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A. Kaur

Publications and source records attributed to A. Kaur.

28 records · Page 2Linked to original sources
arXiv2017

CXO J004318.8+412016, a steady supersoft X-ray source in M 31

We obtained an optical spectrum of a star we identify as the optical counterpart of the M31 Chandra source CXO J004318.8+412016, because of prominent emission lines of the Balmer series, of neutral helium, and a He II line at 4686 Angstrom. The continuum energy distribution and the spectral characteristics demonstrate the presence of a red giant of K or earlier spectral type, so we concluded that the binary is likely to be a symbiotic system. CXO J004318.8+412016 has been observed in X-rays as a luminous supersoft source (SSS) since 1979, with effective temperature exceeding 40 eV and variable X-ray luminosity, oscillating between a few times 10(35) erg/s and a few times 10(37) erg/s. The optical, infrared and ultraviolet colors of the optical object are consistent with an an accretion disk around a compact object companion, which may either be a white dwarf, or a black hole, depending on the system parameters. If the origin of the luminous supersoft X-rays is the atmosphere of a white dwarf that is burning hydrogen in shell, it is as hot and luminous as post-thermonuclear flash novae, yet no major optical outburst has ever been observed, suggesting that the white dwarf is very massive (m>1.2 M(sol)) and it is accreting and burning at the high rate (mdot>10(-8)M(sol)/year) expected for type Ia supernovae progenitors. In this case, the X-ray variability may be due to a very short recurrence time of only mildly degenerate thermonuclear flashes.

astro-ph.HE↗
arXiv2017

High-redshift blazars through nustar eyes

The most powerful sources among the blazar family are MeV blazars. Often detected at $z>2$, they usually display high X- and \gm-ray luminosities, larger-than-average jet powers and black hole masses $\gtrsim 10^9 M_{\odot}$. In the present work we perform a multiwavelength study of three high redshift blazars: 3FGL J0325.5+2223 ($z=2.06$), 3FGL J0449.0+1121 ($z= 2.15$), and 3FGL J0453.2$-$2808 ($z=2.56$), analysing quasi simultaneous data from GROND, \swift-UVOT and XRT, \nustar, and \fermi-LAT. Our main focus is on the hard X-ray band recently unveiled by \nustar~(3$-$79 keV) where these objects show a hard spectrum which enables us to constrain the inverse Compton peak and the jet power. We found that all three targets resemble the most powerful blazars, with the synchrotron peak located in the sub-millimeter range and the inverse Compton peak in the MeV range, and therefore belong to the MeV blazar class. Using a simple one zone leptonic emission model to reproduce the spectral energy distributions, we conclude that a simple combination of synchrotron and accretion disk emission reproduces the infrared-optical spectra while the X-ray to \gm-ray part is well reproduced by the inverse Compton scattering of low energy photons supplied by the broad line region. The black hole masses for each of the three sources are calculated to be $\gtrsim 4 \times 10^{8} M_{\odot}$. The three studied sources have jet power at the level of, or beyond, the accretion luminosity.

astro-ph.HE↗
arXiv2016

New High-z Fermi BL Lacs with the Photometric Dropout Technique

Determining redshifts for BL Lacertae (BL Lac) objects using the traditional spectroscopic method is challenging due to the absence of strong emission lines in their optical spectra. We employ the photometric dropout technique to determine redshifts for this class of blazars using the combined 13 broad-band filters from Swift-UVOT and the multi-channel imager GROND at the MPG 2.2 m telescope at ESO's La Silla Observatory. The wavelength range covered by these 13 filters extends from far ultraviolet to the near-Infrared. We report results on 40 new Fermi detected BL Lacs with the photometric redshifts determinations for 5 sources, with 3FGL J1918.2-4110 being the most distant in our sample at z=2.16. Reliable upper limits are provided for 20 sources in this sample. Using the highest energy photons for these Fermi-LAT sources, we evaluate the consistency with the Gamma-ray horizon due to the extragalactic background light.

astro-ph.HE↗
arXiv2016

M31N 2008-12a - the remarkable recurrent nova in M31: Pan-chromatic observations of the 2015 eruption

The Andromeda Galaxy recurrent nova M31N 2008-12a had been observed in eruption ten times, including yearly eruptions from 2008-2014. With a measured recurrence period of $P_\mathrm{rec}=351\pm13$ days (we believe the true value to be half of this) and a white dwarf very close to the Chandrasekhar limit, M31N 2008-12a has become the leading pre-explosion supernova type Ia progenitor candidate. Following multi-wavelength follow-up observations of the 2013 and 2014 eruptions, we initiated a campaign to ensure early detection of the predicted 2015 eruption, which triggered ambitious ground and space-based follow-up programs. In this paper we present the 2015 detection; visible to near-infrared photometry and visible spectroscopy; and ultraviolet and X-ray observations from the Swift observatory. The LCOGT 2m (Hawaii) discovered the 2015 eruption, estimated to have commenced at Aug. $28.28\pm0.12$ UT. The 2013-2015 eruptions are remarkably similar at all wavelengths. New early spectroscopic observations reveal short-lived emission from material with velocities $\sim13000$ km s$^{-1}$, possibly collimated outflows. Photometric and spectroscopic observations of the eruption provide strong evidence supporting a red giant donor. An apparently stochastic variability during the early super-soft X-ray phase was comparable in amplitude and duration to past eruptions, but the 2013 and 2015 eruptions show evidence of a brief flux dip during this phase. The multi-eruption Swift/XRT spectra show tentative evidence of high-ionization emission lines above a high-temperature continuum. Following Henze et al. (2015a), the updated recurrence period based on all known eruptions is $P_\mathrm{rec}=174\pm10$ d, and we expect the next eruption of M31N 2008-12a to occur around mid-Sep. 2016.

astro-ph.SR↗
arXiv2013

A genome wide dosage suppressor network reveals genetic robustness and a novel mechanism for Huntington's disease

Mutational robustness is the extent to which an organism has evolved to withstand the effects of deleterious mutations. We explored the extent of mutational robustness in the budding yeast by genome wide dosage suppressor analysis of 53 conditional lethal mutations in cell division cycle and RNA synthesis related genes, revealing 660 suppressor interactions of which 642 are novel. This collection has several distinctive features, including high co-occurrence of mutant-suppressor pairs within protein modules, highly correlated functions between the pairs, and higher diversity of functions among the co-suppressors than previously observed. Dosage suppression of essential genes encoding RNA polymerase subunits and chromosome cohesion complex suggest a surprising degree of functional plasticity of macromolecular complexes and the existence od degenerate pathways for circumventing potentially lethal mutations. The utility of dosage-suppressor networks is illustrated by the discovery of a novel connection between chromosome cohesion-condensation pathways involving homologous recombination, and Huntington's disease.

q-bio.MN↗
arXiv2013

Impact of substrate on magnetic phase coexistence in bicritical $Sm_{0.53} Sr_{0.47} Mn O_3$ thin films

Sm0.53Sr0.47MnO3 thin films were deposited on single crystal LaAlO3 (LAO/(001)) and SrTiO3 (STO/(001)) substrates by DC magnetron sputtering. The theta-2theta and omega-2theta scans show that these films have very good crystallinity and the films on LAO and STO are under compressive and tensile strain, respectively. The films on LAO and STO substrates show ferromagnetic (insulator-metal) transition at TC \sim 126 K (at TIM \sim 128 K) and 120 K (TIM \sim 117K), respectively. The magnetic state at T<TC is akin to cluster glass, which is formed by the presence of charge ordered-antiferromagnetic clusters in the ferromagnetic matrix. Huge drop in the resistivity at TIM and the associated hysteresis with respect to cooling and warming cycles reveal the bicritical and the first order nature of phase transition, which is also confirmed by the Banerjee criterion. The differences and similarities in the functional properties of films are explained in terms of substrate modified magnetic phase coexistence.

cond-mat.mtrl-sci↗
arXiv2011

CXOM31 J004253.1+411422: The first ultra-luminous X-ray transient in M 31

We seek clarification of the nature of X-ray sources detected in M 31. Here we focus on CXOM31 J004253.1+411422, the brightness of which suggests that it belongs to the class of ultraluminous X-ray sources. We determine the X-ray properties of sources detected in the XMM-Newton Chandra monitoring program. We investigate spectral properties and search for periodic or quasi-periodic oscillations. A multi-component model is applied to the spectra obtained from XMM-Newton data to evaluate the relative contributions from thermal and non-thermal emission. The time dependence of this ratio is evaluated over a period of forty days. We simultaneously fit data from XMM-Newton EPIC-pn, MOS1 and MOS2 detectors with (non-thermal) powerlaw and (thermal) multicolored blackbody. The X-ray spectrum is best fit by the combination of a thermal component with kT ~ 1 keV and a powerlaw component with photon index approximately 2.6. From combined analysis of Chandra, Swift and XMM-Newton data, the unabsorbed total luminosity of this source decreases from ~ 3.8 x 10^{39} erg s^{-1} in the first observation to ~ 0.5 x 10^{39} ergs s^{-1} over a period of three months. The decay closely follows an exponential decline with a time constant of 32 days. The source spectrum evolves significantly, exhibiting a faster decline of the thermal component. We do not find evidence of any significant temporal features in the power density spectrum. The presence of a thermal component at kT ~ 1 keV in conjunction with a non-thermal high energy tail, is also consistent with spectral properties of other ULXs in the "high state". Our analysis indicates that the underlying source of this first ULX in M~31 is a black hole of mass, M > 13 M_{sun}, accreting near the Eddington limit, that underwent a transient outburst followed by an exponential decay reminiscent of transients associated with galactic X-ray novae.

astro-ph.HE↗