Authors: A.C. Phillips, M.A. Bershady, D.A. Forbes, D.C.Koo, G.D. Illingworth, D.B. Reitzel, R.E. Griffiths, R.A. Windhorst
Accepted: Astrophysical Journal
A set of 100 faint galaxies from nine Hubble Space Telescope Wide Field Camera I-band images have been analyzed as part of the Medium Deep Survey (MDS) Key Project. This sample reaches a depth of I<20.5 (corresponding) to B<22-23) and complements the first set of fainter galaxies analyzed by the MDS team. Images were deconvolved using the Lucy-Reichardson algorithm and a newly-developed procedure designed to yield a more reliable determination of structure in the low S/N regime. These deconvolved images were used to characterize the structure of the galaxies through quantitative measurements of total magnitudes, half-light radii, exponential disk scale lengths and disk-to-total ratios. Extensive testing was done to establish the validity of the procedures used to characterize the degree of systemtic errors present in the analysis techniques.
The observed size--magnitude distribution appears consistent with a scenario in which luminous galaxies have evolved little in intrinsic luminosity, size, or structure over recent epochs in a "normal" cosmology (0
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Title: High Resolution Spectra of Distant Compact Narrow Emission Line Galaxies: Progenitors of Spheroidal Galaxies?
Authors: D. Koo, R. Guzman, S.M. Faber, G. Illingworth, M. Bershady, R. Kron, M. Takamiya
Accepted: Astrophysical Letters
Emission line velocity widths have been determined for 17 faint (B ~ 20-23) very blue, compact galaxies whose redshifts range from z = 0.095 to 0.66. The spectra have a resolution of 8 km/s and were taken with HIRES echelle spectrograph of the Keck 10-m telescope. The galaxies are luminous with all but two within one magnitude of M (B) ~ -21. Yet they exhibit narrow velocity widths between sigma = 28 - 157 km/s, more consistent with typical values of extreme star-forming galaxies than with those of nearby spiral galaxies of similar luminosity. In particular, objects with sigma <= 65 km/s follow the same correlations between sigma and both blue and H-beta luminosities as those of nearby H II galaxies. These results strengthen the identification of H II galaxies as their local counterparts. The blue colors and strong emission lines suggest these compact galaxies are undergoing a recent, strong burst of star formation. Like some H II galaxies, this burst could be a nuclear star-forming event within a much larger, older stellar population. If instead, the burst is a major episode in the total star-forming history, these distant galaxies could fade enough to match the low luminosities and surface brightnesses typical of nearby spheroidals like NGC 185 or NGC 205. Together with evidence for recent star formation, exponential light profiles, and sub-solar metallicities, the post-fading correlations between luminosity and velocity width and luminosity and surface brightness suggest that among the low sigma galaxies, we may be witnessing, in-situ, the progenitors of today's spheroidal galaxies.
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Title: Deep U B V R I Photometric Calibration of High Latitude Fields: SA 57 (1307 + 30) and Hercules (1720+50)
Authors: S.R. Majewski, R. G. Kron, D.C. Koo, M. A. Bershady
Accepted: Astronomical Society of the Pacific
We present CCD photometric calibration sequences in the magnitude range V = 17-22 for two fields at high Galactic latitude: SA 57 (at the North Galactic Pole) and Hercules (1=77, b=35). Photometry to a precision of about 0.02 magnitudes at V=20 and, in general, better than 0.1 magnitudes at V=22 was obtained in the Johnson UBV as well as the Kron-Cousins R and I bands. These data are suitable for setting magnitude zero-points in catalogues of faint stars, galaxies, and QSOs, and we apply them to our own photographic catalogues in these two fields.
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Title: Statistical Consulting Center for Astronomy
Authors: E. Feigelson, M. Akritas, & J. Rosenberger
To appear: Astronomical Data Analysis Software and Systems IV, 1995
We announce the formation of a Statistical Consulting Center for Astronomy (SCCA), designed to provide prompt high-quality advice on Statistical and methodological issues to the astronomical community. Questions should be sent by email to scca@stat.psu.edu. Questions and answers can be examined on the World Wide Web at http:// www.stat.psu.edu/scca/homepage.html.
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Title: Competition of Supermassive Black Holes and Galactic Spheroids in the Destruction of Globular Clusters
Authors: Jane C. Charlton and Pablo Laguna
Accepted: Astrophysical Journal
The globular clusters that we observe in galaxies may be only a fraction of the initial population. Among the evolutionary influences on the population is the destruction of globular clusters by tidal forces as the cluster moves through the field of influence of a disk, a bulge, and/or a putative nuclear component (black hole). We have conducted a series of N-body simulations of globular clusters on bound and marginally bound orbits through potentials that include black hole and spheroidal components. The degree of concentration of the spheroidal component can have a considerable impact on the extent to which a globular cluster is disrupted. If half the mass of a 1010Mo spheroid is concentrated within 800pc, then only black holes with masses > 109Mo can have a significant tidal influence over that already exerted by the bulge. However, if the matter in the spheroidal component is not so strongly concentrated toward the center of the galaxy, a more modest central black hole (down to 108Mo) could have a dominant influence on the globular cluster distribution, particularly if many of the clusters were initially on highly radial orbits. Our simulations show that the stars that are stripped from a globular cluster follow orbits with roughly the same eccentricity as the initial cluster orbit, spreading out along the orbit like a "string of pearls". Since only clusters on close to radial orbits will suffer substantial disruption, the population of stripped stars will be on orbits of high eccentricity.
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Title: Ground-Based Observations of PKS2155-304 IN November 1991
Authors: A. Blecha, T.J.-L Courvoisier, H.D. Aller, M.F. Aller, P. Bouchet,
P. Bratschi, M.T. Carini, M. Donahue, E.D. Feigelson, A.V. Filippenko, I.S. Glass, J. Heidt, PA. Hughes, R.I. Kollgaard, T. Matheson,
H.R. Miller, J.C. Noble, P.S. Smith, and S. Wagner
In: IAV 159: Multi-Wavelength Continuum Emission of AGN
We present ground-based data of the BL Lac object PKS 2155-305 obtained during a large international campaign spanning the electro-magnetic spectrum from the radio waves to X-rays in November 1991. For the complete description of the observations and data analysis we refer to the paper by Courvoisier et al. 1993, and references therein. The ground-based data include radio, infrared JHKL and UBVRI fluxes as well as optical and near IR polarimetry.
The broad-band optical and near IR data from U to I exhibit the same behaviour in all bands: the flux nearly doubled over the well-covered period of 23 days. The cross-correlation function does not reveal any significant changes in the light-curves. Though significant variations in 24 hours have been recorded, the cumulated Fourier power spectrum drops to the noise level for periods shorter than 2.5 days. The spectral index remained constant.
The polarised flux varied by a larger factor than the total flux and did not follow the same pattern. The degree of polarisation and polarisation angle are nearly independent of the wavelength and are strongly correlated in all filters.
In the radio domain the spectral index increased from -0.1 on November 5 to +0.02 on 25-th. The absence of the lag between the optical and infrared bands and the polarisation variations are consistent with a model in which the variability is caused by micro-lensing of the source (Stickel, Fried and Kuhr 1988). One would, however, expect in this model that the variation in the polarisation and the total flux are tightly correlated contrary to what is observed.
The constant shape of the continuum spectral energy suggests that only the number of electrons whose emission is beamed towards the observer changes, rather than the arrival of fresh electrons that are being accelerated.
The variability of the polarisation may be explained by changes in the geometry of the magnetic field (dominant direction). This is consistent with the observed variations of the polarisation angle.
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Title: Differential Evolution of Radio Quiet and Radio Loud Quasars
Authors: M. Schmidt, J.V. Van Gorkom, D. Schneider, and J. Gunn
Accepted: Astronomical Journal
We have observed 40 optically selected high-redshift quasars with the VLA at 1515 MHz (20 cm). Three quasars are detected above our 3s limit of 0.2 mJy. We review all available evidence on radio detections of optically selected quasars in terms of their ratio R of radio to optical luminosity. We find that the quasars in terms of their ratio R of radio to optical luminosity. We find that the quasars ranked in the top 5 - 20 percent in R show strong evolution in their R distribution. We present two interpretations of this phenomenon. The first one poses that the R values decrease by a factor of around 100 from redshift 0.3 to 1.3. In the second interpretation, we consider that there are two populations of quasars, which we call radio-silent and radio-active, separated by R ~ 0.1. The radio-silent fraction of quasars increases from ~ 16% at redshift 0.3, to ~ 75% at redshift 1.3.
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Title: The Interaction of Gamma-ray Bursts Relativistic Winds with the Interstellar Me appear in Pub. Astro. Soc. Pac., 1994, Proc. EIPC Workshop on The Physics of the Interstellar Medium and Intergalactic Medium
A gamma-ray burst event acts as a relativistic gaseous piston ramming into the surrounding interstellar medium. The relativistic wind associated with a burst of typical duration tw ~ seconds can be the result, e.g., of the sudden collapse of a compact object, or the coalescence of a compact binary, The wind is likely to carry a high magnetic field, and its bulk Lorentz factor depends onthe degree of baryon loading (which could be anisotropic). For Lorentz factors ~ 100 non-thermal gamma-ray photons in the MeV range are produced by internal shocks in the wind caused by time variations in the baryon loading. The interaction of the cooled wind ejecta with the interstellar medium also leads to intense gamma-ray emission, the relativistic time-delayed duration of which is td > tw. For Lorentz factors > 100 inverse Compton emission dominates, producing > GeV photons over observer timescales td > 103 s, while for Lorentz factors > 1000 synchrotron emission produces strong MeV emission over timescales td ~ s. This is an attractive mechanism for producing gamma-ray bursts visible at cosmological distances. The delay and duration, the spectrum and the time sub-structure provide diagnostics for the mean density and granularity of the interstellar medium in galaxies out to redshift z > 1.
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Title: The Brightness Distribution of Bursting Sources in Relativistic Cosmologies
Authors: P. Meszaros and A. Meszaros
Accepted: Astrophysical Journal
We present analytical solutions for the integral distribution of arbitrary bursting or steady source counts as a function of peak photon count rate within Friedmann cosmological models. We discuss both the standard candle and truncated power-law luminosity function cases with a power-law density evolution. While the analysis is quite general, the specific example discussed here is that of a cosmological gamma-ray burst distribution. These solutions show quantitatively the degree of dependence of the counts on the density and luminosity function parameters, as well as the weak dependence on the closure parameter and the maximum redshift. An approximate comparison with the publicly available Compton Gamma Ray Observatory data gives an estimate of the maximum source luminosity and an upper limit to the minimum luminosity. We discuss possible ways of further constraining the various parameters.
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Title: The Spectrum and Pulses of 1E2259+586 from ASCA and BBXRT Observations
Authors: R.H.D. Corbet, A.P. Smale, M. Ozaki & K. Koyama, K. Iwasawa
The 7 second X-ray pulsator 1E2259+586 was observed for approximately one day in 1993 with ASCA. Observations were also obtained with BBXRT in 1990 a few months after Ginga had observed 1E2259+586 to be brighter than normal and the BBXRT data show 1E2259+586 to be at an intermediate brightness level. By contrast, the ASCA data appear to have been obtained during a more common lower luminosity state. The pulse-profiles we obtain are consistent with a connection between flux and pulse-shape reported from Ginga data, and the pulsator continues to spin-down. We use our high-spectral resolution data to search for cyclotron lines in the spectrum that were claimed from observations made with other satellites. We find that the ASCA spectra of 1E2259+586 cannot be satisfactorily fitted with either a single power law or a combination of two power laws, and that significant residuals occur around 1.5 and 5 keV. However, a combination of a power law and a black body gives a good fit over the entire ASCA energy band with no evidence of spectral featires/ We have reanalyzed a Ginga LAC spectrum and find that this is also significantly better fit by this two component spectrum than a single power law. A possible explanation for such a two component spectrum is tha the black body emission comes from a neutron star and that the power law component comes, at least in part, from a surrounding nebula.
As there has, so far, been no direct evidence that 1E2259+586 is a binary system we consider whether there are other plausible mechanisms that might power the observed X-ray emission.
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Title: X-ray Morphology, Kinematics and Geometry of The Eridanus Soft X-ray Enhancement
Authors: Z. Guo, D.N. Burrows, W.T. Sanders, S.L. Snowden, and B.E. Penprase
Submitted: The Astrophysical Journal
We present mosaics of X-ray intensity maps and spectral fit results for selected regions of the Eridanus soft X-ray Enhancement (EXE), as well as kinematics of the X-ray absorbing clouds in the EXE region and geometrical properties of this X-ray emitting bubble. The work is based on pointed observations with he ROSAT Position Sensitive Proportional Counter, 21 cm observations with the NRAO 140 foot telescope at Green Bank and interstellar NaD line observations with the NOAO Coud Feed telescope at Kitt Peak. The ROSAT pointed observations examine two regions of the EXE. The first is an X-ray absorption lane produced by an IR filament which is located at galactic coordinates of about (199{, -45{). The second is in the vicinity of the northern (galactic) boundary of the 1/4 keV EXE, at galactic coordinates of about (200{, -25{). Both our spatial and spectral analysis suggest that variations in emission measure and NH are primarily responsible for the observed variations of the X-ray intensity. Using 100 5m intensities obtained from IRAS maps and NH column densities obtained from our X-ray spectral fits, we find 100 5m/NH ratios across the IR filament that are compatible with typical high latitude values. Maps of the X-ray absorbing clouds in the EXE region at 21 cm reveal that these clouds may belong to two different expanding systems, with one possibly associated with our Local Bubble and the other with the boundary of the EXE. Combination of 21 cm data with interstellar Na D line observations toward stars in the directions of some of the X-ray absorbing clouds along (1,b) ~ (200{, -40{) indicates that the near side of the EXE is farther than 151 pc and the distance to the center of the EXE at this latitude is about 226 pc. The density and the thermal pressure found for this X-ray emitting superbubble are 0.015 cm-3 and 4.9 x 104 cm-3 K.
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Title: Relativistic Calculations of Cross Sections for Ionization of U90+ and U91+ Ions
Authors: C. Fontes, D. Sampson and H.L. Zhang
Accepted: Phys. Rev. A
Relativistic distorted wave calculations have been made of the cross sections of electron impact ionization of U90+ and U91+ ions with the generalized Breit interaction included between bound and free electrons. Good agreement is obtained with recent EBIT experiments [Marrs, Elliott and Knapp, Phys. Rev. Lett. 72, 4082 (1994)].
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Title: A Confirmation of 2-40 keV Spectral Complexity in Seyfert Galaxies
Authors: K. A. Weaver, K. A. Arnaud and R. F. Mushotsky
Accepted: The Astrophysical Journal
We have re-analyzed forty spectra of twenty-five Seyfert and narrow emission line galaxies (NELGs) and one QSO, all of which were obtained during the pointed observations phase (1978 May-1979 January) of the HEAO-1 A2 experiment. This investigation was prompted by Ginga results which indicate that Fe Ka lines are common X-ray spectral features of AGN, and that a high energy "bump" associated with reprocessing of nuclear radiation may be common as well. The HEAO-1 data, which cover the 2-40 keV energy band, provide a large sample of AGN with which to test the Ginga results. New analysis techniques are employed which consist of simultaneous fitting of data from both the medium energy (2-20 keV) and high energy (3-40 keV) detectors, as well as the subtracting of summed background spectra (up to 230,000 seconds of data) in order to improve the ratio of signal-to-noise. These improved techniques have resulted in the detection of Fe K features and/or spectral flattening above ~5-10 keV in ~70% of the HEAO-1 sample. This confirms the Ginga result that spectral complexity is common in the X-ray spectra of Seyfert galaxies.
We find that strong FeK lines tend not to be associated with the presence of high-energy .flattening, implying that Fe Ka emission is not entirely the result of fluorescence from accretion disks. Our spectral results typically agree with Ginga for overlapping sources for which we have adequate signal-to-noise, although we find a significantly larger reflection component in both IC 4329A and Mkn 509. For IC 4329A, a time lag in the response of the reflection component to variability in the intrinsic source flux can explain the presence of more reflection, since the source was fainter during the HEAO-1 epoch. A similar effect can explain the larger reflection component in Mkn 509, provided the continuum flux peaked at a higher value prior to the HEAO-1 observation.
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Title: Detection of Compact Ultraviolet Nuclear Emission in LINER Galaxies
Authors: D. Maoz, A. Filippenko, L. Ho, H. Rix, J. Bahcall, D. Schneider, F. Macchetto
Accepted:The Astrophysical Journal
-1), unresolved (FWHM , 0.1") point sources of UV (~ 2300 10-16 erg s-1 cum-2
Low-ionization nuclear emission-line regions (LINERs), which exist in a large fraction of galaxies, may be the least luminous manifestation of quasar activity. As such, they may allow the study of the AGN phenomenon in the nearest galaxies. The nature of LINERs has, however, remained controversial because an AGN-like nonstellar continuum source has not been directly observed in them. We report the detection of bright (< 2 x ) emission in the nuclei of nine nearby galaxies. The galaxies were imaged using the Faint Object Camera on the Hubble Space Telescope (HST), and seven of them are from a complete sample of 110 nearby galaxies that was observed with HST. Ground-based optical spectroscopy reveals that five of the nuclei are LINERs, three are starburst nuclei, and one is a Seyfert nucleus. The observed UV flux in each of the five LINERs implies an inoizing flux that is sufficient to account for the observed emission lines through photoionization. The detection of a strong UV continuum in the LINERs argues against shock excitation as the source of the observed emission lines, and supports the idea that photoionization excites the lines in a least some objects of this class.
We have analyzed ground-based spectra for most of the Northern-hemisphere galaxies in the HST sample, and find that 26 of them are LINERs, among which only the above five LINERs have a detected nuclear UV source. There are no obvious difference in the optical line intensity ratios between the UV-bright and UV-dark LINERs. If all LINERs are photoionized, then the continuum source is unobscured along our line of sight in 5/26 ; 20% of LINERs. Alternatively, it can be argued that spectrally-similar LINERs are produced by various excitation mechanisms, and that photoionization is responsible in only about 20% of the cases. The high angular resolution allows us to set upper limits, typically several parsecs, on the physical size of the compact star-cluster or AGN-type continuum source that is emitting the UV light in these objects.
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Title: Thermal Radiation from Rotating Single Neutron Star: Effect of the magnetic field and surface temperature distribution
Authors: V.E. Zavlin, G.G. Pavlov, Yu. A. Shibanov and J. Ventura
Submitted: Astronomy and Astrophysics
In this paper we investigate the anisotropy of the thermal-like radiation emitted by the atmosphere of a neutron star with a strong magnetic field. Angular dependences of the fluxes at various photon energies, and spectra at various inclinations of the magnetic axis to the line of sight are obtained for different distributions of the magnetic field B and effective temperature Teff over the neutron star surface. We show that the radiation of the magnetized neutron stars is anisotropic even if Teff is constant along the star surface. The shape and degree of modulation of the light curves observed from a rotating neutron star depend essentially on the surface distribution of Teff and B. The pulsations at low-energy and high-energy parts of the spectra may be phase-shifted for not too high effective temperatures. The general properties of the neutron star thermal-like radiation inferred from the theoretical models are in qualitative agreement with the results of recent ROSAT observations of middle-aged pulsars.
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Spring 94
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Title: ROSAT X-ray Sources Embedded in the RHO Ophiuchi Cloud Core
Authors: Sophie Casanova, Thierry Montmerle, Eric Feigelson, Philippe Andr
Submitted: Astrophysical Journal
We present a deep ROSAT PSPC image of the central region of the r Ophiuchi star-forming region. The selected area, about 35' x 35' in size, is rich with dense molecular cores and young stellar objects. Fifty-five reliable X-ray sources are detected ( and up to 50 more may be present), almost doubling the number of Einstein sources in this area. A third of the reliable sources do not have optical counterparts on photographic plates. Most can be cross-identified with Class II and Class III infrared sources, which are embedded T Tauri stars, and four are tentatively identified with Class I sources. The overall detection rate of the bona fide cloud population is very high (73%). the spatial distribution of the X-ray sources closely follows that of the molecular gas. The visual extinctions Av of the ROSAT sources can be as high as 40 or more, confirming that most are embedded in the cloud care, and are presumably very young. The X-ray luminosity function of the embedded sources spans a range of Lx ~ 1028 to 3 1031 erg s-1. It is statistically indistinguishable from that of visible X-ray detected T Tauri stars, which suggests that many undetected weak X-ray sources are also embedded in the r Oph core region. We estimate that up to ~ 300 X-ray sources may be present in the cloud core, with a total X-ray luminosity Lx,Oph = 5 x 1032 erg s-1. We discuss several consequences of a widespread in situ irradiation of molecular clouds by X-rays from embedded young stellar objects. We show in particular that these X-rays are able to provide the bulk of cloud core ionization, and thus must play an important role as a feedback effect on star formation.
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Title: Multiwavelength Study of the Magnetically Active T Tauri Star HD 283447
Authors: E. Feigelson, A. Welty, C. Imhoff, J. Hall, P. Etzel, R. Phillips
Submitted: Astrophysical Journal
We observed the luminous T Tauri star HD 283447 = V773 Tauri simultaneously at X-ray, ultraviolet, optical, optical photometric and spectroscopic, and radio wavelengths for several hours on UT 1992 September 11. ROSAT, IUE, VLA and an intercontinental VLBI network, and three optical observatories participated in the campaign. The star is known for its unusually high and variable northermal radio continuum emission. High level of soft X-ray and Mg II line emission are discovered, with luminosity Lx= 5.5 x 1030 ergs s-1 (0.2-2 keV) and LMgII = 1 x 1029 ergs s-1, respectively. Optically, the spectrum exhibits rather weak characteristics of 'classical' T Tauri stars. A faint broad emission line component, possibly due to a hot wind, is present.
During the campaign, the radio luminosity decreased by a factor of 4, while optical/UV lines and X-ray emission remained strong but constant. The large gyrosynchrotron emitting regions are therefore decoupled from the chromspheric and coronal emission. Five models for the magnetic geometry around the star are discussed: solar-type activity, dipole magnetosphere, stellar-disk magnetic coupling, disk magnetic fields, and close binary interaction. The data suggest that two magnetic geometries are simultaneously present: complex multi-polar fields like those on the Sun, and large-scale field possibly associated with the circumstellar disk.
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Title: Multifrequency Studies of Optically Quiet Quasars
Authors: R.I. Kollgaard, S.A. Laurent-Muehleisen, E.D. Feigelson, H. Spinrad, W. Brinkmann
Presented at: First ROSAT Science Symposium and Data Analysis Workshop
We have made multiband observations of six candidate Optically Quiet Quasars, and confirm the inverted Spectral Energy Distribution in four. OQQs could be ordinary radio-loud AGN whose SED is significantly modified by absorption either intrinsic to the source or along the line of sight, or the X-rays could be dominated by a strong inverse-Compton contribution. Our four OQQs exhibit a range of properties and more than one explanation is possible.
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Title: X-rays and Relativistic Beaming in Radio-Selected BL Lacertae Objects
Authors: R.I. Kollgaard, E.D. Feigelson, D.C. Gabuzda, R.M. Sambruna, C. M. Urry
Presented at: First ROSAT Science Symposium and Data Analysis Workshop
We have used X-ray and VLBI observations of twenty three BL Lac objects from the 1 Jy sample of radio bright sources to test models for the origin of X-ray emission in BL Lacs. We find a correlation between the X-ray luminosity and the parsec scale radio emission, consistent with the idea that the X-rays are relativistically beamed. A simple approximation for the synchrotron self-Compton process has been used to estimate the minimum Doppler factor for bulk relativistic motion. Relativistic motion is implied for ~ 75% of the sources, although there are no clear trends between the minimum Doppler factor and other traits. These results suggest that the soft X-ray spectra do not contain a significant inverse-Compton contribution, but this needs to be tested with more sophisticated models.
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Title: X-rays From the Lobes of Fornax A
Authors: S.A. Laurent-Muehleisen, E.D. Feigelson, R. I. Kollgaard, & E. B. Fomalont
Presented at: First ROSAT Science Symposium and Data Analysis Workshop
We present a deep ROSAT PSPC image of the radio lobes of the nearby galaxy Fornax A (= NGC 1316) and find, after image processing, X-ray emission closely mimicking the radio emission. We argue that this is the long-sought inverse Compton radiation produced when cosmic microwave background photons are upscattered off synchrotron emitting electrons in the radio lobes. The derived magnetic field of the lobes is compared with equipartition values and we also discuss the possibility of thermal Bremsstrahlung emission as a source of lobe X-rays. This observation constitutes the best case for detection of inverse Compton X-rays in radio lobes to date.
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Title: Fast-Timing - A Viable ACT Technique?
Authors: R. Morse, M. Skinner, & S.Z. Tilav
Published: The proceedings of the 23rd International Cosmic Ray Conference (Calgory, 1993)
The Fast-Timing Air Cherenkov technique-the technique of running a multi-photomultiplier system at the single photoelectron level- had as a major design goal the lowering of the detection threshold for gamma-ray showers. It relied on the premise that intrinsic photomultiplier (PMT) pulse-height fluctuations are significantly larger than the Poisson fluctuations of the night sky. It was reasoned that if air showers were observed with a telescope that employed a sufficient number of PMTs such that each PMT only had a 2-5% occupancy probability in a 10 ns window, then these PMTs could be operated at the 1 pe (photoelectron) level. Under such conditions, PMT fluctuations could be eliminated, and the night sky background essentially digitized, and thus minimized. It is suggested here that, in practice, these techniques could also lead to undesired effects, such as solid-angle acceptances that are very energy-dependent near threshold-perhaps being only 5-10% of the expected solid angle, and instrument's threshold behavior that might differ considerably from what would be expected. These deficiencies are not intrinsic to the technique itself, but could result when stringent, and possibly unanticipated, requirements on mirror spot-size, and optical alignment are not met.
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Title: A Doppler Image of the Weak T Tauri Star V410 Tau
Authors: K. G. Strassmeier, A.D. Welty, and J. B. Rice
Submitted: Astronomy and Astrophysics 20.1.1994
We have monitored the weak-lined T Tauri star V410 Tau spectroscopically for six nights in 1992. We detected periodic line-profile variations that are used to obtain a Doppler image of this star, the first of pre-main sequence T Tauri star. Inverse solutions from three moderately-strong absorption lines show consistent surface inhomogeneities of the effective temperature distribution of up to 1200 K cooler and at least 500 K hotter than the nominal photospheric temperature of 4400 K. Our maps thus verify the proposed existence of hot spots on weak-lined T Tauri stars. All features are evident at low and very high latitudes even covering the rotational pole but not a cool polar cap-like spot as seen previously for several other active stars.
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Title: X-ray Selected T Tauri Stars in the Chamaeleon I Cloud
Authors: D.P. Huenemoerder, W.A. Lawson and E.D. Feigelson
Submitted: M.N.R.A.S., February, 1994
We report spectroscopy of 27 stars associated with 24 proposed new X-ray selected cloud members of Chamaeleon I. Fourteen to 20 of the stars are confirmed to be "weak-lined" T Tauri (WTT) stars due to the presence of a emission and strong b $6707 Li I absorption. Of the remainder, we suspect that five of the observed stars are not the X-ray emitters. Only two X-ray emitting stars are found to be unrelated to the cloud. The available evidence suggests that the WTT stars are not systematically older than "classical" T Tauri (CTT) stars in Chamaeleon. The transition between CTT and WTT stages for individual stars must range over most of the Hayashi track.
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Title: Spatial Statistics in Astronomy
Authors: Gutti Jogesh Babu and Eric D. Feigelson
Submitted: Journal of Statistical Planning and Inference, February 1994
Radio pulsars, whicha re rapidly rotating, highly magnetized neutron stars, are known to be extraordinarily precise "celestial clocks" with a variety of applications of astrophysics and fundamental physics. The pulsars found in close binary systems, especially those with massive (>1Mo) compact companions, have proved to be excellent tools of experimental gravitation. Moving in eccentric orbits at mildly relativistic speeds (v/c ~ 10-3), having surface potentials of the order of GM/Rc2~0.2 (compared to ~ 10-6 for the Sun), the high-mass binary pulsars are, in fact, the best known probes of relativistic gravity. Precise timing measurements of the first relativistic binary pulsar, PSR B1913+16, discovered by Hulse and Taylor in 1974, have led to establishing the most convincing evidence so far that the quadrupolar gravitational waves exist. This and several other, later discovered binary pulsars have defined a distinct class of celestial objects characterized by the presence of measurable relativistic effects in their orbital motion. Timing observations of binary pulsars provide a method to detect other
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Title: The Shape of FK Comae Berenices: Evidence for a Recently Coalesced Binary
Authors: Alan D. Welty and Lawrence W. Ramsey
Submitted: The Astrophysical Journal (Letters)
To explain its great rotational speed, it has been argued that FK Com is either a recently coalesced binary or has been spun-up by angular momentum transfer via mass transfer from an unseen companion. Upper limits on radial velocity variation put severe constraints on any binary model. A series of fifty-one spectroscopic observations of FK Com during an eight night observing run in 1989 revealed regular variation of projected rotational speed with rotational phase. These variations are suggestive of strong non-radial pulsations at about half the rotational period. We postulate that the pulsations were excitated by a recent binary merger. Less extensive data obtained in 1987 and 1992 show different patterns of linewidth variation. Thus it appears that the pulsation spectrum is changing on timescales significantly less than two years.
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Title: Soft X-ray Spectral Fits of Geminga with Model Neutron Star Atmospheres
Authors: R.D. Meyer, G.G. Pavlov strophysical Journal
Accepted: The Astrophysical Journal
The spectrum of the soft X-ray pulsar Geminga consists of two components, a softer one which can be interpreted as thermal-like radiation from the surface of the neutron star, and a harder one interpreted as radiation from a polar cap heated by relativistic particles. We have fitted the soft spectrum using a detailed magnetized hydrogen atmosphere model. The fitting parameters are the hydrogen column density, the effective temperature Teff, the gravitational redshift z and the distance to radius ratio, for different values of the magnetic field B. The best fits for this model are obtained when B< 1 x 1012 G and z lies on the upper boundary of explored range (z = 0.45). The values of Teff ~ (2-3) x 105 K are a factor of 2-3 times lower than the value of Teff obtained for blackbody fits with the same z. The lower Teff increases the compatibility with some proposed schemes for fast neutrino cooling of NSs by the direct Urca process or by exotic matter, but conventional cooling cannot be excluded. The hydrogen atmosphere fits also imply a smaller distance to Geminga than that inferred from a blackbody fit. An accurate evaluation of the distance would require a better knowledge of the ROSAT PSPC response to the low-energy region of the incident spectrum. Our modeling of the soft component with a cooler magnetized atmosphere also implies that the hard component fit requires a characteristic temperature which is higher (by a factor of ~ 2-3) and a surface area which is smaller (by a factor of ~ 103), compared to previous blackbody fits.
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Title: X-ray Variability in the Eclipsing Polar DP Leo
Authors: Craig R. Robinson and France A. Csical Journal
Submitted: The Astrophysical Journal
We present an analysis of our ROSAT PSPC observations on the eclipsing magnetic cataclysmic variable DP Leo. The soft X-ray spectrum is modeled by a blackbody of kT = 24.8+2.6 eV. Severe limits are placed upon the flux from any hard bremsstrahlung component. A strong soft x-ray excess, with respect to hard X-ray emission, is found. The soft X-ray blackbody luminosity is lower than the cyclotron luminosity, but significantly larger than an estimate of the bremsstranhlung emission
(Lcyc-~ 6Lbb > 100Lbr).
An upper limit of 500 pc is obtained for the system's distance based upon the X-ray absorbtion (NH < 5 x 1019 cm-2) and an estimate of 260+150 pc is determined from a published measurement of the secondary's flux. For the derived blackbody fit, the bolometric luminosity is found to be Lbb,bol > 1.4 x 1031 (d/260pc)2 erg s -1.
Absorbtion by the accretion stream produces an intensity dip prior to each eclipse. Extreme variability in the shape of the light curve from eclipse to eclipse demonstrates that changes in the rate of accretion onto the white dwarf, the sizes of accretion filaments, or variations in the location or amount of absorbing matter in the system occur on time scales shorter than the orbital period (89.8 min). No evidence exists for accretion onto the stronger (59 MG) magnetic pole in the ROSAT data. A new ephemeris is presented for the eclipse of the white dwarf emission region by the secondary star and another is produced for the orbital conjunction of the two components. The rotation of the white dwarf is shown to be faster than the orbital period by (5.3 1 1.1) x 10-3 seconds.
Modeling of the accretion stream geometry is performed assuming disruption by magnetic field lines from a quadrupolar field. The ROSAT intensity dip is explained using this model and the results are shown to be consistent with contemporaneous optical observations. This provides further evidence that the magnetic field configuration of the white dwarf is more complex than that of a centered dipole.
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Title: The Shape of FK Comae Berenices: Evidence for a Recently Coalesced Binary
Authors: Alan D. Welty and Lawrence W. Ramsey
Submitted: The Astrophysical Journal (Letters)
Sudden collapse of a compact object, or coalesence of a compact binary, can generate an unsteady relativistic wind that lasts for a few seconds. The wind is likely to carry a high magnetic field; and its Lorentz factor depends on the extent to which it is 'loaded' with baryons. If the Lorentz factor is ~100, internal dissipation and shocks in the wind produce a non-thernal gamma-ray burst, detectable in the range 0.1 MeV E 0.1-1 GeV out to cosmologicl distances. The cooled wind ejecta would subsequently be declerated by the external medium. The resultant blast wave and reverse shock can then give rise to a second butst component, mainly detectable in the GeV range, with a time delay relative to the MeV burst ranging from minutes to hours.
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Summer 94
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Title: WFPC-2 Observations Of The Globular Cluster M30
Authors: Brian Yanny, Puragra Guhathakurta, Donald Schneider, John Bahcall
To Appear In: Astrophysical Journal Letters
We describe images of the center of the dense globular cluster M30 obtained with the Hubble Space Telescope Wide Field/Planetary Camera-2. Data taken in the F336W, F439W, and F555W filters (approximately U, B, and V) yield a color-magnitude diagram with photometric errors of 1s = 0.05 mag for stars with V < 17.5, and 1s ~ 0.1 mag at
V = 20, which is 1.5 mag fainter than the main-sequence turnoff. Simulations show that the star identifications are essentially complete for stars with V < 19.5, even in the densest regions of the cluster. The projected radial distribution of stars can be represented by a power law distribution: N(r) ~ ra, with slope a = -0.4 1 0.15, into radii less than 0.4" (0.015 pc). The radial profile is also consistent with at flat core of any radius smaller than about 1.5" (0.05 pc), but inconsistent with any core radius larger than 2.5" (0.09 pc).
A total of 30 blue straggler candidates within 20" of the cluster center have been identified from the three-color data. Their radial distribution is strongly centrally concentrated when compared to the horizontal branch, red giant branch, or main sequence stars in the cluster. The abundance of blue stragglers (relative to red giants and subgiants) within r < 20" of the center of M30 is fBS = 0.19 1 0.04, about twice as high as in other dense clusters.
Subject headings: globular clusters: individual (M30) - techniques: image processing - stars: kinematics - stars: blue stragglers
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Title: Chromospheric Variations in Main-Sequence Stars II.
Authors: Baliunas, S. L., Donahus, R. A., Soon, W. H., Horne, J. H., Frazer, J., Woodard-Eklund, L., Bradford, M., Rao, L. M., Wilson, O. C., Zhang, Q., Bennett, W., Briggs, J., Carroll, S., Duncan, D. K., Figueroa, D., Lanning, H. H., Misch, T., Mueller, J., Noyes, R. W., Poppe, D., Porter, A. C., Robinson, C. R., Russell, J., Shelton, J. C., Soyumer, T., Vaughan, A. H., and Whitney, J.
Submitted: The Astrophysical Journal
The fluxes in passbands 0.1-nm wide and centered on the Ca II H and K emission cores have been monitored in 111 stars of spectral type F2 - M2 on or near the main sequence. Most of the measurements began in 1966 yielding records of chromospheric emission that span 25 years. The records display fluctuations that can be identified with variations on time scales similar to the 11-yr cycle of solar activity as well as axial rotation, and the growth and decay of emitting regions. The records of chromospheric emission and general conclusions about variations in surface magnetic activity on time scales longer than 1 yr but less than a few decades are presented.
The results for stars of spectral type ~ GO - K5V indicate a pattern of changes in rotations and chromospehric activity on an evolutionary time scale, in which (i) young stars exhibit high average levels of activity, rapid rotation rates, no Maunder-minimum phase and only rarely display a smooth, cyclic variation; (ii) stars of intermediate age (~1-2 Gyr for 1Mo) have moderate levels of activity and rotation rates and occasional smooth cycles; and (iii) stars as old as the sun and older have low rotation rates, average activity levels and smooth cycles with occasional Maunder-minimum phases.
This paper is dedicated to the memory of Alain Porter.
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Title: A Sample of Astronomical Time Series
Authors: Paul Hertz and Eric D. Feigelson
Accepted: "Applications of Time Series Analysis in Astronomy and Meteorology"
The observation of variability in astronomical systems provides astronomers with valuable information on the physical nature of the system. The types of variability which astronomers deal with can be periodic, quasi-periodic, or aperiodic. We collect and describe a number of time series data sets which exemplify the types of problems encountered by the modern astronomer and which demonstrate some of the methodological challenges they face. The $Sample$ includes data sets with periodic, quasi-periodic and aperiodic behavior, sparse and unevenly spaced sampling, and paired time series requiring cross-correlation. The data sets in the $Sample$ are available on diskette, over the Internet using the World Wide Web, or by electronic mail. We hope that statisticians and time series analysts will find problems of interest and be stimulated to apply new techniques to astronomical problems.
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