3,300 research outputs found
Blending of Cepheids in M33
A precise and accurate determination of the Hubble constant based on Cepheid
variables requires proper characterization of many sources of systematic error.
One of these is stellar blending, which biases the measured fluxes of Cepheids
and the resulting distance estimates. We study the blending of 149 Cepheid
variables in M33 by matching archival Hubble Space Telescope data with images
obtained at the WIYN 3.5-m telescope, which differ by a factor of 10 in angular
resolution.
We find that 55+-4% of the Cepheids have no detectable nearby companions that
could bias the WIYN V-band photometry, while the fraction of Cepheids affected
below the 10% level is 73+-4%. The corresponding values for the I band are
60+-4% and 72+-4%, respectively. We find no statistically significant
difference in blending statistics as a function of period or surface
brightness. Additionally, we report all the detected companions within 2
arcseconds of the Cepheids (equivalent to 9 pc at the distance of M33) which
may be used to derive empirical blending corrections for Cepheids at larger
distances.Comment: v2: Fixed incorrect description of Figure 2 in text. Accepted for
publication in AJ. Full data tables can be found in ASCII format as part of
the source distribution. A version of the paper with higher-resolution
figures can be found at
http://faculty.physics.tamu.edu/lmacri/papers/chavez12.pd
Cepheid Variables in the Maser-Host Galaxy NGC 4258
We present results of a ground-based survey for Cepheid variables in NGC
4258. This galaxy plays a key role in the Extragalactic Distance Scale due to
its very precise and accurate distance determination via VLBI observations of
water masers. We imaged two fields within this galaxy using the Gemini North
telescope and GMOS, obtaining 16 epochs of data in the SDSS gri bands over 4
years. We carried out PSF photometry and detected 94 Cepheids with periods
between 7 and 127 days, as well as an additional 215 variables which may be
Cepheids or Population II pulsators. We used the Cepheid sample to test the
absolute calibration of theoretical gri Period-Luminosity relations and found
good agreement with the maser distance to this galaxy. The expected data
products from the Large Synoptic Survey Telescope (LSST) should enable Cepheid
searches out to at least 10 Mpc.Comment: Accepted for publication in the Astronomical Journa
Recent laboratory tests of a hard x-ray solar flare polarimeter
We report on the development of a Compton scatter polarimeter for measuring the linear polarization of hard X-rays (50 - 300 keV) from solar flares. Such measurements would be useful for studying the directivity (or beaming) of the electrons that are accelerated in solar flares. We initially used a simple prototype polarimeter to successfully demonstrate the reliability of our Monte Carlo simulation code and to demonstrate our ability to generate a polarized photon source in the lab. We have recently fabricated a science model based on a modular design concept that places a self-contained polarimeter module on the front-end of a 5-inch position- sensitive PMT (PSPMT). The PSPMT is used to determine the Compton interaction location within an annular array of small plastic scintillator elements. Some of the photons that scatter within the plastic scintillator array are subsequently absorbed by a small centrally-located array of CsI(Tl) crystals that is read out by an independent multi-anode PMT. The independence of the two PMT readout schemes provides appropriate timing information for event triggering. We are currently testing this new polarimeter design in the laboratory to evaluate the performance characteristics of this design. Here we present the initial results from these laboratory tests. The modular nature of this design lends itself toward its accommodation on a balloon or spacecraft platform. A small array of such modules can provide a minimum detectable polarization (MDP) of less than 1% in the integrated 50 - 300 keV energy range for X-class solar flares
On the Variation of Fourier Parameters for Galactic and LMC Cepheids at Optical, Near-Infrared and Mid-Infrared Wavelengths
We present a light curve analysis of fundamental-mode Galactic and Large
Magellanic Cloud (LMC) Cepheids based on the Fourier decomposition technique.
We have compiled light curve data for Galactic and LMC Cepheids in optical
({\it VI}), near-infrared ({\it JHK}) and mid-infrared (3.6
4.5-m) bands from the literature and determined the variation of their
Fourier parameters as a function of period and wavelength. We observed a
decrease in Fourier amplitude parameters and an increase in Fourier phase
parameters with increasing wavelengths at a given period. We also found a
decrease in the skewness and acuteness parameters as a function of wavelength
at a fixed period. We applied a binning method to analyze the progression of
the mean Fourier parameters with period and wavelength. We found that for
periods longer than about 20 days, the values of the Fourier amplitude
parameters increase sharply for shorter wavelengths as compared to wavelengths
longer than the -band. We observed the variation of the Hertzsprung
progression with wavelength. The central period of the Hertzsprung progression
was found to increase with wavelength in the case of the Fourier amplitude
parameters and decrease with increasing wavelength in the case of phase
parameters. We also observed a small variation of the central period of the
progression between the Galaxy and LMC, presumably related to metallicity
effects. These results will provide useful constraints for stellar pulsation
codes that incorporate stellar atmosphere models to produce Cepheid light
curves in various bands.Comment: 22 pages, 19 figures, Accepted for publication in Monthly Notices of
the Royal Astronomical Society Main Journa
The Development of GRAPE, a Gamma Ray Polarimeter Experiment
The measurement of hard X‐ray polarization in γ‐ray bursts (GRBs) would add yet another piece of information in our effort to resolve the true nature of these enigmatic objects. Here we report on the development of a dedicated polarimeter design with a relatively large FoV that is capable of studying hard X‐ray polarization (50–300 keV) from GRBs. This compact design, based on the use of a large area position‐sensitive PMT (PSPMT), is referred to as GRAPE (Gamma‐RAy Polarimeter Experiment). The feature of GRAPE that is especially attractive for studies of GRBs is the significant off‐axis polarization response (at angles greater than 60°). For an array of GRAPE modules, current sensitivity estimates give minimum detectable polarization (MDP) levels of a few percent for the brightest GRBs
Hard x-ray polarimeter for gamma-ray bursts and solar flares
We report on the development of a dedicated polarimeter design that is capable of studying the linear polarization of hard X-rays (50-300 keV) from gamma-ray bursts and solar flares. This compact design, based on the use of a large area position-sensitive PMT (PSPMT), is referred to as GRAPE (Gamma-RAy Polarimeter Experiment). The PSPMT is used to determine the Compton interaction location within an array of small plastic scintillator elements. Some of the photons that scatter within the plastic scintillator array are subsequently absorbed by a small centrally-located array of CsI(Tl) crystals that is read out by an independent multi-anode PMT. One feature of GRAPE that is especially attractive for studies of gamma-ray bursts is the significant off-axis response (at angles \u3e 60 degrees). The modular nature of this design lends itself toward its accomodation on a balloon or spacecraft platform. For an array of GRAPE modules, sensitivity levels below a few percent can be achieved for both gamma-ray bursts and solar flares. Here we report on the latest results from the testing of a laboratory science model
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