arXiv subjects

Neeraj Kumari

Publications and source records attributed to Neeraj Kumari.

24 records · Page 2Linked to original sources

arXiv2021

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 $γ$-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 timescales at all of the above wavebands and find its value to be ~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 of the magnetic field to be 0.5 Gauss and the maximum Lorentz factor of the emitting electrons ~1.6 x $10^5$ assuming that synchrotron radiation cooling drives the shortest variability timescale. Blazars vary at a large range 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.

astro-ph.HE↗

arXiv2021

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 {\it 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 and clumpy 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 probably represent its quenching due to the AGN driven outflows emanating from the central engine of NGC 1316.

astro-ph.GA↗

arXiv2021

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 anti-correlation 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 and delayed 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.

astro-ph.HE↗

arXiv2020

Revisiting the spectral and timing properties of 4U 1909+07 with NuSTAR and Astrosat

We present the results obtained from the analysis of high mass X-ray binary pulsar 4U 1909+07 using NuSTAR and Astrosat observations in 2015 and 2017 July, respectively. X-ray pulsations at $\approx$604 s are clearly detected in our study. Based on the long term spin-frequency evolution, the source is found to spun up in the last 17 years. We observed a strongly energy-dependent pulse profile that evolved from a complex broad structure in soft X-rays into a profile with a narrow emission peak followed by a plateau in energy ranges above 20 keV. This behaviour ensured a positive correlation between the energy and pulse fraction. The pulse profile morphology and its energy-evolution are almost similar during both the observations, suggesting a persistent emission geometry of the pulsar over time. The broadband energy spectrum of the pulsar is approximated by an absorbed high energy exponential cutoff power law model with iron emission lines. In contrast to the previous report, we found no statistical evidence for the presence of cyclotron absorption features in the X-ray spectra. We performed phase-resolved spectroscopy by using data from the NuSTAR observation. Our results showed a clear signature of absorbing material at certain pulse-phases of the pulsar. These findings are discussed in terms of stellar wind distribution and its effect on the beam geometry of this wind-fed accreting neutron star. We also reviewed the subsonic quasi-spherical accretion theory and its implication on the magnetic field of 4U 1909+07 depending on the global spin-up rate.

astro-ph.HE↗

arXiv2020

Probing Nuclear and Circumnuclear Properties of NGC 6300 using X-ray Observations

We present the results obtained from a detailed X-ray timing and spectral analysis of Seyfert 2 galaxy NGC 6300 by using observations with the {\it Suzaku}, {\it Chandra} and {\it NuSTAR} observatories between 2007 and 2016. We calculate variance, rms fractional variability of the source in different energy bands and find variabilities in various energy bands. Spectral properties of the source are studied by using various phenomenological and physical models. The properties of the Compton clouds, reflection, Fe K$α$ line emission and soft X-ray excess are studied in detail. Several physical parameters of the source are extracted and investigated to establish the presence/absence of any correlation between them. We also investigate the nature of the circumnuclear `torus' and find that the torus is not uniform, rather clumpy. The observed changes in the line-of-sight column density can be explained in terms of transiting clouds. The iron line emitting region is found to be different in the different epoch of observations. We also observe that the torus and the nucleus independently evolve over the years.

astro-ph.HE↗

arXiv2020

Accretion Flow Evolution of a New Black Hole Candidate MAXI J1348-630 During the 2019 Outburst

Galactic black hole candidate MAXI~J1348-630 was recently discovered by {\it MAXI} and {\it Swift}/BAT satellites during its first outburst in 2019 January which continued for $\sim4$ months. We study the spectral and timing properties of the source in detail. The combined $1-150$ keV {\it Swift}/XRT, {\it Swift}/BAT and {\it MAXI}/GSC spectra are investigated with the two component advective flow (TCAF) solution. Physical flow parameters of TCAF, such as the Keplerian disk accretion rate, the sub-Keplerian halo accretion rate, the shock location and the shock compression ratio are estimated from our spectral fits. Based on the variation of flux in soft and hard X-ray ranges, the hardness ratio, TCAF model fitted accretion rates and accretion rate ratio (ARR), we show how the sources evolved through four spectral states viz. hard, hard-intermediate, soft-intermediate and soft states in rising and declining states. Low-frequency quasi-periodic oscillations (QPO) are observed in two observations during the rising phase of the outburst. From the spectral analysis, we estimate the mass of the black hole to be $9.1^{+1.6}_{-1.2}$ $M_{\odot}$. We also find that the viscous timescale in this outburst is $\sim 3.5$ days. The distance of the source is also estimated as $5-10$ kpc from state transition luminosity.

astro-ph.HE↗