arXiv subjects
Publications and source records attributed to S. Bhattacharyya.
This is a White Paper in support of the mission concept of the Large Observatory for X-ray Timing (LOFT), proposed as a medium-sized ESA mission. We discuss the potential of LOFT for the study of the physics of accretion and ejection around compact objects. For a summary, we refer to the paper.
This is a White Paper in support of the mission concept of the Large Observatory for X-ray Timing (LOFT), proposed as a medium-sized ESA mission. We discuss the potential of LOFT for the study of thermonuclear X-ray bursts on accreting neutron stars. For a summary, we refer to the paper.
Excitation functions were measured by stacked-foil activation technique for the $^{150}$Nd(p, xpyn) reaction using 97.65$%$ enriched $^{150}$Nd target. Measurement up to $\sim$50$%$ above barrier and down to 18$%$ below the barrier was performed using proton beam energy (E$_p$) of 7 - 15 MeV from VECC Cyclotron. The yield of suitable $γ$ rays emitted following the decay of relevant evaporation residues was determined using a 50$%$ High Purity Germanium (HPGe) detector.(p,n) cross section was found to follow the expected trend with a maximum value of 63.7(4.9)mb at E$_p$ $\sim$ 8.6 MeV. (p,2n) cross section gradually increased with E$_p$ and had maximum contribution to the total reaction cross section after E$_p$ $\sim$ 9.0 MeV. (p, p$^{\prime}$n) reaction channel also showed a reasonable yield with a threshold of E$_p$ $\sim$ 12.0 MeV. The experimental data were corroborated with statistical model calculations using different codes, viz., CASCADE, ALICE/91 and EMPIRE3.1. All the calculations using a suitable set of global parameters could reproduce the excitation function fairly well in the present energy range.
High-spin states in the doubly-odd $^{198}$Bi nucleus have been studied by using the $^{185,187}$Re($^{16}$O, xn) reactions at the beam energy of 112.5 MeV. $γ-γ$ coincidence were measured by using the INGA array with 15 Compton suppressed clover HPGe detectors. The observed levels have been assigned definite spin-parity. The high spin structure is grouped into three bands (B1, B2 and B3), of which two (B1 and B2) exhibit the properties of magnetic rotation (MR). Tilted axis cranking calculations were carried out to explain the MR bands having large multi-quasiparticle configurations. The calculated results explain the bands B1 and B2 very nicely, confirming the shears mechanism and suggest a crossing of two MR bands in both the cases. The crossing is from 6-qp to 8-qp in band B1 and from 4-qp to 6-qp in band B2, a very rare finding. A semiclassical model has also been used to obtain the particle-hole interaction strengths for the bands B1 and B2, below the band crossing.
The Large Observatory For x-ray Timing (LOFT) was studied within ESA M3 Cosmic Vision framework and participated in the final down-selection for a launch slot in 2022-2024. Thanks to the unprecedented combination of effective area and spectral resolution of its main instrument, LOFT will study the behaviour of matter under extreme conditions, such as the strong gravitational field in the innermost regions of accretion flows close to black holes and neutron stars, and the supra-nuclear densities in the interior of neutron stars. The science payload is based on a Large Area Detector (LAD, 10 m 2 effective area, 2-30 keV, 240 eV spectral resolution, 1 deg collimated field of view) and a WideField Monitor (WFM, 2-50 keV, 4 steradian field of view, 1 arcmin source location accuracy, 300 eV spectral resolution). The WFM is equipped with an on-board system for bright events (e.g. GRB) localization. The trigger time and position of these events are broadcast to the ground within 30 s from discovery. In this paper we present the status of the mission at the end of its Phase A study.
The $γ$-ray flare of PKS 1222+216, observed in June 2010, is interpreted as an outcome of jet dynamics at recollimation zone. We obtained the $γ$-ray light-curves in three different energy bands, namely, 100--300 MeV, 300 MeV--1 GeV and 1--3 GeV from observations by the \emph{Fermi} Large Area Telescope (LAT). We also use the \emph{Swift}--XRT flux from 0.3--10 keV obtained from archival data. We supplement these with the 0.07--0.4 TeV observations with MAGIC telescope, available in the literature. The detection of source at very high energy (VHE, $E>100$ GeV) with a differential photon spectral index of $2.7\pm0.3$ and the rapid variability associated with it suggests that the emission arises from a compact region located beyond the broad line emitting region. The plausible $γ$-ray emission mechanism can then be inverse Compton scattering of IR photons from obscuring torus. Further, the decay time of LAT flare cannot be explained by considering simple radiative loss mechanisms. Hence, to interpret the LAT light curves, we develop a model where the broadband emission originates from a compact region, arising plausibly from the compression of jet matter at the recollimation zone. The flare is then expressed as an outcome of jet deceleration probably associated with this focusing effect. The parameters of the model are further constrained by reproducing the broadband spectral energy distribution of the source obtained during the flare episode. Our study suggests that the particle energy density exceeds magnetic energy density by a large factor which in turn may cause rapid expansion of the emission region. However, near equipartition can be achieved towards the end of LAT flare during which the compact emission region would have expanded to the size of jet cross-section.
We would attempt here to understand some properties of the transverse momentum ($p_T$) and rapidity ($y$) spectra on production of deuteron ($d$) and antideuteron($\overline{d}$) in lead-lead $(Pb+Pb)$ collisions at 158A GeV recently reported by NA49 collaboration. Starting from some basic properties of $p+p$ reactions for production of secondary proton-antiprotons the cases of production of the composite set of particles, like $d$ and $\overline{d}$, would be analysed. Some ratio-behaviours would also be dealt with in the light of the same approaches. It is found that the combination of the models put into use here capture modestly well the trends of the data on some important observables. Some limitations of the approach would also be pointed out in the end.
The excited states in the odd-$A$ $^{197}$Tl nucleus have been studied by populating them using the $^{197}$Au($α$, 4$n$)$^{197}$Tl reaction at the beam energy of 48 MeV. The $γ-γ$ coincidence data were taken using a combination of clover, LEPS and single crystal HPGe detectors. Precise spin and parity assignments of the excited states have been done through the polarization and the DCO measurements. A new band structure has been identified and the evidence for a possible intruder $πi_{13/2}$ state has been found for the first time. Possible configurations of the observed bands have been discussed. The total Routhian surface calculations have been performed to study the shape of $^{197}$Tl for different configurations.
Compilation of CTA contributions to the proceedings of the 33rd International Cosmic Ray Conference (ICRC2013), which took place in 2-9 July, 2013, in Rio de Janeiro, Brazil
Electron Capture (EC) decay of $^{146}$Gd($\it{t_{1/2}}$ = 48d) to the low lying states of $^{146}$Eu has been studied using high-resolution $γ$ ray spectroscopy. The $^{146}$Gd activity was produced by ($α$, 2n) reaction at E$_α$ = 32 MeV using 93.8% enriched $^{144}$Sm target. The level structure has been considerably modified from the measurement of $γ$ ray singles, $γγ$ coincidences and decay half lives. Lifetime measurement has been performed for the 3$^-$ (114.06 keV) and 2$^-$ (229.4 keV) levels of $^{146}$Eu using Mirror Symmetric Centroid Difference (MSCD) method with LaBr$_3$ (Ce) detectors. The lifetimes for these two states have been found to be 5.38 $\pm$ 2.36 ps and 8.38 $\pm$ 2.19 ps respectively. Shell model calculation has been performed using OXBASH code in order to interpret the results.
Linear polarization measurements have been performed for $γ$-rays in $^{91}$Ru produced with the $^{58}$Ni($^{36}$Ar, $2p1n$$γ$)$^{91}$Ru reaction at a beam energy of 111 MeV. The EXOGAM Ge clover array has been used to measure the $γ$-$γ$ coincidences, $γ$-ray linear polarization and $γ$-ray angular distributions. The polarization sensitivity of the EXOGAM clover detectors acting as Compton polarimeters has been determined in the energy range 0.3$-$1.3 MeV. Several transitions have been observed for the first time. Measurements of linear polarization and angular distribution have led to the firm assignments of spin differences and parity of high-spin states in $^{91}$Ru. More specifically, calculations using a semi-empirical shell model were performed to understand the structures of the first and second (21/2$^{+}$) and (17/2$^{+}$) levels. The results are in good agreement with the experimental data, supporting the interpretation of the non yrast (21/2$^{+}$) and (17/2$^{+}$) states in terms of the $J_{\rm max}$ and $J_{\rm max}-2$ members of the seniority-three $ν(g_{9/2})^{-3}$ multiplet.
Exclusive measurements of prompt $γ$-rays from the heavy-residues with various light charged particles in the $^7$Li + $^{198}$Pt system, at an energy near the Coulomb barrier (E/$V_b$ $\sim$ 1.6) are reported. Recent dynamic classical trajectory calculations, constrained by the measured fusion, $α$ and $t$ capture cross-sections have been used to explain the excitation energy dependence of the residue cross-sections. These calculations distinctly illustrate a two step process, breakup followed by fusion in case of the capture of $t$ and $α$ clusters; whereas for $^{6}$He + $p$ and $^{5}$He + $d$ configurations, massive transfer is inferred to be the dominant mechanism. The present work clearly demonstrates the role played by the cluster structures of $^7$Li in understanding the reaction dynamics at energies around the Coulomb barrier.
In continuation of their earlier measurements, the PAMELA group reported data on antiproton flux and $\bar{P}/P$ ratios in 2010 at much higher energies. In past we had dealt with these specific aspects of PAMELA data in great detail and each time we captured the contemporary data-trends quite successfully with the help of a multiple production model of secondary antiprotons with some non-standard ilk and with some other absolutely standard assumptions and approximations. In this work we aim at presenting a comprehensive and valid description of all the available data on antiproton flux and the nature of $\bar{P}/P$ ratios at the highest energies reported so far by the PAMELA experiment in 2010. The main physical implication of all this would, in the end, be highlighted.
The high-spin states in odd-odd $^{194}$Tl nucleus have been studied by populating them using the $^{185,187}$Re($^{13}$C, xn) reactions at 75 MeV of beam energy. $γ-γ$ coincidence measurement has been performed using the INGA array with a digital data acquisition system to record the time stamped data. Definite spin-parity assignment of the levels was made from the DCO ratio and the IPDCO ratio measurements. The level scheme of $^{194}$Tl has been extended up to 4.1 MeV in excitation energy including 19 new gamma ray transitions. The $πh_{9/2} \otimes νi_{13/2}$ band, in the neighboring odd-odd Tl isotopes show very similar properties in both experimental observables and calculated shapes. Two new band structures, with 6-quasiparticle configuration, have been observed for the first time in $^{194}$Tl. One of these bands has the characteristics of a magnetic rotational band. The cranked shell model calculations, using a deformed Woods-Saxon potential, have been performed to obtain the total Routhian surfaces in order to study the shapes of the bands and the band crossing in $^{194}$Tl. The semiclassical formalism has been used to describe the magnetic rotational band.
We attempt here to understand successfully some crucial aspects of $J/Ψ$-production in some high energy nuclear collisions in the light of a non-standard framework outlined in the text. It is found that the results arrived at with this main working approach here is fairly in good agreement with both the measured data and the results obtained on the basis of some other models of the `standard' variety. Impact and implications of this comparative study have also been precisely highlighted in the end.
The high spin states in $^{195}$Bi has been studied by $γ$-ray spectroscopic method using the $^{181}$Ta($^{20}$Ne, 6n) fusion evaporation reaction at 130 MeV. The $γγ$ coincidence data were taken using an array of 8 clover HPGe detectors. The spin and parity assignments of the excited states have been made from the measured directional correlation from oriented states (DCO) ratios and integrated polarization asymmetry (IPDCO) ratios. The results show, for the first time, the evidence of a rotational like band based on a 13/2$^+$ band head in this nucleus, indicating the onset of deformation at neutron number $N = 112$ for the Bismuth isotopes. The results obtained were found to be consistent with the prediction of the total Routhian surface calculations using Woods Saxon potential. The same calculations also predict a change in shape from oblate to triaxial in $^{195}$Bi at high rotational frequency.
We would attempt, in this work, at dwelling upon some crucial aspects of $J/Ψ$-production in a few high energy nuclear collisions in the light of a non-standard model which is outlined in the text. The underlying physical ideas, assumptions and ansatzs have also been enunciated in some detail. It is found that the results arrived at with this main working approach here are fairly in good agreement with both the measured data and the results obtained on the basis of some other models of the `standard' variety. Impact and implications of this comparative study have also been precisely highlighted in the end.
With the help of a phenomenological approach outlined in the text in some detail, we have dealt here with the description of the plots on rapidity and pseudorapidity spectra of some hadron-secondaries produced in various nucleus-nucleus interactions at high energies. The agreement between the measured data and the attempted fits are, on the whole, modestly satisfactory excepting a very narrow central region in the vicinity of y=$η$=0. At last, hints to how the steps suggested in the main body of the text to proceed with the description of the measured data given in the plots could lead finally to a somewhat systematic methodology have also been made.