arXiv subjects

J. C. Pandey

Publications and source records attributed to J. C. Pandey.

At least 55 records · Page 3Linked to original sources

arXiv2011

FR Cnc Revisited: Photometry, Polarimetry and Spectroscopy

Here we present the study of FR Cnc, a young, active and spotted star. We performed analysis of ASAS-3 (The All Sky Automated Survey) data for the years 2002-2008 and amended the value of the rotational period to be 0.826518 d. The amplitude of photometric variations decreased abruptly in the year 2005, while the mean brightness remained the same, which was interpreted as a quick redistribution of spots. BVRc and Ic broad band photometric calibration was performed for 166 stars in FR Cnc vicinity. The photometry at Terskol Observatory shows two brightening episodes, one of which occurred at the same phase as the flare of 2006 November 23. Polarimetric BVR observations indicate the probable presence of a supplementary source of polarization. We monitored FR Cnc spectroscopically during the years 2004-2008. We concluded that the RV changes cannot be explained by the binary nature of FR Cnc. We determined the spectral type of FR Cnc as K7V. Calculated galactic space-velocity components (U, V, W) indicate that FR Cnc belongs to the young disc population and might also belong to the IC 2391 moving group. Based on LiI 6707.8 measurement, we estimated the age of FR Cnc to be between 10-120 Myr. Doppler Tomography was applied to create a starspot image of FR Cnc. We optimized the goodness of fit to the deconvolved profiles for axial inclination, equivalent width and v sin i, finding v sin i=46.2 km s^-1 and i=55 degrees. The starspot distribution of FR Cnc is also of interest since it is one of the latest spectral types to have been imaged. No polar spot was detected on FR Cnc.

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arXiv2011

A study of X-ray flares - II. RS CVn type Binaries

We present an analysis of seven flares detected from five RS CVn-type binaries (UZ Lib, σGem, λAnd, V711 Tau and EI Eri) observed with XMM-Newton observatory. The quiescent state X-ray luminosities in the energy band of 0.3-10.0 keV of these stars were found to be 10^{30.7-30.9} erg/s. The exponential decay time in all the sample of flares range from ~ 1 to 8 hrs. The luminosity at peak of the flares in the energy band of 0.3-10.0 keV were found to be in the range of 10^{30.8} - 10^{31.8} erg/s. The great sensitivity of the XMM-EPIC instruments allowed us to perform time resolved spectral analysis during the flares and also in the subsquent quiescent phases. The derived metal abundances of coronal plasma were found to vary during the flares observed from σGem, V771 Tau and EI Eri. In these flares elemental abundances found to be enhanced by factors of ~ 1.3-1.5 to the quiescent states. In most of the flares, the peak temperature was found to be more than 100 MK whereas emission measure increased by factors of 1.5 - 5.5. Significant sustained heating was present in the majority of flares. The loop lengths (L) derived for flaring structure were found to be of the order of 10^{10 -11} cm and are smaller than the stellar radii (R*) i.e. L/R* \lesssim 1. The flare from σGem showed a high and variable absorption column density during the flare.

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arXiv2011

Stellar contents and Star formation in the young cluster Stock 18

We have carried out deep (V$\sim$21 mag) \ubvri photometric study of the star cluster Stock 18. These along with archival Infrared data have been used to derive the basic cluster parameters and also to study the star formation processes in and around the cluster region. The distance to the cluster is derived as 2.8$\pm$0.2 kpc while its age is estimated as $6.0\pm2.0$ Myr. Present study indicates that interstellar reddening is normal in the direction of the cluster. The mass function slope is found to be -1.37$\pm$0.27 for the mass range 1$<M/M_\odot<$11.9. There is no evidence found for the effect of mass segregation in main-sequence stars of the cluster. A young stellar population with age between 1-2 Myr have been found in and around the cluster region. The presence of IRAS and AKARI sources with MSX intensity map also show the youth of the Sh2-170 region.

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arXiv2011

Observations of X-ray flares and associated MHD oscillations in star XI Boo

Using data from observations made with XMM-Newton, we present an X-ray analysis of two flares observed in $ξ$ Boo. The flare loop parameters are derived using various loop models including state-of-the-art hydrodynamic flare model. The loop lengths derived for the flaring loop structure are found to be less than the stellar radius. The exponential decay of the X-ray light curves, and time evolution of the plasma temperature and emission measure are similar to those observed in compact solar flares. The X-ray light curve of post flare phase is investigated with wavelet analysis. Wavelet analysis clearly show oscillations of the period of 1019 s. Using the observationally estimated loop length, density and magnetic field, the theoretically derived oscillation period for fast-kink mode approximately matches with the observationally estimated period. This is the first likely observational evidence of fundamental fast-kink mode of magnetoacoustic waves in the stellar loops during the post-flare phase of heightened emission.

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arXiv2010

Multi-wavelength polarimetric study towards the open cluster NGC 1893

We present multi-wavelength linear polarimetric observations for 44 stars of the NGC 1893 young open cluster region along with V-band polarimetric observations of stars of other four open clusters located between l ~160 to ~175 degree. We found evidence for the presence of two dust layers located at a distance of ~170 pc and ~360 pc. The dust layers produce a polarization Pv ~2.2%. It is evident from the clusters studied in the present work that, in the Galactic longitude range l ~160 to 175 degree and within the Galactic plane (|b| < 2 degree), the polarization angles remain almost constant, with a mean ~163 degree and a dispersion of 6 degree. The small dispersion in polarization angle could be due to the presence of uniform dust layer beyond 1 kpc. Present observations reveal that in case of NGC 1893, the foreground two dust layers, in addition to the intracluster medium, seems to be responsible for the polarization effects. It is also found that towards the direction of NGC 1893, the dust layer that exists between 2-3 kpc has a negligible contribution towards the total observed polarization. The weighted mean for percentage of polarization (Pmax) and the wavelength at maximum polarization (λmax) are found to be 2.59 \pm 0.02% and 0.55 \pm 0.01 \mum respectively. The estimated mean value of λmax indicates that the average size of the dust grains within the cluster is similar to that in the general interstellar medium. The spatial variation of the polarization is found to decrease towards the outer region of the cluster. In the present work, we support the notion, as already has been shown in previous studies, that polarimetry, in combination with (U-B)/(B-V) colour-colour diagram, is a useful tool for identifying non-members in a cluster.

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arXiv2010

X-ray emission from O-type stars : DH Cep and HD 97434

We present X-ray emission characteristics of the massive O-type stars DH Cep and HD 97434 using archival XMM-Newton observations. There is no convincing evidence for short term variability in the X-ray intensity during the observations. However, the analysis of their spectra reveals X-ray structure being consistent with two-temperature plasma model. The hydrogen column densities derived from X-ray spectra of DH Cep and HD 97434 are in agreement with the reddening measurements for their corresponding host clusters NGC 7380 and Trumpler 18, indicating that the absorption by stellar wind is negligible. The X-ray emission from these hot stars is interpreted in terms of the standard instability-driven wind shock model.

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arXiv2009

Polarization in young open cluster NGC 6823

We present multiwavelength linear polarimetric observations of 104 stars towards the region of young open cluster NGC 6823. The polarization towards NGC 6823 is dominated by foreground dust grains and we found the evidence for the presence of several layers of dust towards the line of sight. The first layer of dust is located approximately within 200 pc towards the cluster, which is much closer to the Sun than the cluster (~ 2.1 kpc). The radial distribution of the position angles for the member stars are found to show a systematic change while the polarization found to reduce towards the outer parts of the cluster and the average position angle of coronal region of the cluster is very close to the inclination of the Galactic parallel (~ 32 degree). The size distribution of the grains within NGC 6823 is similar to those in general interstellar medium. The patchy distribution of foreground dust grains are suggested to be mainly responsible for the both differential reddening and polarization towards NGC 6823. The majority of the observed stars do not show the evidence of intrinsic polarization in their light.

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arXiv2009

X-ray emission characteristics of two WR binaries : V444 Cyg and CD Cru

We present X-ray analysis of two Wolf-Rayet (WR) binaries: V444 Cyg and CD Cru using the data from observations with XMM-Newton. The X-ray light curves show the phase locked variability in both binaries, where the flux increased by a factor of $\sim 2$ in the case of V444 Cyg and $\sim 1.5$ in the case of CD Cru from minimum to maximum. The maximum luminosities in the 0.3--7.5 keV energy band were found to be $5.8\times10^{32}$ and $2.8\times10^{32}$ erg s$^{-1}$ for V444 Cyg and CD Cru, respectively. X-ray spectra of these stars confirmed large extinction and revealed hot plasma with prominent emission line features of highly ionized Ne, Mg, Si, S, Ar, Ca and Fe, and are found to be consistent with a two-temperature plasma model. The cooler plasma at a temperature of $\sim$ 0.6 keV was found to be constant at all phases of both binaries, and could be due to a distribution of small-scale shocks in radiation-driven outflows. The hot components in these binaries were found to be phase dependent. They varied from 1.85 to 9.61 keV for V444 Cyg and from 1.63 to 4.27 keV for CD Cru. The absorption of the hard component varied with orbital phase and found to be maximum during primary eclipse of V444 Cyg. The high plasma temperature and variability with orbital phase suggest that the hard-component emission is caused by a colliding wind shock between the binary components.

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arXiv2009

Observations of X-ray oscillations in XI Boo: Evidence of a fast kink mode in the stellar loops

We report the observations of X-ray oscillations during the flare in a cool active star XI Boo for the first time. XI Boo was observed by EPIC/MOS of the XMM-Newton satellite. The X-ray light curve is investigated with wavelet and periodogram analyses. Both analyses clearly show oscillations of the period of 1019 s. We interpret these oscillations as a fundamental fast-kink mode of magnetoacoustic waves.

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arXiv2009

LO Pegasi: An investigation of multi-band optical polarization

We present BVR polarimetric study of the cool active star LO Peg for the first time. LO Peg was found to be highly polarized among the cool active stars. Our observations yield average values of polarization in LO Peg: $P_B =0.387\pm 0.004%$, $θ_B = 88°\pm1°$; $P_V=0.351\pm0.004%$, $θ_V=91°\pm1°$; and $P_R= 0.335\pm0.003%$, $θ_R = 91°\pm1°$. Both the degree of polarization and the position angle are found to be variable. The semi-amplitude of the polarization variability in B, V and R bands are found to be $0.18\pm0.02$%, $0.13\pm0.01$% and $0.10\pm0.02$%, respectively. We suggest that the levels of polarization observed in LO Peg could be the result of scattering of an anisotropic stellar radiation field by an optically thin circumstellar envelope or scattering of the stellar radiation by prominence-like structures.

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arXiv2008

Multi-wavelength study of a young open cluster NGC 7419

Using UBVRI Halpha CCD photometric observations and the archival NIR and X-ray data, we have carried out a multi-wavelength study of a young star cluster NGC 7419. An age of 22.5+/-3.0 Myr and a distance of 3230^{+330}_{-430} pc are derived for the cluster with a higher value of color excess ratio E(U-B)/E(B-V) than the normal one. There is an evidence for mass segregation in this dynamically relaxed cluster with mass function slope is in agreement with the Salpeter value. NIR and Halpha excess support the existence of a young (< 2 Myr) stellar population of Herbig Ae/Be stars (> 3.0 M_sun) indicating a second episode of star formation in the cluster region. Using XMM-Newton observations, we found several X-ray sources in the cluster region but none of the Herbig Ae/Be stars is detected in X-rays. We compare the distribution of upper limits for Herbig Ae/Be stars with the X-ray distribution functions of the T-Tauri and the Herbig Ae/Be stars from previous studies, and found that the X-ray emission level of these Herbig Ae/Be stars is not more than L_X ~5.2 x 10^{30} erg/s, which is not significantly higher than for the T-Tauri stars. Therefore, X-ray emission from Herbig Ae/Be stars could be the result of either unresolved companion stars or a process similar to T-Tauri stars. We report an extended X-ray emission from the cluster region NGC 7419, with a total L_X estimate of ~ 1.8 x 10^31 erg/s/arcmin^2. Investigation of dust and CO map of 1 degree region around the cluster indicates the presence of a foreground dust cloud which is most likely associated with star forming region Sh2-154. This cloud harbors uniformly distributed pre main sequence stars (0.1-2.0M_sun) and the star formation in this cloud depend mostly upon the primordial fragmentation.

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arXiv2008

A study of X-ray flares - I. Active late-type dwarfs

We present temporal and spectral characteristics of X-ray flares observed from six late-type G-K active dwarfs (V368 Cep, XI Boo, IM Vir, V471 Tau, CC Eri and EP Eri) using data from observations with the XMM-Newton observatory. All the stars were found to be flaring frequently and altogether a total of seventeen flares were detected above the ``quiescent'' state X-ray emission which varied from 0.5 to 8.3 x 10^{29} erg/s. The largest flare was observed in a low activity dwarf XI Boo with a decay time of 10 ks and ratio of peak flare luminosity to ``quiescent'' state luminosity of 2. We have studied the spectral changes during the flares by using colour-colour diagram and by detailed spectral analysis during the temporal evolution of the flares. The exponential decay of the X-ray light curves, and time evolution of the plasma temperature and emission measure are similar to those observed in compact solar flares. We have derived the semiloop lengths of flares based on the hydrodynamic flare model. The size of the flaring loops is found to be less than the stellar radius. The hydrodynamic flare decay analysis indicates the presence of sustained heating during the decay of most flares.

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arXiv2008

Optical polarimetric study of open clusters: Distribution of Interstellar matter towards NGC 654

We present new B, V and R linear polarimetric observations for 61 stars towards the region of the young open cluster NGC 654. In this study we found evidence for the presence of at least two layers of dust along the line of sight to the cluster. The distances to the two dust layers are estimated to be ~ 200 pc and ~ 1 kpc which are located much closer to the Sun than the cluster (~ 2.4 kpc). Both the dust layers have their local magnetic field orientation nearly parallel to the direction of the Galactic plane. The foreground dust layer is found to have a ring morphology with the central hole coinciding with the center of the cluster. The foreground dust grains are suggested to be mainly responsible for both the observed differential reddening and the polarization towards the cluster.

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arXiv2007

Broad Band Optical Polarimetric Study of IC 1805

We present the BVR broad band polarimetric observations of 51 stars belonging to the young open cluster IC 1805. Along with the photometric data from the literature we have modeled and subtracted the foreground dust contribution from the maximum polarization (P_{max}) and colour excess (E_{B-V}). The mean value of the P_max for intracluster medium and the foreground are found to be 5.008 +/-0.005 % and 4.865 +/-0.022 % respectively. Moreover, the mean value of the wavelength of maximum polarization (lambda_{max}) for intracluster medium is 0.541 +/- 0.003 micro m, which is quite similar as the general interstellar medium (ISM). The resulting intracluster dust component is found to have negligible polarization efficiency as compared to interstellar dust. Some of the observed stars in IC 1805 have shown the indication of intrinsic polarization in their measurements.

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arXiv2005

Unravelling the nature of HD 81032 - a new RS CVn Binary

BVR photometric and quasi-simultaneous optical spectroscopic observations of the star HD 81032 have been carried out during the years 2000 - 2004. A photometric period of $18.802 \pm 0.07$ d has been detected for this star. A large group of spots with a migration period of $7.43 \pm 0.07$ years is inferred from the first three years of the data. H$α$ and Ca II H and K emissions from the star indicate high chromospheric activity. The available photometry in the BVRIJHK bands is consistent with spectral type of K0 IV previously found for this star. We have also examined the spectral energy distribution of HD 81032 for the presence of an infrared colour excess using the 2MASS JHK and IRAS photometry, but found no significant excess in any band abovethe normal values expected for a star with this spectral type. We have also analyzed the X-ray emission properties of this star using data obtained by the ROSAT X-ray observatory during its All-Sky Survey phase. An X-ray flare of about 12 hours duration was detected during the two days of X-ray coverage obtained for this star. Its X-ray spectrum, while only containing 345 counts, is inconsistent with a single-temperature component solar-abundance coronal plasma model, but implies either the presence of two or more plasma components, non-solar abundances, or a combination of both of these properties. All of the above properties of HD 81032 suggest that it is a newly identified, evolved RS CVn binary.

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arXiv2005

Optical and X-ray studies of chromospherically active stars : FR Cancri, HD 95559 and LO Pegasi

We present a multiwavelength study of three chromospherically active stars, namely FR Cnc (= BD +16\degr 1753), HD 95559 and LO Peg (=BD +22\degr 4409), including newly obtained optical photometry, low-resolution optical spectroscopy for FR Cnc, as well as archival IR and X-ray observations. The BVR photometry carried out during the years 2001 - 2004 has found significant photometric variability to be present in all three stars. For FR Cnc, a photometric period $0.8267 \pm 0.0004 \rm {d}$ has been established. The strong variation in the phase and amplitude of the FR Cnc light curves when folded on this period implies the presence of evolving and migrating spots or i spot groups on its surface. Two independent spots with migration periods of 0.97 and 0.93 years respectively are inferred. The photometry of HD 95559 suggests the formation of a spot (group) during the interval of our observations. We infer the existence of two independent spots or groups in the photosphere of LO Peg, one of which has a migration period of 1.12 years. The optical spectroscopy of FR Cnc carried out during 2002-2003, reveals the presence of strong and variable Ca II H and K, H$_β$ and $H_α$ emission features indicative of high level of chromospheric activity. The value of 5.3 for the ratio of the excess emission in $H_α$ to $H_β$, $E_{H_α}/E_{H_β}$, suggests that the chromospheric emission may arise from an extended off-limb region. We have also analyzed archival X-ray observations of HD 95559 and LO Peg carried out by with the ROSAT observatory. The best fit models to their X-ray spectra imply the presence of two coronal plasma components of differing temperatures and with sub-solar metal abundances.

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arXiv2005

SS 433: Results of a Recent Multi-wavelength Campaign

We conducted a multi-wavelength campaign in September-October, 2002, to observe SS 433. We used 45 meter sized 30 dishes of Giant Meter Radio Telescope (GMRT) for radio observation, 1.2 meter Physical Research Laboratory Infra-red telescope at Mt Abu for IR, 1 meter Telescope at the State Observatory, Nainital for Optical photometry, 2.3 meter optical telescope at the Vainu Bappu observatory for spectrum and Rossi X-ray Timing Explorer (RXTE) Target of Opportunity (TOO) observation for X-ray observations. We find sharp variations in intensity in time-scales of a few minutes in X-rays, IR and radio wavelengths. Differential photometry at the IR observation clearly indicated significant intrinsic variations in short time scales of minutes throughout the campaign. Combining results of these wavelengths, we find a signature of delay of about two days between IR and Radio. The X-ray spectrum yielded double Fe line profiles which corresponded to red and blue components of the relativistic jet. We also present the broadband spectrum averaged over the campaign duration.

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arXiv2003

Optical observations of the bright long duration peculiar GRB 021004 afterglow

The CCD magnitudes in Johnson $B,V$ and Cousins $R$ and $I$ photometric passbands are determined for the bright long duration GRB 021004 afterglow from 2002 October 4 to 16 starting $\sim$ 3 hours after the $γ-$ray burst. Light curves of the afterglow emission in $B$,$V$,$R$ and $I$ passbands are obtained by combining these measurements with other published data. The earliest optical emission appears to originate in a revese shock. Flux decay of the afterglow shows a very uncommon variation relative to other well-observed GRBs. Rapid light variations, especially during early times ($Δt < 2$ days) is superposed on an underlying broken power law decay typical of a jetted afterglow. The flux decay constants at early and late times derived from least square fits to the light curve are $0.99\pm0.05$ and $2.0\pm0.2$ respectively, with a jet break at around 7 day. Comparison with a standard fireball model indicates a total extinction of $E(B-V)=0.20$ mag in the direction of the burst. Our low-resolution spectra corrected for this extinction provide a spectral slope $β= 0.6\pm0.02$. This value and the flux decay constants agree well with the electron energy index $p\sim 2.27$ used in the model. The derived jet opening angle of about $7^{\circ}$ implies a total emitted gamma-ray energy $E_γ = 3.5\times10^{50}$ erg at a cosmological distance of about 20 Gpc. Multiwavelength observations indicate association of this GRB with a star forming region, supporting the case for collapsar origin of long duration GRBs.

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