Karolina Rożko

THERMAL ABSORPTION AS THE CAUSE OF GIGAHERTZ-PEAKED SPECTRA IN PULSARS AND MAGNETARS

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.

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

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 1000–20 000 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 and Thermal Absorption Model

We present the results of our radio interferometric observations of pulsars at 325 MHz and 610 MHz using the Giant Metrewave Radio Telescope (GMRT). We used the imaging method to estimate the flux densities of several pulsars at these radio frequencies. The analysis of the shapes of the pulsar spectra allowed us to identify five new gigahertz-peaked spectra (GPS) pulsars. Using the hypothesis that the spectral turnovers are caused by thermal free-free absorption in the interstellar medium, we modeled the spectra of all known objects of this kind. Using the model, we were able to put some observational constrains on the physical parameters of the absorbing matter, which allows us to distinguish between the possible sources of absorption. We also discuss the possible effects of the existence of GPS pulsars on future search surveys, showing that the optimal frequency range for finding such objects would be from a few GHz (for regular GPS sources) to possibly 10 GHz for pulsars and radio-magnetars exhibiting very strong absorption.

Modelling of the radio spectrum evolution in the binary pulsar B1259−63

In this paper we give the first attempt to model the evolution of the spectrum of PSR B1259−63 radio emission while the pulsar orbits the companion Be star. As suggested by Kijak et al. (Mon. Not. R. Astron. Soc. 418:L114, 2011a) this binary system can be useful in understanding the origin of the gigahertz-peaked spectrum of pulsars. The model explains, at least qualitatively, the observed alterations of the spectral shape depending on the orbital phases of this pulsar. Thus, our results support the hypothesis that the external factors have a significant impact on the observed radio emission of a pulsar. The model can also contribute to our understanding of the origin of some non-typical spectral shapes (e.g. flat or broken spectra).

Gigahertz-peaked spectra pulsars in Pulsar Wind Nebulae

We have carried out a detailed study of the spectral nature of six pulsars surrounded by Pulsar wind nebulae (PWN). The pulsar flux density were estimated using the interferometric imaging technique of the Giant Metrewave Radio Telescope at three frequencies 325 MHz, 610 MHz and 1280 MHz. The spectra showed a turnover around gigahertz frequency in four out of six pulsars. It has been suggested that the gigahertz peaked spectra (GPS) in pulsars arises due to thermal absorption of the pulsar emission in surrounding medium like PWN, HII regions, Supernova remnants, etc. The relatively high incidence of GPS behaviour in pulsars surrounded by PWN impart further credence to this view. The pulsar J1747

Multi-frequency observations and spectral analysis of two gigahertz-peaked spectra pulsars

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The study of multi-frequency scattering of 10 radio pulsars

2958 associated with the well known Mouse nebula was also observed in our sample and exhibited GPS behaviour. The pulsar was detected as a point source in the high resolution images. However, the pulsed emission was not seen in the phased array mode. It is possible that the pulsed emission was affected by extreme scattering causing considerable smearing of the emission at low radio frequencies. The GPS spectra were modeled using the thermal free-free absorption and the estimated absorber properties were largely consistent with PWN. The spatial resolution of the images made it unlikely that the point source associated with J1747

Gigahertz-peaked spectra pulsars

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Is anomalous scattering typical for pulsars

2958 was the compact head of the PWN, but the synchrotron self-absorption seen in such sources was a better fit to the estimated spectral shape.

Analysis and discussion of a sample of 25 gigahertz-peaked spectra pulsars

We report the multi-frequency observations of two pulsars: J1740+1000 and B1800-21, using the Giant Metrewave Radio Telescope and the Green Bank Telescope. The main aim of these observations was to estimate the flux density spectrum of these pulsars, as both of them were previously reported to exhibit gigahertz-peaked spectra. J1740+1000 is a young pulsar far from the Galactic plane and the interpretation of its spectrum was inconclusive in the light of the recent flux density measurements. Our result supports the gigahertz-peaked interpretation of the PSR J1740+1000 spectrum. B1800-21 is a Vela-like pulsar near the W30 complex, whose spectrum exhibit a significant change between 2012 and 2014 year. Our analysis shows that the current shape of the spectrum is similar to that observed before 2009 and confirms that the observed spectral change happen in a time-scale of a few years.