Sourav Palit

Instruments of RT-2 experiment onboard CORONAS-PHOTON and their test and evaluation III: Coded Aperture Mask and Fresnel Zone Plates in RT-2/CZT payload

Imaging in hard X-rays of any astrophysical source with high angular resolution is a challenging job. Shadow-casting technique is one of the most viable options for imaging in hard X-rays. We have used two different types of shadow-casters, namely, Coded Aperture Mask (CAM) and Fresnel Zone Plate (FZP) pair and two types of pixellated solid-state detectors, namely, CZT and CMOS in RT-2/CZT payload, the hard X-ray imaging instrument onboard the CORONAS-PHOTON satellite. In this paper, we present the results of simulations with different combinations of coders (CAM & FZP) and detectors that are employed in the RT-2/CZT payload. We discuss the possibility of detecting transient Solar flares with good angular resolution for various combinations. Simulated results are compared with laboratory experiments to verify the consistency of the designed configuration.

Modeling of temporal variation of Very Low Frequency (VLF) radio waves over long paths as observed from Indian Antarctic stations.

Characteristics Very Low Frequency (VLF) signal depends on solar illumination across the propagation path. For a long path, solar zenith angle varies widely over the path and this has a significant influence on the propagation characteristics. To study the effect, Indian Center for Space Physics participated in the 27th and 35th Scientific Expedition to Antarctica. VLF signals transmitted from the transmitters, namely, VTX (18.2 kHz), Vijayanarayanam, India and NWC (19.8 kHz), North-west Cape, Australia were recorded simultaneously at Indian permanent stations Maitri and Bharati having respective geographic coordinates 70.75∘S, 11.67∘E and 69.4∘S, 76.17∘E. A very stable diurnal variation of the signal has been obtained from both the stations. We reproduced the signal variations of VLF signal using solar zenith angle model coupled with Long Wavelength Propagation Capability (LWPC) code. We divided the whole path into several segments and computed the solar zenith angle (χ) profile. We assumed a linear relationship between the Waits exponential model parameters effective reflection height (h′), steepness parameter (β) and solar zenith angle. The h′ and β values were later used in the LWPC code to obtain the VLF signal amplitude at a particular time. The same procedure was repeated to obtain the whole day signal. Nature of the whole day signal variation from the theoretical modeling is also found to match with our observation to some extent.

Inverse problem in ionospheric science: prediction of solar soft-X-ray spectrum from very low frequency radiosonde results

X-rays and gamma-rays from astronomical sources such as solar flares are mostly absorbed by the Earth’s atmosphere. Resulting electron-ion production rate as a function of height depends on the intensity and wavelength of the injected spectrum and therefore the effects vary from one source to another. In other words, the ion density vs. altitude profile has the imprint of the incident photon spectrum. In this paper, we investigate whether we can invert the problem uniquely by deconvolution of the VLF amplitude signal to obtain the details of the injected spectrum. We find that it is possible to do this up to a certain accuracy. This leads us to the possibility of uninterrupted observation of X-ray photon spectra of solar flares that are often hindered by the restricted observation window of space satellites to avoid charge particle damages. Such continuous means of observation are essential in deriving information on time evolution of physical processes related to electron acceleration and interaction with plasma in solar atmosphere. Our method is useful to carry out a similar exercise to infer the spectra of more energetic events such as the Gamma Ray Bursts (GRBs), Soft Gamma-ray Repeaters (SGRs) etc., by probing even the lower part of the Earth’s atmosphere. We thus show that to certain extent, the Earth’s atmosphere could be used as a gigantic detector of relatively strong astronomical events.

Fresnel zone plate telescopes for X-ray imaging II: numerical simulations with parallel and diverging beams

We present the results of simulations of shadows cast by a zone plate telescope which may have one to four pairs of zone plates. From the shadows we reconstruct the images under various circumstances. We discuss physical basis of the resolution of the telescope and demonstrate this by our simulations. We allow the source to be at a finite distance (diverging beam) as well as at an infinite distance (parallel beam) and show that the resolution is worsened when the source is nearby. By reconstructing the zone plates in a way that both the zone plates subtend the same solid angles at the source, we obtain back high resolution even for sources at a finite distance. We present simulated results for the observation of the galactic center and show that the sources of varying intensities may be reconstructed with accuracy. Results of these simulations would be of immense use in interpreting the X-ray images from recently launched CORONAS-PHOTON satellite.

Effect of solar flares on ionospheric VLF radio wave propagation, modeling with GEANT4 and LWPC and determination of effective reflection height

Solar flares are among the most prominent transient events affecting the radio wave propagation in the ionosphere. In this paper we model the ionizing effect on VLF wave, reflected from D-layer of the ionosphere during different classes of flares and compare with observed VLF modulation. A well known detector simulation software GEANT4 is used to calculate the ionization produced in different heights (~ 60-80 km) of the D layer. The chemical balancing between the ions and neutrals in the region is taken into account with a simplified chemical model, the GPI model. The VLF modulation is reproduced with LWPC. We have seen that the peaks of electron density arise at different times at different heights. Matching with the time delay of the VLF signals we have proposed an alternative effective reflection height during such events.

Fresnel zone plates for Achromatic Imaging Survey of X‐ray sources

A telescope with Fresnel Zone Plates has been contemplated to be an excellent imaging mask in X‐rays and gamma‐rays for quite some time. With a proper choice of zone plate material, spacing and an appropriate readout system it is possible to achieve any theoretical angular resolution. We provide the results of numerical simulations of how a large number of X‐ray sources could be imaged at a high resolution. We believe that such an imager would be an excellent tool for a future survey mission for X‐ray and gamma‐ray sources which we propose.