arXiv subjects

S. Chandra

Publications and source records attributed to S. Chandra.

69 records · Page 4Linked to original sources

arXiv2019

H.E.S.S. observations of the flaring gravitationally lensed galaxy PKS 1830-211

PKS 1830-211 is a known macrolensed quasar located at a redshift of z=2.5. Its high-energy gamma-ray emission has been detected with the Fermi-LAT instrument and evidence for lensing was obtained by several authors from its high-energy data. Observations of PKS 1830-211 were taken with the H.E.S.S. array of Imaging Atmospheric Cherenkov Telescopes in August 2014, following a flare alert by the Fermi- LAT collaboration. The H.E.S.S observations were aimed at detecting a gamma-ray flare delayed by 20-27 days from the alert flare, as expected from observations at other wavelengths. More than twelve hours of good quality data were taken with an analysis threshold of $\sim67$ GeV. The significance of a potential signal is computed as a function of the date as well as the average significance over the whole period. Data are compared to simultaneous observations by Fermi-LAT. No photon excess or significant signal is detected. An upper limit on PKS 1830-211 flux above 67 GeV is computed and compared to the extrapolation of the Fermi-LAT flare spectrum.

astro-ph.HE↗

arXiv2019

The 2014 TeV Gamma-ray Flare of Mrk 501 Seen with H.E.S.S.: Temporal and Spectral Constraints on Lorentz Invariance Violation

The blazar Mrk 501 (z=0.034) was observed at very-high-energy (VHE, $E\gtrsim 100$~GeV) gamma-ray wavelengths during a bright flare on the night of 2014 June 23-24 (MJD 56832) with the H.E.S.S. phase-II array of Cherenkov telescopes. Data taken that night by H.E.S.S. at large zenith angle reveal an exceptional number of gamma-ray photons at multi-TeV energies, with rapid flux variability and an energy coverage extending significantly up to 20 TeV. This data set is used to constrain Lorentz invariance violation (LIV) using two independent channels: a temporal approach considers the possibility of an energy dependence in the arrival time of gamma rays, whereas a spectral approach considers the possibility of modifications to the interaction of VHE gamma rays with extragalactic background light (EBL) photons. The non-detection of energy-dependent time delays and the non-observation of deviations between the measured spectrum and that of a supposed power-law intrinsic spectrum with standard EBL attenuation are used independently to derive strong constraints on the energy scale of LIV ($E_{\rm{QG}}$) in the subluminal scenario for linear and quadratic perturbations in the dispersion relation of photons. For the case of linear perturbations, the 95% confidence level limits obtained are $E_{\rm{QG},1} > 3.6 \times 10^{17} \ \rm{GeV} $ using the temporal approach and $E_{\rm{QG},1} > 2.6 \times 10^{19} \ \rm{GeV}$ using the spectral approach. For the case of quadratic perturbations, the limits obtained are $E_{\rm{QG},2} > 8.5 \times 10^{10} \ \rm{GeV} $ using the temporal approach and $E_{\rm{QG},2} > 7.8 \times 10^{11} \rm{ GeV}$ using the spectral approach.

astro-ph.HE↗

arXiv2018

Particle Transport within the Pulsar Wind Nebula HESS J1825-137

Aims: We present a detailed view of the pulsar wind nebula (PWN) HESS J1825-137. We aim to constrain the mechanisms dominating the particle transport within the nebula, accounting for its anomalously large size and spectral characteristics. Methods: The nebula is studied using a deep exposure from over 12 years of H.E.S.S. I operation, together with data from H.E.S.S. II improving the low energy sensitivity. Enhanced energy-dependent morphological and spatially-resolved spectral analyses probe the Very High Energy (VHE, E > 0.1 TeV) gamma-ray properties of the nebula. Results: The nebula emission is revealed to extend out to 1.5 degrees from the pulsar, ~1.5 times further than previously seen, making HESS J1825--137, with an intrinsic diameter of ~100 pc, potentially the largest gamma-ray PWN currently known. Characterisation of the nebula's strongly energy-dependent morphology enables the particle transport mechanisms to be constrained. A dependence of the nebula extent with energy of R $\propto$ E^αwith α= -0.29 +/- 0.04 (stat) +/- 0.05 (sys) disfavours a pure diffusion scenario for particle transport within the nebula. The total gamma-ray flux of the nebula above 1~TeV is found to be (1.12 +/- 0.03 (stat) +/- 0.25 (sys)) $\times 10^{-11}$ cm$^{-2}$ s$^{-1}$, corresponding to ~64% of the flux of the Crab Nebula. Conclusions: HESS J1825-137 is a PWN with clear energy-dependent morphology at VHE gamma-ray energies. This source is used as a laboratory to investigate particle transport within middle-aged PWNe. Deep observations of this highly spatially-extended PWN enable a spectral map of the region to be produced, providing insights into the spectral variation within the nebula.

astro-ph.HE↗

arXiv2018

VHE $γ$-ray discovery and multi-wavelength study of the blazar 1ES 2322-409

A hotspot at a position compatible with the BL Lac object 1ES 2322-409 was serendipitously detected with H.E.S.S. during observations performed in 2004 and 2006 on the blazar PKS 2316-423. Additional data on 1ES 2322-409 were taken in 2011 and 2012, leading to a total live-time of 22.3h. Point-like very-high-energy (VHE; E>100GeV) $γ$-ray emission is detected from a source centred on the 1ES 2322-409 position, with an excess of 116.7 events at a significance of 6.0$σ$. The average VHE $γ$-ray spectrum is well described with a power law with a photon index $Γ=3.40\pm0.66_{\text{stat}}\pm0.20_{\text{sys}}$ and an integral flux $Φ(E>200GeV) = (3.11\pm0.71_{\rm stat}\pm0.62_{\rm sys})\times10^{-12} cm^{-2} s^{-1}$, which corresponds to 1.1$\%$ of the Crab nebula flux above 200 GeV. Multi-wavelength data obtained with Fermi LAT, Swift XRT and UVOT, RXTE PCA, ATOM, and additional data from WISE, GROND and Catalina, are also used to characterise the broad-band non-thermal emission of 1ES 2322-409. The multi-wavelength behaviour indicates day-scale variability. Swift UVOT and XRT data show strong variability at longer scales. A spectral energy distribution (SED) is built from contemporaneous observations obtained around a high state identified in Swift data. A modelling of the SED is performed with a stationary homogeneous one-zone synchrotron-self-Compton (SSC) leptonic model. The redshift of the source being unknown, two plausible values were tested for the modelling. A systematic scan of the model parameters space is performed, resulting in a well-constrained combination of values providing a good description of the broad-band behaviour of 1ES 2322-409.

astro-ph.HE↗

arXiv2018

Searches for gamma-ray lines and `pure WIMP' spectra from Dark Matter annihilations in dwarf galaxies with H.E.S.S

Dwarf spheroidal galaxies are among the most promising targets for detecting signals of Dark Matter (DM) annihilations. The H.E.S.S. experiment has observed five of these systems for a total of about 130 hours. The data are re-analyzed here, and, in the absence of any detected signals, are interpreted in terms of limits on the DM annihilation cross section. Two scenarios are considered: i) DM annihilation into mono-energetic gamma-rays and ii) DM in the form of pure WIMP multiplets that, annihilating into all electroweak bosons, produce a distinctive gamma-ray spectral shape with a high-energy peak at the DM mass and a lower-energy continuum. For case i), upper limits at 95\% confidence level of about $\langle σv \rangle \lesssim 3 \times 10^{-25}$ cm$^3$ s$^{-1}$ are obtained in the mass range of 400 GeV to 1 TeV. For case ii), the full spectral shape of the models is used and several excluded regions are identified, but the thermal masses of the candidates are not robustly ruled out.

astro-ph.HE↗

arXiv2018

Long term X-ray variability characteristics of the narrow-line Seyfert 1 galaxy RE~J1034+396

We present the results of our study of the long term X-ray variability characteristics of the Narrow Line Seyfert 1 galaxy RE J1034+396. We use data obtained from the AstroSat satellite along with the light curves obtained from XMM-Newton and Swift-XRT. We use the 0.3 - 7.0 keV and 3 - 20 keV data, respectively, from the SXT and the LAXPC of AstroSat. The X-ray spectra in the 0.3 - 20 keV region are well fit with a model consisting of a power-law and a soft excess described by a thermal-Compton emission with a large optical depth, consistent with the earlier reported results. We have examined the X-ray light curves in the soft and hard X-ray bands of SXT and LAXPC, respectively, and find that the variability is slightly larger in the hard band. To investigate the variability characteristics of this source at different time scales, we have used X-ray light curves obtained from XMM-Newton data (200 s to 100 ks range) and Swift-XRT data (1 day to 100 day range) and find that there are evidences to suggest that the variability sharply increases at longer time scales. We argue that the mass of the black hole in RE J1034+396 is likely to be $\sim$3 $\times$ 10$^6$ M$_\odot$, based on the similarity of the observed QPO to the high frequency QPO seen in the Galactic black hole binary, GRS 1915+105.

astro-ph.HE↗

arXiv2018

The starburst galaxy NGC 253 revisited by H.E.S.S. and Fermi-LAT

(Abridged) Context. NGC 253 is one of only two starburst galaxies found to emit $γ$-rays from hundreds of MeV to multi-TeV energies. Accurate measurements of the very-high-energy (VHE) (E $>$ 100 GeV) and high-energy (HE) (E $>$ 60 MeV) spectra are crucial to study the underlying particle accelerators and cosmic-ray interaction and transport. Aims. The measurement of the VHE $γ$-ray emission of NGC 253 published in 2012 by H.E.S.S. was limited by large systematic uncertainties. Here, a measurement of the $γ$-ray spectrum of NGC 253 is investigated in both HE and VHE $γ$-rays. Methods. The data of H.E.S.S. observations are reanalysed using an updated calibration and analysis chain. The $Fermi$-LAT analysis employs more than 8 years of data processed using pass 8. The cosmic-ray particle population is evaluated from the combined HE--VHE $γ$-ray spectrum using NAIMA. Results. The VHE $γ$-ray energy spectrum is best fit by a power-law with a flux normalisation of $(1.34\,\pm\,0.14^{\mathrm{stat}}\,\pm\,0.27^{\mathrm{sys}}) \times 10^{-13} \mathrm{cm^{-2} s^{-1} TeV^{-1}}$ at 1 TeV -- about 40 \% above, but compatible with the value obtained in Abramowski et al. (2012). The spectral index $Γ= 2.39 \pm 0.14^{\mathrm{stat}} \pm 0.25^{\mathrm{sys}}$ is slightly softer than but consistent with the previous measurement. At energies above $\sim$3 GeV the HE spectrum is consistent with a power-law ranging into the VHE part of the spectrum measured by H.E.S.S. Conclusions. Two scenarios for the starburst nucleus are tested, in which the gas in the starburst nucleus acts as a target for hadronic cosmic rays. In these two models, the level to which NGC\,253 acts as a calorimeter is estimated to a range of $f_{\rm cal} = 0.1$ to $1$ while accounting for the measurement uncertainties.

astro-ph.HE↗

arXiv2018

Optical variability in IBL S5 0716+714 during the 2013-2015 outburst

With an aim to explore optical variability at diverse timescales in BL Lac source S5 0716+714, it was observed for 46 nights during 2013 January 14 to 2015 June 01 when it underwent two major outbursts. The observations were made using the 1.2-m Mount Abu InfraRed Observatory telescope mounted with a CCD camera. On 29 nights, the source was monitored for more than two hours, resulting in 6256 data points in R-band, to check for the intra-night variability. Observations in B, V and I bands with 159, 214, and 177 data points, respectively, along with daily averaged R-band data are used to address inter-night and long-term variability and the color behavior of S5 0716+71. The study suggests that the source shows significant intra-night variability with a duty cycle of more than 31% and night-to-night variations. The average brightness magnitudes in B, V, R & I bands were found to be 14.42(0.02), 14.02(0.01), 13.22(0.01) & 13.02(0.03), respectively, while S5 0716+714 was historically brightest with R = 11.68 mag on 2015 January 18, indicating that source was in relatively high state during this period. A mild bluer when brighter behavior, typical of BL Lacs, supports the shock-in-jet model. We notice larger amplitudes of variation when the source was relatively brighter. Based on the shortest time scale of variability and causality argument, upper bound on the size of the emission region is estimated to be 9:32x 10^{14} cm and the mass of the black hole to be 5.6 x10^8Ms.

astro-ph.GA↗

arXiv2017

A precise measurement of the orbital period parameters of Cygnus X-3

We present X-ray light curves of Cygnus X-3 as measured by the recently launched AstroSat satellite. The light curve folded over the binary period of 4.8 hours shows a remarkable stability over the past 45 years and we find that we can use this information to measure the zero point to better than 100 s. We revisit the historical binary phase measurements and examine the stability of the binary period over 45 years. We present a new binary ephemeris with the period and period derivative determined to an accuracy much better than previously reported. We do not find any evidence for a second derivative in the period variation. The precise binary period measurements, however, indicate a hint of short term episodic variations in periods. Interestingly, these short term period variations coincide with the period of enhanced jet activity exhibited by the source. We discuss the implications of these observations on the nature of the binary system.

astro-ph.HE↗

arXiv2017

Multi-wavelength study of flaring activity in HBL 1ES 1959+650 during 2015-16

We present the results from a multiwavelength study of the flaring activity in HBL, 1ES 1959+650, during January 2015-June 2016. The source underwent significant flux enhancements showing two major outbursts (March 2015 and October 2015) in optical, UV, X-rays and gamma-rays. Normally, HBLs are not very active but 1ES 1959+650 has shown exceptional outburst activity across the whole electromagnetic spectrum (EMS). We used the data from Fermi-LAT, Swift-XRT & UVOT and optical data from Mt. Abu InfraRed Observatory (MIRO) along with archival data from Steward Observatory to look for possible connections between emissions at different energies and the nature of variability during flaring state. During October 2015 outburst, thirteen nights of optical follow-up observations showed brightest and the faintest nightly averaged V-band magnitudes as 14.45(0.03) and 14.85(0.02), respectively. In optical, the source showed a hint of optical intra-night variability during the outburst. A significant short-term variability in optical during MJD 57344 to MJD 57365 and in gamma-rays during MJD 57360 and MJD 57365 was also noticed. Multiwavelength study suggests the flaring activity at all frequencies to be correlated in general, albeit with diverse flare durations. We estimated the strength of the magnetic field as 4.21 G using the time-lag between optical and UV bands as synchrotron cooling time scale (2.34 hrs). The upper limits on the sizes of both the emission regions, gamma-ray and optical, are estimated to be of the order of 10^16cm using shortest variability time scales. The quasi-simultaneous flux enhancements in 15 GHz and VHE gamma-ray emissions indicates to a fresh injection of plasma into the jet, which interacts with a standing sub-mm core resulting in co-spatial emissions across the EMS. The complex and prolonged behavior of the second outburst in October 2015 is discussed in detail.

astro-ph.GA↗

arXiv2016

Electrochemical supercapacitor performance of SnO2 quantum dots

Metal oxide nanostructures are widely used in energy applications like super capacitors and Li-on battery. Smaller size nanocrystals show better stability, low ion diffusion time, higher-ion flux and low pulverization than bigger size nanocrystals during electrochemical operation. Studying the distinct properties of smaller size nanocrystals such as quantum dots (QDs) can improve the understanding on reasons behind the better performance and it will also help in using QDs or smaller size nanoparticles (NPs) more efficiently in different applications. Aqua stable pure SnO2 QDs with compositional stability and high surface to volume ratio are studied as an electrochemical super capacitor material and compared with bigger size NPs of size 25 nm. Electron energy-loss spectroscopic study of the QDs revealed dominant role of surface over the bulk. Temperature dependent study of low frequency Raman mode and defect Raman mode of QDs indicated no apparent volume change in the SnO2 QDs within the temperature range of 80-300 K. The specific capacitance of these high surface area and stable SnO2 QDs has showed only 9% loss while increasing the scan rate from 20 mV/S to 500 mV/S. Capacitance loss for the QDs is less than 2% after 1000 cycles of charging discharging, whereas for the 25 nm SnO2 NPs, the capacitance loss is 8% after 1000 cycles. Availability of excess open volume in QDs leading to no change in volume during the electro-chemical operation and good aqua stability is attributed to the better performance of QDs over bigger sized NPs.

cond-mat.mtrl-sci↗

arXiv2014

Two-state Weiss model for the anomalous thermal expansion in EuNi${}_{2}$P${}_{2}$

It has recently been found that the single crystalline EuNi${}_{2}$P${}_{2}$ shows an anormalous thermal expansion and its scaling relation to the average 4$f$ electron number. We point out that a phenomenological 2-state Weiss model can explain these behaviors. We also show that the model leads to the observed effective Bohr magneton number per atom (7.4 $μ_{\rm B}$) in the high-temperature limit, and is consistent with the temperature dependence of experimental susceptibility. Moreover the model predicts that the 4$f$ contribution to the thermal expansion coefficient is in proportional to that of the specific heat. The results suggest that the strong temperature dependence found in these experimental data originates in the excitations from the heavy-fermion ground state to the independent 4$f^{7}$ atomic states.

cond-mat.str-el↗

arXiv2013

Time resolved spectral analysis of the prompt emission of long gamma ray bursts with GeV Emission

We make a detailed time resolved spectroscopy of bright long gamma ray bursts (GRBs) which show significant GeV emissions (GRB 080916C, GRB 090902B, and GRB 090926A). In addition to the standard Band model, we also use a model consisting of a blackbody and a power-law to fit the spectra. We find that for the latter model there are indications for an additional soft component in the spectra. While previous studies have shown that such models are required for GRB 090902B, here we find that a composite spectral model consisting of two black bodies and a power law adequately fit the data of all the three bright GRBs. We investigate the evolution of the spectral parameters and find several generic interesting features for all three GRBs, like a) temperatures of the black bodies are strongly correlated to each other, b) flux in the black body components are strongly correlated to each other, c) the temperatures of the black body trace the profile of the individual pulses of the GRBs, and d) the characteristics of the power law component like the spectral index and the delayed onset bear a close similarity to the emission characteristics in the GeV regions. We discuss the implications of these results to the possibility of identifying the radiation mechanisms during the prompt emission of GRBs.

astro-ph.HE↗

arXiv2012

Precursor flares in OJ 287

We have studied three most recent precursor flares in the light curve of the blazar OJ 287 while invoking the presence of a precessing binary black hole in the system to explain the nature of these flares. Precursor flare timings from the historical light curves are compared with theoretical predictions from our model that incorporate effects of an accretion disk and post-Newtonian description for the binary black hole orbit. We find that the precursor flares coincide with the secondary black hole descending towards the accretion disk of the primary black hole from the observed side, with a mean z-component of approximately z_c = 4000 AU. We use this model of precursor flares to predict that precursor flare of similar nature should happen around 2020.96 before the next major outburst in 2022.

astro-ph.HE↗

arXiv2010

Elastic and electronic properties of PbO-type FeSe1-xTex (x = 0 - 1.0): A first-principles study

The effect of doping on electronic and other related properties of PbO-type FeSe1-xTex has been investigated theoretically using density functional method. The elastic properties for mono- and poly-crystalline FeSe1-xTex system are predicted for the first time and the results discussed. Analysis of doping dependent band characteristics in conjunction with previous studies reveal that favorable nesting of Fermi surface indicates a possible basis for understanding why Tc in FeSe1-xTex is maximum for x ~ 0.5. Keywords: FeSe superconductors, Substitution effect, Elastic properties, Electronic properties. PACS: 74.62.Dh, 74.70.Dd, 74.20.Pq, 74.25.Ld

cond-mat.supr-con↗