Satyendra Thoudam

Polarized radio emission from extensive air showers measured with LOFAR

We present LOFAR measurements of radio emission from extensive air showers. We find that this emission is strongly polarized, with a median degree of polarization of nearly 99%, and that the angle between the polarization direction of the electric field and the Lorentz force acting on the particles, depends on the observer location in the shower plane. This can be understood as a superposition of the radially polarized charge-excess emission mechanism, first proposed by Askaryan and the geomagnetic emission mechanism proposed by Kahn and Lerche. We calculate the relative strengths of both contributions, as quantified by the charge-excess fraction, for 163 individual air showers. We find that the measured charge-excess fraction is higher for air showers arriving from closer to the zenith. Furthermore, the measured charge-excess fraction also increases with increasing observer distance from the air shower symmetry axis. The measured values range from (3.3± 1.0)% for very inclined air showers at 25 m to (20.3± 1.3)% for almost vertical showers at 225 m. Both dependencies are in qualitative agreement with theoretical predictions.

Method for high precision reconstruction of air shower Xmax using two-dimensional radio intensity profiles

The mass composition of cosmic rays contains important clues about their origin. Accurate measurements are needed to resolve longstanding issues such as the transition from Galactic to extra-Galact ...

LOFAR tied-array imaging of Type III solar radio bursts

The Sun is an active source of radio emission which is often associated with energetic phenomena such as solar flares and coronal mass ejections (CMEs). At low radio frequencies (<100 MHz), the Sun has not been imaged extensively because of the instrumental limitations of previous radio telescopes. Here, the combined high spatial, spectral and temporal resolution of the Low Frequency Array (LOFAR) was used to study solar Type III radio bursts at 30-90 MHz and their association with CMEs. The Sun was imaged with 126 simultaneous tied-array beams within 5 solar radii of the solar centre. This method offers benefits over standard interferometric imaging since each beam produces high temporal (83 ms) and spectral resolution (12.5 kHz) dynamic spectra at an array of spatial locations centred on the Sun. LOFARs standard interferometric output is currently limited to one image per second. Over a period of 30 minutes, multiple Type III radio bursts were observed, a number of which were found to be located at high altitudes (4 solar radii from the solar center at 30 MHz) and to have non-radial trajectories. These bursts occurred at altitudes in excess of values predicted by 1D radial electron density models. The non-radial high altitude Type III bursts were found to be associated with the expanding flank of a CME. The CME may have compressed neighbouring streamer plasma producing larger electron densities at high altitudes, while the non-radial burst trajectories can be explained by the deflection of radial magnetic fields as the CME expanded in the low corona.

Nearby supernova remnants and the cosmic ray spectral hardening at high energies

Recent measurements of cosmic ray spectra of several individual nuclear species by the CREAM, TRACER and ATIC experiments indicate a change in the spectral index of the power laws at TeV energies. Possible explanations among others include non-linear diffusive shock acceleration of cosmic rays, different cosmic ray propagation properties at higher and lower energies in the Galaxy and the presence of nearby sources. In this paper, we show that if supernova remnants are the main sources of cosmic rays in our Galaxy, the effect of the nearby remnants can be responsible for the observed spectral changes. Using a rigidity-dependent escape of cosmic rays from the supernova remnants, we explain the apparent observed property that the hardening of the helium spectrum occurs at relatively lower energies as compared to the protons and also that the spectral hardening does not persist beyond ∼(20–30) TeV energies.

The TACTIC atmospheric Cherenkov imaging telescope

The TACTIC (TeV Atomospheric Cherenkov Telescope with Imaging Camera) γγ-ray telescope, equipped with a light collector of area ∼9.5m2 and a medium resolution imaging camera of 349 pixels, has been in operation at Mt. Abu, India, since 2001. This paper describes the main features of its various subsystems and its overall performance with regard to (a) tracking accuracy of its two-axes drive system, (b) spot size of the light collector, (c) back-end signal processing electronics and topological trigger generation scheme, (d) data acquisition and control system and (e) relative and absolute gain calibration methodology. Using a trigger field-of-view of 11×1111×11 pixels (∼3.4∘×3.4∘)(∼3.4∘×3.4∘), the telescope records a cosmic ray event rate of ∼2.5Hz at a typical zenith angle of 15∘15∘. Monte Carlo simulation results are also presented in the paper for comparing the expected performance of the telescope with actual observational results. The consistent detection of a steady signal from the Crab Nebula above ∼1.2TeV energy, at a sensitivity level of ∼5.0σ∼5.0σ in ∼25h, alongwith excellent matching of its energy spectrum with that obtained by other groups, reassures that the performance of the TACTIC telescope is quite stable and reliable. Furthermore, encouraged by the detection of strong γγ-ray signals from Mrk 501 (during 1997 and 2006 observations) and Mrk 421 (during 2001 and 2005–2006 observations), we believe that there is considerable scope for the TACTIC telescope to monitor similar TeV γγ-ray emission activity from other active galactic nuclei on a long-term basis.

Calibrating the absolute amplitude scale for air showers measured at LOFAR

Air showers induced by cosmic rays create nanosecond pulses detectable at radio frequencies. These pulses have been measured successfully in the past few years at the LOw-Frequency ARray (LOFAR) and are used to study the properties of cosmic rays. For a complete understanding of this phenomenon and the underlying physical processes, an absolute calibration of the detecting antenna system is needed. We present three approaches that were used to check and improve the antenna model of LOFAR and to provide an absolute calibration of the whole system for air shower measurements. Two methods are based on calibrated reference sources and one on a calibration approach using the diffuse radio emission of the Galaxy, optimized for short data-sets. An accuracy of 19% in amplitude is reached. The absolute calibration is also compared to predictions from air shower simulations. These results are used to set an absolute energy scale for air shower measurements and can be used as a basis for an absolute scale for the measurement of astronomical transients with LOFAR.

Probing atmospheric electric fields in thunderstorms through radio emission from cosmic-ray induced air showers

We present measurements of radio emission from cosmic ray air showers that took place during thunderstorms. The intensity and polarization patterns of these air showers are radically different from those measured during fair-weather conditions. With the use of a simple two-layer model for the atmospheric electric field, these patterns can be well reproduced by state-of-the-art simulation codes. This in turn provides a novel way to study atmospheric electric fields.

The shape of the radio wavefront of extensive air showers as measured with LOFAR

Extensive air showers, induced by high energy cosmic rays impinging on the Earths atmosphere, produce radio emission that is measured with the LOFAR radio telescope. As the emission comes from a finite distance of a few kilometers, the incident wavefront is non-planar. A spherical, conical or hyperbolic shape of the wavefront has been proposed, but measurements of individual air showers have been inconclusive so far. For a selected high-quality sample of 161 measured extensive air showers, we have reconstructed the wavefront by measuring pulse arrival times to sub-nanosecond precision in 200 to 350 individual antennas. For each measured air shower, we have fitted a conical, spherical, and hyperboloid shape to the arrival times. The fit quality and a likelihood analysis show that a hyperboloid is the best parametrization. Using a non-planar wavefront shape gives an improved angular resolution, when reconstructing the shower arrival direction. Furthermore, a dependence of the wavefront shape on the shower geometry can be seen. This suggests that it will be possible to use a wavefront shape analysis to get an additional handle on the atmospheric depth of the shower maximum, which is sensitive to the mass of the primary particle.

Observations of TeV γ-rays from Mrk 421 during December 2005 to April 2006 with the TACTIC telescope

The TACTIC

Wide-Band, Low-Frequency Pulse Profiles of 100 Radio Pulsars with LOFAR

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