arXiv subjects

A. Basak

Publications and source records attributed to A. Basak.

5 recordsLinked to original sources

arXiv2021

NICER uncovers the transient nature of the type-B quasi-periodic oscillation in the black hole candidate MAXI J1348-630

We present a systematic spectral-timing analysis of a fast appearance/disappearance of a type-B quasi-periodic oscillation (QPO), observed in four NICER observations of MAXI J1348-630. By comparing the spectra of the period with and without the type-B QPO, we found that the main difference appears at energy bands above ~2 keV, suggesting that the QPO emission is dominated by the hard Comptonised component. During the transition, a change in the relative contribution of the disk and Comptonised emission was observed. The disk flux decreased while the Comptonised flux increased from non-QPO to type-B QPO. However, the total flux did not change too much in the NICER band. Our results reveal that the type-B QPO is associated with a redistribution of accretion power between the disk and Comptonised emission. When the type-B QPO appears, more accretion power is dissipated into the Comptonised region than in the disk. Our spectral fits give a hint that the increased Comptonised emission may come from an additional component that is related to the base of the jet.

astro-ph.HE↗

arXiv2020

NICER observations reveal that the X-ray transient MAXI J1348-630 is a Black Hole X-ray binary

We studied the outburst evolution and timing properties of the recently discovered X-ray transient MAXI J1348-630 as observed with NICER. We produced the fundamental diagrams commonly used to trace the spectral evolution, and power density spectra to study the fast X-ray variability. The main outburst evolution of MAXI J1348-630 is similar to that commonly observed in black hole transients. The source evolved from the hard state, through hard- and soft-intermediate states, into the soft state in the outburst rise, and back to the hard state in reverse during the outburst decay. At the end of the outburst, MAXI J1348-630 underwent two reflares with peak fluxes ~1 and ~2 orders of magnitude fainter than the main outburst, respectively. During the reflares, the source remained in the hard state only, without undergoing any state transitions, which is similar to the so-called "failed outbursts". Different types of quasi-periodic oscillations (QPOs) are observed at different phases of the outburst. Based on our spectral-timing results, we conclude that MAXI J1348-630 is a black hole candidate.

astro-ph.HE↗

arXiv2020

A NICER look at the state transitions of the black hole candidate MAXI J1535-571 during its reflares

The black hole candidate and X-ray binary MAXI J1535-571 was discovered in September 2017. During the decay of its discovery outburst, and before returning to quiescence, the source underwent at least four reflaring events, with peak luminosities of $\sim$10$^{35-36}$ erg s$^{-1}$ (d/4.1 kpc)$^2$. To investigate the nature of these flares, we analysed a sample of NICER observations taken with almost daily cadence. In this work we present the detailed spectral and timing analysis of the evolution of the four reflares. The higher sensitivity of NICER at lower energies, in comparison with other X-ray detectors, allowed us to constrain the disc component of the spectrum at $\sim$0.5 keV. We found that during each reflare the source appears to trace out a q-shaped track in the hardness-intensity diagram similar to those observed in black hole binaries during full outbursts. MAXI J1535-571 transits between the hard state (valleys) and softer states (peaks) during these flares. Moreover, the Comptonised component is undetected at the peak of the first reflare, while the disc component is undetected during the valleys. Assuming the most likely distance of 4.1 kpc, we find that the hard-to-soft transitions take place at the lowest luminosities ever observed in a black hole transient, while the soft-to-hard transitions occur at some of the lowest luminosities ever reported for such systems.

astro-ph.HE↗

arXiv2018

Probing strain modulation in a gate-defined one dimensional electron system

Gate patterning on semiconductors is routinely used to electrostatically restrict electron movement into reduced dimensions. At cryogenic temperatures, where most studies are carried out, differential thermal contraction between the patterned gate and the semiconductor often lead to an appreciable strain modulation. The impact of such modulated strain to the conductive channel buried in a semiconductor has long been recognized, but measuring its magnitude and variation is rather challenging. Here we present a way to measure that modulation in a gate-defined GaAs-based one-dimensional channel by applying resistively-detected NMR (RDNMR) with in-situ electrons coupled to quadrupole nuclei. The detected strain magnitude, deduced from the quadrupole-split resonance, varies spatially on the order of $10^{-4}$, which is consistent with the predicted variation based on an elastic strain model. We estimate the initial lateral strain $\epsilon_{xx}$ developed at the interface to be about $3.5 \times 10^{-3}$.

cond-mat.mes-hall↗

arXiv2018

Determination of the mass and energy of primary cosmic rays above 100 TeV

This analysis aims to determine the mass composition and energy of cosmic rays at energies above 100 TeV based on the lateral distribution of extensive air showers. Here, we propose quite a few air shower observables for reconstructing the mass and energy of the primary particles. The present reconstruction uses a detailed Monte Carlo simulation for cosmic ray induced air showers in KASCADE and NBU types surface arrays of particle detectors. Some of the observables obtained from this analysis of simulated data are used to infer the nature of the primary particles from a comparison with KASCADE and/or NBU data. It is expected that the determination of primary energy of a cosmic-ray shower may deliver a better accuracy compared to standalone analysis using shower size or S600 or S500 or $N_{\rm {pe}}$ etc, owing to strong fluctuations in the EAS development. Moreover, the present study might be useful to discriminate between hadronic cosmic rays and primary gamma rays, and to measure the cosmic ray all-particle energy spectrum.

astro-ph.HE↗