arXiv subjects

A. Chakraborty

Publications and source records attributed to A. Chakraborty.

At least 19 recordsLinked to original sources

arXiv2026

Acceleration radiation and HBAR thermodynamics for atoms falling into a BTZ black hole: A CQM quantum-optics approach

Atoms falling freely into a Ba\~{n}ados-Teitelboim-Zanelli (BTZ) black hole in a Boulware-like vacuum are shown to emit radiation with a Planck spectrum at the Hawking temperature $T_{H}$. This leads to thermal Hawking-like radiation for a cloud of falling atoms prepared with random initial times. Moreover, the radiation is related to the relative equivalence principle, with the vacuum field modes accelerated with respect to the falling atom. The physics of the atom-field interactions is most easily described within a quantum optics approach, where each atom can be interpreted as a detector. Despite the topological nature of gravity in $(2+1)$ dimensions, the thermodynamic and radiation properties of BTZ black holes are still universally governed by the same near-horizon conformal quantum mechanics (CQM) applicable to higher-dimensional gravity. This universal conformal behavior is exhibited by all fields in the background of generic black holes, and generates an HBAR entropy $S_{\mathcal P}$ associated with the photon radiation field that mimics the Bekenstein-Hawking entropy $S_{\mathrm{BH}}=A/4$, proportional to the black-hole horizon area, and with the correct $1/4$ proportionality factor.

gr-qc↗

arXiv2026

Energies and lifetimes of the 9p and 10p excited states in atomic francium

We present the first measurement of 9p 2P1/2,3/2 and 10p 2P1/2,3/2 excited levels absolute wavenumbers and radiative lifetime in francium. We used the Collinear Resonance Ionization Spectroscopy (CRIS) technique, applied on a beam of 221Fr atoms. Prior to this work, no experimental data existed for francium p-states with n > 8. The results provide a precision experimental test of relativistic coupled-cluster theory for the heaviest alkali, showing good agreement for lifetimes and relative excitation energies, despite a residual global offset in absolute energies.

physics.atom-ph↗

arXiv2026

X-ray polarization in the soft state of Cyg X-1

We aim to identify the physical mechanism responsible for the observed X-ray emission and polarization of Cyg X-1 in the soft state. We performed a detailed spectral analysis of X-ray data obtained with NICER, NuSTAR, and INTEGRAL during observations simultaneous with IXPE on 2023 June 20, supplemented at higher energies with archival CGRO data. We developed a new model, retBB, which describes thermal disk emission and its returning reflection, and applied it together with accurate models of Comptonization and relativistically broadened reflection. Using the resulting spectral solution, we computed the expected polarization signal and compared it with the IXPE measurements. Our spectropolarimetric modeling shows that the observed polarization is in agreement with being produced by Comptonization in a corona undergoing a semi-relativistic outflow at a velocity of ~0.3c. Our spectral solutions admit either low or high black-hole spin values, depending on the adopted model setup. However, the observed polarization strongly favors a low spin. At high spin, the polarization angle would inevitably rotate significantly across the energy band, which is inconsistent with the observations. Apart from this rotation of the polarization angle, general relativistic effects do not play a significant role in producing the observed polarization. In particular, we find that, at most, returning disk radiation contributes only a minor amount.

astro-ph.HE↗

arXiv2026

Fast, dust-poor outflows in the local candidate dual AGN MCG-03-34-64 observed with VLT/ERIS

We present VLT/ERIS IFU J-band observations of MCG-03-34-64, a nearby (z = 0.0167) Seyfert galaxy hosting a candidate dual AGN system with a separation of ~100 pc between the nuclei. The observations cover, among others, the HeI1.083um, [FeII]1.257um and Pa$\beta$ emission lines, over a FoV of 3"x3"(~1x1 kpc$^2$). We analyse the ionised gas kinematics and identify two regions with enhanced velocity dispersion (W80~1500 km/s), suggestive of fast outflowing gas, spatially coincident with the position of the two candidate active nuclei. The spectra of the two outflows show a prominent blueshifted wing with velocities vmax ~ -1700 km/s corresponding to the highest 2-5 percentiles of samples of local AGN with similar bolometric luminosities. For the ionised phase of the two outflows, we derive comparable masses of $(4\pm1)\times 10^5$ $M_{\odot}$ and mass outflow rates of $20\pm5$ $M_{\odot}$/yr. The two distinct outflows could be associated with the two nuclei, or be generated by the interaction of the radio jet with the ISM. We also analyse the peculiar profile of the [Fe VII]6087A optical coronal line from an archival VLT/X-shooter spectrum. The comparison with the [NeV]3425A and [FeII]1.257um profiles indicates that [Fe VII] emission likely arises only from the outflow. The absence of the systemic component in [Fe VII] - unlike in [NeV], which has similar ionisation potential and critical density - suggests suppression of [Fe VII] due to iron depletion onto dust grains, while its detection in the outflow implies a lower dust content than in the host ISM. The additional information gained from the ERIS data are consistent with the scenario of a dual AGN, however further observations are required to confirm its nature.

astro-ph.GA↗

arXiv2026

Investigating Roles of Triple Excitations for High-precision Determination of Clock Properties of Alkaline Earth Metal Singly Charged Ions

High-accuracy calculations of electric dipole polarizabilities and quadrupole moments ($Θ$) of the clock states of the singly charged calcium (Ca$^+$), strontium (Sr$^+$) and barium (Ba$^+$) alkaline-earth ions are estimated by employing relativistic coupled-cluster (RCC) theory. It demonstrates importance of the triple excitations in the RCC method for precise determination of the above quantities. We also observe a different trend of correlations in the $Θ$ values than an earlier study with respect to orbitals from higher angular momenta. Reliability of the results is verified by comparing the calculated energies, magnetic dipole hyperfine structure constants, and lifetimes of the atomic states with the experimental values of the $^{43}$Ca$^+$, $^{87}$Sr$^+$ and $^{137}$Ba$^+$ ions. Nuclear quadrupole moments of these isotopes are also estimated by combining calculations with the measured electric quadrupole hyperfine structure constants, showing large deviations from the literature values.

physics.atom-ph↗

arXiv2026

M^3D: Mosaicking M33 with MUSE datacubes I. Unveiling the diversity of HII regions in M33 with MUSE

We present new VLT/MUSE observations of a 3 $\times$ 8 arcmin$^2$ mosaic along the southern major axis of the Local Group galaxy M33. These data provide an unprecedented view of the galaxy's interstellar medium (ISM) and allow us to resolve ionised nebulae at a spatial scale of $\approx$5 pc. We identified and catalogued 131 HII regions, down to Hαluminosities of $\approx 5\times$10$^{35}$ erg s$^{-1}$, one order of magnitude fainter than previous surveys on nearby galaxies beyond the Local Group, and we compared these regions with the spatial distribution of ionising stars and embedded star clusters. For each region, we extracted the corresponding integrated optical spectra and measured the intensity of key optical emission lines (Hβ, [OIII], [NII], Hα, [SII], [SIII]), other weaker optical lines when detectable, and Paschen lines to characterise the physical properties of the ioinised gas, such as density, dust attenuation, and metallicity. Our spatially resolved line ratio and flux maps reveal a remarkable diversity in ionisation properties, from dust-obscured regions hosting young stellar objects to highly ionised bubbles exhibiting high [OIII]/Hβratios. Our data reveal a diversity of ionisation fronts, ranging from well-defined to partial to absent. The radial profiles we obtained indicate the presence of both optically thin (density-bounded) HII regions permitting the escape of ionising photons and fully ionised, optically thick (ionisation-bounded) HII regions. The richness of this MUSE mosaic offers an unprecedented view of the ionised ISM at $\approx$5 pc resolution, providing direct insight into how stellar feedback shapes its environment.

astro-ph.GA↗

arXiv2026

Quantum aspects of spacetime: A quantum optics view of acceleration radiation and black holes

For the centennial of quantum mechanics, we offer an overview of the central role played by quantum information and thermalization in problems involving fundamental properties of spacetime and gravitational physics. This is an open area of research still a century after the initial development of formal quantum mechanics, highlighting the effectiveness of quantum physics in the description of all natural phenomena. These remarkable connections can be highlighted with the tools of modern quantum optics, which effectively addresses the three-fold interplay of interacting atoms, fields, and spacetime backgrounds describing gravitational fields and noninertial systems. In this review article, we select aspects of these phenomena centered on quantum features of the acceleration radiation of particles in the presence of black holes. The ensuing horizon-brightened radiation (HBAR) provides a case study of the role played by quantum physics in nontrivial spacetime behavior, and also shows a fundamental correspondence with black hole thermodynamics.

gr-qc↗

arXiv2025

Cosmic Duets I. High-spatial resolution spectroscopy of dual and lensed AGN with MUSE-NFM

We present the first-year results of the MUSE Large Program "Cosmic Duets", whose goal is to obtain adaptive-optics assisted MUSE observations with an angular resolution of 0.1"-0.2" in order to provide integral-field spectroscopy of sub-arcsec separation dual and lensed active galactic nucleus (AGN) candidates. These observations reveal previously unexplored properties of dual and lensed systems that are key to understanding galaxy evolution, supermassive black hole mergers, and strong-lensing modeling. Targets were efficiently selected using the Gaia multipeak (GMP) technique, which identifies pairs of point-like sources with separations below 0.8" in the Gaia catalog. MUSE spatially resolved spectroscopy provides accurate redshifts, ionization diagnostics, and identification of absorption systems along the line of sight. We report results for 30 GMP targets at z=0.5-3.5. All systems show at least 2 spatially resolved components. 19 objects are confirmed as AGN multiplets, including 6 dual AGN, 10 doubly lensed quasars, and 3 quadruply lensed systems, while the remaining 11 correspond to chance alignments with foreground stars. Among all the spectroscopically confirmed dual AGN in the literature, 27 pairs have projected separations below 7kpc in this redshift regime, and our sample accounts for 22% of the total. We studied dual and lensed AGN distributions as a function of redshift, magnitude, and projected separation while accounting for selection effects, and we find that bright systems are dominated by lensed quasars, whereas the relative fraction of dual AGN increases at fainter magnitudes. This first-year sample demonstrates the high efficiency of GMP selection combined with MUSE spectroscopy for identifying sub-arcsec dual and lensed AGN. The full program, targeting 150 systems, will enable statistical studies of dual AGN and constraints on mass distribution in lensing galaxies.

astro-ph.GA↗

arXiv2025

Tracing nitrogen enrichment across cosmic time with JWST

We present a comprehensive analysis of the nitrogen-to-oxygen (N/O) abundance ratio in a sample of ~ 660 star-forming galaxies at redshift z ~ 1-6, with a median redshift of z = 3.0, using deep JWST/NIRSpec spectroscopy. Leveraging detections of faint auroral emission lines in 92 galaxies at z>1 from both the MARTA survey and a large compilation of high-redshift literature objects, we derive direct electron temperature-based abundances for nitrogen and oxygen using rest-frame optical lines. We establish the first high-redshift calibrations of strong-line N/O diagnostics based on 'direct' abundance measurements, finding a mild evolution for the N2O2 = log([NII]6585/[OII]3727,3729) diagnostic and no clear evolution for the N2S2 = log([NII]6585/[SII]6717,6731) diagnostic compared to local realisations. We then investigate the N/O-O/H relation across cosmic time using both 'direct' abundances and strong-lines based measurements (additional 535 galaxies). We find evidence for mild but systematic nitrogen enhancement at high redshift: galaxies at z>1 exhibit N/O ratios elevated by ~0.18 dex (median offset) at fixed O/H compared to local trends, with a more pronounced enhancement at low metallicity (i.e. 12 + log(O/H) < 8.1), where the offset reaches up to ~0.4-0.5 dex. We consider several scenarios to explain the observed trends, including bursty star formation, differential metal loading, and inflows of pristine gas. Our results provide the most extensive confirmation of elevated N/O ratios at high-redshift to date based on rest-optical diagnostics and within a self-consistent frame.

astro-ph.GA↗

arXiv2025

Comprehensive Assessment of $\mathrm{Th}^{3+}$ Properties for Nuclear Clock and Fundamental Physics Applications

By employing singles, doubles, and triples excitations within the relativistic coupled-cluster framework, we perform comprehensive calculations of a wide range of atomic properties for the Th$^{3+}$ ion. These properties are essential for advancing nuclear clock technology and probing fundamental physics. Combining our isotope shift parameters with experimental data, we estimate highly accurate values of the differential nuclear charge radii for $^{232,229}$Th and $^{229m,229}$Th. Additionally, we determine the nuclear magnetic dipole and electric quadrupole moments for both the ground and isomeric states of $^{229}$Th by combining measured hyperfine structure constants with our theoretical calculations. Our precise evaluations of electric dipole polarizabilities and hyperfine-induced quadrupole moments are critical for assessing systematic uncertainties in $^{229}$Th$^{3+}$-based nuclear clock. Notably, we observe unexpectedly significant contributions from higher-order relativistic effects and excitations involving orbitals with higher angular momentum, which markedly influence the energies of the ground state and its fine-structure partner. These results highlight the substantial challenges in achieving highly accurate predictions for these properties.

physics.atom-ph↗

arXiv2025

Comprehensive Inclusion of Higher-order Ca$^+$ Isotope Shifts in the King's Plot Yields an Order Improvement on the $e^-$-$n$ Coupling Limit

By critically evaluating higher-order nonlinear effects to the isotope shifts (ISs) in the low-lying transition frequencies of the singly charged calcium ion, stringent constraint on the electron-neutron coupling due to a hypothetical boson describing physics beyond the Standard Model is inferred. It shows an order magnitude difference compared to the previously reported limit demonstrating importance of higher-order effects in the analysis of nonlinearity in the King's plot. The first-order IS parameters and enhancement factor ($D$) were evaluated using two complementary approaches in the relativistic coupled-cluster theory framework: namely finite-field (FF) and analytical response (AR) approaches. Extraction of the second-order IS parameters in the FF approach show numerical instabilities, so they are determined in the AR approach. Comparison of these factors with previous calculation shows substantial differences in the magnitudes. However, $D$ values from both the FF and AR approaches display excellent agreement. We also show explicitly roles of electron correlation effects in the evaluation of $D$ values accurately.

physics.atom-ph↗

arXiv2025

Euclid: A machine-learning search for dual and lensed AGN at sub-arcsec separations

Cosmological models of hierarchical structure formation predict the existence of a widespread population of dual accreting supermassive black holes (SMBHs) on kpc-scale separations, corresponding to projected distances < 0".8 at redshifts higher than 0.5. However, close companions to known active galactic nuclei (AGN) or quasars (QSOs) can also be multiple images of the object itself, strongly lensed by a foreground galaxy, as well as foreground stars in a chance superposition. Thanks to its large sky coverage, sensitivity, and high spatial resolution, Euclid offers a unique opportunity to obtain a large, homogeneous sample of dual/lensed AGN candidates with sub-arcsec projected separations. Here we present a machine learning approach, in particular a Convolutional Neural Network (CNN), to identify close companions to known QSOs down to separations of $\sim\,$0".15 comparable to the Euclid VIS point spread function (PSF). We studied the effectiveness of the CNN in identifying dual AGN and demonstrated that it outperforms traditional techniques. Applying our CNN to a sample of $\sim\,$6000 QSOs from the Q1 Euclid data release, we find a fraction of about 0.25% dual AGN candidates with separation $\sim\,$0".4 (corresponding to $\sim$3 kpc at z=1). Estimating the foreground contamination from stellar objects, we find that most of the pair candidates with separation higher than 0".5 are likely contaminants, while below this limit, contamination is expected to be less than 20%. For objects at higher separation (>0".5, i.e. 4 kpc at z=1), we performed PSF subtraction and used colour-colour diagrams to constrain their nature. We present a first set of dual/lensed AGN candidates detected in the Q1 Euclid data, providing a starting point for the analysis of future data releases.

astro-ph.GA↗

arXiv2025

MARTA: Temperature-temperature relationships and strong-line metallicity calibrations from multiple auroral-line detections at cosmic noon

We present the first results from MARTA (Measuring Abundances at high Redshift with the T$_e$ Approach), a programme leveraging ultra-deep, medium-resolution JWST/NIRSpec spectroscopy to probe the interstellar medium (ISM) of star-forming galaxies at $z \sim 2 - 3$. We report detections of one or more auroral lines, including [O III]$\lambda4363$, [O II]$λ\lambda7320,7330$, [S II] $\lambda4068$, and [S III] $\lambda6312$ for 16 galaxies in the sample, providing measurements of multiple ionic temperatures. We tested the validity of the T[O II]-T[O III] relation at high redshift considering a total sample of 21 objects including literature data, and obtained a shallower slope than in the low-$z$ literature. However, such a slope is consistent with low-redshift data when ultra-low metallicity objects are considered. We assessed the correlation of the T[O II]-T[O III] relationship and its scatter on different physical parameters, finding a mild correlation with the ionisation parameter and radiation field hardness, while no significant correlation with gas density. The location of high-redshift data is also consistent with the low-$z$ literature in the T[O II]-T[S II], and T[S III]-T[O III] relations, although this conclusion is limited with low-number statistics. Finally, we leveraged our sample together with a comprehensive compilation of galaxies with [O III]$\lambda4363$ detections from the literature to recalibrate classical strong-line diagnostics at high redshift. MARTA represents a key addition in this space because it provides direct metallicities at moderately high oxygen abundances (12+log(O/H) $\sim8.0-8.4$).

astro-ph.GA↗

arXiv2025

Demonstrating Correlation Trends in the Electric Dipole Polarizabilities of Many Low-lying States in Cs I through First-principle Calculations

Electron correlation and higher-order relativistic effects are probed in the evaluation of scalar and tensor static electric dipole (E1) polarizabilities ($α_d$) of several even- and odd-parity states in cesium (Cs) using the Dirac-Hartree-Fock (DHF) method, second-order perturbation theory (MBPT(2)), third-order perturbation theory (MBPT(3)), random phase approximation (RPA), and singles and doubles approximated relativistic coupled-cluster (RCCSD) method. To account for perturbation due to odd-parity E1 operator on the atomic orbitals, calculations are carried out in the linear response approach. Our final $α_d$ values, with the estimated uncertainties, show reasonably good agreement with the previous calculations and available experimental results. Differences among the DHF, MBPT(2), MBPT(3) and RPA results indicate pair-correlation (PC) effects play major roles than the core-polarization (CP) effects in the determination of $α_d$ values in Cs. From the differences among the MBPT(3) and RCC results, we find correlations among the PC and CP effects and double CP effects together are also significant in these calculations. Contributions from the Breit interactions are found to be quite large in the high-lying states.

physics.atom-ph↗

arXiv2025

Investigating the $4D_{3/2}|3,\pm2\rangle$--$4D_{5/2}|3,\pm2\rangle$ transition in Nb$^{4+}$ for a THz atomic clock

In this work, the $4D_{3/2}|3,\pm2\rangle \rightarrow 4D_{5/2}|3,\pm2\rangle$ transition in the Nb$^{4+}$ ion is identified as a promising candidate for a terahertz (THz) atomic clock, with the transition frequency occurring at 56.0224 THz. This transition is primarily driven by the magnetic dipole decay channel, which can easily be accessed by a laser. We focus on the stable $^{93}$Nb isotope, which has 100\% natural abundance and a nuclear spin of $I=9/2$ for experimental advantage. Our data analysis allows us to estimate potential systematic shifts in the proposed clock system, including those due to blackbody radiation, electric quadrupole, second-order Zeeman, and second-order Doppler {shifts}. {The scheme presented in this study can help suppress the AC Stark and electric quadrupole shifts in the clock frequency measurement.} {All these analyses} suggest that the proposed THz atomic clock using Nb$^{4+}$ could be valuable in both quantum thermometry and frequency metrology.

physics.atom-ph↗

arXiv2025

Ab initio Calculations of Electric Dipole Polarizabilities in the Li, Na and K Atoms

We carry out first-principle calculations of scalar and tensor components of the static electric dipole polarizabilities of six low-lying states of lithium (Li), sodium (Na) and potassium (K) alkali atoms in the linear response approach. Results are compared from the Dirac-Hartree-Fock (DHF) method, third-order many-body perturbation theory (MBPT(3) method), random phase approximation (RPA) and singles and doubles approximated relativistic coupled-cluster theory (RCCSD method). We find the DHF and RPA results are close to each other, while the MBPT(3) and RCCSD results are close to each other. This suggests that pair-correlation effects play significant roles over core-polarization effects to determine these quantities accurately in the above alkali atoms. We also compare contributions arising through the core, core-valence and valence correlations through all the methods in Li, Na and K, which show that the core-valence contributions are negligibly small in all the methods and there is no particular trends of the core and valence correlation contributions with the size of the atom. Uncertainties to the RCCSD results are estimated to quote the final values, and they are compared with the previous calculations and experimental results.

physics.atom-ph↗

arXiv2025

Square Kilometre Array Science Data Challenge 3a: foreground removal for an EoR experiment

We present and analyse the results of the Science data challenge 3a (SDC3a, https://sdc3.skao.int/challenges/foregrounds), an EoR foreground-removal community-wide exercise organised by the Square Kilometre Array Observatory (SKAO). The challenge ran for 8 months, from March to October 2023. Participants were provided with realistic simulations of SKA-Low data between 106 MHz and 196 MHz, including foreground contamination from extragalactic as well as Galactic emission, instrumental and systematic effects. They were asked to deliver cylindrical power spectra of the EoR signal, cleaned from all corruptions, and the corresponding confidence levels. Here we describe the approaches taken by the 17 teams that completed the challenge, and we assess their performance using different metrics. The challenge results provide a positive outlook on the capabilities of current foreground-mitigation approaches to recover the faint EoR signal from SKA-Low observations. The median error committed in the EoR power spectrum recovery is below the true signal for seven teams, although in some cases there are some significant outliers. The smallest residual overall is $4.2_{-4.2}^{+20} \times 10^{-4}\,\rm{K}^2h^{-3}$cMpc$^{3}$ across all considered scales and frequencies. The estimation of confidence levels provided by the teams is overall less accurate, with the true error being typically under-estimated, sometimes very significantly. The most accurate error bars account for $60 \pm 20$\% of the true errors committed. The challenge results provide a means for all teams to understand and improve their performance. This challenge indicates that the comparison between independent pipelines could be a powerful tool to assess residual biases and improve error estimation.

astro-ph.IM↗

arXiv2025

Conformal quantum mechanics of causal diamonds: Quantum instability and semiclassical approximation

Causal diamonds are known to have thermal behavior that can be probed by finite-lifetime observers equipped with energy-scaled detectors. This thermality can be attributed to the time evolution of observers within the causal diamond, governed by one of the conformal quantum mechanics (CQM) symmetry generators: the noncompact hyperbolic operator $S$. In this paper, we show that the unbounded nature of $S$ endows it with a quantum instability, which is a generalization of a similar property exhibited by the inverted harmonic oscillator potential. Our analysis is semiclassical, including a detailed phase-space study of the classical dynamics of $S$ and its dual operator $R$, and a general semiclassical framework yielding basic instability and thermality properties that play a crucial role in the quantum behavior of the theory. For an observer with a finite lifetime $\mathcal{T}$, the detected temperature $T_D = 2 \hbar/(π\mathcal{T})$ is associated with a Lyapunov exponent $λ_L = πT_D/\hbar$, which is half the upper saturation bound of the information scrambling rate.

quant-ph↗