Thermal Absorption as the Cause of Gigahertz-Peaked Spectra in Pulsars and Magnetars

Abstract

We present a model that explains the observed deviation of the spectra of some pulsars and magnetars from the power-law spectra which are seen in the bulk of the pulsar population. Our model is based on the assumption that the observed variety of pulsar spectra can be naturally explained by the thermal free-free absorption that takes place in the surroundings of the pulsars. In this context, the variety of the pulsar spectra can be explained according to the shape, density and temperature of the absorbing media and the optical path of the line-of-sight across that. We have put specific emphasis on the case of the radio magnetar SGR J1745-2900 (also known as Sgr A* magnetar), modeling the rapid variations of the pulsar spectrum after the outburst of Apr 2013 as due to the free-free absorption of the radio emission in the electron material ejected during the magnetar outburst. The ejecta expands with time and consequently the absorption rate decreases and the shape of the spectrum changes in such a way that the peak frequency shifts towards the lower radio frequencies. In the hypothesis of an absorbing medium, we also discuss the similarity between the spectral behaviour of the binary pulsar B1259−63 and the spectral peculiarities of isolated pulsars.

Gigahertz-peaked spectra pulsars in Pulsar Wind Nebulae

“…Finally, the most probable absorber are the PWN since their presence is known around these pulsars. The ne values for the four pulsars in this case are between 50 and 200 cm −3 , and the corresponding Te are between 150 and 2500 K. Lewandowski et al (2015a) explored a simplified model of the asymmetric bow-shocked PWN, assuming an uniform electron density and temperature across the medium, and showed that the GPS behaviour is affected by the line of sight towards the PWN. For instance the line of sights passing through the tail would cover the maximum absorber size and result in GPS spectra.…”

“…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.…”

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

“…In all three cases the spectra exhibit GPS characteristics. The thermal free-free absorption in the intervening medium first proposed by Kijak et al (2011aKijak et al ( , 2013 to explain GPS behaviour and further extended by Lewandowski et al (2015b) has been utilized to model the spectrum in each case with the spectral fits and errors during fitting shown as solid and broken lines respectively, the details of which are presented in the next section. The GPS behaviour is characterized by the turnover in the pulsar radio spectra around the GHz frequency range.…”

Detecting fast radio bursts at decametric wavelengths

“…6. We consider two cases for absorption: (i) cold, molecular clouds with ionization fronts for which Te = 200 K and EM = 1000 cm −6 (Lewandowski et al 2015) (hereafter, model B); (ii) hot, ionized magnetar ejecta/circum-burst medium for which Te = 8000 K and EM = 1.5 × 10 6 cm −6 (hereafter, model C). The value of EM for model C has been chosen from a range of values reported in Rajwade et al (2016), Kulkarni et al (2015) and Lewandowski et al (2015).…”