arXiv subjects

P. M. Garnavich

Publications and source records attributed to P. M. Garnavich.

81 records · Page 5Linked to original sources

arXiv2000

The Optical Spectroscopic Evolution of V1974 Cygni (Nova Cygni 1992)

Optical observations of nova V1974 Cygni (Cygni 1992), spanning a 4 year period, have been used to study its spectroscopic evolution. The data cover a wavelength range from $\sim 3200 - 8000$ Åand follows the nebular evolutionary phase of the ejecta. We have modeled the integrated fluxes by means of the photoionization code CLOUDY. The models were run at a {\it fixed} abundance value for the most prominent elements (i.e. H, He, C, O, N, Ne, Fe, etc) over the entire time sequence. It is possible to constrain from this simple model some of the physical conditions of the gaseous emitting region, like temperature and density. Compared with previous studies of the gas abundances of the heavy elements, we found that smaller enhancements of S, N and Ar, and comparable values for O and Fe, are able to reproduce the observations. The time evolution of the surface temperature of the ionizing source and the high-ionization iron lines [Fe VII] 6087 Åand [Fe X] 6374 Å, is similar to what it is observed in the soft X-rays. The early line profiles can be reproduced using a simple kinematical model consisting of an equatorial ring and polar caps, expanding at a velocity of $\sim 1100$ km/s. This simple model also approximates the structure of the resolved shell observed by HST. Considering the complicated structure of the shell, the lack of well defined values of its gas density and our limited knowledge of the time evolution of the surface temperature of the photoionization source, the comparison between models and observations agrees remarkably well.

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arXiv2000

Resolving Gamma-Ray Burst 000301C with a Gravitational Microlens

The afterglow of the Gamma-Ray Burst (GRB) 000301C exhibited achromatic, short time-scale variability that is difficult to reconcile with the standard relativistic shock model. We interpret the observed light curves as a microlensing event superimposed on power-law flux decays typical of afterglows. In general, a relativistic GRB shock appears on the sky as a thin ring expanding at a superluminal speed. Initially the ring is small relative to its angular separation from the lens and so its flux is magnified by a constant factor. As the ring grows and sweeps across the lens its magnification reaches a maximum. Subsequently, the flux gradually recovers its unlensed value. This behavior involves only three free parameters in its simplest formulation and was predicted theoretically by Loeb & Perna (1998). Fitting the available R-band photometric data of GRB 000301C to a simple model of the microlensing event and a broken power-law for the afterglow, we find reasonable values for all the parameters and a reduced chi^2/DOF parameter of 1.48 compared with 2.99 for the broken power-law fit alone. The peak magnification of ~2 occurred 3.8 days after the burst. The entire optical-IR data imply a width of the GRB ring of order 10% of its radius, similar to theoretical expectations. The angular resolution provided by microlensing is better than a micro-arcsecond. We infer a mass of approximately 0.5 M_Sun for a lens located half way to the source at z_s=2.04. A galaxy 2'' from GRB 000301C might be the host of the stellar lens, but current data provides only an upper-limit on its surface brightness at the GRB position.

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arXiv2000

RJK Observations of the Optical Afterglow of GRB 991216

We present near-infrared and optical observations of the afterglow to the Gamma-Ray Burst (GRB) 991216 obtained with the F. L. Whipple Observatory 1.2-m telescope and the University of Hawaii 2.2-m telescope. The observations range from 15 hours to 3.8 days after the burst. The temporal behavior of the data is well described by a single power-law decay with index -1.36 +/-0.04, independent of wavelength. The optical spectral energy distribution, corrected for significant Galactic reddening of E(B-V)=0.626, is well fitted by a single power-law with index -0.58 +/- 0.08. Combining the IR/optical observations with a Chandra X-ray measurement gives a spectral index of -0.8 +/- 0.1 in the synchrotron cooling regime. A comparison between the spectral and temporal power-law indices suggest that a jet is a better match to the observations than a simple spherical shock.

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arXiv2000

Optical and Near Infrared Study of the Cepheus E outflow, a very low excitation object

We present images and spectra of the Cepheus E (Cep E) region at both optical and infrared wavelengths. Only the brightest region of the southern lobe of the Cep E outflow reveals optical emission, suggesting that the extinction close to the outflow source plays an important rôle in the observed difference between the optical and IR morphologies. Cep E is a unique object since it provides a link between the spectroscopic properties of the optical Herbig-Haro (HH) objects and those of deeply embedded outflows.

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arXiv1999

BVRI Observations of the Optical Afterglow of GRB 990510

We present $BVRI$ observations of the optical counterpart to the Gamma-Ray Burst (GRB) 990510 obtained with the Las Campanas 1.0-m telescope between 15 and 48 hours after the burst. The temporal analysis of the data indicates steepening decay, independent of wavelength, approaching asymptotically $t^{-0.76\pm 0.01}$ at early times ($t\ll 1 day)$ and $t^{-2.40\pm 0.02}$ at late times, with the break time at $t_0=1.57\pm 0.03 days$. GRB 990510 is the most rapidly fading of the well-documented GRB afterglows. It is also the first observed example of broad-band break for a GRB optical counterpart. The optical spectral energy distribution, corrected for significant Galactic reddening, is well fitted by a single power-law with $ν^{-0.61\pm 0.12}$. However, when the $B$-band point is dropped from the fit, the power-law becomes $ν^{-0.46\pm 0.08}$, indicating a possible deviation from the power-law in the spectrum, either intrinsic or due to additional extinction near the source or from an intervening galaxy at $z=1.62$. Broad-band break behavior broadly similar to that observed in GRB 990510 has been predicted in some jet models of GRB afterglows, thus supporting the idea that the GRB energy is beamed, at least in some cases.

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arXiv1998

Supernova Limits on the Cosmic Equation of State

We use Type Ia supernovae studied by the High-Z Supernova Search Team to constrain the properties of an energy component which may have contributed to accelerating the cosmic expansion. We find that for a flat geometry the equation of state parameter for the unknown component, alpha_x=P_x/rho_x, must be less than -0.55 (95% confidence) for any value of Omega_m and is further limited to alpha_x<-0.60 (95%) if Omega_m is assumed to be greater than 0.1 . These values are inconsistent with the unknown component being topological defects such as domain walls, strings, or textures. The supernova data are consistent with a cosmological constant (alpha_x=-1) or a scalar field which has had, on average, an equation of state parameter similar to the cosmological constant value of -1 over the redshift range of z=1 to the present. Supernova and cosmic microwave background observations give complementary constraints on the densities of matter and the unknown component. If only matter and vacuum energy are considered, then the current combined data sets provide direct evidence for a spatially flat Universe with Omega_tot=Omega_m+Omega_Lambda = 0.94 +/- 0.26 (1-sigma).

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arXiv1998

Distance to M31 With the HST and Hipparcos Red Clump Stars

Following an approach by Paczyński & Stanek we compare red clump stars with parallaxes known to better than 10% in the Hipparcos catalog with the red clump stars observed in three fields in M31 using the HST. There are $\sim 600$ and $\sim 6,300$ such stars in the two data sets, respectively. The local red clump luminosity function is well represented by a Gaussian with the peak at $M_{I,m}=-0.23$, and the dispersion $σ_{RC}\approx0.2 $mag. This allows a single step determination of the distance modulus to M31 $μ_{0,M31} = 24.471\pm 0.035 \pm 0.045 $mag (statistical plus systematic error) and the corresponding distance $R_{M31}= 784\pm 13\pm 17 kpc$. The number of red clump stars is large enough that the formal statistical error in the distance is only $\lesssim 2$%. We also correct the treatment of the local interstellar extinction by Paczyński & Stanek and we obtain the Galactocentric distance modulus $μ_{0,GC}=14.57 \pm0.04 \pm 0.04 $mag (statistical plus systematic error), and the corresponding Galactocentric distance $R_0=8.2 \pm0.15 \pm 0.15 kpc$.

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arXiv1997

Constraints on Cosmological Models from Hubble Space Telescope Observations of High-z Supernovae

We have coordinated Hubble Space Telescope photometry with ground-based discovery for three supernovae: two SN Ia near z~0.5 (SN 1997ce, SN 1997cj) and a third event at z=0.97 (SN 1997ck). The superb spatial resolution of HST separates each supernova from its host galaxy and leads to good precision in the light curves. The HST data combined with ground-based photometry provide good temporal coverage. We use these light curves and relations between luminosity, light curve shape, and color calibrated from low-z samples to derive relative luminosity distances which are accurate to 10% at z~0.5 and 20% at z=1. The redshift-distance relation is used to place constraints on the global mean matter density, Omega_matter, and the normalized cosmological constant, Omega_Lambda. When the HST sample is combined with the distance to SN 1995K (z=0.48), analyzed by the same precepts, it suggests that matter alone is insufficient to produce a flat Universe. Specifically, for Omega_matter+Omega_Lambda=1, Omega_matter is less than 1 with >95% confidence, and our best estimate of Omega_matter is -0.1 +/- 0.5 if Omega_Lambda=0. Although the present result is based on a very small sample whose systematics remain to be explored, it demonstrates the power of HST measurements for high redshift supernovae.

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arXiv1997

Two Confirmed Cataclysmic Variables in the Old Stellar Cluster NGC 6791

Based on new CCD photometry and spectroscopy we confirm the presence of two cataclysmic variables (CVs) in the very old open cluster NGC 6791. One of these variables, known as B8, was observed by Rucinski, Kaluzny & Hilditch (1996) to undergo a large magnitude outburst in 1995. The spectrum of this star outside the outburst, obtained by us with MMT, shows clearly the emission lines characteristic for dwarf novae. We observed the second star, known as B7, to undergo large ($\sim3 mag$) drop in its brightness over $\sim 10 days$. The spectrum of B7 obtained in the high state resembles spectra of nova-like CVs. This star a likely member of UX~UMa subtype of CVs. Variables B7 and B8 represent only the second and the third cataclysmic variables known in open clusters. Variable B7 has observational characteristics which would make it difficult to identify as a CV with some of the methods being currently used in surveys for CVs in globular clusters.

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