arXiv subjects

G. Gyuk

Publications and source records attributed to G. Gyuk.

86 records · Page 5Linked to original sources

arXiv2008

VERITAS Discovery of >200GeV Gamma-ray Emission from the Intermediate-frequency-peaked BL Lac Object W Comae

We report the detection of very high-energy gamma-ray emission from the intermediate-frequency-peaked BL Lacertae object W Comae (z=0.102) by VERITAS. The source was observed between January and April 2008. A strong outburst of gamma-ray emission was measured in the middle of March, lasting for only four days. The energy spectrum measured during the two highest flare nights is fit by a power-law and is found to be very steep, with a differential photon spectral index of Gamma = 3.81 +- 0.35_stat +- 0.34_syst. The integral photon flux above 200GeV during those two nights corresponds to roughly 9% of the flux from the Crab Nebula. Quasi-simultaneous Swift observations at X-ray energies were triggered by the VERITAS observations. The spectral energy distribution of the flare data can be described by synchrotron-self-Compton (SSC) or external-Compton (EC) leptonic jet models, with the latter offering a more natural set of parameters to fit the data.

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arXiv2008

Additional Ultracool White Dwarfs Found in the Sloan Digital Sky Survey

We identify seven new ultracool white dwarfs discovered in the Sloan Digital Sky Survey (SDSS). The SDSS photometry, spectra, and proper motions are presented, and additional BVRI data are given for these and other previously discovered ultracool white dwarfs. The observed colors span a remarkably wide range, qualitatively similar to colors predicted by models for very cool white dwarfs. One of the new stars (SDSS J1251+44) exhibits strong collision-induced absorption (CIA) in its spectra, while the spectra and colors of the other six are consistent with mild CIA. Another of the new discoveries (SDSS J2239+00A) is part of a binary system -- its companion is also a cool white dwarf, and other data indicate that the companion exhibits an infrared flux deficiency, making this the first binary system composed of two CIA white dwarfs. A third discovery (SDSS J0310-00) has weak Balmer emission lines. The proper motions of all seven stars are consistent with membership in the disk or thick disk.

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arXiv2008

Observation of gamma-ray emission from the galaxy M87 above 250 GeV with VERITAS

The multiwavelength observation of the nearby radio galaxy M87 provides a unique opportunity to study in detail processes occurring in Active Galactic Nuclei from radio waves to TeV gamma-rays. Here we report the detection of gamma-ray emission above 250 GeV from M87 in spring 2007 with the VERITAS atmospheric Cherenkov telescope array and discuss its correlation with the X-ray emission. The gamma-ray emission is measured to be point-like with an intrinsic source radius less than 4.5 arcmin. The differential energy spectrum is fitted well by a power-law function: dPhi/dE=(7.4+-1.3_{stat}+-1.5_{sys})(E/TeV)^{-2.31+-0.17_{stat}+-0.2_{sys}} 10^{-9}m^{-2}s^{-1}TeV^{-1}. We show strong evidence for a year-scale correlation between the gamma-ray flux reported by TeV experiments and the X-ray emission measured by the ASM/RXTE observatory, and discuss the possible short-time-scale variability. These results imply that the gamma-ray emission from M87 is more likely associated with the core of the galaxy than with other bright X-ray features in the jet.

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arXiv2006

GRB 060121: Implications of a Short/Intermediate Duration Gamma-Ray Burst at High Redshift

Since the discovery of the first short-hard gamma-ray burst afterglows in 2005, the handful of observed events have been found to be embedded in nearby (z < 1), bright underlying galaxies. We present multiwavelength observations of the short-duration burst GRB 060121, which is the first observed to clearly outshine its host galaxy (by a factor >10^2). A photometric redshift for this event places the progenitor at a most probable redshift of z = 4.6, with a less probable scenario of z = 1.7. In either case, GRB 060121 could be the farthermost short-duration GRB detected to date and implies an isotropic-equivalent energy release in gamma-rays comparable to that seen in long-duration bursts. We discuss the implications of the released energy on the nature of the progenitor. These results suggest that GRB 060121 may belong to a family of energetic short-duration events, lying at z > 1 and whose optical afterglows would outshine their host galaxies, unlike the first short-duration GRBs observed in 2005. The possibility of GRB 060121 being an intermediate duration burst is also discussed.

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arXiv2006

The First VERITAS Telescope

The first atmospheric Cherenkov telescope of VERITAS (the Very Energetic Radiation Imaging Telescope Array System) has been in operation since February 2005. We present here a technical description of the instrument and a summary of its performance. The calibration methods are described, along with the results of Monte Carlo simulations of the telescope and comparisons between real and simulated data. The analysis of TeV $γ$-ray observations of the Crab Nebula, including the reconstructed energy spectrum, is shown to give results consistent with earlier measurements. The telescope is operating as expected and has met or exceeded all design specifications.

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arXiv2000

Microlensing in M31 - The MEGA Survey's Prospects and Initial Results

January 2000 completes the first season of intensive, wide-field observations of microlensing and stellar variability in M31 by MEGA (Microlensing Exploration of the Galaxy and Andromeda) at the Isaac Newton 2.5m Telescope, the KPNO 4m, and the 1.3m and 2.5m telescopes of MDM Observatory. In preliminary analysis, we detect ~50000 variable objects, including some consistent with microlensing events. We present the level of sensitivity to be reached in our planned three-year program to test for the presence of a significant halo microlensing population in M31, as well as its spatial distribution and mass-function. We also discuss our application of image subtraction to these wide fields and HST WFPC2 Snapshot followup observations to confirm candidates identified from previous years' surveys. We present intermediate results from our smaller-field survey, on the MDM 1.3m and Vatican Advanced Technology 1.8m Telescope, from 1994-1998, wherein we have discovered 8 additional probable microlensing events, over about one-half the time base of the project, in addition to confirming three of our original 6 microlensing candidates from 1995.

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arXiv1999

MEGA, a Wide-Field Survey of Microlensing in M31

We detail how microlensing internal to M31 could be used to test whether a large fraction of the matter in spiral galaxy haloes is composed of dark objects with masses comparable to those of stars, and in the process show how the microlensing matter is distributed in space and as a function of mass. A survey of small fields in M31 has revealed several such candidate microlensing events, at roughly the predicted rate. We discuss what efforts have been required to further verify the microlensing nature of these events. We show how halo lensing optical depth varies over the face of M31 depending on the shape of the microlensing halo, and show how this shape could be determined in representative results from a simulated survey in which roughly one-half square degree is imaged for six hours every three nights on a two-meter telescope, in 1-arcsec seeing. After two or three seasons of such observation, the flattening and core radius of the microlensing halo can be measured to an interesting level of accuracy.

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arXiv1999

Self-Lensing Models of the LMC

All of the proposed explanations for the microlensing events observed towards the LMC have difficulties. One of these proposed explanations, LMC self-lensing, which invokes ordinary LMC stars as the long sought-after lenses, has recently gained considerable popularity as a possible solution to the microlensing conundrum. In this paper, we carefully examine the set of LMC self-lensing models. In particular, we review the pertinent observations made of the LMC, and show how these observations place limits on such self-lensing models. We find that, given current observational constraints, no purely LMC disk models are capable of producing optical depths as large as that reported in the MACHO collaboration 2-year analysis. Besides pure disk, we also consider alternate geometries, and present a framework which encompasses the previous studies of LMC self-lensing. We discuss which model parameters need to be pushed in order for such models to succeed. For example, like previous workers, we find that an LMC halo geometry may be able to explain the observed events. However, since all known LMC tracer stellar populations exhibit disk-like kinematics, such models will have difficulty being reconciled with observations. For SMC self-lensing, we find predicted optical depths differing from previous results, but more than sufficient to explain all observed SMC microlensing. In contrast, for the LMC we find a self-lensing optical depth contribution between 0.47e-8 and 7.84e-8, with 2.44e-8 being the value for the set of LMC parameters most consistent with current observations.

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arXiv1998

The LMC Microlensing Events: Evidence for a Warped and Flaring Milky Way Disk?

The simplest interpretation of the microlensing events towards the Large Magellanic Cloud detected by the MACHO and EROS collaborations is that about one third of the halo of our own Milky Way galaxy exists in the form of objects of around 0.5 solar mass. There are grave problems with this interpretation. A normal stellar population of 0.5 solar mass stars should be visible. The other obvious candidate for the lenses is a population of white dwarfs. But, the precursor population must have polluted the interstellar medium with metals, in conflict with current population II abundance. Here, we propose a more conventional, but at the moment more speculative, explanation. Some of the lenses are stars in the disk of the Milky Way. They lie along the line of sight to the LMC because of warping and flaring of the Galactic disk. Depending on its scalelength and ellipticity, the disk's optical depth may lie anywhere between $0.2 \times 10^{-7}$ and $0.9 \times 10^{-7}$. Together with contributions from the LMC disk and bar and perhaps even intervening stellar contaminants, the total optical depth may match the data within the uncertainties. Microlensing towards the LMC may be telling us more about the distorted structure and stellar populations of the outer Milky Way disk than the composition of the dark halo.

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arXiv1998

Brown Dwarfs, White Knights and Demons

This paper investigates the hypothesis that the lensing objects towards the Large Magellanic Cloud (LMC) are brown dwarfs by analysing the effects of velocity anisotropy on the inferred microlensing masses. To reduce the masses, the transverse velocity of the lenses with respect to the microlensing tube must be minimised. In the outer halo, radial anisotropy is best for doing this; closer to the solar circle, azimuthal anisotropy is best. By using a constraint on the total kinetic energy of the tracer population from the Jeans equations, the microlensing mass is minimised over orientations of the velocity dispersion tensor. This minimum mass is about a tenth of a solar mass, which lies above the hydrogen burning limit. This demonstrates explicitly that populations of brown dwarfs with smoothly decreasing densities and dynamically mixed velocity distributions cannot be responsible for the microlensing events. Brown dwarfs are no white knights! There is one caveat. If there are demons sitting on the microlensing tube, they can drop brown dwarfs so as to reproduce the microlensing data-set exactly. Such a distribution is not smooth and does not give well-mixed velocities in phase space. It is a permissible solution only if the outer halo is dynamically young and lumpy. In such a case, theorists cannot rule out brown dwarfs. Only exorcists can!

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arXiv1997

Rotating Halos and the Microlensing MACHO Mass Estimate

We investigate the implications of a bulk rotational component of the halo velocity distribution for MACHO mass estimates. We find that for a rotating halo to yield a MACHO mass estimate significantly below that of the standard spherical case, its microlensing must be highly concentrated close to the Sun. We examine two classes of models fitting this criteria: a highly flattened $1/r^2$ halo, and a spheroid-like population with whose density falls off as $1/r^{3.5}$. The highly flattened $1/r^2$ models can decrease the implied average MACHO mass only marginally and the spheroid models not at all. Generally, rotational models cannot bring the MACHO mass implied by the current microlensing data down to the substellar range.

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arXiv1995

The Local Halo Density

For almost twenty years models of the Galaxy have included a dark halo responsible for supporting a substantial fraction of the local rotation velocity and a flat rotation curve at large distances. Estimates of the local halo density range from $2\times 10^{-25}\gcmm3$ to $10\times 10^{-25}\gcmm3$. By careful modeling of the Galaxy, taking account of the evidence that dark halos are flattened and recent microlensing data, we arrive at a more quantitative estimate, $9.2^{+3.8}_{-3.1}\times 10^{-25}\gcmm3$. Microlensing toward the LMC indicates that only a small fraction, less than about 30\%, can be in the form of MACHOs, consistent with the idea that most of the halo consists of cold dark matter particles.

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arXiv1994

Relaxing the Big-bang Bound to the Baryon Density

In the standard picture of big-bang nucleosynthesis the yields of D, $^3$He, $^4$He, and $^7$Li only agree with their inferred primordial abundances if the fraction of critical density contributed by baryons is between $0.01h^{-2}$ and $0.02h^{-2}$ ($h$ is the present value of the Hubble constant in units of $100\kms\Mpc^{-1}$). This is the basis of the very convincing and important argument that baryons can contribute at most 10\% of critical density and thus cannot close the Universe. Nonstandard scenarios involving decaying particles,$^1$ inhomogeneities in the baryon density,$^2$ and even more exotic ideas$^3$ put forth to evade this bound have been largely unsuccessful.$^4$ We suggest a new way of relaxing the bound: If the tau neutrino has a mass of $20\MeV-30\MeV$ and lifetime of $200\sec -1000\sec$, and its decay products include electron neutrinos, the bound to the baryon mass density can be loosened by a about factor of $10$. The key is the decay-generated electron antineutrinos: around the time of nucleosynthesis they are captured by protons to produce neutrons, thereby changing the outcome of nucleosynthesis. Experiments at $e^\pm$ colliders should soon be sensitive to a tau-neutrino mass in the required range.

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arXiv1993

Primordial Nucleosynthesis with a Decaying Tau Neutrino

A comprehensive study of the effect of an unstable tau neutrino on primordial nucleosynthesis is presented. The standard code for nucleosynthesis is modified to allow for a massive decaying tau neutrino whose daughter products include neutrinos, photons, $e^\pm$ pairs, and/or noninteracting (sterile) daughter products. Tau-neutrino decays influence primordial nucleosynthesis in three distinct ways: (i) the energy density of the decaying tau neutrino and its daughter products affect the expansion rate tending to increase $^4$He, D, and $^3$He production; (ii) electromagnetic (EM) decay products heat the EM plasma and dilute the baryon-to-photon ratio tending to decrease $^4$He production and increase D and $^3$He production; and (iii) electron neutrinos and antineutrinos produced by tau-neutrino decays increase the weak rates that govern the neutron-to-proton ratio, leading to decreased $^4$He production for short lifetimes ($\la 30\sec$) and masses less than about $10\MeV$ and increased $^4$He production for long lifetimes or large masses. The precise effect of a decaying tau neutrino on the yields of primordial nucleosynthesis and the mass-lifetime limits that follow depend crucially upon decay mode. We identify four generic decay modes that serve to bracket the wider range of possibilities:

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