz, but the upper limits indicate the spectral shape to be much flatter.…”

“…The observing mode during each session is also shown in the table with IA corresponding to interferometric array and PA to phased-array observations. The imaging analysis was carried out using the Astronomical Image Processing System (AIPS) similar to Dembska et al (2015a); Basu et al (2016). Initially all the six sources were observed at 610 MHz to look for GPS nature.…”

“…Dembska et al (2015a) showed that interferometric imaging provides the only possible way to securely measure the flux density in highly scattered pulsar profiles. Additionally, Basu et al (2016) also demonstrated that the interferometric techniques were vastly superior to the standard pulsar flux density measurements from a phased-array owing to the possibility of self calibration. In this work we attempt to look for GPS behaviour in six pulsars with associated PWN by characterising their spectral nature around gic 0000 RAS gahertz frequencies.…”

“…These sources are called gigahertz-peaked spectrum (GPS) pulsars and only a handful of such sources are known. A systematic search for GPS pulsars have been conducted since the initial discovery (Kijak et al 2011a(Kijak et al ,b, 2013Dembska et al 2014Dembska et al , 2015aBasu et al 2016;Kijak et al 2017;Jankowski et al 2017) with seventeen pulsars confirmed to exhibit GPS behaviour. The GPS pulsars are usually associated with young energetic sources which are found in peculiar environments like Pulsar wind nebulae (PWN), HII regions, Supernova remnants (SNR), etc.…”

“…This motivated Kijak et al (2011b); Lewandowski et al (2015a); Rajwade, Lorimer & Anderson (2016) to suggest the thermal absorption model based on the initial ideas of Sieber (1973), where the pulsar emission is absorbed in the specialized environments, ⋆ e-mail: rahulbasu.astro@gmail.com as a likely candidate for GPS behaviour in pulsars. The model has been subsequently used by Basu et al (2016); Kijak et al (2017) to successfully explain the spectral behaviour in a majority of GPS pulsars. The environmental effect on the pulsar spectrum is further highlighted by the observation that out of seventeen known GPS pulsars four pulsars are associated with PWN.…”