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Now showing items 1 - 8 of 8

  • Radio-loud Active Galactic Nucleus Variability from Three-dimensional Propagating Relativistic Jets

    Li, Yutong   Wiita, Paul J.   Schuh, Terance   Elghossain, Geena   Hu, Shaoming  

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  • Comprehensive photometric study of an extremely low mass ratio deep contact binary in the globular cluster M4

    Li, Kai   Hu, Shaoming   Chen, Xu   Guo, Difu  

    A comprehensive photometric study and an investigation of the orbital period variation of V53 in the globular cluster M4 are presented. The photometric study reveals that the mass ratio and the contact degree of V53 are q similar to 0.078 and f similar to 69%, respectively. The observed variation in the light curve can be explained by adjusting the spot parameters. V53 belongs to extreme mass ratio (q <=3D 0.25), deep contact (f >=3D 50%) binaries, and its mass ratio is close to the minimum mass ratio predicted by theoretical studies, making it a potential object for studying the evolution of binaries and the formation of blue stragglers and FK Com-type stars. The orbital period of V53 shows a long-term decrease at a rate of dp/dt =3D 5.89(+/- 0.02) x 10(-8) dyr(-1). This secular period decrease may be caused by the combination of mass transfer from the more massive component to the less massive component and an angular momentum loss via magnetic braking. As this mass transfer and angular momentum loss continues, V53 will ultimately evolve into a single fast-rotation star. By studying the statistics of all the contact binaries in globular clusters that have been analyzed, we found a possible correlation between the contact degree and whether or not a contact binary is a blue straggler. A contact binary is likely to become a blue straggler when its fill-out factor is more than 46.25(+/- 2.05)%. More samples should be introduced to confirm this preliminary result in the future.
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  • A possible giant planet orbiting the cataclysmic variable LX Ser

    Li, Kai   Hu, Shaoming   Zhou, Jilin   Wu, Donghong   Guo, Difu   Jiang, Yunguo   Gao, Dongyang   Chen, Xu   Wang, Xianyu  

    LX Ser is a deeply eclipsing cataclysmic variable with an orbital period of 0.1584325 d. 62 new eclipse times were determined by our observations and the AAVSO International Data base. Combining all available eclipse times, we analyzed the O-C behavior of LX Ser. We found that the O-C diagram of LXSer shows a sinusoidal oscillation with a period of 22.8 yr and an amplitude of 0.00035 d. Two mechanisms (i.e., the Applegate mechanism and the light-travel time effect) are applied to explain the cyclic modulation. We found that it is difficult to apply the Applegate mechanism to explain the cyclic oscillation in the orbital period. Therefore, the cyclic period change is most likely to be caused by the light-travel time effect due to the presence of a third body. The mass of the tertiary component was determined to be M-3 similar to 7.5M(Jup). We supposed that the tertiary companion is plausibly a giant planet. The stability of the giant planet was checked, and we found that the multiple system is stable.
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  • Variability of OI 090.4

    Hu, ShaoMing   Chen, Xu   Guo, DiFu  

    OI 090.4 was monitored on 21 nights from 2006 to 2012 for studying the variability. Strong variations occurred during the past 6 years. The long-term variability amplitude is consistent with previous results. Microvariability was analysed for 43 intra-night light curves. 30 out of 43 light curves showed microvariability by C and F test analysis.
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  • Gaia17biu/SN 2017egm in NGC 3191:The Closest Hydrogen-poor Superluminous Supernova to Date Is in a "Normal," Massive,Metal-rich Spiral Galaxy

    Bose, Subhash   Dong, Subo   Pastorello, A.   Filippenko, Alexei V.   Kochanek, C. S.   Mauerhan, Jon   Romero-Canizales, C.   Brink, Thomas G.   Chen, Ping   Prieto, J. L.   Post, R.   Ashall, Christopher   Grupe, Dirk   Tomasella, L.   Benetti, Stefano   Shappee, B. J.   Stanek, K. Z.   Cai, Zheng   Falco, E.   Lundqvist, Peter   Mattila, Seppo   Mutel, Robert   Ochner, Paolo   Pooley, David   Stritzinger, M. D.   Villanueva, S., Jr.   Zheng, WeiKang   Beswick, R. J.   Brown, Peter J.   Cappellaro, E.   Davis, Scott   Fraser, Morgan   de Jaeger, Thomas   Elias-Rosa, N.   Gall, C.   Gaudi, B. Scott   Herczeg, Gregory J.   Hestenes, Julia   Holoien, T. W. -S.   Hosseinzadeh, Griffin   Hsiao, E. Y.   Hu, Shaoming   Jaejin, Shin   Jeffers, Ben   Koff, R. A.   Kumar, Sahana   Kurtenkov, Alexander   Lau, Marie Wingyee   Prentice, Simon   Reynolds, T.   Rudy, Richard J.   Shahbandeh, Melissa   Somero, Auni   Stassun, Keivan G.   Thompson, Todd A.   Valenti, Stefano   Woo, Jong-Hak   Yunus, Sameen  

    Hydrogen-poor superluminous supernovae (SLSNe-I) have been predominantly found in low-metallicity, star-forming dwarf galaxies. Here we identify Gaia17biu/SN 2017egm as an SLSN-I occurring in a "normal" spiral galaxy (NGC 3191) in terms of stellar mass (several times 10(10) M-circle dot) and metallicity (roughly solar). At redshift z =3D 0.031, Gaia17biu is also the lowest-redshift SLSN-I to date, and the absence of a larger population of SLSNe-I in dwarf galaxies of similar redshift suggests that metallicity is likely less important to the production of SLSNe-I than previously believed. With the smallest distance and highest apparent brightness for an SLSN-I, we are able to study Gaia17biu in unprecedented detail. Its pre-peak near-ultraviolet to optical color is similar to that of Gaia16apd and among the bluest observed for an SLSN-I, while its peak luminosity (M-g =3D -21 mag) is substantially lower than that of Gaia16apd. Thanks to the high signal-to-noise ratios of our spectra, we identify several new spectroscopic features that may help to probe the properties of these enigmatic explosions. We detect polarization at the similar to 0.5% level that is not strongly dependent on wavelength, suggesting a modest, global departure from spherical symmetry. In addition, we put the tightest upper limit yet on the radio luminosity of an SLSN-I with < 5.4 x 10(26) erg s(-1) Hz(-1) at 10 GHz, which is almost a factor of 40 better than previous upper limits and one of the few measured at an early stage in the evolution of an SLSN-I. This limit largely rules out an association of this SLSN-I with known populations of gamma-ray-burst-like central engines.
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  • Radio-loud Active Galactic Nucleus Variability from Three-dimensional Propagating Relativistic Jets

    Li, Yutong   Wiita, Paul J.   Schuh, Terance   Elghossain, Geena   Hu, Shaoming  

    The enormous sizes and variability of emission of radio-loud active galactic nuclei arise from the relativistic flows of plasma along two oppositely directed jets. We use the Athena hydrodynamics code to simulate an extensive suite of 54 propagating three-dimensional relativistic jets with wide ranges of input jet velocities and jet-to-ambient matter density ratios. We determine which parameter sets yield unstable jets that produce jet-dominated Fanaroff-Riley I (FR I) radio galaxy morphologies and which tend to produce stable jets with hot spots and FR II morphologies. Nearly all our simulations involve jets with internal pressures matched to those of the ambient medium but we also consider over-pressured jets and discuss differences from the standard ones. We also show that the results are not strongly dependent on the adiabatic index of the fluid. We focus on simulations that remain stable for extended distances (60-240 times the initial jet radius). Scaled to the much smaller sizes probed by very long baseline interferometry observations, the fluctuations in such simulated flows yield variability in the observed emissivity on timescales from months. Adopting results for the densities, pressures, and velocities from these simulations, we estimate normalized rest frame synchrotron emissivities from individual cells in the jets. The observed emission from each cell is strongly dependent upon its variable Doppler boosting factor. We sum the fluxes from thousands of zones around the primary reconfinement shock. The light curves and power spectra, with red-noise slopes between -2.1 and -2.5, so produced are similar to those observed from blazars.
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  • Statistical analysis of variability properties of the Kepler blazar W2R 1926+42

    Li, Yutong   Hu, Shaoming   Wiita, Paul J.   Gupta, Alok C.  

    We analysed Kepler light curves of the blazar W2R 1926+42 that provided nearly continuous coverage from quarter 11 to quarter 17 (589 d between 2011 and 2013) and examined some of their flux variability properties. We investigate the possibility that the light curve is dominated by a large number of individual flares and adopt exponential rise and decay models to investigate the symmetry properties of flares. We found that those variations of W2R 1926+42 are predominantly asymmetric with weak tendencies towards positive asymmetry (rapid rise and slow decay). The durations (D) and the amplitudes (F-0) of flares can be fit with lognormal distributions. The energy (E) of each flare is also estimated for the first time. There are positive correlations between logD and logE with a slope of 1.36, and between logF(0) and logE with a slope of 1.12. Lomb-Scargle periodograms are used to estimate the power spectral density shape. It is well described by a power law with an index ranging between -1.1 and -1.5. The sizes of the emission regions, R, are estimated to be in the range of 1.1 x 10(15)-6.6 x 10(16) cm. The flare asymmetry is difficult to explain by a light travel time effect but may be caused by differences between the time-scales for acceleration and dissipation of high-energy particles in the relativistic jet. A jet-in-jet model could also produce the observed lognormal distributions.
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  • NLTE Line Formation for MgI and MgII in the Atmospheres of B-A-F-G-K Stars

    Alexeeva, Sofya   Ryabchikova, Tatiana   Mashonkina, Lyudmila   Hu, Shaoming  

    Non-local thermodynamical equilibrium (NLTE) line formation for Mg I and Mg II lines is considered in classical 1D LTE model atmospheres of the Sun and 17 stars with reliable atmospheric parameters and in a broad range of spectral types: 3900 K <=3D T-eff <=3D 17,500 K, 1.1 <=3D log g <=3D 4.7, and -2.6 <=3D [Fe/H] <=3D +0.4. We find that, for each star, NLTE leads to smaller line-to-line scatter. For 10 stars, NLTE leads to consistent abundances of Mg I and Mg II, while the difference in LTE abundance varies between -0.21 and +0.23. dex. We obtain an abundance discrepancy between Mg I and Mg II in two very metal-poor stars, HD 140283 and HD 84937. The origin of these abundance differences remains unclear. Our standard NLTE modeling predicts Mg I emission lines at 7.736, 11.789, 12.224, and 12.321 mu m in the atmospheres with T-eff <=3D 7000 K. We reproduce well the Mg I 12.2 and 12.3 mu m emission lines in Procyon. However, for the Sun and three K-giants, the predicted Mg I emission lines are too weak compared with the observations. For stars with 7000 K <=3D T-eff <=3D 17,500 K, we recommend the Mg II 3848, 3850, 4384, 4390, 4427, and 4433. A lines for Mg abundance determinations even at the LTE assumption due to their small NLTE effects. The Mg I 4167, 4571, 4702, 5528, 5167, 5172, and 5183 angstrom lines can be safely used in the LTE analysis of stars with 7000 K <=3D T-eff <=3D 8000 K. For the hotter stars, with T-eff from 8000 K to 9500 K, the NLTE effects are minor only for Mg I 4167, 4702, and 4528 angstrom.
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