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Publications:
Publications:
Dutta, S., and Gupta, N. Multiple Emission Regions in Jets of the Low-Luminosity Active Galactic Nucleus in NGC 4278, The Astrophysical Journal (ApJ) 2024, 974, 56.
Abstract:
Abstract:
The Large High Altitude Air Shower Observatory (LHAASO) has detected very-high-energy gamma rays from the low-ionization nuclear emission-line region galaxy NGC 4278, which has a low-luminosity active galactic nucleus (LLAGN) and symmetric, mildly relativistic S-shaped twin jets detected by radio observations. Few LLAGNs have been detected in gamma rays due to their faintness. Earlier, several radio-emitting components were detected in the jets of NGC 4278. We model their radio emission with synchrotron emission of ultra-relativistic electrons to estimate the strength of the magnetic field inside these components within a time-dependent framework after including the ages of the different components. We show that the synchrotron and synchrotron self-Compton emission by these components cannot explain the Swift X-ray data and the LHAASO gamma-ray data from NGC 4278. We suggest that a separate component in one of the jets is responsible for the high-energy emission, whose age, size, magnetic field, and the spectrum of the ultra-relativistic electrons inside it have been estimated after fitting the multiwavelength data of NGC 4278 with the sum of the spectral energy distributions from the radio components and the high-energy component. We note that the radio components of NGC 4278 are larger than the high-energy component, which has also been observed in several high-luminosity active galactic nuclei.
Dutta, S., Chhabra, A., Banerjee, A., Mukherjee, S. and Mohanty, S. The curious case of parabolic encounters: gravitational waves with linear & non-linear memory, arXiv:2511.01968.
Abstract:
Abstract:
The memory effect is known to introduce a permanent displacement in the gravitational wave (GW) detectors after the passage of a GW signal. While the linear memory adheres to the source properties, the non-linear memory is a secondary effect sourced by the GW itself. In the present work, we discuss GW signals with both these kinds of memory effects, while focusing on the parabolic limit of an encounter. This special case is theoretically intriguing and emerges as a limiting situation for both eccentric and hyperbolic events. However, in this paper, we argue that a simple extrapolation of memory calculations for eccentric or hyperbolic cases to the parabolic case may lead to incorrect estimations. Therefore, we treat the parabola as a special case and use an intrinsic parameterization, with which we calculate gravitational wave signals and their energy spectrum via an effective field theory formalism. Unlike the hyperbolic case, which is known to have linear memory, we notice that parabolic encounters bring out new features in the zero frequency limit (ZFL). Our work highlights some of the key challenges and salient aspects of these encounters, and paves the way to study such binary evolution with nonzero memory.
Conference Presentations:
Conference Presentations:
1. Conference on Blazars and Restless AGN (COBRA): A High Energy View
Duration: July 22 - 26, 2024
Organized by: Presidency University, Kolkata & Inter-University Centre for Astronomy and Astrophysics (IUCAA), Pune
Title of Presentation: Multiple Emission Regions in Jets of Low Luminosity Active Galactic Nucleus in NGC 4278
Type of Presentation: Poster
COBRA Certificate-Samik.pdfBRICS - Samik Dutta.pdf2. 4th Symposium of BRICS Association on Gravity, Astrophysics and Cosmology
Duration: December 17 - 19, 2024
Organized by: The BRICS Association on Gravity, Astrophysics and Cosmology (AGAC) & Centre for Cosmology and Science Popularization (CCSP) of SGT University
Title of Presentation: Understanding the Very High Energy (VHE) Gamma-ray emission from NGC 4278
Type of Presentation: Parallel Session Talk
3. 33rd Meeting of Indian Association for General Relativity and Gravitation (IAGRG)
Duration: January 2 - 4, 2025
Organized by: Birla Institute of Technology and Science - Pilani, Pilani campus
Title of Presentation: Multiple Emission Regions in Jets of the Low-Luminosity Active Galactic Nucleus in NGC 4278
Type of Presentation: Parallel Session Talk
Title of Presentation: Gravitational Wave Signals with Memory from Parabolic Encounters
Type of Presentation: Poster
Member of the Local Organising Committee (LOC)
IAGRG - Samik Dutta.pdfSamik_Dutta_ASI2025_certificate.pdf4. 43rd Annual Meeting of the Astronomical Society of India (ASI)
Duration: February 15 - 19, 2025
Organized by: National Institute of Technology (NIT) Rourkela
Title of Presentation: Multiple Emission Regions in Jets of the Low-Luminosity Active Galactic Nucleus in NGC 4278
Type of Presentation: Poster
4. CLAF/ICTP-SAIFR Latin-American Astroparticle Physics School
Duration: August 11 - 15, 2025
Organized by: ICTP - South American Institute for Fundamental Research (SAIFR) & IFT - Universidade Estadual Paulista (UNESP)
Title of Presentation: Multiple Emission Regions in Jets of the Low-Luminosity Active Galactic Nucleus in NGC 4278
Type of Presentation: Poster
Certificate Poster - Samik Dutta.pdf
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