255,538 research outputs found
A 2 h periodic variation in the low-mass X-ray binary Ser X-1
No full textSpectroscopy of the low-mass X-ray binary Ser X-1 using the Gran Telescopio Canarias have revealed a ?2 h periodic variability that is present in the three strongest emission lines. We tentatively interpret this variability as due to orbital motion, making it the first indication of the orbital period of Ser X-1. Together with the fact that the emission lines are remarkably narrow, but still resolved, we show that a main-sequence K dwarf together with a canonical 1.4 M? neutron star gives a good description of the system. In this scenario, the most likely place for the emission lines to arise is the accretion disc, instead of a localized region in the binary (such as the irradiated surface or the stream-impact point), and their narrowness is due instead to the low inclination (?10°) of Ser X-1
Letter from Curtis C. Ray to C. T. Profitt
No full textLetter from Curtis C. Ray to C. T. Profitt, requesting vacation time
Letter from Curtis C. Ray to W. T. Johnson
No full textLetter from Curtis C. Ray to W. T. Johnson, concerning program of work
Frank (Fred?) McDonald and T. T at Foret, Romare
No full textThis record was harvested from a previous catalogue system and will be withdrawn in 2025. Information in this record may be superseded or incomplete. Visit this record in UMA's new catalogue at: https://archives.library.unimelb.edu.au/nodes/view/247625170908
Item: [1981.0081.00020] "Frank (Fred?) McDonald and T. T at Foret, Romare
Recommended from our members
Series 1: Papers: Mary Lou Ray Documents, 1945-1984
No full textLetter from Dr. J. T. McRee to Mary Lou Ray McCain discussing proof of Typhoid inoculations in Longview, Texas, July 14, 1945. An envelop from Dr. J. T. McRee, M. D. from 415 - 416 Southwest Reserve Life Building
[Tribute by Ray Kendrick for T. N. Carswell]
No full textA tribute to T. N. Carswell by Ray Kendrick regarding the resignation of Carswell from the Abilene Chamber of Commerce
Active X-ray optics for the next generation of X-ray space telescopes
No full textDescribed within is the design, manufacture, metrology and X-ray testing of an active X-ray
prototype intended for the next generation of X-ray telescopes. One of the challenges faced by
the X-ray telescope community is how to combine high resolution and high sensitivity into one
system, as weight limitations place constraints on the optics that can be launched. Therefore the
mandate of the active X-ray prototype is to provide high sensitivity through the ability of the optics
to be nested and to deliver high angular resolution through the active control of the optic’s form.
Piezoelectric unimorph actuators provide the active component: it is intended that they will correct
for figure errors within the optic and therefore increase the angular resolution capability.
The prototype’s design is based upon an ellipsoidal segment which provides point-to-point
focussing of an X-ray source. The prototype itself is composed of an electroformed nickel optic
where the non-reflective surface is populated with 30 piezoelectric actuators and it is the production
of the prototype that is the core of the presented research. Metrology of the actuators’ influence
functions is presented and highlight the prototype’s ability to deform its optic surface by microns.
In addition, the measured influence functions are compared against finite element models and a
distinct similarity between the functions is observed.
The prototype was tested at an X-ray beamline facility in November 2008 and the results
showed the prototype’s ability to correct the optic to achieve an improved angular resolution: from
0.786 arc-minutes to 0.686 arc-minutes in terms of full width half maximum. Finally, difficulties
in the manufacture of the prototype and X-ray testing shall be presented alongside future work in
conclusion to this thesis
Frank (Fred?) McDonald, M. W. and T. T at Foret, Romare
No full textThis record was harvested from a previous catalogue system and will be withdrawn in 2025. Information in this record may be superseded or incomplete. Visit this record in UMA's new catalogue at: https://archives.library.unimelb.edu.au/nodes/view/247626170909
Item: [1981.0081.00021] "Frank (Fred?) McDonald, M. W. and T. T at Foret, Romare
Temperaments of young stars : rapid mass accretion rate changes in T Tauri and Herbig Ae stars
No full textFunding: JSV would like to thank the UK Science and Technologies Facility Council (STFC) and the Northern Ireland Department of Culture Arts and Leisure (DCAL) for financial support. AS and TR would like to thank the Science Foundation of Ireland (SFI) for their support under grant numbers 11/RFP/AST/3331 and 10/RFP/AST2780.Variability in emission lines is a characteristic feature in young stars and can be used as a tool to study the physics of the accretion process. Here, we present a study of Hα variability in 15 T Tauri and Herbig Ae stars (K7 - B2) over a wide range of time windows, from minutes, to hours, to days, and years. We assess the variability using linewidth measurements and the time series of line profiles. All objects show gradual, slow profile changes on time-scales of days. In addition, in three cases there is evidence for rapid variations in Hα with typical time-scales of 10 min, which occurs in 10 per cent of the total covered observing time. The mean accretion rate changes, inferred from the line fluxes, are 0.01–0.07 dex for time-scales of <1 h, 0.04–0.4 dex for time-scales of days, and 0.13–0.52 dex for time-scales of years. In Costigan et al., we derived an upper limit finding that the intermediate (days) variability dominated over longer (years) variability. Here, our new results, based on much higher cadence observations, also provide a lower limit to accretion rate variability on similar time-scales (days), thereby constraining the accretion rate variability physics in a much more definitive way. A plausible explanation for the gradual variations over days is an asymmetric accretion flow resulting in a rotational modulation of the accretion-related emission, although other interpretations are possible as well. In conjunction with our previous work, we find that the time-scales and the extent of the variability is similar for objects ranging in mass from ∼0.1 to ∼5 M⊙. This confirms that a single mode of accretion is at work from T Tauri to Herbig Ae stars – across a wide range of stellar masses.Peer reviewe
X-ray emission from faint galaxies and quasars
No full textIn this thesis a study of faint X-ray sources is undertaken to understand the nature and origin of the Cosmic X-ray Background (XRB). A study of the X-ray variability characteristics of the QSO population is also presented. The optical identification of faint ROSAT sources is described. A large fraction of these sources are identified as QSOs. A number of the remaining sources appear to be associated with faint galaxies with a range of morphological types. The discovery of a rare, high redshift obscured QSO is also described and discussed. A method is developed to study the X-ray variability of faint QSOs. Low amplitude variability (~ 25% rms) on timescales of hours to days appears to be characteristic. Contrary to the trends apparent in local AGN, the QSO population show no evidence for a decrease in variability amplitude with luminosity. There is also no trend in variability with redshift, which may favour a short lived model for QSO evolution. A bright, highly variable Seyfert galaxy is analysed in detail. To constrain the galaxy contribution to the XRB a cross-correlation analysis is presented. Cross-correlating faint galaxy catalogues with unidentified X-ray sources suggests that faint b(_i) < 23 galaxies account for ~ 21 ± 6% of all X-ray sources to a flux limit S(0.5 - 2.0 keV) ~ 4 x l0(^-15)ergs(^-1)cm(^-2). Galaxies are then cross-correlated with the remaining unresolved XRB. A highly significant signal is obtained on 3 deep ROSAT fields. Using a formalism to model the galaxy population and its clustering, an estimate of the local X-ray volume emissivity is obtained. The results suggest that b(_j) < 23 galaxies contribute ~ 20% of the unresolved XRB. Comparing the cross-correlation of faint and bright galaxies with theoretical models reveals strong evolution in the X-ray luminosity of galaxies with the form L(_x) oc (1 + z)(^3±1). Extrapolation to high redshift suggests that faint galaxies contribute 50 - 100% of the unresolved XRB. Finally, a study of the X-ray spectra of faint ROSAT sources is presented. The mean source spectra harden significantly at lower flux. Separating the QSOs from the unidentified sources and galaxies, we find no trend in the 0.5 - 2 keV spectra of QSOs. The remaining population produces the spectral hardening. Taking a subset of the most probable X-ray luminous galaxies, these show significantly harder spectra than QSOs with a mean photon index of Γ ~ 1.5 ± 0.1. X-ray luminous galaxies could therefore account for the missing component of the XRB
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