arXiv subjects

K. P. Singh

Publications and source records attributed to K. P. Singh.

At least 55 records · Page 3Linked to original sources

arXiv2015

Study of X-ray emission from the old open cluster, M67

We present an X-ray analysis of a 4 Gyr old open cluster, M67, using archival XMM-Newton data. The aim of this study was to find new X-ray members of M67, and to use the updated member list for studying X-ray variability, and derive the X-ray luminosity functions (XLFs) of different stellar types and compare them with other star clusters of similar age. We report the detection of X-ray emission from 25 members of M67, with membership based primarily on their proper motion, of which one X-ray source is a new member. Supplementing this study with previous ROSAT and Chandra studies of M67, and using the most recent proper motion study by Vereshchagin et al., we have compiled a revised list of X-ray emitting members of M67 consisting of 43 stars. Sixteen of these are known RS CVn type binaries having orbital periods $<$ 10 days, and near-circular orbits, 5 are contact binaries with orbital periods $<$ 6 hours, 5 are yellow and blue stragglers, 2 are Algol-type binaries, and one source is a cataclysmic variable. Fourteen members do not have any orbital information and cannot be classified. Fourteen of the X-ray sources detected do not have any optical counterpart down to a magnitude of $V\simeq22$, and their membership is uncertain. Finally, we report the X-ray luminosity functions of RS CVn type and other types of stars in M67 and compare them with other open clusters of intermediate-to-old age.

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arXiv2015

X-raying the coronae of HD~155555

We present an analysis of the high-resolution Chandra observation of the multiple system, HD 155555 (an RS CVn type binary system, HD 155555 AB, and its spatially resolved low-mass companion HD 155555 C). This is an intriguing system which shows properties of both an active pre-main sequence star and a synchronised (main sequence) binary. We obtain the emission measure distribution, temperature structures, plasma densities, and abundances of this system and compare them with the coronal properties of other young/active stars. HD 155555 AB and HD 155555 C produce copious X-ray emission with log Lx of 30.54 and 29.30, respectively, in the 0.3-6.0 keV energy band. The light curves of individual stars show variability on timescales of few minutes to hours. We analyse the dispersed spectra and reconstruct the emission measure distribution using spectral line analysis. The resulting elemental abundances exhibit inverse first ionisation potential effect in both cases. An analysis of He-like triplets yields a range of coronal electron densities ~10^10-10^13 cm-3. Since HD 155555 AB is classified both as an RS CVn and a PMS star, we compare our results with those of other slightly older active main-sequence stars and T Tauri stars, which indicates that the coronal properties of HD 155555 AB closely resemble that of an older RS CVn binary rather than a younger PMS star. Our results also suggests that the properties of HD 155555 C is very similar to those of other active M dwarfs.

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arXiv2014

Brightest Fermi-LAT Flares of PKS 1222+216: Implications on Emission and Acceleration Processes

We present a high time resolution study of the two brightest $γ$-ray outbursts from a blazar PKS 1222+216 observed by the \textit{Fermi} Large Area Telescope (LAT) in 2010. The $γ$-ray light-curves obtained in four different energy bands: 0.1--3, 0.1--0.3, 0.3--1 and 1--3 GeV, with time bin of 6 hr, show asymmetric profiles with a similar rise time in all the bands but a rapid decline during the April flare and a gradual one during the June. The light-curves during the April flare show $\sim 2$ days long plateau in 0.1--0.3 GeV emission, erratic variations in 0.3--1 GeV emission, and a daily recurring feature in 1--3 GeV emission until the rapid rise and decline within a day. The June flare shows a monotonic rise until the peak, followed by a gradual decline powered mainly by the multi-peak 0.1--0.3 GeV emission. The peak fluxes during both the flares are similar except in the 1--3 GeV band in April which is twice the corresponding flux during the June flare. Hardness ratios during the April flare indicate spectral hardening in the rising phase followed by softening during the decay. We attribute this behavior to the development of a shock associated with an increase in acceleration efficiency followed by its decay leading to spectral softening. The June flare suggests hardening during the rise followed by a complicated energy dependent behavior during the decay. Observed features during the June flare favor multiple emission regions while the overall flaring episode can be related to jet dynamics.

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arXiv2014

X-ray emission from the super-Earth host GJ 1214

Stellar activity can produce large amounts of high-energy radiation, which is absorbed by the planetary atmosphere leading to irradiation-driven mass-loss. We present the detection and an investigation of high-energy emission in a transiting super-Earth host system, GJ 1214, based on an XMM-Newton observation. We derive an X-ray luminosity LX=7.4E25 erg/s and a corresponding activity level of log(LX/Lbol)~ -5.3. Further, we determine a coronal temperature of about -3.5 MK, which is typical for coronal emission of moderately active low-mass stars. We estimate that GJ 1214 b evaporates at a rate of 1.3E10 g/s and has lost a total of ~2-5.6 MEarth.

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arXiv2014

Gamma-ray Flare of PKS 1222+216 in 2010: Effect of Jet Dynamics at the Recollimation Zone

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.

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arXiv2014

Dynamics of ten clusters of galaxies with substructures

We present a detailed Chandra study of a sample of ten clusters of galaxies selected based on the presence of substructures in their optical images. The X-ray surface brightness maps of most of these clusters show anisotropic morphologies, especially in the central regions. A total of 22 well resolved significantly bright X-ray peaks (corresponding with high-density regions) are seen in the central parts (within r$_{\rm c}/2$) of the clusters. Multiple peaks are seen in central parts of six clusters. Eleven peaks are found to have optical counterparts (10 coinciding with the BCGs of the 10 clusters and one coinciding with the second brightest galaxy in A539). For most of the clusters, the optical substructures detected in the previous studies are found to be outside the field of view of Chandra. In the spectroscopically produced 2-D temperature maps, significantly lower temperatures are seen at the location of three peaks (two in A539 and one in A376). The centres of five clusters in our sample also host regions of higher temperature compared to the ambient medium, indicating the presence of galaxy scale mergers. The X-ray luminosity, gas mass and central cooling time estimates for all the clusters are presented. The radial X-ray surface-brightness profiles of all but one of the clusters are found to be best-fitted with a double-$β$ model, pointing towards the presence of double-phased central gas due to cool-cores. The cooling time estimates of all the clusters, however, indicate that none of them hosts a strong cool-core, although the possibility of weak cool-cores cannot be ruled out.

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arXiv2013

X-ray flares observed from six young stars located in the region of star clusters NGC 869 and IC 2602

We present an analysis of seven intense X-ray flares observed from six stars (LAV 796, LAV 1174, SHM2002 3734, 2MASS 02191082+5707324, V553 Car, V557 Car) for the first time. These stars are located in the region of young open star clusters NGC 869 and IC 2602. These flares detected in the XMM-Newton data show a rapid rise (10-40 minutes) and a slow decay (20-90 minutes). The X-ray luminosities during the flares in the energy band 0.3-7.5 keV are in the range of $10^{29.9}$ to $10^{31.7}$ erg s$^{-1}$. The strongest flare was observed with the ratio $\sim 13$ for count rates at peak of the flare to the quiescent intensity. The maximum temperature during the flares has been found to be $\sim$100 MK. The semi loop lengths for the flaring loops are estimated to be of the order of $\rm{10^{10}}$ cm. The physical parameters of the flaring structure, the peak density, pressure, and minimum magnetic field required to confine the plasma have been derived and found to be consistent with flares from pre-main sequence stars in the Orion and the Taurus-Auriga-Perseus region.

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arXiv2013

X-ray observations of eight young open star clusters : I. Membership and X-ray Luminosity

We present a detailed investigation of X-ray source contents of eight young open clusters with ages between 4 to 46 Myr using archival X-ray data from XMM-Newton. The probable cluster memberships of the X-ray sources have been established on the basis of multi-wavelength archival data, and samples of 152 pre-main sequence (PMS) low mass (< 2 M_\odot), 36 intermediate mass (2 - 10 M_\odot) and 16 massive (> 10 M_\odot) stars have been generated. X-ray spectral analyses of high mass stars reveal the presence of high temperature plasma with temperature <2 keV, and mean L_X/L_{bol} of 10^{-6.9}. In the case of PMS low mass stars, the plasma temperatures have been found to be in the range of 0.2 keV to 3 keV with a median value of ~1.3 keV, with no significant difference in plasma temperatures during their evolution from 4 to 46 Myr. The X-ray luminosity distributions of the PMS low mass stars have been found to be similar in the young star clusters under study. This may suggest a nearly uniform X-ray activity in the PMS low mass stars of ages ~4--14 Myr. These observed values of L_X/L_{bol} are found to have a mean value of 10^{-3.6\pm0.4}, which is below the X-ray saturation level.The L_X/L_{bol} values for the PMS low mass stars are well correlated with their bolometric luminosities, that implies its dependence on the internal structure of the low mass stars. The difference between the X-ray luminosity distributions of the intermediate mass stars and the PMS low mass stars has not been found to be statistically significant. Their L_X/L_{bol} values, however have been found to be significantly different from each other with a confidence level greater than 99.999%. The possibility of X-ray emission from the intermediate mass stars due to a low mass star in close proximity of the intermediate mass star can not be ruled out due to the limited spatial resolution of XMM-Newton.

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arXiv2013

An X-ray view of quasars

I present an overview of observational studies of quasars of all types, with particular emphasis on X-ray observational studies. The presentation is based on the most popularly accepted unified picture of quasars - collectively referred to as AGN (active galactic nuclei) in this review. Characteristics of X-ray spectra and X-ray variability obtained from various X-ray satellites over the last 5 decades have been presented and discussed. The contribution of AGN in understanding the cosmic X-ray background is discussed very briefly. Attempt has been made to provide up-to-date information; however, this is a vast subject and this presentation is not intended to be comprehensive.

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arXiv2013

X-ray Emission from Magnetic Cataclysmic Variables

I present a brief review of the present status of X-ray emission from Magnetic Cataclysmic Variables (MCVs). A short introduction to the types of Cataclysmic Variables (CVs) is followed by a presentation of some of the properties of the two types of MCVs - Polars and Intermediate Polars (IPs) as seen in X-rays. Finally X-ray spectra of MCVs and future prospects of their studies are discussed.

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arXiv2013

A multi-wavelength study of nuclear activity and environment of a low power radio galaxy CTD 86

We present an X-ray study of the nuclear and extended emission of a nearby Fanaroff & Riley class I (FR-I) radio galaxy CTD 86 based on the \xmm{} observations. Two different components observed are : diffuse thermal emission from hot gas ($kT\sim 0.79\kev$, $n_e\sim 10^{-3}{\rm cm^{-3}}$, $L_X \sim 5\times10^{42}{\rm erg s^{-1}}$ extended over $\sim 186{\rm kpc}$), and unresolved nuclear emission exhibiting mild activity. The hot gaseous environment of CTD 86 is similar to that found in groups of galaxies or in bright early-type galaxies. No clear signatures of radio-lobe interaction with the diffuse hot gas is evident in this case. X-ray emission from the nucleus is well constrained by an intrinsically absorbed ($N_H \sim 5.9\times10^{22}{\rm cm^{-2}}$) power law ($Γ\sim 1.5$) with $2-10\kev$ luminosity $L_X \sim 2.1\times10^{42}{\rm erg s^{-1}}$. We have measured the stellar velocity dispersion, $σ=182\pm8\kms$, for the CTD 86 and estimated a mass $M_{BH}\sim 9\times 10^7{\rm M_\odot}$ with $L_{bol}/L_{Edd} \sim 4\times10^{-3}$. The low $L_{bol}/L_{Edd}$ rate and high $L_X/L_{[O III]}$ ratio suggest that the central engine of CTD 86 consists of a truncated accretion disk lacking a strong ionizing UV radiation and an inner hot flow producing the X-ray emission. The truncated disk is likely to be inclined with ($i\sim40^\circ-50^\circ$) such that our line of sight passes through the outer regions of a putative torus and thus results in high X-ray absorption. We have also identified two bright X-ray sources, SDSS J142452.11+263715.1 and SDSS J142443.78+263616.2, near CTD 86. SDSS J142452.11+263715.1 is a type 1 active galactic nucleus at $z=0.3761$ and unabsorbed $0.3-10\kev$ X-ray luminosity $L_X\sim 8 \times 10^{43}{\rm erg s^{-1}}$, while SDSS J142443.78+263616.2 is probably a galaxy with an active nucleus.

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arXiv2013

High Energy Emission Processes in OJ 287 during 2009 Flare

The broadband spectrum of a BL Lac object, OJ 287, from radio to $γ$-rays obtained during a major $γ$-ray flare detected by \emph{Fermi} in 2009 are studied to understand the high energy emission mechanism during this episode. Using a simple one-zone leptonic model, incorporating synchrotron and inverse Compton emission processes, we show that the explanation of high energy emission from X-rays to $γ$-rays, by considering a single emission mechanism, namely, synchrotron self-Compton (SSC) or external Compton (EC) requires unlikely physical conditions. However, a combination of both SSC and EC mechanisms can reproduce the observed high energy spectrum satisfactorily. Using these emission mechanisms we extract the physical parameters governing the source and its environment. Our study suggests that the emission region of OJ 287 is surrounded by a warm infrared (IR) emitting region of $\sim 250 \, K$. Assuming this region as a spherical cloud illuminated by an accretion disk, we obtain the location of the emission region to be $\sim 9 pc$. This supports the claim that the $γ$-ray emission from OJ 287 during the 2009 flare arises from a location far away from the central engine as deduced from millimeter-gamma ray correlation study and very long baseline array images.

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arXiv2013

A cluster pair : A3532 and A3530

We present a detailed study of a close pair of clusters of galaxies, A3532 and A3530, and their environments. The \textit{Chandra} X-ray image of A3532 reveals presence of substructures on scales of $\sim$20$^{\prime\prime}$ in its core. XMM-Newton maps of the clusters show excess X-ray emission from an overlapping region between them. Spectrally determined projected temperature and entropy maps do not show any signs of cluster scale mergers either in the overlapping region or in any of the clusters. In A3532, however, some signs of the presence of galaxy scale mergers are visible e.g., anisotropic temperature variations in the projected thermodynamic maps, a wide angled tailed (WAT) radio source in the brighter nucleus of its dumbbell Brightest Cluster Galaxy (BCG), and a candidate X-ray cavity coincident with the northwestern extension of the WAT source in the low-frequency radio observations. The northwestern extension in A3532 seems either a part of the WAT or an unrelated diffuse source in A3532 or in the background. There is an indication that the cool core in A3532 has been disrupted by the central AGN activity. A reanalysis of the redshift data reinforces the close proximity of the clusters. The excess emission in the overlapping region appears to be a result of tidal interactions as the two clusters approach each other for the first time. However, we can not rule out the possibility of the excess being due to the chance superposition of their X-ray halos.

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arXiv2012

X-ray Observations of INTEGRAL Discovered Cataclysmic Variable IGR J17195-4100

We present analysis of archival X-ray data obtained with the XMM-Newton and Suzaku for a new Intermediate Polar identified as a counterpart of an INTEGRAL discovered gamma-ray source, IGR J17195-4100. We report a new period of 1053.7\pm12.2 s in X-rays. A new binary orbital period of 3.52+1.43-0.80 h is strongly indicated in the power spectrum of the time series. An ephemeris of the new period proposed as the spin period of the system has also been obtained. The various peaks detected in the power spectrum suggest a probable disc-less accretion system. The soft X-rays (<3 keV) dominate the variability seen in the X-ray light curves. The spin modulation shows energy dependence suggesting the possibility of a variable partial covering accretion column. The averaged spectral data obtained with XMM-Newton EPIC cameras show a multi temperature spectra with a soft excess. The latter can be attributed to the varying coverage of accretion curtains.

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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

Intracluster medium of the merging cluster Abell 3395

We present a detailed imaging and spectral analysis of the merging environment of the bimodal cluster A3395 using X-ray and radio observations. X-ray images of the cluster show five main constituents of diffuse emission : A3395 NE, A3395 SW, A3395 NW, A3395 W, and a filament connecting NE to W. X-ray surface-brightness profiles of the cluster did not show any shock fronts in the cluster. Temperature and entropy maps show high temperature and high entropy regions in the W, the NW, the filament and between the NE and SW subclusters. The NE, SW and W components have X-ray bolometric luminosities similar to those of rich clusters of galaxies but have relatively higher temperatures. Similarly, the NW component has X-ray bolometric luminosity similar to that of isolated groups but with much higher temperature. It is, therefore, possible that all the components of the cluster have been heated by the ongoing mergers. The NE subcluster is the most massive and luminous constituent and other subclusters are found to be gravitationally bound to it. The W component is most probably either a clump of gas stripped off the SW due to ram pressure or a separate subcluster that has merged or is merging with the SW. No X-ray cavities are seen associated with the Wide Angle Tailed (WAT) radio source near the centre of the SW subcluster. Minimum energy pressure in the radio emission-peaks of the WAT galaxy is comparable with the external thermal pressure. The radio spectrum of the WAT suggests a spectral age of ~10Myr.

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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

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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