Search

The LUX-ZEPLIN experiment is a dark matter detector centered on a dual-phase xenon time projection chamber operating at the Sanford Underground Research Facility in Lead, South Dakota, USA. This Letter reports results from LUX-ZEPLIN's first search for weakly interacting massive particles (WIMPs) with an exposure of 60~live days using a fiducial mass of 5.5 t. A profile-likelihood ratio analysis shows the data to be consistent with a background-only hypothesis, setting new limits on spin-independent WIMP-nucleon, spin-dependent WIMP-neutron, and spin-dependent WIMP-proton cross sections for WIMP masses above 9 GeV/c$^2$. The most stringent limit is set for spin-independent scattering at 36 GeV/c$^2$, rejecting cross sections above 9.2$\times 10^{-48}$ cm$^2$ at the 90% confidence level., Comment: 9 pages, 8 figures. See https://doi.org/10.1103/PhysRevLett.131.041002 for a data release related to this paper

LUX-ZEPLIN (LZ) is a second-generation direct dark matter experiment with spin-independent WIMP-nucleon scattering sensitivity above $1.4 \times 10^{-48}$ cm$^{2}$ for a WIMP mass of 40 GeV/c$^{2}$ and a 1000 d exposure. LZ achieves this sensitivity through a combination of a large 5.6 t fiducial volume, active inner and outer veto systems, and radio-pure construction using materials with inherently low radioactivity content. The LZ collaboration performed an extensive radioassay campaign over a period of six years to inform material selection for construction and provide an input to the experimental background model against which any possible signal excess may be evaluated. The campaign and its results are described in this paper. We present assays of dust and radon daughters depositing on the surface of components as well as cleanliness controls necessary to maintain background expectations through detector construction and assembly. Finally, examples from the campaign to highlight fixed contaminant radioassays for the LZ photomultiplier tubes, quality control and quality assurance procedures through fabrication, radon emanation measurements of major sub-systems, and bespoke detector systems to assay scintillator are presented., Comment: 45 pages (79 inc. tables), 7 figures, 9 tables

We describe the design and assembly of the LUX-ZEPLIN experiment, a direct detection search for cosmic WIMP dark matter particles. The centerpiece of the experiment is a large liquid xenon time projection chamber sensitive to low energy nuclear recoils. Rejection of backgrounds is enhanced by a Xe skin veto detector and by a liquid scintillator Outer Detector loaded with gadolinium for efficient neutron capture and tagging. LZ is located in the Davis Cavern at the 4850' level of the Sanford Underground Research Facility in Lead, South Dakota, USA. We describe the major subsystems of the experiment and its key design features and requirements.

LUX-ZEPLIN (LZ) is a next generation dark matter direct detection experiment that will operate 4850 feet underground at the Sanford Underground Research Facility (SURF) in Lead, South Dakota, USA. Using a two-phase xenon detector with an active mass of 7~tonnes, LZ will search primarily for low-energy interactions with Weakly Interacting Massive Particles (WIMPs), which are hypothesized to make up the dark matter in our galactic halo. In this paper, the projected WIMP sensitivity of LZ is presented based on the latest background estimates and simulations of the detector. For a 1000~live day run using a 5.6~tonne fiducial mass, LZ is projected to exclude at 90\% confidence level spin-independent WIMP-nucleon cross sections above $1.4 \times 10^{-48}$~cm$^{2}$ for a 40~$\mathrm{GeV}/c^{2}$ mass WIMP. Additionally, a $5\sigma$ discovery potential is projected reaching cross sections below the exclusion limits of recent experiments. For spin-dependent WIMP-neutron(-proton) scattering, a sensitivity of $2.3 \times 10^{-43}$~cm$^{2}$ ($7.1 \times 10^{-42}$~cm$^{2}$) for a 40~$\mathrm{GeV}/c^{2}$ mass WIMP is expected. With underground installation well underway, LZ is on track for commissioning at SURF in 2020., Comment: 14 pages, 11 figures

In this Technical Design Report (TDR) we describe the LZ detector to be built at the Sanford Underground Research Facility (SURF). The LZ dark matter experiment is designed to achieve sensitivity to a WIMP-nucleon spin-independent cross section of three times ten to the negative forty-eighth square centimeters., Comment: 392 pages. Submitted to the Department of Energy as part of the documentation for the Critical Decision Numbers Two and Three (CD-2 and CD-3) management processes. Report also available by chapter at this URL

The LUX-ZEPLIN (LZ) experiment will search for dark matter particle interactions with a detector containing a total of 10 tonnes of liquid xenon within a double-vessel cryostat. The large mass and proximity of the cryostat to the active detector volume demand the use of material with extremely low intrinsic radioactivity. We report on the radioassay campaign conducted to identify suitable metals, the determination of factors limiting radiopure production, and the selection of titanium for construction of the LZ cryostat and other detector components. This titanium has been measured with activities of $^{238}$U$_{e}$~$<$1.6~mBq/kg, $^{238}$U$_{l}$~$<$0.09~mBq/kg, $^{232}$Th$_{e}$~$=0.28\pm 0.03$~mBq/kg, $^{232}$Th$_{l}$~$=0.25\pm 0.02$~mBq/kg, $^{40}$K~$<$0.54~mBq/kg, and $^{60}$Co~$<$0.02~mBq/kg (68\% CL). Such low intrinsic activities, which are some of the lowest ever reported for titanium, enable its use for future dark matter and other rare event searches. Monte Carlo simulations have been performed to assess the expected background contribution from the LZ cryostat with this radioactivity. In 1,000 days of WIMP search exposure of a 5.6-tonne fiducial mass, the cryostat will contribute only a mean background of $0.160\pm0.001$(stat)$\pm0.030$(sys) counts., Comment: 13 pages, 3 figures, accepted for publication in Astroparticle Physics

The design and performance of the LUX-ZEPLIN (LZ) detector is described as of March 2015 in this Conceptual Design Report. LZ is a second-generation dark-matter detector with the potential for unprecedented sensitivity to weakly interacting massive particles (WIMPs) of masses from a few GeV/c2 to hundreds of TeV/c2. With total liquid xenon mass of about 10 tonnes, LZ will be the most sensitive experiment for WIMPs in this mass region by the end of the decade. This report describes in detail the design of the LZ technical systems. Expected backgrounds are quantified and the performance of the experiment is presented. The LZ detector will be located at the Sanford Underground Research Facility in South Dakota. The organization of the LZ Project and a summary of the expected cost and current schedule are given., Comment: 278 pages. Submitted to the Department of Energy as part of the documentation for the Critical Decision Number One (CD-1) management process. Report also available by chapter at http://hep.ucsb.edu/LZ/CDR. This version includes corrections of minor typographic errors

In order to reduce backgrounds from radon-daughter plate-out onto detector surfaces, an ultra-low-radon cleanroom is being commissioned at the South Dakota School of Mines and Technology. An improved vacuum-swing-adsorption radon mitigation system and cleanroom build upon a previous design implemented at Syracuse University that achieved radon levels of $\sim$0.2$\,$Bq$\,$m$^{-3}$. This improved system will employ a better pump and larger carbon beds feeding a redesigned cleanroom with an internal HVAC unit and aged water for humidification. With the rebuilt (original) radon mitigation system, the new low-radon cleanroom has already achieved a $>$$\,$300$\times$ reduction from an input activity of $58.6\pm0.7$$\,$Bq$\,$m$^{-3}$ to a cleanroom activity of $0.13\pm0.06$$\,$Bq$\,$m$^{-3}$., Comment: 5 pages, 4 figures, Proceedings of Low Radioactivity Techniques (LRT) 2015, Seattle, WA, March 18-20, 2015

We have demonstrated the feasibility of performing high-frame-rate, fast neutron radiography of air-water two-phase flows in a thin channel with rectangular cross section. The experiments have been carried out at the accelerator facility of the Physikalisch-Technische Bundesanstalt. A polychromatic, high-intensity fast neutron beam with average energy of 6 MeV was produced by 11.5 MeV deuterons hitting a thick Be target. Image sequences down to 10 millisecond exposure times were obtained using a fast-neutron imaging detector developed in the context of fast-neutron resonance imaging. Different two-phase flow regimes such as bubbly slug and churn flows have been examined. Two phase flow parameters like the volumetric gas fraction, bubble size and bubble velocities have been measured. The first results are promising, improvements for future experiments are also discussed.

About 50% of all known hot subdwarf B stars (sdBs) reside in close (short period) binaries, for which common envelope ejection is the most likely formation mechanism. However, Han et al. 2003 predict that the majority of sdBs should form through stable mass transfer leading to long period binaries. Determining orbital periods for these systems is challenging and while the orbital periods of ~100 short period systems have been measured, there are no periods measured above 30 days. As part of a large program to characterise the orbital periods of subdwarf B binaries and their formation history, we have found that PG1018-047 has an orbital period of 760\pm6 days, easily making it the longest period ever detected for a subdwarf B binary. Exploiting the Balmer lines of the subdwarf primary and the narrow absorption lines of the companion present in the spectra, we derive the radial velocity amplitudes of both stars, and estimate the mass ratio M_{MS}/M_{sdB} = 1.6\pm0.2. From the combination of visual and infrared photometry, the spectral type of the companion star is determined to be mid K., Comment: Submitted for publication in MNRAS, 12 pages, 6 figures, 8 tables

Following the recent discovery of gamma rays from the radio-loud narrow-line Seyfert 1 galaxy PMN J0948+0022 (z=0.5846), we started a multiwavelength campaign from radio to gamma rays, which was carried out between the end of March and the beginning of July 2009. The source displayed activity at all the observed wavelengths: a general decreasing trend from optical to gamma-ray frequencies was followed by an increase of radio emission after less than two months from the peak of the gamma-ray emission. The largest flux change, about a factor of about 4, occurred in the X-ray band. The smallest was at ultraviolet and near-infrared frequencies, where the rate of the detected photons dropped by a factor 1.6-1.9. At optical wavelengths, where the sampling rate was the highest, it was possible to observe day-scale variability, with flux variations up to a factor of about 3. The behavior of PMN J0948+0022 observed in this campaign and the calculated power carried out by its jet in the form of protons, electrons, radiation and magnetic field are quite similar to that of blazars, specifically of flat-spectrum radio quasars. These results confirm the idea that radio-loud narrow-line Seyfert 1 galaxies host relativistic jets with power similar to that of average blazars., Comment: 39 pages, 12 figures. Accepted for publication on ApJ Main Journal. Corresponding author: Luigi Foschini

The hot subdwarf (sd) stars in the Palomar Green (PG) catalog of ultraviolet excess (UVX) objects play a key role in investigations of the frequency and types of binary companions and the distribution of orbital periods. These are important for establishing whether and by which channels the sd stars arise from interactions in close binary systems. It has been suggested that the list of PG sd stars is biased by the exclusion of many stars in binaries, whose spectra show the Ca II K line in absorption. A total of 1125 objects that were photometrically selected as candidates were ultimately rejected from the final PG catalog using this K-line criterion. We study 88 of these "PG-Rejects" (PGRs), to assess whether there are significant numbers of unrecognized sd stars in binaries among the PGR objects. The presence of a sd should cause a large UVX. We assemble GALEX, Johnson V, and 2MASS photometry and compare the colors of these PGR objects with those of known sd stars, cool single stars, and hot+cool binaries. Sixteen PGRs were detected in both the far- and near- ultraviolet GALEX passbands. Eleven of these, plus the 72 cases with only an upper limit in the far-ultraviolet band, are interpreted as single cool stars. Of the remaining five stars, three are consistent with being sd stars paired with a cool main sequence companion, while two may be single stars or composite systems of another type. We discuss the implications of these findings for the 1125 PGR objects as a whole. (slightly abridged), Comment: 32 pages with 3 figures and 4 tables. Uses AASTEX style files. To be published in The Astronomical Journal (August 2009 issue)

The 11th magnitude star LS IV -08 3 has been classified previously as an OB star in the Luminous Stars survey, or alternatively as a hot subdwarf. It is actually a binary star. We present spectroscopy, spectroscopic orbital elements, and time series photometry, from observations made at the Kitt Peak National Observatory 2.1m, Steward Observatory 2.3m, MDM Observatory 1.3m and 2.4m, Hobby-Eberly 9.2m, and Michigan State University 0.6m telescopes. The star exhibits emission of varying strength in the cores of H and He I absorption lines. Emission is also present at 4686 Angstroms (He II) and near 4640/4650 Angstroms (N III/C III). Time-series spectroscopy collected from 2005 July to 2007 June shows coherent, periodic radial velocity variations of the H-alpha line, which we interpret as orbital motion with a period of 0.1952894(10) days. High-resolution spectra show that there are two emission components, one broad and one narrow, moving in antiphase, as might arise from an accretion disk and the irradiated face of the mass donor star. Less coherent, low-amplitude photometric variability is also present on a timescale similar to the orbital period. Diffuse interstellar bands indicate considerable reddening, which however is consistent with a distance of ~100-200 pc. The star is the likely counterpart of a weak ROSAT X-ray source, whose properties are consistent with accretion in a cataclysmic variable (CV) binary system. We classify LS IV -08 3 as a new member of the UX UMa subclass of CV stars., Comment: To be published in AJ, 16 pages, 6 figures. Uses AAS Latex

SAX J2103.5+4545 has been continuously monitored for $\sim $ 900 days by Rossi X-ray Timing Explorer (RXTE) since its outburst in July 2002. Using these observations and previous archival RXTE observations of SAX J2103.5+4545, we refined the binary orbital parameters and find the new orbital period as P= (12.66536 $\pm $ 0.00088) days and the eccentricity as 0.4055$\pm$ 0.0032. With these new orbital parameters, we constructed the pulse frequency and pulse frequency derivative histories of the pulsar and confirmed the correlation between X-ray flux and pulse frequency derivative presented by Baykal, Stark and Swank (2002). We constructed the power spectra for the fluctuations of pulse frequency derivatives and found that the power law index of the noise spectra is 2.13 $\pm$ 0.6. The power law index is consistent with random walk in pulse frequency derivative and is the steepest among the HMXRBs. X-ray spectra analysis confirmed the inverse correlation trend between power-law index and X-ray flux found by Baykal, Stark and Swank (2002)., Comment: 23 pages, 7 figures, revised version accepted for publication in MNRAS

Following the suggestion of Zuckerman et al. (2001, ApJ, 562, L87), we consider the evidence that 51 Eri (spectral type F0) and GJ 3305 (M0), historically classified as unrelated main sequence stars in the solar neighborhood, are instead a wide physical binary system and members of the young beta Pic moving group (BPMG). The BPMG is the nearest (d < 50 pc) of several groups of young stars with ages around 10 Myr that are kinematically convergent with the Oph-Sco-Cen Association (OSCA), the nearest OB star association. Combining SAAO optical photometry, Hobby-Eberly Telescope high-resolution spectroscopy, Chandra X-ray data, and UCAC2 catalog kinematics, we confirm with high confidence that the system is indeed extremely young. GJ 3305 itself exhibits very strong magnetic activity but has rapidly depleted most of its lithium. The 51 Eri/GJ 3305 system is the westernmost known member of the OSCA, lying 110 pc from the main subgroups. The system is similar to the BPMG wide binary HD 172555/CD -64d1208 and the HD 104237 quintet, suggesting that dynamically fragile multiple systems can survive the turbulent environments of their natal giant molecular cloud complexes, while still being imparted high dispersion velocities. Nearby young systems such as these are excellent targets for evolved circumstellar disk and planetary studies, having stellar ages comparable to that of the late phases of planet formation., Comment: 27 pages, 7 figures. Accepted for publication in the Astronomical Journal. For a version with high resolution figures, see http://www.astro.psu.edu/users/edf/51Eri.pdf

We present the calibration and background model for the Proportional Counter Array (PCA) aboard the Rossi X-ray Timing Explorer (RXTE). The energy calibration is systematics limited below 10 keV with deviations from a power-law fit to the Crab nebula plus pulsar less than 1%. Unmodelled variations in the instrument background amount to less than 2% of the observed background below 10 keV and less than 1% between 10 and 20 keV. Individual photon arrival times are accurate to 4.4 micro-seconds at all times during the mission and to 2.5 micro-seconds after 29 April 1997. The peak pointing direction of the five collimators is known to a precision of a few arc-seconds, Comment: 77 pages, 34 figures. accepted by Ap J Supplement for April 2006 issue. This paper and additional calibration information are available from http://universe.gsfc.nasa.gov/xrays/programs/rxte/pca/ pcapage.html. Revised version contains minor changes to match accepted version

We present the results of a study of the late-type companions in hot subdwarf composite spectrum binaries. The exact nature of these late-type companions has been disputed in the literature -- some argue that they are main sequence stars, and others have claimed they are subgiants. To determine the properties of the late-type companions, we first conducted a survey utilizing the Two Micron All Sky Survey (2MASS) All-Sky Data Release Catalog to identify composite-colored binaries in the "Catalogue of Spectroscopically Identified Hot Subdwarfs" (Kilkenny, Heber, & Drilling 1988, 1992). We then conducted a spectroscopic study of a sub-sample of the 2MASS composite-colored hot subdwarfs. The sample consists of photometrically and spectroscopically single and composite hot subdwarfs (14 single and 51 composite). We also obtained spectra of 59 single late-type stars with Hipparcos parallaxes for calibration. We used measured equivalent width (EW) indices from the composite systems to estimate the temperature and gravity of the late-type star, taking into account the dilution of its spectral features by light from the hot subdwarf. Results from combining the spectroscopic data with model energy distributions indicate that the late-type companions in composite-spectrum systems are best described by main sequence companions overall., Comment: eight pages, six figures, Proceedings of Second Meeting on Hot Subdwarf Stars and Related Objects, 6-10 June 2005 (La Palma, Canary Islands, Spain) to appear in Baltic Astronomy

Many EHB stars have been found in short-period binaries, where the companions in these post-common envelope systems are either white dwarfs or dM stars; these systems are catalogued as hot subdwarfs because the subdwarf is the more luminous component. Hypothesized Roche-lobe overflow systems (with more massive companions) may largely be uncatalogued, since the G band or Ca II K-line from the companion may have caused them to be overlooked or discarded. In particular, many candidate objects were excluded from the PG catalog because of such spectroscopic indicators. Could these rejects include large numbers of "missing" hot subdwarfs? We have examined 2MASS, SDSS, and GALEX archival data for large subsets of these rejected stars, and conclude that only a handful (about 3%) show indications of binarity; most are consistent with (single) metal-poor F stars, as was originally supposed., Comment: four pages, one figure, Proceedings of Second Meeting on Hot Subdwarf Stars and Related Objects, 6-10 June 2005 (La Palma, Canary Islands, Spain) to appear in Baltic Astronomy

We present first competitive results on WIMP dark matter using the phonon-light-detection technique. A particularly strong limit for WIMPs with coherent scattering results from selecting a region of the phonon-light plane corresponding to tungsten recoils. The observed count rate in the neutron band is compatible with the rate expected from neutron background. CRESST is presently being upgraded with a 66 channel SQUID readout system, a neutron shield and a muon veto system. This results in a significant improvement in sensitivity., Comment: 6 pages, 3 figures, to be published in the proceedings of the 5th International Workshop on the Identification and Detection of Dark Matter IDM 2004, Edinburgh, Sept. 2004, World Scientific

The Palomar-Green (PG) survey for UV-excess objects selected objects with U-B<-0.46 for spectroscopic follow-up. The color selection was done from photographic photometry, with typical error sigma_(U-B)=0.38. Spectroscopic detection of the Ca II K line in color-selected candidates was thought to indicate that a metal-weak cool star (sdF-sdG) had entered the list owing to photometric errors. About 1100 such "K-line" stars were rejected on this basis from the final published PG catalogue, as not having "genuine" UV excesses. However, another possibility is that some of these objects are composite (binary) stars, consisting of a hot subdwarf (sdB or sdO) and a cool companion (F-G-K). Recent interest in binary-star formation channels for sdB stars in particular has brought renewed attention to the issue of completeness of lists of known hot subdwarfs, especially ones with cool companions. We have studied the nature of the "rejected" PG candidate stars, by assembling available information for a subset of 173 stars between r magnitudes 14.0 and 16.0 that have photometric data from both Data Release 2 of the Sloan Digital Sky Survey (SDSS) and the Two-Micron All Sky Survey (2MASS). Two of the stars have SDSS spectra. Both from their location in (u-g,g-r) and (g-r,r-K_s) two-color diagrams and from detailed fitting of single-star models to the spectral energy distributions, we conclude that the vast majority of these stars can be interpreted as metal-poor F and G subdwarfs, consistent with the original interpretation by the authors of the PG survey. We discuss the seven outliers individually; these may plausibly be binary systems that include a hot subdwarf star as a member, or they are hot stars that entered the list of rejected stars by accident., Comment: 4 pages, to appear in 14th European Workshop on White Dwarfs, ASP Conference Series, eds. D. Koester, S. Moehler. Revised to reflect editorial changes made for conference book. Abstract and references updated

We give preliminary results from a spectroscopic study of composite spectrum hot subdwarfs (sd+late-type). We obtained spectra of a sample of hot subdwarfs selected from the "Catalogue of Spectroscopically Identified Hot Subdwarfs" on the basis of near-infrared photometry from the Two Micron All Sky Survey (2MASS). The sample consists of 20 photometric and spectroscopic single and 54 composite hot subdwarfs, 6 resolved (or barely resolved) visual doubles, and 5 objects with emission lines or broad absorption lines with emission cores. Spectra of 84 "standard" (single late-type) stars with Hipparcos parallaxes were also obtained for calibration. These observations cover 4600-8900 Ang with 3 Ang resolution. We measured equivalent width-like indices around Mg I b, Na I D, the Ca II infrared triplet, H-alpha, and H-beta. Using the single late-type star observations combined with model energy distributions, we explore how the measured indices of a composite spectrum vary as the temperature and luminosity of the late-type companion are varied and as the temperature and radius of the hot subdwarf are varied. We use the measured indices of the composite systems to estimate the temperature and gravity of the late-type star, taking into account the dilution of its spectral features by light from the hot subdwarf., Comment: 4 pages, to appear in 14th European Workshop on White Dwarfs, ASP Conference Series, eds. D. Koester, S. Moehler

The natural alpha decay of 180W has been unambiguously detected for the first time. The alpha peak is found in a (gamma,beta and neutron)-free background spectrum. This has been achieved by the simultaneous measurement of phonon and light signals with the CRESST cryogenic detectors. A half-life of T1/2 = (1.8 +- 0.2) x 10^18 y and an energy release of Q = (2516.4 +- 1.1 (stat.) +- 1.2 (sys.)) keV have been measured. New limits are also set on the half-lives of the other naturally occurring tungsten isotopes., Comment: Submitted to Physical Review C Revised version

We present first significant limits on WIMP dark matter by the phonon-light technique, where combined phonon and light signals from a scintillating cryogenic detector are used. Data from early 2004 with two 300 g CRESST-II prototype detector modules are presented, with a net exposure of 20.5 kg days. The modules consist of a CaWO4 scintillating ``target'' crystal and a smaller cryogenic light detector. The combination of phonon and light signals leads to a strong suppression of non-nuclear recoil backgrounds. Using this information to define an acceptance region for nuclear recoils we have 16 events from the two modules, corresponding to a rate for nuclear recoils between 12 and 40 keV of (0.87 +- 0.22) events/(kg day). This is compatible with the rate expected from neutron background, and most of these events lie in the region of the phonon-light plane anticipated for neutron-induced recoils. A particularly strong limit for WIMPs with coherent scattering results from selecting a region of the phonon-light plane corresponding to tungsten recoils, where the best module shows zero events., Comment: 24 pages, 10 figures, version v2 includes figures with improved print quality, v3 accepted for publication in astroparticle physics

We present spectroscopy of the microquasar SS 433 obtained near primary eclipse and disk precessional phase Psi = 0.0, when the accretion disk is expected to be most ``face-on''. The likelihood of observing the spectrum of the mass donor is maximized at this combination of orbital and precessional phases since the donor is in the foreground and above the extended disk believed to be present in the system. The spectra were obtained over four different runs centered on these special phases. The blue spectra show clear evidence of absorption features consistent with a classification of A3-7 I. The behavior of the observed lines indicates an origin in the mass donor. The observed radial velocity variations are in anti-phase to the disk, the absorption lines strengthen at mid-eclipse when the donor star is expected to contribute its maximum percentage of the total flux, and the line widths are consistent with lines created in an A supergiant photosphere. We discuss and cast doubt on the possibility that these lines represent a shell spectrum rather than the mass donor itself. We re-evaluate the mass ratio of the system and derive masses of 10.9 +/- 3.1 Msun and 2.9 +/- 0.7 Msun for the mass donor and compact object plus disk, respectively. We suggest that the compact object is a low mass black hole. In addition, we review the behavior of the observed emission lines from both the disk/wind and high velocity jets., Comment: submitted to ApJ, 24 pages, 7 figures

XMM-Newton observed SAX J2103.5+4545 on January 6, 2003, while RXTE was monitoring the source. Using RXTE-PCA dataset between December 3, 2002 and January 29, 2003, the spin period and average spin-up rate during the XMM-Newton observations were found to be $354.7940\pm0.0008$ s and $(7.4\pm0.9)\times10^{-13}$Hz s$^{-1}$ respectively. In the power spectrum of the 0.9-11 keV EPIC-PN lightcurve, we found quasi periodic oscillations around 0.044 Hz (22.7 s) with an rms fractional amplitude $\sim $6.6 %. We interpreted this QPO feature as the Keplerian motion of inhomogenuities through the inner disk. In the X-ray spectrum, in addition to the power law component with high energy cutoff and $\sim6.4$ keV fluorescent iron emission line (Baykal et al., 2002), we discovered a soft component consistent with a blackbody emission with ${\rm{kT}}\sim1.9$keV. The pulse phase spectroscopy of the source revealed that the blackbody flux peaked at the peak of the pulse with an emission radius $\sim 0.3$ km, suggesting the polar cap on the neutron star surface as the source of blackbody emission. The flux of the iron emission line at $\sim 6.42$ keV was shown to peak at the off-pulse phase, supporting the idea that this feature arises from fluorescent emission of the circumstellar material around the neutron star rather than the hot region in the vicinity of the neutron star polar cap., Comment: 16 pages 4 figures. Major revisions present. Accepted for publication in ApJ

CRESST II is an experiment for direct WIMP search, using cryogenic detectors. The ratio of the two signals (temperature rise and scintillation light) measured for each interaction is an excellent parameter for discrimination of radioactive background. The main remaining background is the neutron flux present at the experimental site, since neutrons produce the same signals as WIMPs do. Based on Monte Carlo simulations the present work shows how neutrons from different origins affect CRESST and which measures have to be taken into account to reach the goal sensitivity., Comment: 2 pages, to appear in IAU Symposium 220, "Dark Matter in Galaxies", eds. Stuart Ryder, D.J. Pisano, Mark Walker & Ken Freeman, Publ. Astron. Soc. Pac

We discuss whether the hypothesis that "all (or most) subdwarfs are in close binaries" is supported by the frequently reported observations of photometrically or spectroscopically composite character of many hot subdwarf stars. By way of a possible counter-argument, we focus on resolved companions (optical pairs) of hot subdwarf stars. On a statistical basis, many of these are physically associated with the hot subdwarfs, i.e. are common proper motion pairs. These resolved pairs make a several percent contribution to the catalog of hot subdwarf stars per decade in projected separation. If they are just the relatively wide members of a binary population similar to the local G-dwarf binary population (Duquennoy & Mayor 1991), which has a very wide distribution of orbital separations, then many of the unresolved but composite hot subdwarf binaries may not be "close" in the astrophysical sense. In that case, binary channels for hot subdwarf formation may be less important than thought, or must involve companions (white dwarfs) that do not result in a composite spectrum system., Comment: 4 pages, 1 figure. To appear in "Extreme Horizontal Branch Stars and Related Objects", Astrophysics and Space Science, Kluwer Academic Publishers, proceedings of the meeting held in Keele, UK, June 16-20, 2003

Utilizing the Two Micron All Sky Survey (2MASS) All-Sky Data Release Catalog, we have retrieved useful near-IR J, H, and Ks magnitudes for more than 800 hot subdwarfs (sdO and sdB stars) drawn from the "Catalogue of Spectroscopically Identified Hot Subdwarfs" (Kilkenny, Heber, & Drilling 1988, 1992). This sample size greatly exceeds previous studies of hot subdwarfs. We find that, of the hot subdwarfs in Kilkenny et al., ~40% in a magnitude limited sample have colors that are consistent with the presence of an unresolved late type (FGK) companion. Binary stars are over-represented in a magnitude limited sample. In an approximately volume limited sample the fraction of composite-color binaries is ~25%., Comment: 4 pages, 1 figure. To appear in "Extreme Horizontal Branch Stars and Related Objects", Astrophysics and Space Science, Kluwer Academic Publishers, proceedings of the meeting held in Keele, UK, June 16-20, 2003

Utilizing the Two Micron All Sky Survey (2MASS) Second Incremental Data Release Catalog, we have retrieved near-IR magnitudes for several hundred hot subdwarfs (sdO and sdB stars) drawn from the "Catalogue of Spectroscopically Identified Hot Subdwarfs" (Kilkenny, Heber, & Drilling 1988, 1992). This sample size greatly exceeds that of previous studies of hot subdwarfs. Examining 2MASS photometry alone or in combination with visual photometry (Johnson BV or Stromgren uvby) available in the literature, we show that it is possible to identify hot subdwarf stars that exhibit atypically red IR colors that can be attributed to the presence of an unresolved late type companion. Utilizing this large sample, we attempt for the first time to define an approximately volume limited sample of hot subdwarfs. We discuss the considerations, biases, and difficulties in defining such a sample. We find that, of the hot subdwarfs in Kilkenny et al., about 40% in a magnitude limited sample have colors that are consistent with the presence of an unresolved late type companion. Binary stars are over-represented in a magnitude limited sample. In an approximately volume limited sample the fraction of composite-color binaries is about 30%., Comment: to appear in Sept 2003 AJ, 41 pages total, 12 figures, 2 tables are truncated (full tables to appear in electronic journal or available by request)

The November 1999 outburst of the transient pulsar SAX J2103.5+4545 was monitored with the large area detectors of the Rossi X-Ray Timing Explorer until the pulsar faded after a year. The 358 s pulsar was spun up for 150 days, at which point the flux dropped quickly by a factor of 7, the frequency saturated and, as the flux continued to decline, a weak spin-down began. The pulses remained strong during the decay and the spin-up/flux correlation can be fit to the Ghosh and Lamb derivations for the spin-up caused by accretion from a thin, pressure-dominated disk, for a distance 3.2 kpc and a surface magnetic field 1.2 10^{13} Gauss. During the bright spin-up part of the outburst, the flux was subject to strong orbital modulation, peaking 3 days after periastron of the eccentric 12.68 day orbit, while during the faint part, there was little orbital modulation. The X-ray spectra were typical of accreting pulsars, describable by a cut-off power-law, with an emission line near the 6.4 keV of K alpha fluorescence from cool iron. The equivalent width of this emission did not share the orbital modulation, but nearly doubled during the faint phase, despite little change in the column density. The outburst could have been caused by an episode of increased wind from a Be star, such that a small accretion disk is formed during each periastron passage. A change in the wind and disk structure apparently occurred after 5 months such that the accretion rate was no longer modulated or the diffusion time was longer. The distance estimate implies the X-ray luminosity observed was between 1 10^{36} ergs s^{-1} and 6 \times 10^{34} ergs s^{-1}, with a small but definite correlation of the intrinsic power-law spectral index., Comment: Accepted for publication in Astrophysical Journal

Data taken by CRESST in 2000 with a cryogenic detector system based on 262 g sapphire crystals is used to place limits on WIMP dark matter in the Galactic Halo. The detector is especially sensitive for low-mass WIMPS with spin-dependent cross sections and improves on existing limits in this region. CRESST is now preparing for a second phase, which will use a 10 kg detector consisting of 300 g CaWO4 crystals with simultaneous detection of phonons and scintillation light to reduce background., Comment: 23 pages, 15 figures, Paper contributed to the X International Symposium on Lepton and Photon Interactions at High Energies, 23rd-28th July 2001, Rome, Italy

The CRESST experiment seeks hypothetical WIMP particles that could account for the bulk of dark matter in the Universe. The detectors are cryogenic calorimeters in which WIMPs would scatter elastically on nuclei, releasing phonons. The first phase of the experiment has successfully deployed several 262 g sapphire devices in the Gran Sasso underground laboratories. A main source of background has been identified as microscopic mechanical fracturing of the crystals, and has been eliminated, improving the background rate by up to three orders of magnitude at low energies, leaving a rate close to one count per day per kg and per keV above 10 keV recoil energy. This background now appears to be dominated by radioactivity, and future CRESST scintillating calorimeters which simultaneously measure light and phonons will allow rejection of a great part of it., Comment: To appear in the proceedings of the CAPP2000 Conference, Verbier, Switzerland, July, 2000 (eds J. Garcia-Bellido, R. Durrer, and M. Shaposhnikov)

Using X-ray data from the Rossi X-Ray Timing Explorer (RXTE), we carried out pulse timing analysis of the transient X-ray pulsar SAX J2103.5+4545. An outburst was detected by All Sky Monitor (ASM) October 25 1999 and reached a peak X-ray brightness of 27 mCrab October 28. Between November 19 and December 27, the RXTE/PCA carried out pointed observations which provided us with pulse arrival times. These yield an eccentric orbit (e= 0.4 \pm 0.2) with an orbital period of 12.68 \pm 0.25 days and light travel time across the projected semimajor axis of 72 \pm 6 sec. The pulse period was measured to be 358.62171 \pm 0.00088 s and the spin-up rate (2.50 \pm 0.15) \times 10^{-13} Hz s^{-1}. The ASM data for the February to September 1997 outburst in which BeppoSAX discovered SAX J2103.5+4545 (Hulleman, in't Zand and Heise 1998) are modulated at time scales close to the orbital period. Folded light curves of the 1997 ASM data and the 1999 PCA data are similar and show that the intensity increases at periastron passages., Comment: To appear in The Astrophysical Journal (Letters)

We analysed the RXTE archival data of 1E 1048.1-5937 covering a time span of more than one year. The spin down rate of this source decreases by 30 percent during the observation. We could not resolve the X-ray flux variations because of contamination by Eta Carinae. We find that the level of pulse frequency fluctuations of 1E 1048.1-5937 is consistent with typical noise levels of accretion powered pulsars. Recent RXTE observations of 1E 2259+586 have shown a constant spin down with a very low upper limit on timing noise. We used the RXTE archival X-ray observations of 1E 2259+586 to show that the intrinsic X-ray luminosity times series is also stable, with an rms fractional variation of less than 15 percent. The source could have been in a quiet phase of accretion with a constant X-ray luminosity and spin down rate., Comment: MNRAS in press

We present new X-ray observations of the X-ray pulsar 1E 2259+586, obtained during March 1997, with the Rossi X-Ray Timing Explorer (RXTE). We have measured the pulse frequency derivative $\dot \nu = (-1.08 \pm 0.04) \times 10^{-14}$ Hz s$^{-1}$ from pulse arrival times obtained in a sequence of 5 observations spread over one month. This $\dot\nu$ is consistent with the long term spin-down trend. We also found that the observed X-ray luminosity is consistent with that measured at quiescent X-ray flux levels by previous missions. Our observations imply that 1E 2259+586 was spinning down steadily without exhibiting any stochastic torque noise fluctuations during the month covered by our observations., Comment: 4 pages, Latex (l-aa), Accepted for publication in Astronomy and Astrophysics

GRO J1744-28, discovered by BATSE in December 95, is the second neutron star system known to produce frequent accretion powered bursts. The system has been regularly monitored with the RXTE PCA since the peak of the first outburst in January 96 at which time the observed persistent and bursting count rates were ~25,000 ct/sec and ~150,000 ct/sec, with corresponding instrumental deadtimes of ~10% and > 50%. We present a model which allows the reconstruction of the true incident count rate in the presence of enormous deadtime and use the model to derive a history of the peak luminosities and fluences of the bursts as a function of time. During the peak of the January 1996 and January 1997 outbursts, when the persistent emission was > 1 Crab, we infer peak luminosities of ~100 times the Eddington luminosity, and a ratio of persistent emission to integrated burst emission of ~34., Comment: 6 pages, To Appear in the Proceedings of the Symposium "The Active X-ray Sky", held October 21-24, 1997, Rome