• SACHINDRA NAIK

Articles written in Journal of Astrophysics and Astronomy

• Imaging and photometric studies of NGC 1316 (Fornax A) using Astrosat/UVIT

We present imaging and photometric studies of the radio galaxy NGC 1316 (Fornax A) using high spatial resolution near-ultraviolet (NUV) and far-ultraviolet (FUV) imaging telescopes of the first Indian multi-wavelength space observatory AstroSat. The residual maps of UV emission obtained from the subtraction of smooth models witness peculiar features within the central few kpc (1–2 kpc) region. The spatial correspondence between the radio emission maps and FUV imaging study reveal that the UV emitting sources are displaced away from the centre by the AGN outburst (radio jet). The presence of rims andclumpy structures in the outskirt of this galaxy delineate that the galaxy has acquired a large fraction of gas through merger-like events and is still in the process of settling. The estimates of the star formation rates (SFR) using FUV and NUV luminosities are found to be 0.15 $M_{\odot}$ yr$^{-1}$ and 0.36 $M_{\odot}$ yr$^{-1}$, respectively, and provide the lower limit due to the screen effect. The estimated lower rates of SFR in this galaxy probablyrepresent its quenching due to the AGN driven outflows emanating from the central engine of NGC 1316.

• Detection of X-ray pulsations at the lowest observed luminosity of Be/X-ray binary pulsar EXO 2030+375 with AstroSat

We present the results obtained from timing and spectral studies of Be/X-ray binary pulsar EXO 2030$+$375 using observations with the Large Area Xenon Proportional Counters and Soft X-ray Telescope of AstroSat, at various phases of its Type-I outbursts in 2016, 2018, and 2020. The pulsar was faint during these observations as compared to earlier observations with other observatories. At the lowest luminosity of $2.5\times10^{35}$ erg s$^{-1}$ in 0.5–30 keV energy range, $\approx$41.3 s pulsations were clearly detected in the X-ray light curves. This finding establishes the first firm detection of pulsations in EXO 2030$+$375 at anextremely low mass accretion rate to date. The shape of the pulse profiles is complex due to the presence of several narrow dips. Though pulsations were detected up to $\sim$80 keV when the source was brighter, pulsations were limited up to $\sim$25 keV during the third AstroSat observation at lowest source luminosity. A search for quasi-periodic oscillations in $2\times 10^4$ Hz to 10 Hz yielded a negative result. Spectral analysis of the AstroSat data showed that the spectrum of the pulsar was steep with a power-law index of $\sim$2. Thevalues of photon-indices at observed low luminosities follow the known pattern in sub-critical regime of the pulsar.

• AstroSat observations of eclipsing high mass X-ray binary pulsar OAO 1657-415

We present the results obtained from analysis of two AstroSat observations of the high mass Xray binary pulsar OAO 1657-415. The observations covered 0.681–0.818 and 0.808–0.968 phases of the $\sim$10.4 day orbital period of the system, in March and July 2019, respectively. Despite being outside theeclipsing regime, the power density spectrum from the first observation lacks any signature of pulsation or quasi-periodic oscillations. However, during July observation, X-ray pulsations at a period of 37.0375 s were clearly detected in the light curves. The pulse profiles from the second observation consist of a broad single peak with a dip-like structure in the middle across the observed energy range. We explored evolution of the pulse profile in narrow time and energy segments. We detected pulsations in the light curves obtained from0.808–0.92 orbital phase range, which is absent in the remaining part of the observation. The spectrum of OAO 1657-415 can be described by an absorbed power-law model along with an iron fluorescent emission line and a blackbody component for out-of-eclipse phase of the observation. Our findings are discussed in the frame of stellar wind accretion and accretion wake at late orbital phases of the binary.

• Short-timescale variability of the blazar Mrk 421 from AstroSat and simultaneous multi-wavelength observations

We study the multi-wavelength variability of the blazar Mrk 421 at minutes to days timescales using simultaneous data at c-rays from Fermi, 0.7–20 keV energies from AstroSat, and optical and near infrared (NIR) wavelengths from ground based observatories. We compute the shortest variability timescalesat all of the above wave bands and find its value to be $\sim$1.1 ks at the hard X-ray energies and increasingly longer at soft X-rays, optical and NIR wavelengths as well as at the GeV energies. We estimate the value ofthe magnetic field to be 0.5 Gauss and the maximum Lorentz factor of the emitting electrons $\sim 1.6 \times 10^5$ assuming that synchrotron radiation cooling drives the shortest variability timescale. Blazars vary at a largerange of timescales often from minutes to years. These results, as obtained here from the very short end of the range of variability timescales of blazars, are a confirmation of the leptonic scenario and in particular the synchrotron origin of the X-ray emission from Mrk 421 by relativistic electrons of Lorentz factor as high as $10^5$. This particular mode of confirmation has been possible using minutes to days timescale variability data obtained from AstroSat and simultaneous multi-wavelength observations.

• Spectro-timing analysis of a highly variable narrow-line Seyfert 1 galaxy NGC 4748 with AstroSat and XMM-Newton

We present a detailed timing and spectral study of an extremely variable narrow-line Seyfert 1 galaxy NGC 4748 using observations in the year 2017 and 2014 performed with AstroSat and XMM-Newton, respectively. Both observations show extremely variable soft and hard X-ray emission that are correlated with each other. In the 2014 data set, the source retains its general behaviour of ‘‘softer when brighter’’ while the 2017 observation exhibits a ‘‘harder when brighter’’ nature. Such changing behaviour is rare in AGNs and is usually observed in the black hole binary systems. The ‘‘harder when brighter’’ is confirmed with the anticorrelation between the photon index and the 0.3–10 keV power-law flux. This suggests a possible change in the accretion mode from standard to the advection-dominated flow. Additionally, both the observations show soft X-ray excess below 2 keV over the power-law continuum. This excess was fitted with a single or multiple blackbody component(s). The origin of soft excess during the 2017 observation is likely due to the cool Comptonization as the photon index changes with time. On the other hand, the broad iron line anddelayed UV emission during the 2014 observation strongly suggest that X-ray illumination onto the accretion disk and reflection and reprocessing play a significant role in this AGN.

• # Journal of Astrophysics and Astronomy

Volume 42, 2021
All articles
Continuous Article Publishing mode

• # Continuous Article Publication

Posted on January 27, 2016

Since January 2016, the Journal of Astrophysics and Astronomy has moved to Continuous Article Publishing (CAP) mode. This means that each accepted article is being published immediately online with DOI and article citation ID with starting page number 1. Articles are also visible in Web of Science immediately. All these have helped shorten the publication time and have improved the visibility of the articles.

• # Editorial Note on Continuous Article Publication

Posted on July 25, 2019