arXiv subjects

Santanu Mondal

Publications and source records attributed to Santanu Mondal.

At least 55 records · Page 3Linked to original sources

arXiv2022

Study of accretion flows around an ultraluminous X-ray source M82 X-1 using NuSTAR data

We study the spectral properties and accretion flow behavior of an ultraluminous X-ray source M82\,X-1 using {\it NuSTAR} observations. We use the physical two component advective flow (TCAF) model to fit the data and to derive the accretion flow properties of the source. From the model fitted parameters, we found that M82\,X-1 is harboring an intermediate mass black hole at its centre, where the mass varies from $156.04^{+13.51}_{-15.30}$ to $380.96^{28.38}_{-29.76}$ M$_\odot$. The error weighted average mass of the black hole is $273\pm43$ M$_\odot$, which accreted in nearly super-Eddington rate. The Compton cloud was compact with a size of $\sim13 r_g$ and the shock compression ratio had \textcolor{black}{nearly intermediate values except for the epoch four}. These indicate a possible significant mass outflow from the inner region of the disk. The quasi periodic oscillation (QPO) frequencies estimated from the model fitted parameters can reproduce the observed QPOs. The robustness of the model parameters is verified by drawing the confidence contours among them.

astro-ph.HE↗

arXiv2022

Extremal sequences for the unit-weighted Gao constant of $\mathbb Z_n$

For $A\subseteq \mathbb Z_n$, the $A$-weighted Gao constant $E_A(n)$ is defined to be the smallest natural number $k$, such that any sequence of $k$ elements in $\mathbb Z_n$ has a subsequence of length $n$, whose $A$-weighted sum is zero. Sequences of length $E_A(n)-1$ in $\mathbb Z_n$, which do not have any $A$-weighted zero-sum subsequence of length $n$ are called $A$-extremal sequences for the Gao constant. Such a sequence which has $n-1$ zeroes is said to be of the standard type. When $A=U(n)$ (units in $\mathbb Z_n$) where $n$ is odd, we characterize all such sequences and show that they are of the standard type. When $n$ is even, we give examples of such sequences which are not of the standard type. We also characterize the $U(n)$-extremal sequences for the Gao constant, when $n=2^rp$, where $p$ is an odd prime.

math.NT↗

arXiv2022

Flux and spectral variability of Mrk 421 during its moderate activity state using NuSTAR: Possible accretion disc contribution?

The X-ray emission in BL Lac objects is believed to be dominated by synchrotron emission from their relativistic jets. However, when the jet emission is not strong, one could expect signatures of X-ray emission from inverse Compton scattering of accretion disc photons by the corona. Moreover, the observed X-ray variability can also originate in the disc, and gets propagated and amplified by the jet. Here, we present results on the BL Lac object Mrk 421 using the NuSTAR data acquired during 2017 when the source was in a moderate X-ray brightness state. For comparison with high jet activity, we also considered one epoch data in April 2013 during a very high X-ray brightness state. Our aim is to explore the possibility of the signature of accretion disc emission in the overall X-ray emission from Mrk 421. The spectral fitting of the data using the two component advective flow model shows gives (a) the size of the dynamic corona at the base of the jet from ~28 to 10 r$_s$, (b) the disc mass accretion rate from 0.021 to 0.051 $\dot M_{\rm Edd}$, (c) the halo mass accretion rate from 0.22 to 0.35 $\dot M_{\rm Edd}$, and (d) the viscosity parameter of the Keplerian accretion disc from 0.18 $-$ 0.25. In the assumed model, the total flux, disc and jet flux correlate with the radio flux observed during these epochs. We conclude that the spectra of all the epochs of Mrk 421 in 2017 are well described by the accretion disc based two component advective flow model. The estimated disc and jet flux relations with radio flux show that accretion disc can contribute to the observed X-ray emission, when X-ray data (that covers a small portion of the broad band spectral energy distribution of Mrk 421) is considered in isolation. However, the present disc based models are disfavoured with respect to the relativistic jet models when considering the X-ray data in conjunction with data at other wavelengths.

astro-ph.HE↗

arXiv2022

Variable mass accretion and failed wind explain changing look phenomena in NGC 1365

Changing look active galactic nuclei (CLAGNs) show complex nature in their X-ray spectral shape and line of sight column density variation. The physical mechanisms responsible for these variations are unclear. Here, we study the spectral properties of a CLAGN, NGC\,1365 using combined {\it XMM-Newton} and {\it NuSTAR} observations to understand the CL behavior. The model fitted mass accretion rate varied between $0.003\pm 0.001$ and $0.009\pm0.002$ $\dot M_{\rm Edd}$ and the dynamic corona changed from $28\pm 3$ to $10\pm1$ $r_g$. We found that the variable absorption column density correlates with the mass accretion rate and the geometry of the corona. The derived wind velocity was sufficiently low compared to the escape velocity to drive the wind away from the disc for the epochs when column densities were high. This suggests that the high and variable absorption can be due to failed winds from the disc. Our estimated ratio of mass outflow to inflow rate from the inner region of the disc lies between $0.019\pm0.006$ and $0.12\pm0.04$. From spectral fitting of the combined data, we found the mass of the central black hole to be constant $4.38\pm0.34 - 4.51\pm0.29 \times10^{6} M_\odot$, consistent with earlier findings. The confidence contours of $N_H$ with other model parameters show that the model fitted parameters are robust and non-degenerate. Our study construed that the changing accretion rate, which is a fundamental physical quantity and the geometry of the corona driving the CL phenomena in NGC\,1365. The physical picture considered in this work connects both variable continuum and variable absorbing medium scenarios.

astro-ph.HE↗

arXiv2022

Flavor Diagonal Nucleon Charges

This talk provides an update on the calculation of matrix elements of flavor diagonal axial, scalar and tensor quark bilinear operators between the nucleon ground state. The simulations are done using Wilson-clover fermions on a sea of eight 2+1+1-flavor HISQ ensembles generated by the MILC collaboration. We discuss the signal in the connected and disconnected contributions, calculation of the renormalization constants and mixing in the RI-sMOM scheme, and control over the simultaneous chiral-continuum-finite-volume fit used to extract the final charges.

hep-lat↗

arXiv2022

Excited states and precision results for nucleon charges and form factors

The exponentially falling signal-to-noise ratio in all nucleon correlation functions, and the presence of towers of multihadron excited states with relatively small mass gaps makes extraction of matrix elements of various operators within the ground state nucleon challenging. Theoretically, the allowed positive parity states with the smallest mass gaps are the $N(\bm p)π(-\bm p)$, $N(\bm 0)π(\bm 0)π(\bm 0)$, $N(\bm p)π(\bm 0)$, $N(\bm 0)π(\bm p),\ \ldots$, states. A priori, the contribution of these states arises at one loop in chiral perturbation theory ($χ$PT), however, in many cases the contributions are enhanced. In this talk, I will review four such cases: the correlation functions from which the axial form factors, electric and magnetic form factors, the $Θ$-term contribution to neutron electric dipole moment (nEDM), and the pion-nucleon sigma term are extracted. Including appropriate multihadron states in the analysis can lead to significantly different results compared to standard analyses with the mass gaps taken from fits to 2-point functions. The $χ$PT case for $N π$ states is the most clear in the axial/pseudoscalar form factors which need to satisfy the PCAC relation between them. Our analyses, supported by $χ$PT, suggests similarly large effects in the calculations of the $Θ$-term and the pion-nucleon sigma term that have significant phenomenological implications.

hep-lat↗

arXiv2022

Precision Nucleon Charges and Form Factors Using 2+1-flavor Lattice QCD

We present high statistics results for the isovector nucleon charges and form factors using seven ensembles of 2+1-flavor Wilson-clover fermions. The axial and pseudoscalar form factors obtained on each ensemble satisfy the PCAC relation once the lowest energy $Nπ$ excited state is included in the spectral decomposition of the correlation functions used for extracting the ground state matrix elements. Similarly, we find evidence that the $Nππ$ excited state contributes to the correlation functions with the vector current, consistent with the vector meson dominance model. The resulting form factors are consistent with the Kelly parameterization of the experimental electric and magnetic data. Our final estimates for the isovector charges are $g_{A}^{u-d} = 1.31(06)(05)_{sys}$, $g_{S}^{u-d} = 1.06(10)(06)_{sys}$, and $g_{T}^{u-d} = 0.95(05)(02)_{sys}$, where the first error is the overall analysis uncertainty and the second is an additional combined systematic uncertainty. The form factors yield: (i) the axial charge radius squared, ${\langle r_A^2 \rangle}^{u-d}=0.428(53)(30)_{sys}\ {\rm fm}^2$, (ii) the induced pseudoscalar charge, $g_P^\ast=7.9(7)(9)_{sys}$, (iii) the pion-nucleon coupling $g_{π{\rm NN}} = 12.4(1.2)$, (iv) the electric charge radius squared, ${\langle r_E^2 \rangle}^{u-d} = 0.85(12)(19)_{sys} \ {\rm fm}^2$, (v) the magnetic charge radius squared, ${\langle r_M^2 \rangle}^{u-d} = 0.71(19)(23)_{\rm sys} \ {\rm fm}^2$, and (vi) the magnetic moment $μ^{u-d} = 4.15(22)(10)_{\rm sys}$. All our results are consistent with phenomenological/experimental values but with larger errors. Lastly, we present a Padé parameterization of the axial, electric and magnetic form factors over the range $0.04< Q^2 <1$ GeV${}^2$ for phenomenological studies.

hep-lat↗

arXiv2022

Parton physics from a heavy-quark operator product expansion: Lattice QCD calculation of the second moment of the pion distribution amplitude

The pion light-cone distribution amplitude (LCDA) is a central non-perturbative object of interest for high-energy exclusive processes in quantum chromodynamics. In this article, the second Mellin moment of the pion LCDA is determined as a proof-of-concept calculation for the first numerical implementation of the heavy-quark operator product expansion (HOPE) method. The resulting value for the second Mellin moment, determined in quenched QCD at a pion mass of $m_π=550$ MeV at a factorization scale of 2 GeV is $ \langle ξ^2 \rangle = 0.210 \pm 0.013\text{ (stat.)} \pm 0.034\text{ (sys.)}$. This result is compatible with those from previous determinations of this quantity.

hep-lat↗

arXiv2021

Nucleon isovector momentum fraction, helicity and transversity moment using Lattice QCD

We present our recent high precision calculations (Phys. Rev. D102 (2020) no.5, 054512 and JHEP 04 (2021) 044, JHEP 21 (2020) 004) of the first moment of nucleon isovector polarized, unpolarized and transversity distributions, i.e., momentum fraction, helicity and transversity moment, respectively. We use the standard method for the calculation of these moments (via matrix elements of twist two operators), and carry out a detailed analysis of the sources of systematic uncertainty, in particular of excited state contributions. Our calculations have been performed using two different lattice setups (Clover-on-HISQ and Clover-on-Clover), each with several ensembles. They give consistent results that are in agreement with global fit analyses.

hep-lat↗

arXiv2021

Progress in the determination of Mellin moments of the pion LCDA using the HOPE method

The pion light-cone distribution amplitude (LCDA) is a central non-perturbative object of interest for the calculation of high-energy exclusive processes in quantum chromodynamics. In this article, we discuss the calculation of the second and fourth Mellin moment of the pion LCDA using a heavy-quark operator product expansion. The resulting value for the second Mellin moment is $ \langle{ ξ^2 }\rangle(μ= 2~\text{GeV})= 0.210 \pm 0.013\text{ (stat.)} \pm 0.034\text{ (sys.)}$. This result is compatible with those from previous determinations of this quantity.

hep-lat↗

arXiv2021

Low Frequency Quasi Periodic Oscillations and Shocks in Accretion onto Black Hole

Low-frequency quasi-periodic oscillations (LFQPOs) have been routinely observed in black hole X-ray binaries (BHXRBs). These LFQPOs can be explained by axisymmetric shock oscillation in accretion flow around a rotating black hole. We address the physical origin of Type-C LFQPOs in BHXRBs observed by the Rossi X-ray Timing Explorer satellite considering a minimum number of free parameters, namely, specific energy and specific angular momentum of the infalling matter for a given set of BH mass and spin parameter. We apply the solution for a large number of BH candidates to further strengthen the scenario of an anti-correlation between the QPO frequency and the location of the shock. Our study also confirms that Compton cooling can be sufficient to explain the observed QPOs.

astro-ph.HE↗

arXiv2021

Fermi bubbles: the collimated outburst needed to explain forward-shock edges

The bipolar, nonthermal, high-latitude lobes known as the Fermi bubbles (FBs) are thought to originate from a massive energy release near the Galactic centre (GC). We constrain the FB engine and the circumgalactic medium (CGM) by analytically and numerically modeling the FB edges as strong forward shocks, as inferred from recent observations. A non-directed energy release produces shocks too spherical to account for observations even for a maximally massive Galactic disc, critical CGM rotation, or injection effectively offset from the GC. In contrast, collimated injection nearly perpendicular to the disc can account for observations in both ballistic (free expansion) and slowdown regimes, as we show using a simple stratified evolution model verified by hydrodynamic simulations. FBs still in their ballistic regime require injection (at $z\simeq100$ pc heights in our model) with a half-opening angle $θ\simeq4^\circ$, a normalized velocity $β_{-2}\equiv v/(0.01c)\gtrsim 0.4$, and an energy $E\gtrsim2β_{-2}^2\times 10^{55}$ erg, launched $\mathbb{T}\simeq 3.3β_{-2}^{-1}$ Myr ago, showing a distinctive low-pressure region behind the bubble head. Slowing-down (mass accumulated) FBs require a faster injection, a thinner jet, a smaller $E/(β_{-2}θ)^{2}$, and a comparable $\mathbb{T}$, and follow a ballistic stage that must reach a height $z_{s}\gtrsim 5$ kpc.

astro-ph.HE↗

arXiv2021

Strange and charm contributions to nucleon charges and moments

We present preliminary results for strange and charm contributions to nucleon charges and moments. The scalar, axial and tensor charges, and unpolarized first moments are calculated using clover-on-HISQ formulation and cover four lattice spacings, $a=\{0.06,0.09,0.12, 0.15\}$~fm, and three pion masses, $M_π=\{310,220,130\}$~MeV. The renormalization factors are calculated nonperturbatively using the RI-sMOM scheme. We carry out a chiral and continuum extrapolation to obtain physical results.

hep-lat↗

arXiv2021

Spectral signature of mass outflow in Two Component Advective Flow Paradigm

Outflows are common in many astrophysical systems. In the Two Component Advective Flow ({\fontfamily{qcr}\selectfont TCAF}) paradigm which is essentially a generalized Bondi flow including rotation, viscosity and cooling effects, the outflow is originated from the hot, puffed up, post-shock region at the inner edge of the accretion disk. We consider this region to be the base of the jet carrying away matter with high velocity. In this paper, we study the spectral properties of black holes using {\fontfamily{qcr}\selectfont TCAF} which includes also a jet ({\fontfamily{qcr}\selectfont JeTCAF}) in the vertical direction of the disk plane. Soft photons from the Keplerian disk are up-scattered by the post-shock region as well as by the base of the jet and are emitted as hard radiation. We also include the bulk motion Comptonization effect by the diverging flow of jet. Our self-consistent accretion-ejection solution shows how the spectrum from the base of the jet varies with accretion rates, geometry of the flow and the collimation factor of the jet. We apply the solution to a jetted candidate GS\,1354-64 to estimate its mass outflow rate and the geometric configuration of the flow during 2015 outburst using {\it NuSTAR} observation. The estimated mass outflow to mass inflow rate is $0.12^{+0.02}_{-0.03}$. From the model fitted accretion rates, shock compression ratio and the energy spectral index, we identify the presence of hard and intermediate spectral states of the outburst. Our model fitted jet collimation factor ($f_{\rm col}$) is found to be $0.47^{+0.09}_{-0.09}$.

astro-ph.HE↗

arXiv2021

Emission lines from X-ray illuminated accretion disc in black hole binaries

X-ray flux from the inner hot region around central compact object in a binary system illuminates the upper surface of an accretion disc and it behaves like a corona. This region can be photoionised by the illuminating radiation, thus can emit different emission lines. We study those line spectra in black hole X-ray binaries for different accretion flow parameters including its geometry. The varying range of model parameters captures maximum possible observational features. We also put light on the routinely observed Fe line emission properties based on different model parameters, ionization rate, and Fe abundances. We find that the Fe line equivalent width $W_{\rm E}$ decreases with increasing disc accretion rate and increases with the column density of the illuminated gas. Our estimated line properties are in agreement with observational signatures.

astro-ph.HE↗

arXiv2021

Changing Accretion Geometry of Seyfert 1 Mrk 335 with {\it NuSTAR}: A Comparative Study

We present a detailed spectral study of the narrow-line Seyfert~1 galaxy, Markarian~335, using eight epoch observations made between 2013 and 2020 with the Nuclear Spectroscopic Telescope Array. The source was variable during this period both in spectral flux and flow geometry. We estimated the height of the Compton cloud from the model fitted parameters for the whole observation period. This allowed us to investigate the underlying physical processes that drive the variability in X-rays. Our model fitted mass varies in a narrow range, between $(2.44\pm0.45-3.04\pm0.56)\times10^{7}M_\odot$, however, given the large error bars, it is consistent with being constant and is in agreement with that known from optical reverberation mapping observations. The disk mass accretion rate reached a maximum of 10\% of the Eddington rate during June 2013. Our study sheds light on mass outflows from the system and also compares different aspects of accretion with X-ray binaries.

astro-ph.HE↗

arXiv2021

Moments of nucleon isovector structure functions in $2+1+1$-flavor QCD

We present results on the isovector momentum fraction, $\langle x \rangle_{u-d}$, helicity moment, $\langle x \rangle_{Δu-Δd}$, and the transversity moment, $\langle x \rangle_{δu-δd}$, of the nucleon obtained using nine ensembles of gauge configurations generated by the MILC collaboration using $2+1+1$-flavors of dynamical highly improved staggered quarks (HISQ). The correlation functions are calculated using the Wilson-Clover action and the renormalization of the three operators is carried out nonperturbatively on the lattice in the RI${}^\prime$-MOM scheme. The data have been collected at lattice spacings $a \approx 0.15,\ 0.12,\ 0.09,$ and 0.06 fm and $M_π\approx 310,\ 220$ and 135 MeV, which are used to obtain the physical values using a simultaneous chiral-continuum-finite-volume fit. The final results, in the $\overline{MS}$ scheme at 2 GeV, are $\langle x \rangle_{u-d} = 0.173(14)(07)$, $\langle x \rangle_{Δu-Δd} = 0.213(15)(22)$ and $\langle x \rangle_{δu-δd} = 0.208(19)(24)$, where the first error is the overall analysis uncertainty and the second is an additional systematic uncertainty due to possible residual excited-state contributions. These results are consistent with other recent lattice calculations and phenomenological global fit values.

hep-lat↗

arXiv2020

Nucleon Momentum Fraction, Helicity and Transversity from 2+1-flavor Lattice QCD

High statistics results for the isovector momentum fraction, $\langle x \rangle_{u-d}$, helicity moment, $\langle x \rangle_{Δu-Δd}$, and the transversity moment, $\langle x\rangle_{δu-δd}$, of the nucleon are presented using seven ensembles of gauge configurations generated by the JLab/W&M/LANL/MIT collaborations using $2+1$-flavors of dynamical Wilson-clover quarks. Attention is given to understanding and controlling the contributions of excited states. The final results are obtained using a simultaneous fit in the lattice spacing $a$, pion mass $M_π$ and the finite volume parameter $M_πL$ keeping leading order corrections. The data show no significant dependence on the lattice spacing and some evidence for finite-volume corrections. The main variation is with $M_π$, whose magnitude depends on the mass gap of the first excited state used in the analysis. Our final results, in the $\overline{\rm MS}$ scheme at 2 GeV, are $\langle x \rangle_{u-d} = 0.160(16)(20)$, $\langle x \rangle_{Δu-Δd} = 0.192(13)(20)$ and $\langle x \rangle_{δu-δd} = 0.215(17)(20)$, where the first error is the overall analysis uncertainty assuming excited-state contributions have been removed, and the second is an additional systematic uncertainty due to possible residual excited-state contributions. These results are consistent with other recent lattice calculations and phenomenological global fit values.

hep-lat↗