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Papers by Biplab Raychaudhuri

Research paper thumbnail of Evidence of hard power-law spectral cutoff and disc reflection features from the X-ray transient XTE J1739–285

Monthly Notices of the Royal Astronomical Society

We report on the nearly simultaneous NICER and NuSTAR observations of the known X-ray transient X... more We report on the nearly simultaneous NICER and NuSTAR observations of the known X-ray transient XTE J1739–285. These observations provide the first sensitive hard X-ray spectrum of this neutron star X-ray transient. The source was observed on 2020 February 19 in the hard spectral state with a luminosity of 0.007 of the Eddington limit. The broadband 1−70 keV NICER and NuSTAR observation clearly detects a cutoff of the hard spectral component around 34−40 keV when the continuum is fitted by a soft thermal component and a hard power-law component. This feature has been detected for the first time in this source. Moreover, the spectrum shows evidence for disc reflection – a relativistically broadened Fe K α line around 5−8 keV and a Compton hump in the 10−20 keV energy band. The accretion disc reflection features have not been identified before from this source. Through accretion disc reflection modelling, we constrain the radius of the inner disc to be $R_{\rm in}=3.1_{-0.5}^{+1.8}R_{...

Research paper thumbnail of Accretion disc/corona emission from a radio-loud narrow-line Seyfert 1 galaxy PKS 0558–504

Monthly Notices of the Royal Astronomical Society, 2015

Research paper thumbnail of Evidence of disc reflection in the X-ray spectrum of the neutron star low-mass X-ray binary 4U 1636−536

Monthly Notices of the Royal Astronomical Society, 2021

We present a broad-band spectral analysis of the atoll source 4U 1636−536 observed for ∼92 ks wit... more We present a broad-band spectral analysis of the atoll source 4U 1636−536 observed for ∼92 ks with NuSTAR. The source was found to be in a low-luminosity state during this observation with 3–79 keV X-ray luminosity of L3–79 keV = (1.03 ± 0.01) × 1037 erg s−1, assuming a distance of 6 kpc. We have identified and removed 12 type-I X-ray bursts during this observation to study the persistent emission. The continuum is well described by a thermal Comptonization model nthcomp with Γ ∼ 1.9, kTe ∼ 28 keV, and kTs ∼ 0.9 keV. The NuSTAR data reveal a clear signature of disc reflection, a significantly broad Fe–K emission line (around 5–8 keV), and the corresponding reflection hump (around 15–30 keV). We have modelled the data with two relativistically blurred reflection models. Both families of reflection models prefer truncated inner disc radii prior to the ISCO. We find that the inner disc is truncated with an inner radius of Rin = 3.2–4.7 RISCO ($\simeq 16\!-\!24\, R_{\mathrm{ g}}\: \text...

Research paper thumbnail of Study of the reflection spectrum of the bright atoll source GX 3 + 1 with NuSTAR

Monthly Notices of the Royal Astronomical Society, 2019

We report on the NuSTAR observation of the atoll type neutron star (NS) low-mass X-ray binary GX ... more We report on the NuSTAR observation of the atoll type neutron star (NS) low-mass X-ray binary GX 3 + 1 performed on 2017 October 17. The source was found in a soft X-ray spectral state with 3–70 keV luminosity of LX ∼ 3 × 1037 erg s−1 (${\sim } 16{{\ \rm per\ cent}}$ of the Eddington luminosity), assuming a distance of 6 kpc. A positive correlation between intensity and hardness ratio suggests that the source was in the banana branch during this observation. The broad-band 3–70 keV NuSTAR spectral data can be described by a two-component continuum model consisting of a disc blackbody (kTdisc ∼ 1.8 keV) and a single temperature blackbody model (kTbb ∼ 2.7 keV). The spectrum shows a clear and robust indication of relativistic reflection from the inner disc which is modelled with a self-consistent relativistic reflection model. The accretion disc is viewed at an inclination of i ≃ 22°–26° and extended close to the NS, down to $R_\text{in}=(1.2\!-\!1.8) R_\text{ISCO}\:(\simeq 6.1\!-\!9....

Research paper thumbnail of On the disc reflection spectroscopy of NS LMXB Serpens X-1: analysis of a recent NuSTAR observation

Monthly Notices of the Royal Astronomical Society, 2020

We present NuSTAR observation of the atoll type neutron star (NS) low-mass X-ray binary (LMXB) Se... more We present NuSTAR observation of the atoll type neutron star (NS) low-mass X-ray binary (LMXB) Serpens X-1 (Ser X-1) performed on 2018 February 17. We observed Ser X-1 in a soft X-ray spectral state with 3–79 keV luminosity of LX ∼ 0.4 × 1038 erg s−1 (${\sim}23{{\ \rm per\ cent}}$ of the Eddington luminosity), assuming a distance of 7.7 kpc. A positive correlation between intensity and hardness ratio suggests that the source was in the banana branch during this observation. The broad-band 3–30 keV NuSTAR energy spectrum can be well described either by a three-component continuum model consisting of a disc blackbody, a single temperature blackbody, and a power law or by a two-component continuum model consisting of a disc blackbody and a Comptonization component. A broad iron line ∼5–8 keV and the Compton back-scattering hump peaking at ∼10–20 keV band are clearly detected in the X-ray spectrum. These features are best interpreted by a self-consistent relativistic reflection model. F...

Research paper thumbnail of Traversable wormholes supported by dark matter and monopoles with semiclassical effects

The European Physical Journal C

We present a new traversable wormhole explication of Einstein’s field equations supported by the ... more We present a new traversable wormhole explication of Einstein’s field equations supported by the profile of Einasto Dark Matter densities (Einasto in Trudy Inst Astrofiz Alma-Ata 51:87, 1965; PTarO 36:414, 1968; Astron Nachr 291:97, 1969) and global monopole charges along with semiclassical effects in the local universe as the galactic halo. The Einasto DM density profile produces a suitable shape function that meets all the requirements for presenting the wormhole geometries. The Null Energy Condition (NEC) is violated by the obtained solution with different redshift functions i.e. the Einasto profile representing DM candidate within the wormholes gives the fuel to sustain these wormhole structures in the galactic halo. Moreover, the reported wormhole geometries are getting asymptotically flat and non-flat depending only on the choices of redshift function whereas all the wormhole structures are maintaining their balance of equilibrium under the action of different forces.

Research paper thumbnail of Erratum: A quest for the origin of the Sagnac effect

The European Physical Journal C

In this article, in both Eq. (2) and Eq. (3), the numerator should read "4L-q" instead of "4Lv-qc... more In this article, in both Eq. (2) and Eq. (3), the numerator should read "4L-q" instead of "4Lv-qc". The original article has been corrected.

Research paper thumbnail of Spacelike trajectories in BTZ spacetime and comparison with timelike and lightlike trajectories

Physica Scripta

Spacelike trajectories or the motions of tachyons in (2+1)(2+1)(2+1) dimensional BTZ spacetime are discus... more Spacelike trajectories or the motions of tachyons in (2+1)(2+1)(2+1) dimensional BTZ spacetime are discussed. A study of the effective potential in this case has also been done and it is shown that unlike that for tardyons or photons it does not have any local or global extremum. It rather possesses an inflection point that depends, along with the blackhole parameters, on the integrals of motion. Tachyons are, however, found to be sensitive, like photons and tardyons, to the usual frame dragging effect due to the rotational feature of the BTZ spacetime and thus pure radial motion cannot persist. It is further found that circular orbits exist nowhere except at the horizon. That the timelike object shows the same behaviour in this case is a remarkable feature of (2+1)(2+1)(2+1) dimensional gravity. Moreover, the trapping of tachyons at the horizon is also indicated. Comparison of the spacelike trajectories with the timelike and the lightlike trajectories and the corresponding effective potentials hav...

Research paper thumbnail of A quest for the origin of the Sagnac effect

The European Physical Journal C

In the literature, there is no consensus on the origin of the relativistic Sagnac effect, particu... more In the literature, there is no consensus on the origin of the relativistic Sagnac effect, particularly from the standpoint of the rotating observer. The experiments of Wang et al. (Phys Lett A 312(1–2):7, 2003; Phys Rev Lett 93(14):143901, 2004) has, however, questioned the pivotal role of rotation of the platform in Sagnac effect. Recently, the relative motion between the reflectors which force light to propagate along a closed path and the observer has been ascribed as the cause of the Sagnac effect. Here, we propose a thought experiment on linear Sagnac effect and explore another one proposed earlier to demonstrate that the origin of the Sagnac effect is neither the rotation of frame affecting clock synchronization nor the relative motion between the source and the observer; Sagnac effect originates purely due to asymmetric position of the observer with respect to the light paths. Such a conclusion is validated by analysis of a gedanken Sagnac kind experiment involving rotation.

Research paper thumbnail of Scalar Perturbation of Black Holes

Scalar perturbation represented by the Klein-Gordon field equation of the system of black holes i... more Scalar perturbation represented by the Klein-Gordon field equation of the system of black holes in 2+1 dimensional Static charged dilaton gravity proposed by Chan and Mann is investigated analytically. The radial equation is analyzed near horizon and at the asymptotic region to show the absence of superradiance instability.

Research paper thumbnail of NuSTAR view of the Z-type neutron star low-mass X-ray binary Cygnus X-2

Monthly Notices of the Royal Astronomical Society, 2017

We report on the NuSTAR observation of the Z-type neutron star low-mass X-ray binary Cygnus X-2 p... more We report on the NuSTAR observation of the Z-type neutron star low-mass X-ray binary Cygnus X-2 performed on 2015 January 7. During this observation, the source exhibited a sudden decrease in count rate (dips) and stronger variability in 3-79 keV X-ray light curve. The hardness-intensity diagram shows that the source remained in the so-called normal branch of the Z-track, although an extended 'flaring branch' is observed during the dips. The source was in a soft spectral state with the 3-45 keV luminosity of L (0.5-1.1) × 10 38 erg s −1 , assuming a distance of 8 kpc. Both the non-dip and dip X-ray spectra are well represented by models in which the soft band is dominated by the emission from the disc, while the hard X-ray band is dominated by the Comptonized emission from the boundary layer/corona and its reflected emission from the disc. The X-ray spectrum also revealed a broad Fe K α emission line which is nearly symmetric at the higher flux and asymmetric when the flux is reduced by a factor of ∼2. The relativistic reflection model predicts the inner radius of the accretion disc as R in 2.5-6.0 R ISCO (30-73 km) for the non-dip state and R in 2.0-2.6 R ISCO (24-32 km) for the dip state. If the inner disc is truncated due to the pressure arising from a magnetic field, this implies an upper limit of the magnetic field strength of ≤7.6 × 10 9 G at the magnetic poles which is consistent with other estimates.

Research paper thumbnail of NuSTAR and SWIFT joint view of neutron star X-ray binary 4U 1728–34 : Disc reflection in the island and lower banana states

Monthly Notices of the Royal Astronomical Society, 2017

We analyze two simultaneous NuSTAR and SWIFT data of the Atoll type neutron star (NS) X-ray binar... more We analyze two simultaneous NuSTAR and SWIFT data of the Atoll type neutron star (NS) X-ray binary 4U 1728-34 observed on 1 and 3 October, 2013. Based on power density spectra, hardness ratio and colour-colour diagram, we infer that the first and second observations belong to the island state and lower banana state, respectively. During island state, a low-frequency quasi-periodic oscillation (LFQPO) at ∼ 4.53 Hz is observed along with four type-I X-ray bursts (∼ 10 − 15 s duration each). The X-ray luminosity of the source during the island and lower banana states are found to be L X = 1.1 and 1.6 × 10 37 erg s −1 , respectively which correspond to ∼ 6% and ∼ 9% of the Eddington luminosity. The combined burst spectrum is well represented by a blackbody with a characteristic temperature of 2.22 ± 0.05 keV and the blackbody radius of 9.87 ± 1.86 km which is consistent with the typical radius of the neutron star. The persistent energy spectra from both observations in the energy band 1 − 79 keV are well described with thermal emission from the NS surface (kT bb ≃ 1 − 2.5 keV), Comptonized emission of thermal seed photons from the hot boundary layer/corona and the strong reflection component from the accretion disc. We detect a broad Iron line in the 5 − 8 keV band and reflection hump in the 15 − 30 keV band. These features are well modelled by the relxill reflection model. From joint spectral fitting, we constrain the inclination angle of the binary system and inner disc radius to be 22 • − 40 • and (2.8 − 4.3) × R ISCO , respectively. From inner disc radius, we estimate the magnetic field to be (3.3 − 6.5) × 10 8 Gauss.

Research paper thumbnail of A New Thought Experiment on Relativistic Length Contraction

The Physics Educator, 2021

Relativistic length contraction is revisited and a simple but new thought experiment is proposed ... more Relativistic length contraction is revisited and a simple but new thought experiment is proposed in which an apparent asymmetric situation is developed between two different inertial frames regarding detection of light that comes from a chamber to an adjacent chamber through a movable slit. The proposed experiment does not involve gravity, rigidity or any other dynamical aspect apart from the kinematics of relative motion; neither does it involve any kind of nonuniformity in motion. The resolution of the seemingly paradoxical situation has finally been discussed.

Research paper thumbnail of AVS synchronization gauge field and standing waves

We discuss the consequences of Anderson-Vetharaniam-Stedman (AVS) synchronization gauge on the st... more We discuss the consequences of Anderson-Vetharaniam-Stedman (AVS) synchronization gauge on the standing wave formation. We found that although the position of the node (s) is gauge invariant and remain the same as in the standard case of the stationary wave formation following the Einstein synchronization, the anti-node (s) becomes a gauge dependent (conventional) element and the resulting wave travels between two nodes, contrary to the experimental observation. The experimental detection of standing wave substantiates that the one-way velocity is equal to the round-trip velocity implying the uniqueness of the Einstein synchronization convention. ∗aru bhadra@yahoo.com †abhichak7@gmail.com ‡dr.souvikghose@gmail.com §biplabphy@visva-bharati.ac.in 1 ar X iv :2 11 1. 12 28 5v 1 [ ph ys ic s. cl as sph ] 2 4 N ov 2 02 1

Research paper thumbnail of Broad-band X-ray emission and the reality of the broad iron line from the neutron star–white dwarf X-ray binary 4U 1820−30

Monthly Notices of the Royal Astronomical Society, 2016

Broad relativistic iron lines from neutron star X-ray binaries are important probes of the inner ... more Broad relativistic iron lines from neutron star X-ray binaries are important probes of the inner accretion disc. The X-ray reflection features can be weakened due to strong magnetic fields or very low iron abundances such as is possible in X-ray binaries with low mass, first generation stars as companions. Here, we investigate the reality of the broad iron line detected earlier from the neutron-star low-mass X-ray binary 4U 1820−30 with a degenerate helium dwarf companion. We perform a comprehensive, systematic broad-band spectral study of the atoll source using Suzaku and simultaneous NuSTAR and Swift observations. We have used different continuum models involving accretion disc emission, thermal blackbody and thermal Comptonization of either disc or blackbody photons. The Suzaku data show positive and negative residuals in the region of Fe K band. These features are well described by two absorption edges at 7.67 ± 0.14 keV and 6.93 ± 0.07 keV or partial covering photoionized absorption or by blurred reflection. Though, the simultaneous Swift and NuSTAR data do not clearly reveal the emission or absorption features, the data are consistent with the presence of either absorption or emission features. Thus, the absorption based models provide an alternative to the broad iron line or reflection model. The absorption features may arise in winds from the inner accretion disc. The broad-band spectra appear to disfavour continuum models in which the blackbody emission from the neutron-star surface provides the seed photons for thermal Comptonization. Our results suggest emission from a thin accretion disc (kT disc ∼ 1 keV), Comptonization of disc photons in a boundary layer most likely covering a large fraction of the neutron-star surface and innermost parts of the accretion disc, and blackbody emission (kT bb ∼ 2 keV) from the polar regions.

Research paper thumbnail of Broad-band spectral study of the jet-disc emission in the radio-loud narrow-line Seyfert 1 galaxy 1H 0323+342

Monthly Notices of the Royal Astronomical Society

Research paper thumbnail of Discussions on a special static spherically symmetric perfect fluid solution of Einstein's equations

Research paper thumbnail of XMM-Newton view of a hard X-ray transient IGR J17497-2821

Monthly Notices of the Royal Astronomical Society, 2015

Research paper thumbnail of Rotating Disc Problem and Sagnac Phase-Shift Formula

Research paper thumbnail of Scalar Perturbation of Black Holes in (2+1)-Dimensional Dilaton Gravity

Scalar perturbation represented by the Klein-Gordon field equation of the system of black holes i... more Scalar perturbation represented by the Klein-Gordon field equation of the system of black holes in 2+1 dimensional Static charged dilaton gravity proposed by Chan and Mann is investigated analytically. The radial equation is analyzed near horizon and at the asymptotic region to show the absence of superradiance instability.

Research paper thumbnail of Evidence of hard power-law spectral cutoff and disc reflection features from the X-ray transient XTE J1739–285

Monthly Notices of the Royal Astronomical Society

We report on the nearly simultaneous NICER and NuSTAR observations of the known X-ray transient X... more We report on the nearly simultaneous NICER and NuSTAR observations of the known X-ray transient XTE J1739–285. These observations provide the first sensitive hard X-ray spectrum of this neutron star X-ray transient. The source was observed on 2020 February 19 in the hard spectral state with a luminosity of 0.007 of the Eddington limit. The broadband 1−70 keV NICER and NuSTAR observation clearly detects a cutoff of the hard spectral component around 34−40 keV when the continuum is fitted by a soft thermal component and a hard power-law component. This feature has been detected for the first time in this source. Moreover, the spectrum shows evidence for disc reflection – a relativistically broadened Fe K α line around 5−8 keV and a Compton hump in the 10−20 keV energy band. The accretion disc reflection features have not been identified before from this source. Through accretion disc reflection modelling, we constrain the radius of the inner disc to be $R_{\rm in}=3.1_{-0.5}^{+1.8}R_{...

Research paper thumbnail of Accretion disc/corona emission from a radio-loud narrow-line Seyfert 1 galaxy PKS 0558–504

Monthly Notices of the Royal Astronomical Society, 2015

Research paper thumbnail of Evidence of disc reflection in the X-ray spectrum of the neutron star low-mass X-ray binary 4U 1636−536

Monthly Notices of the Royal Astronomical Society, 2021

We present a broad-band spectral analysis of the atoll source 4U 1636−536 observed for ∼92 ks wit... more We present a broad-band spectral analysis of the atoll source 4U 1636−536 observed for ∼92 ks with NuSTAR. The source was found to be in a low-luminosity state during this observation with 3–79 keV X-ray luminosity of L3–79 keV = (1.03 ± 0.01) × 1037 erg s−1, assuming a distance of 6 kpc. We have identified and removed 12 type-I X-ray bursts during this observation to study the persistent emission. The continuum is well described by a thermal Comptonization model nthcomp with Γ ∼ 1.9, kTe ∼ 28 keV, and kTs ∼ 0.9 keV. The NuSTAR data reveal a clear signature of disc reflection, a significantly broad Fe–K emission line (around 5–8 keV), and the corresponding reflection hump (around 15–30 keV). We have modelled the data with two relativistically blurred reflection models. Both families of reflection models prefer truncated inner disc radii prior to the ISCO. We find that the inner disc is truncated with an inner radius of Rin = 3.2–4.7 RISCO ($\simeq 16\!-\!24\, R_{\mathrm{ g}}\: \text...

Research paper thumbnail of Study of the reflection spectrum of the bright atoll source GX 3 + 1 with NuSTAR

Monthly Notices of the Royal Astronomical Society, 2019

We report on the NuSTAR observation of the atoll type neutron star (NS) low-mass X-ray binary GX ... more We report on the NuSTAR observation of the atoll type neutron star (NS) low-mass X-ray binary GX 3 + 1 performed on 2017 October 17. The source was found in a soft X-ray spectral state with 3–70 keV luminosity of LX ∼ 3 × 1037 erg s−1 (${\sim } 16{{\ \rm per\ cent}}$ of the Eddington luminosity), assuming a distance of 6 kpc. A positive correlation between intensity and hardness ratio suggests that the source was in the banana branch during this observation. The broad-band 3–70 keV NuSTAR spectral data can be described by a two-component continuum model consisting of a disc blackbody (kTdisc ∼ 1.8 keV) and a single temperature blackbody model (kTbb ∼ 2.7 keV). The spectrum shows a clear and robust indication of relativistic reflection from the inner disc which is modelled with a self-consistent relativistic reflection model. The accretion disc is viewed at an inclination of i ≃ 22°–26° and extended close to the NS, down to $R_\text{in}=(1.2\!-\!1.8) R_\text{ISCO}\:(\simeq 6.1\!-\!9....

Research paper thumbnail of On the disc reflection spectroscopy of NS LMXB Serpens X-1: analysis of a recent NuSTAR observation

Monthly Notices of the Royal Astronomical Society, 2020

We present NuSTAR observation of the atoll type neutron star (NS) low-mass X-ray binary (LMXB) Se... more We present NuSTAR observation of the atoll type neutron star (NS) low-mass X-ray binary (LMXB) Serpens X-1 (Ser X-1) performed on 2018 February 17. We observed Ser X-1 in a soft X-ray spectral state with 3–79 keV luminosity of LX ∼ 0.4 × 1038 erg s−1 (${\sim}23{{\ \rm per\ cent}}$ of the Eddington luminosity), assuming a distance of 7.7 kpc. A positive correlation between intensity and hardness ratio suggests that the source was in the banana branch during this observation. The broad-band 3–30 keV NuSTAR energy spectrum can be well described either by a three-component continuum model consisting of a disc blackbody, a single temperature blackbody, and a power law or by a two-component continuum model consisting of a disc blackbody and a Comptonization component. A broad iron line ∼5–8 keV and the Compton back-scattering hump peaking at ∼10–20 keV band are clearly detected in the X-ray spectrum. These features are best interpreted by a self-consistent relativistic reflection model. F...

Research paper thumbnail of Traversable wormholes supported by dark matter and monopoles with semiclassical effects

The European Physical Journal C

We present a new traversable wormhole explication of Einstein’s field equations supported by the ... more We present a new traversable wormhole explication of Einstein’s field equations supported by the profile of Einasto Dark Matter densities (Einasto in Trudy Inst Astrofiz Alma-Ata 51:87, 1965; PTarO 36:414, 1968; Astron Nachr 291:97, 1969) and global monopole charges along with semiclassical effects in the local universe as the galactic halo. The Einasto DM density profile produces a suitable shape function that meets all the requirements for presenting the wormhole geometries. The Null Energy Condition (NEC) is violated by the obtained solution with different redshift functions i.e. the Einasto profile representing DM candidate within the wormholes gives the fuel to sustain these wormhole structures in the galactic halo. Moreover, the reported wormhole geometries are getting asymptotically flat and non-flat depending only on the choices of redshift function whereas all the wormhole structures are maintaining their balance of equilibrium under the action of different forces.

Research paper thumbnail of Erratum: A quest for the origin of the Sagnac effect

The European Physical Journal C

In this article, in both Eq. (2) and Eq. (3), the numerator should read "4L-q" instead of "4Lv-qc... more In this article, in both Eq. (2) and Eq. (3), the numerator should read "4L-q" instead of "4Lv-qc". The original article has been corrected.

Research paper thumbnail of Spacelike trajectories in BTZ spacetime and comparison with timelike and lightlike trajectories

Physica Scripta

Spacelike trajectories or the motions of tachyons in (2+1)(2+1)(2+1) dimensional BTZ spacetime are discus... more Spacelike trajectories or the motions of tachyons in (2+1)(2+1)(2+1) dimensional BTZ spacetime are discussed. A study of the effective potential in this case has also been done and it is shown that unlike that for tardyons or photons it does not have any local or global extremum. It rather possesses an inflection point that depends, along with the blackhole parameters, on the integrals of motion. Tachyons are, however, found to be sensitive, like photons and tardyons, to the usual frame dragging effect due to the rotational feature of the BTZ spacetime and thus pure radial motion cannot persist. It is further found that circular orbits exist nowhere except at the horizon. That the timelike object shows the same behaviour in this case is a remarkable feature of (2+1)(2+1)(2+1) dimensional gravity. Moreover, the trapping of tachyons at the horizon is also indicated. Comparison of the spacelike trajectories with the timelike and the lightlike trajectories and the corresponding effective potentials hav...

Research paper thumbnail of A quest for the origin of the Sagnac effect

The European Physical Journal C

In the literature, there is no consensus on the origin of the relativistic Sagnac effect, particu... more In the literature, there is no consensus on the origin of the relativistic Sagnac effect, particularly from the standpoint of the rotating observer. The experiments of Wang et al. (Phys Lett A 312(1–2):7, 2003; Phys Rev Lett 93(14):143901, 2004) has, however, questioned the pivotal role of rotation of the platform in Sagnac effect. Recently, the relative motion between the reflectors which force light to propagate along a closed path and the observer has been ascribed as the cause of the Sagnac effect. Here, we propose a thought experiment on linear Sagnac effect and explore another one proposed earlier to demonstrate that the origin of the Sagnac effect is neither the rotation of frame affecting clock synchronization nor the relative motion between the source and the observer; Sagnac effect originates purely due to asymmetric position of the observer with respect to the light paths. Such a conclusion is validated by analysis of a gedanken Sagnac kind experiment involving rotation.

Research paper thumbnail of Scalar Perturbation of Black Holes

Scalar perturbation represented by the Klein-Gordon field equation of the system of black holes i... more Scalar perturbation represented by the Klein-Gordon field equation of the system of black holes in 2+1 dimensional Static charged dilaton gravity proposed by Chan and Mann is investigated analytically. The radial equation is analyzed near horizon and at the asymptotic region to show the absence of superradiance instability.

Research paper thumbnail of NuSTAR view of the Z-type neutron star low-mass X-ray binary Cygnus X-2

Monthly Notices of the Royal Astronomical Society, 2017

We report on the NuSTAR observation of the Z-type neutron star low-mass X-ray binary Cygnus X-2 p... more We report on the NuSTAR observation of the Z-type neutron star low-mass X-ray binary Cygnus X-2 performed on 2015 January 7. During this observation, the source exhibited a sudden decrease in count rate (dips) and stronger variability in 3-79 keV X-ray light curve. The hardness-intensity diagram shows that the source remained in the so-called normal branch of the Z-track, although an extended 'flaring branch' is observed during the dips. The source was in a soft spectral state with the 3-45 keV luminosity of L (0.5-1.1) × 10 38 erg s −1 , assuming a distance of 8 kpc. Both the non-dip and dip X-ray spectra are well represented by models in which the soft band is dominated by the emission from the disc, while the hard X-ray band is dominated by the Comptonized emission from the boundary layer/corona and its reflected emission from the disc. The X-ray spectrum also revealed a broad Fe K α emission line which is nearly symmetric at the higher flux and asymmetric when the flux is reduced by a factor of ∼2. The relativistic reflection model predicts the inner radius of the accretion disc as R in 2.5-6.0 R ISCO (30-73 km) for the non-dip state and R in 2.0-2.6 R ISCO (24-32 km) for the dip state. If the inner disc is truncated due to the pressure arising from a magnetic field, this implies an upper limit of the magnetic field strength of ≤7.6 × 10 9 G at the magnetic poles which is consistent with other estimates.

Research paper thumbnail of NuSTAR and SWIFT joint view of neutron star X-ray binary 4U 1728–34 : Disc reflection in the island and lower banana states

Monthly Notices of the Royal Astronomical Society, 2017

We analyze two simultaneous NuSTAR and SWIFT data of the Atoll type neutron star (NS) X-ray binar... more We analyze two simultaneous NuSTAR and SWIFT data of the Atoll type neutron star (NS) X-ray binary 4U 1728-34 observed on 1 and 3 October, 2013. Based on power density spectra, hardness ratio and colour-colour diagram, we infer that the first and second observations belong to the island state and lower banana state, respectively. During island state, a low-frequency quasi-periodic oscillation (LFQPO) at ∼ 4.53 Hz is observed along with four type-I X-ray bursts (∼ 10 − 15 s duration each). The X-ray luminosity of the source during the island and lower banana states are found to be L X = 1.1 and 1.6 × 10 37 erg s −1 , respectively which correspond to ∼ 6% and ∼ 9% of the Eddington luminosity. The combined burst spectrum is well represented by a blackbody with a characteristic temperature of 2.22 ± 0.05 keV and the blackbody radius of 9.87 ± 1.86 km which is consistent with the typical radius of the neutron star. The persistent energy spectra from both observations in the energy band 1 − 79 keV are well described with thermal emission from the NS surface (kT bb ≃ 1 − 2.5 keV), Comptonized emission of thermal seed photons from the hot boundary layer/corona and the strong reflection component from the accretion disc. We detect a broad Iron line in the 5 − 8 keV band and reflection hump in the 15 − 30 keV band. These features are well modelled by the relxill reflection model. From joint spectral fitting, we constrain the inclination angle of the binary system and inner disc radius to be 22 • − 40 • and (2.8 − 4.3) × R ISCO , respectively. From inner disc radius, we estimate the magnetic field to be (3.3 − 6.5) × 10 8 Gauss.

Research paper thumbnail of A New Thought Experiment on Relativistic Length Contraction

The Physics Educator, 2021

Relativistic length contraction is revisited and a simple but new thought experiment is proposed ... more Relativistic length contraction is revisited and a simple but new thought experiment is proposed in which an apparent asymmetric situation is developed between two different inertial frames regarding detection of light that comes from a chamber to an adjacent chamber through a movable slit. The proposed experiment does not involve gravity, rigidity or any other dynamical aspect apart from the kinematics of relative motion; neither does it involve any kind of nonuniformity in motion. The resolution of the seemingly paradoxical situation has finally been discussed.

Research paper thumbnail of AVS synchronization gauge field and standing waves

We discuss the consequences of Anderson-Vetharaniam-Stedman (AVS) synchronization gauge on the st... more We discuss the consequences of Anderson-Vetharaniam-Stedman (AVS) synchronization gauge on the standing wave formation. We found that although the position of the node (s) is gauge invariant and remain the same as in the standard case of the stationary wave formation following the Einstein synchronization, the anti-node (s) becomes a gauge dependent (conventional) element and the resulting wave travels between two nodes, contrary to the experimental observation. The experimental detection of standing wave substantiates that the one-way velocity is equal to the round-trip velocity implying the uniqueness of the Einstein synchronization convention. ∗aru bhadra@yahoo.com †abhichak7@gmail.com ‡dr.souvikghose@gmail.com §biplabphy@visva-bharati.ac.in 1 ar X iv :2 11 1. 12 28 5v 1 [ ph ys ic s. cl as sph ] 2 4 N ov 2 02 1

Research paper thumbnail of Broad-band X-ray emission and the reality of the broad iron line from the neutron star–white dwarf X-ray binary 4U 1820−30

Monthly Notices of the Royal Astronomical Society, 2016

Broad relativistic iron lines from neutron star X-ray binaries are important probes of the inner ... more Broad relativistic iron lines from neutron star X-ray binaries are important probes of the inner accretion disc. The X-ray reflection features can be weakened due to strong magnetic fields or very low iron abundances such as is possible in X-ray binaries with low mass, first generation stars as companions. Here, we investigate the reality of the broad iron line detected earlier from the neutron-star low-mass X-ray binary 4U 1820−30 with a degenerate helium dwarf companion. We perform a comprehensive, systematic broad-band spectral study of the atoll source using Suzaku and simultaneous NuSTAR and Swift observations. We have used different continuum models involving accretion disc emission, thermal blackbody and thermal Comptonization of either disc or blackbody photons. The Suzaku data show positive and negative residuals in the region of Fe K band. These features are well described by two absorption edges at 7.67 ± 0.14 keV and 6.93 ± 0.07 keV or partial covering photoionized absorption or by blurred reflection. Though, the simultaneous Swift and NuSTAR data do not clearly reveal the emission or absorption features, the data are consistent with the presence of either absorption or emission features. Thus, the absorption based models provide an alternative to the broad iron line or reflection model. The absorption features may arise in winds from the inner accretion disc. The broad-band spectra appear to disfavour continuum models in which the blackbody emission from the neutron-star surface provides the seed photons for thermal Comptonization. Our results suggest emission from a thin accretion disc (kT disc ∼ 1 keV), Comptonization of disc photons in a boundary layer most likely covering a large fraction of the neutron-star surface and innermost parts of the accretion disc, and blackbody emission (kT bb ∼ 2 keV) from the polar regions.

Research paper thumbnail of Broad-band spectral study of the jet-disc emission in the radio-loud narrow-line Seyfert 1 galaxy 1H 0323+342

Monthly Notices of the Royal Astronomical Society

Research paper thumbnail of Discussions on a special static spherically symmetric perfect fluid solution of Einstein's equations

Research paper thumbnail of XMM-Newton view of a hard X-ray transient IGR J17497-2821

Monthly Notices of the Royal Astronomical Society, 2015

Research paper thumbnail of Rotating Disc Problem and Sagnac Phase-Shift Formula

Research paper thumbnail of Scalar Perturbation of Black Holes in (2+1)-Dimensional Dilaton Gravity

Scalar perturbation represented by the Klein-Gordon field equation of the system of black holes i... more Scalar perturbation represented by the Klein-Gordon field equation of the system of black holes in 2+1 dimensional Static charged dilaton gravity proposed by Chan and Mann is investigated analytically. The radial equation is analyzed near horizon and at the asymptotic region to show the absence of superradiance instability.