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Kashyap V.L.,Smithsonian Astrophysical Observatory | Van Dyk D.A.,University of California at Irvine | Connors A.,Eureka Scientific | Freeman P.E.,Carnegie Mellon University | And 3 more authors.
Astrophysical Journal | Year: 2010

A common problem in astrophysics is determining how bright a source could be and still not be detected in an observation. Despite the simplicity with which the problem can be stated, the solution involves complicated statistical issues that require careful analysis. In contrast to the more familiar confidence bound, this concept has never been formally analyzed, leading to a great variety of often ad hoc solutions. Here we formulate and describe the problem in a self-consistent manner. Detection significance is usually defined by the acceptable proportion of false positives (background fluctuations that are claimed as detections, or Type I error), and we invoke the complementary concept of false negatives (real sources that go undetected, or Type II error), based on the statistical power of a test, to compute an upper limit to the detectable source intensity. To determine the minimum intensity that a source must have for it to be detected, we first define a detection threshold and then compute the probabilities of detecting sources of various intensities at the given threshold. The intensity that corresponds to the specified Type II error probability defines that minimum intensity and is identified as the upper limit. Thus, an upper limit is a characteristic of the detection procedure rather than the strength of any particular source. It should not be confused with confidence intervals or other estimates of source intensity. This is particularly important given the large number of catalogs that are being generated from increasingly sensitive surveys. We discuss, with examples, the differences between these upper limits and confidence bounds. Both measures are useful quantities that should be reported in order to extract the most science from catalogs, though they answer different statistical questions: an upper bound describes an inference range on the source intensity, while an upper limit calibrates the detection process. We provide a recipe for computing upper limits that applies to all detection algorithms. © 2010. The American Astronomical Society. All rights reserved.

Miles B.E.,University of California at Los Angeles | Roberge A.,NASA | Welsh B.,Eureka Scientific
Astrophysical Journal | Year: 2016

We present the analysis of time-variable Doppler-shifted absorption features in far-UV spectra of the unusual 49 Ceti debris disk. This nearly edge-on disk is one of the brightest known and is one of the very few containing detectable amounts of circumstellar (CS) gas as well as dust. In our two visits of Hubble Space Telescope STIS spectra, variable absorption features are seen on the wings of lines arising from Cii and Civ but not for any of the other CS absorption lines. Similar variable features have long been seen in spectra of the well-studied Pictoris debris disk and attributed to the transits of star-grazing comets. We calculated the velocity ranges and apparent column densities of the 49 Cet variable gas, which appears to have been moving at velocities of tens to hundreds of km s-1 relative to the central star. The velocities in the redshifted variable event seen in the second visit show that the maximum distances of the infalling gas at the time of transit were about 0.05-0.2 au from the central star. A preliminary attempt at a composition analysis of the redshifted event suggests that the C/O ratio in the infalling gas is super-solar, as it is in the bulk of the stable disk gas. © 2016. The American Astronomical Society. All rights reserved.

Blackburne J.A.,Ohio State University | Blackburne J.A.,Massachusetts Institute of Technology | Pooley D.,Eureka Scientific | Rappaport S.,Massachusetts Institute of Technology | Schechter P.L.,Massachusetts Institute of Technology
Astrophysical Journal | Year: 2011

Microlensing perturbations to the flux ratios of gravitationally lensed quasar images can vary with wavelength because of the chromatic dependence of the accretion disk's apparent size. Multiwavelength observations of microlensed quasars can thus constrain the temperature profiles of their accretion disks, a fundamental test of an important astrophysical process which is not currently possible using any other method. We present single-epoch broadband flux ratios for 12 quadruply lensed quasars in 8 bands ranging from 0.36 to 2.2 μm, as well as Chandra 0.5-8keV flux ratios for five of them. We combine the optical/IR and X-ray ratios, together with X-ray ratios from the literature, using a Bayesian approach to constrain the half-light radii of the quasars in each filter. Comparing the overall disk sizes and wavelength slopes to those predicted by the standard thin accretion disk model, we find that on average the disks are larger than predicted by nearly an order of magnitude, with sizes that grow with wavelength with an average slope of ∼0.2 rather than the slope of 4/3 predicted by the standard thin disk theory. Though the error bars on the slope are large for individual quasars, the large sample size lends weight to the overall result. Our results present severe difficulties for a standard thin accretion disk as the main source of UV/optical radiation from quasars. © 2011. The American Astronomical Society. All rights reserved.

Liao J.-Y.,CAS Institute of High Energy Physics | Zhang S.-N.,CAS Institute of High Energy Physics | Zhang S.-N.,CAS National Astronomical Observatories | Yao Y.,Eureka Scientific | Yao Y.,University of Colorado at Boulder
Astrophysical Journal | Year: 2013

Accurate atomic transition data are important in many astronomical research areas, especially for studies of line spectroscopy. Whereas transition data of He-like and H-like ions (i.e., ions in high-charge states) have been accurately calculated, the corresponding data of K transitions of neutral or low-ionized metal elements are still very uncertain. Spectroscopy of absorption lines produced in the interstellar medium (ISM) has been proven to be an effective way to measure the central wavelengths of these atomic transitions. In this work, we analyze 36 Chandra High Energy Transmission Grating observations to search for and measure the ISM absorption lines along sight lines to 11 low-mass X-ray binaries. We correct the Galactic rotation velocity to the rest frame for every observation and then use two different methods to merge all the corrected spectra to a co-added spectrum. However, the co-added spectra obtained by this method exhibit biases, toward to either observations with high counts or lines with high signal-to-noise ratios. We do a Bayesian analysis of several significantly detected lines to obtain the systematic uncertainty and the bias correction for other lines. Compared to previous studies, our results improve the wavelength accuracy by a factor of two to five and significantly reduce the systematic uncertainties and biases. Several weak transitions (e.g., 1s-2p of Mg IV and Mg V; 1s-3p of Mg III and Mg V) are also detected for the first time, albeit with low significance; future observations with improved accuracy are required to confirm these detections. © 2013. The American Astronomical Society. All rights reserved.

Stark C.C.,NASA | Schneider G.,University of Arizona | Weinberger A.J.,Carnegie Institution of Washington | Debes J.H.,US Space Telescope Science Institute | And 3 more authors.
Astrophysical Journal | Year: 2014

New multi-roll coronagraphic images of the HD 181327 debris disk obtained using the Space Telescope Imaging Spectrograph on board the Hubble Space Telescope reveal the debris ring in its entirety at high signal-to-noise ratio and unprecedented spatial resolution. We present and apply a new multi-roll image processing routine to identify and further remove quasi-static point-spread function-subtraction residuals and quantify systematic uncertainties. We also use a new iterative image deprojection technique to constrain the true disk geometry and aggressively remove any surface brightness asymmetries that can be explained without invoking dust density enhancements/deficits. The measured empirical scattering phase function for the disk is more forward scattering than previously thought and is not well-fit by a Henyey-Greenstein function. The empirical scattering phase function varies with stellocentric distance, consistent with the expected radiation pressured-induced size segregation exterior to the belt. Within the belt, the empirical scattering phase function contradicts unperturbed debris ring models, suggesting the presence of an unseen planet. The radial profile of the flux density is degenerate with a radially varying scattering phase function; therefore estimates of the ring's true width and edge slope may be highly uncertain. We detect large scale asymmetries in the disk, consistent with either the recent catastrophic disruption of a body with mass >1% the mass of Pluto, or disk warping due to strong interactions with the interstellar medium. © 2014. The American Astronomical Society. All rights reserved.

Maccarone T.J.,University of Southampton | Kundu A.,Eureka Scientific | Zepf S.E.,Michigan State University | Rhode K.L.,Indiana University
Monthly Notices of the Royal Astronomical Society: Letters | Year: 2010

We present the results of new X-ray observations of XMMU 122939.7+075333, the black hole (BH) in the globular cluster RZ 2109 in the Virgo cluster galaxy NGC 4472. A combination of non-detections and marginal detections in several recent Swift and Chandra observations show that the source has varied by at least a factor of 20 in the past 6 yr, and that the variations seem not just to be 'flickering'. This variation could be explained with changes in the absorption column intrinsic to the source no larger than those which were previously seen near the peak of the 1989 outburst of the Galactic BH X-ray binary V404 Cyg. The large amplitude variations are also a natural expectation from a hierarchical triple system with Kozai cycles - the mechanism recently proposed to produce BH-white dwarf (WD) binaries in globular clusters. On the other hand, variation by such a large factor on time-scales of years, rather than centuries, is very difficult to reconcile with the scenario in which the X-ray emission from XMMU 122939.7+075333 is due to fallback of material from a tidally destroyed or detonated WD. © 2010 The Authors. Journal compilation © 2010 RAS.

Maccarone T.J.,University of Southampton | Kundu A.,Eureka Scientific | Zepf S.E.,Michigan State University | Rhode K.L.,Indiana University
Monthly Notices of the Royal Astronomical Society | Year: 2011

We discuss CXOU 1229410+075744, a new black hole candidate in a globular cluster in the elliptical galaxy NGC 4472. By comparing two Chandra observations of the galaxy, we find a source that varies by at least a factor of 4, and has a peak luminosity of at least 2×1039 erg s-1. As such, the source varies by significantly more than the Eddington luminosity for a single neutron star, and is a strong candidate for being a globular cluster black hole. The source's X-ray spectrum also evolves in amanner consistent with what would be expected from a single accreting stellar mass black hole. We consider the properties of the host cluster of this source and the six other strong black hole X-ray binary candidates, and find that there is suggestive evidence that black hole X-ray binary formation is favoured in bright and metal-rich clusters, just as is the case for bright X-ray sources in general. © 2010 The Authors. Journal compilation © 2010 RAS.

Wheatley J.,University of California at Berkeley | Welsh B.Y.,University of California at Berkeley | Browne S.E.,Eureka Scientific
Publications of the Astronomical Society of the Pacific | Year: 2012

We present well-sampled ultraviolet (UV) light curves of seven RR Lyrae stars (SW Aqr, HL Her, SDSS J100133:3+014328:1, SDSS J104844:1+581539:4, SDSS J105513:7+564747:0, SDSS J105622:2+570520:6, and USNO-B1.0 1024-0010686) serendipitously obtained with the NASA GALEX satellite that show changes of up to AB = 6:5 mag in the far-ultraviolet (FUV; 1350-1800 Å) wavelength band. For the test case SW Aqr, we combine the GALEX FUV and NUV (1800-2800 Å) observations with visible U, B, and V photometry to determine the variation of physical parameters such as temperature, surface gravity, and radius during the RR Lyrae pulsation cycle of 0.459299 days. Assuming that SWAqr changes temperature from 6100 K to 8400 K over this cycle, we find that the FUVand NUV light curves can be well matched using the 2004 stellar atmosphere models of Castelli and Kurucz (CK) and a best-fit metallicity of [Fe=H] = -1:5. Pulsation periods are also found for the remaining six stars from their UV light curves, and we have shown that the UV data are consistent with the predictions of CK models using an assumed temperature variation of 6100 K to 8000 K and a log g variation of 2.15 to 3.75 over the pulsation cycle for assumed metallicity values of -2:0 < [Fe=H] < 0:0. Our main finding from the present analysis is that CK models can be successfully used to predict the light-curve behavior of RR Lyrae stars at UV wavelengths if the minimum and maximum temperatures are known from (preferably) visible light-curve observations, such that the stellar metallicity values can be derived. Finally, we note that the model fits to the GALEX data are quite sensitive to the light curves around minimum light, such that the best-fit metallicity is well constrained over this region of the pulsation cycle. © 2012. The Astronomical Society of the Pacific. All rights reserved.

Roberts M.S.E.,Eureka Scientific | Roberts M.S.E.,Ithaca College
Proceedings of the International Astronomical Union | Year: 2012

Over the last few years, the number of known eclipsing radio millisecond pulsar systems in the Galactic field has dramatically increased, with many being associated with Fermi gamma-ray sources. All are in tight binaries (orbital period < 24 hr) with many being classical black widows which have very low mass companions (companion mass Mc ≠0.1 M ⊙) but some are redbacks with low mass (Mc ∼ 0.2-0.4 M ⊙) companions which are probably non-degenerate. These latter are systems where the mass transfer process may have only temporarily halted, and so are transitional systems between low mass X-ray binaries and ordinary binary millisecond pulsars. Here we review the new discoveries and their multi-wavelength properties, and briefly discuss models of shock emission, mass determinations, and evolutionary scenarios. © 2013 International Astronomical Union.

Rappaport S.,Kavli Institute for Astrophysics and Space Research | Levine A.,Kavli Institute for Astrophysics and Space Research | Pooley D.,Eureka Scientific | Steinhorn B.,Kavli Institute for Astrophysics and Space Research
Astrophysical Journal | Year: 2010

The Chandra X-Ray Observatory was used to image the collisional ring galaxy Arp 147 for 42 ks. We detect nine X-ray sources with luminosities in the range of (1.4-7) × 1039 erg s-1 (assuming that the sources emit isotropically) in or near the blue knots of star formation associated with the ring. A source with an X-ray luminosity of 1.4 × 1040 erg s-1 is detected in the nuclear region of the intruder galaxy. X-ray sources associated with a foreground star and a background quasar are used to improve the registration of the X-ray image with respect to Hubble Space Telescope (HST) high-resolution optical images. The intruder galaxy, which apparently contained little gas before the collision, shows no X-ray sources other than the one in the nuclear bulge which may be a poorly fed supermassive black hole. These observations confirm the conventional wisdom that collisions of gas-rich galaxies trigger large rates of star formation which, in turn, generate substantial numbers of X-ray sources, some of which have luminosities above the Eddington limit for accreting stellar-mass black holes (i.e., ultraluminous X-ray sources, "ULXs").We also utilize archival Spitzer and Galex data to help constrain the current star formation rate in Arp 147 to ∼7 M⊙ yr-1. All of these results, coupled with binary evolution models for ULXs, allow us to tentatively conclude that the most intense star formation may have ended some 15 Myr in the past. © 2010. The American Astronomical Society. All rights reserved.

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