Pulsars and Electron Distribution in the Galaxy

Cover picture from X-Ray NASA/CXC/ASU/J. Hester et al., Public domain, via Wikimedia Commons

Astronomical observations are fundamentally different as compared to laboratory experiments. In the former case, it is not possible to change the physical conditions of the objects under examination nor to fix their physical parameters, whereas in the latter case, controlled experiments can be done. Even the whole set-up can be changed or the object under examination can be replaced with another, more suitable one and the measurements can be repeated under more or less desired conditions. On the other hand, the maximum values of physical variables that can be attained and maintained for long time intervals are highly limited in laboratory conditions. This fact is especially important in checking and finding the limitations and the boundaries of applicability of some fundamental theories in physics. Checking the limit of Einstein’s theory of gravitation for strong gravitational fields is not possible in laboratory experiments done on Earth. The only hope to have some progress in this area of exploration is related to indirect observations of black holes.

Neutron stars are next to black holes in having extremely high values of some physical quantities, especially the density and the magnetic field, and they give the advantage of direct observations. The main difficulty in determining some of their intrinsic and extrinsic properties is due to the fact that determining the distances to astronomical objects is a complicated problem in many cases. The neutron star distances need to be known accurately throughout the Galaxy for example to calculate their radiative power.

There are some standard methods of distance measurements applicable to a large number of astronomical objects. Spectroscopic parallax is one of them. However, it can only be used for ordinary stars by classifying them based on different phases of thermonuclear evolution i.e. reactions in their cores. Trigonometric parallax is another standard method to measure distances with high precision but only for nearby sources (up to about 1 kpc from the Sun).

Observations of supernovae type Ia, used as standard ‘candles’, and redshift measurements provide distances to galaxies which is important in cosmology, but these methods do not supply information on the distributions of different types of sources within the galaxies. Recent studies on supernovae type Ia (see e.g. Moreno-Raya et al. 2016 and references therein) also point out to the possibility that these sources may not be as standard as we used to assume and it may be needed to make some subclassifications based on the metallicity and the single – double system scenarios.

Some general relations between visual absorption (\(A_V\)), and neutral hydrogen column density (\(N_H\)), which is calculated from the X-ray absorption, are given in the literature based on observational data of supernova remnants (SNRs), X-ray binaries, molecular (dust) clouds and HI clouds (Reina and Tarenghi 1973, Gorenstein 1975, Predehl and Schmitt 1995, Guver and Ozel 2009). In principle, the measured NH values of X-ray sources or the measured AV values of optical sources can be used to put limits on the distances using such relations. However, there are some issues related to these relations. Both \(N_H\) and, in particular, \(A_V\) values for SNRs are position dependent and can vary significantly within a single SNR due to both intrinsic and extrinsic contributions to \(N_H\) and \(A_V\). Similarly, X-ray binaries can have considerable intrinsic \(N_H\) , comparable to or even exceeding the contributions to \(N_H\) of HI clouds in some cases (see e.g. White et al. 1995, Ankay and Guseinov 1998). Changes in the surface temperature of companion stars in X-ray binaries due to mass loss or effects of irradiation and gravitation of the primary component increase the uncertainty in the \(A_V\) values. HI clouds and especially dust clouds have inhomogeneous and anisotropic distributions in the Galactic disk with different scale heights (see e.g. Diplas and Savage 1994a,b, Fruscione et al. 1994, Levine et al. 2006, Hou et al. 2009, Wienen et al. 2015). Some attempts to improve this method were made in the past (Aydin et al. 1997, Ankay and Guseinov 1998) constructing \(A_V − N_H\) relations for different solid angle intervals as a function of distance using AV values of ordinary stars and NH values of some X-ray sources. Yet, the uncertainties in the distances obtained by this method are still large. As the number of observed clouds, \(A_V\) measurements of ordinary stars and \(N_H\) measurements of X-ray sources increase, the uncertainties will surely decrease, but this method may never become a standard one because of the significant discrepancy between the two distributions.

For the SNRs formed as a result of core-collapse supernova explosions (see Yazgan 2007 for a review on core-collapse supernovae) producing neutron stars and possibly black holes, there are two basic approaches to distance determination; constructing Galactic rotation models and establishing surface brightness versus linear diameter (\(\Sigma – D\)) relations. The former method is a generally applicable one which can be used as a rule for any astronomical object if its radial speed is measurable. The model requires two parameters: the distance of the Sun from the Galactic center and the rotational speed of the Sun around the Galactic center. This method has two major disadvantages. In directions towards the Galactic center, the radial speed corresponds to two possible distances; one beyond the center and the other closer to the Sun than the Galactic center. The orbital speed and the distance of the Sun from the Galactic center adopted differently in the literature (see e.g. Hou et al. 2009, Reid et al. 2014) leads to differences in the distance of the same astronomical object on the order of 100 pc to 1 kpc. The other general distance determination method applicable for Galactic SNRs (excluding plerionic types which is only about 3% of all the Galactic SNRs observed up to date) is to construct relations between their surface brightness and diameter values (e.g. Guseinov et al. 2003a, Pavlovi ́c et al. 2013, 2014). Eventually, these relations may effectively classify SNRs based on some intrinsic (e.g. supernova explosion energy) and extrinsic (e.g. density distribution in the ambient medium) parameters. Different types of the supernovae should also be considered in relation to these parameters which is not a trivial problem (see Ankay et al. 2007 for a review). This method was also applied to Galactic planetary nebulae (Vukotic and Urosevic 2012). (The above text is taken from Ankay, Yazgan, Kutukcu, SAJ 2016 - see below for the full reference.)

Here are some of the papers related to our work on Pulsars and Electron Distribution in the Galaxy: