Robert E. Rutledge

Crustal Heating and Quiescent Emission from Transiently Accreting Neutron Stars

Nuclear reactions occurring at densities ≈ 1012 g cm-3 in the crust of a transiently accreting neutron star efficiently maintain the core at a temperature ≈ (5-10)×107 K. When accretion halts, the envelope relaxes to a thermal equilibrium set by the flux from the hot core, as if the neutron star were newly born. For the time-averaged accretion rates (10-10 M☉ yr-1) typical of low-mass X-ray transients, standard neutrino cooling is unimportant and the core thermally reradiates the deposited heat. The resulting luminosity is ~5×1032-5×1033 ergs s-1 and agrees with many observations of transient neutron stars in quiescence. Confirmation of this mechanism would strongly constrain rapid neutrino cooling mechanisms for neutron stars (e.g., a pion condensate). Thermal emission had previously been dismissed as a predominant source of quiescent emission since blackbody spectral fits implied an emitting area much smaller than a neutron stars surface. However, as with thermal emission from radio pulsars, fits with realistic emergent spectra will imply a substantially larger emitting area. Other emission mechanisms, such as accretion or a pulsar shock, can also operate in quiescence and generate intensity and spectral variations over short timescales. Indeed, quiescent accretion may produce gravitationally redshifted metal photoionization edges in the quiescent spectra (detectable with AXAF and XMM). We discuss past observations of Aql X-1 and note that the low-luminosity (less than 1034 ergs s-1) X-ray sources in globular clusters and the Be star/X-ray transients are excellent candidates for future study.

XID: Cross-association of rosat/bright source catalog X-ray sources with USNO A-2 optical point sources

We quantitatively cross-associate the 18,811 ROSAT Bright Source Catalog (RASS/BSC) X-ray sources with optical sources in the USNO A-2 catalog, calculating the probability of unique association (Pid) between each candidate within 75 of the X-ray source position, on the basis of optical magnitude and proximity. We present catalogs of RASS/BSC sources for which P_(id) > 98%, P_(id) > 90%, and P_(id) > 50%, which contain 2705, 5492, and 11,301 unique USNO A-2 optical counterparts respectively down to the stated level of significance. Together with identifications of objects not cataloged in USNO A-2 due to their high surface brightness (M31, M32, ...) and optical pairs, we produced a total of 11,803 associations to a probability of P_(id) > 50%. We include in this catalog a list of objects in the SIMBAD database within 10 of the USNO A-2 position, as an aid to identification and source classification. This is the first RASS/BSC counterpart catalog which provides a probability of association between each X-ray source and counterpart, quantifying the certainty of each individual association. The catalog is more useful than previous catalogs which either rely on plausibility arguments for association or do not aid in selecting a counterpart between multiple off-band sources in the field. Sources of high probability of association can be separated out, to produce high-quality lists of classes (Seyfert 1/2s, QSOs, RS CVns) desired for targeted study, or for discovering new examples of known classes (or new classes altogether) through the spectroscopic classification of securely identified but unclassified USNO A-2 counterparts. Low P_(id) associations can be used for statistical studies and follow-on investigation—for example, performing follow-up spectroscopy of the many low-mass stars to search for signatures of coronal emission, or to investigate the relationship between X-ray emission and classes of sources not previously well-studied for their X-ray emissions (such as pulsating variable stars). We find that a fraction ~65.8% of RASS/BSC sources have an identifiable optical counterpart, down to the magnitude limit of the USNO A-2 catalog which could be identified by their spatial proximity and high optical brightness.

Quasi-periodic oscillations in black hole candidates as an indicator of transition between low and high states

By comparing positions on a spectral color-color diagram from 10 black hole candidates (BHCs) observed with Ginga (1354-64, 1826-24, 1630-47, LMC X-1, LMC X-3, GS 2000+25, GS 2023+33, GS 1124-68, Cyg X-1, and GX 339-4) with the observed broadband noise (BBN) (0.001-64 Hz) and quasi-periodic oscillation (QPO) variability, we find that the very high state is spectrally intermediate to the soft/high state and hard/low state. We find a transition point in spectral hardness where the dependence of the BHC QPO centroid frequency (of GS 1124-68 and GX 339-4) on spectral hardness switches from a correlation to an anticorrelation; where the BBN variability switches from high state to low state; and where the spectral hardness of the QPO relative to that of the BBN variability is a maximum. This coincidence of changing behavior in both the QPO and the broadband variability leads us to hypothesize that the QPO is due to interaction between the physical components which dominate the behaviors of BHCs when they occupy the hard/low and soft/high states. We conclude that these QPOs should be observed from BHCs during transition between these two states. Comparison with QPO and BBN behavior observed during the 1996 transition of Cyg X-1 supports this hypothesis. We also report 1-3 Hz QPOs observed in GS 2000+25 and Cyg X-1 in the hard/low state, and we compare these to the QPOs observed in GS 1124-68 and GX 339-4.

Sheets and Filaments as the Origin of the High-Velocity Tail of the Lyα Forest

Simulations of large-scale structure formation predict the formation of sheet and filamentary structures, which are often invoked as the origin of the Lyα forest. In their simplest description, these sheets and filaments require a differential distribution of observed line-of-sight velocity widths (b), which will decrease as power laws at velocities well above the observed peak in this distribution; for filaments, the differential distribution is dN/db ∝ b-3, while for sheets it is dN/db ∝ b-2. These functional dependences on b arise a priori because of the geometry of these absorbing structures—assuming random orientations relative to the line of sight—and are otherwise unrelated to the physical state in the absorbing structure. We find the distribution at b > 35 km s-1 in three previously published data sets to be steeper than dN/dB ∝ b-3 (99.99% confidence). This implies that evidence of the finite length of these kinds of absorbing structures is present in the b-distribution data.

On the galactic distribution of gamma‐ray bursts

Quashnock & Lamb (hereafter QL) defined a sub‐sample of Gamma Ray Bursts (GRBs) from the publicly availably BATSE database which shows clumping toward the galactic plane, and they concluded that all GRBs are galactic in origin. The selection of these bursts involved a peak count‐rate [B, in counts (1024 ms)−1] which is uncorrected for aspect. Using, as limits, the corresponding peak fluxes (in photons cm−2u2009s−1) for the bursts in the QL sample, we find an additional 24 bursts, which we include in a new sample (Sample 2). We assert that the peak flux of a burst is physically more meaningful than peak count‐rate, as used by QL. We find that the significance of deviation from isotropy due to a possible galactic population in Sample 2 is much less than QL’s sample, which does not support QL’s interpretation of the anisotropies as being due to a galactic population. To make meaningful statistical statements regarding isotropy, burst samples must have peak fluxes above a minimum flux (Ipeak,LL1024), which is set...