Subhashis Roy

An improved map of the galactic Faraday sky

We aim to summarize the current state of knowledge regarding Galactic Faraday rotation in an all-sky map of the Galactic Faraday depth. For this we have assembled the most extensive catalog of Faraday rotation data of compact extragalactic polarized radio sources to date. In the map-making procedure we used a recently developed algorithm that reconstructs the map and the power spectrum of a statistically isotropic and homogeneous field while taking into account uncertainties in the noise statistics. This procedure is able to identify some rotation angles that are offset by an integer multiple of π. The resulting map can be seen as an improved version of earlier such maps and is made publicly available, along with a map of its uncertainty. For the angular power spectrum we find a power law behavior C� ∝ � −2.17 for a Faraday sky where an overall variance profile as a function of Galactic latitude has been removed, in agreement with earlier work. We show that this is in accordance with a 3D Fourier power spectrum P(k) ∝ k −2.17 of the underlying

Extragalactic sources towards the central region of the Galaxy

We have observed a sample of 64 small-diameter sources towards the central -6° < l < 6°, -2° < b < 2° of the Galaxy with the aim of studying the Faraday rotation measure near the Galactic Centre region. All the sources were observed at 6- and 3.6-cm wavelengths using the ATCA and the VLA. 59 of these sources are inferred to be extragalactic. The observations presented here constitute the first systematic study of the radio polarization properties of the background sources towards this direction and increase the number of known extragalactic radio sources in this part of the sky by almost an order of magnitude. Based on the morphology, spectral indices and lack of polarized emission, we identify four Galactic H II regions in the sample.

Magnetic fields in nearby normal galaxies: energy equipartition

We present maps of total magnetic field using ‘equipartition’ assumptions for five nearby normal galaxies at sub-kpc spatial resolution. The mean magnetic field is found to be ∼11 μG. The field is strongest near the central regions where mean values are ∼20−25 μG and fall to ∼15 μG in disc and ∼10 μG in the outer parts. There is little variation in the field strength between arm and interarm regions, such that, in the interarms, the field is 20 per cent weaker than in the arms. There is no indication of variation in magnetic field as one moves along arm or interarm after correcting for the radial variation of magnetic field. We also studied the energy densities in gaseous and ionized phases of the interstellar medium and compared to the energy density in the magnetic field. The energy density in the magnetic field was found to be similar to that of the gas within a factor of 2 at sub-kpc scales in the arms, and thus magnetic field plays an important role in pressure balance of the interstellar medium. Magnetic field energy density is seen to dominate over the kinetic energy density of gas in the interarm regions and outer parts of the galaxies and thereby helps in maintaining the large-scale ordered fields seen in those regions.

Sgr A* at low radio frequencies: Giant Metrewave Radio Telescope observations

The central region of the Galaxy has been observed at 580, 620 and 1010 MHz with the Giant Metrewave Radio Telescope (GMRT). We detect emission from Sgr A*, the compact object at the dynamical centre of the Galaxy, and estimate it s flux density at 620 MHz to be 0.5±0.1 Jy. This is the first detection of Sgr A* below 1 GHz (Roy & Ra o 2002, 2003), which along with a possible detection at 330 MHz (Nord et al. 2004) p rovides its spectrum below 1 GHz. Comparison of the 620 MHz map with maps made at other freq uencies indicates that most parts of the Sgr A West HII region have optical depth ∼2. However, Sgr A*, which is seen in the same region in projection, shows a slightly inver ted spectral index between 1010 MHz and 620 MHz. This is consistent with its high frequency sp ectral index, and indicates that Sgr A* is located in front of the Sgr A West complex, and ru les out any low frequency turnover around 1 GHz, as suggested by Davies et al. (1976).

Synchrotron spectral index and interstellar medium densities of star-forming galaxies

The spectral index of synchrotron emission is an important parameter in understanding the properties of cosmic ray electrons (CREs) and the interstellar medium (ISM). We determine the synchrotron spectral index (

Constraints on distances to Galactic Centre non-thermal filaments from HI absorption

\alpha_{\rm nt}

GMRT observations of four suspected supernova remnants near the Galactic Centre

) of four nearby star-forming galaxies, namely NGC 4736, NGC 5055, NGC 5236 and NGC 6946 at sub-kpc linear scales. The

NuSTAR + XMM-Newton monitoring of the neutron star transient AX J1745.6-2901

\alpha_{\rm nt}

325 and 610 MHz radio counterparts of SNR G353.6−0.7 also known as HESS J1731−347

was determined between 0.33 and 1.4 GHz for all the galaxies. We find the spectral index to be flatter (

Radio sources at low Galactic latitudes

\gtrsim -0.7