Jeremy S. Heyl

Autofib Redshift Survey - I. Evolution of the galaxy luminosity function

We present a detailed determination of the restframe B-band galaxy luminosity function (LF) as a function of redshift and star formation activity from z=0 to z=0.75. The dataset used for this purpose is a combined sample of over 1700 redshifts spanning a wide range in apparent magnitude, 11.5<B<24.0, which we term the Autofib Redshift Survey. The sample includes various earlier magnitude-limited surveys constructed by our team as well as a new survey of 1026 redshifts measured for galaxies at intermediate magnitudes. The large range in apparent magnitude sampled allows us to investigate both the nature of the LF at low redshift (z<0.1) and possible evolution in its shape to z=0.75. We find that earlier bright surveys have underestimated the absolute normalisation of the LF. Because the shape of the local LF does not change with the survey apparent magnitude limit, it seems unlikely that the local deficiency arises from an underestimated population of low luminosity galaxies. Furthermore, surface brightness losses cannot be significant unless they conspire to retain the LF shape over a variety of detection thresholds. Our data directly demonstrates that the B-band LF evolves with redshift. This evolution is best represented as a steepening of the faint-end slope of the LF, from alpha=-1.1 at low redshift to alpha=-1.5 at z=0.5. Using [OII] emission as an indicator of star formation activity, we show that the LF of quiescent galaxies has remained largely unchanged since z=0.5, whereas the luminosity density of star-forming galaxies has declined by nearly a factor of 2. The steepening of the overall LF with lookback time is a direct consequence of the increasing space density of blue star-forming galaxies at moderate redshifts.

Changes in the X-ray Emission from the Magnetar Candidate 1E 2259+586 during its 2002 Outburst

An outburst of more than 80 individual bursts, similar to those seen from Soft Gamma Repeaters (SGRs), was detected from the anomalous X-ray pulsar (AXP) 1E 2259+586 in 2002 June. Coincident with this burst activity were gross changes in the pulsed flux, persistent flux, energy spectrum, pulse profile, and spin-down of the underlying X-ray source. We present Rossi X-Ray Timing Explorer and X-Ray Multi-Mirror Mission observations of 1E 2259+586 that show the evolution of the aforementioned source parameters during and following this episode and identify recovery timescales for each. Specifically, we observe an X-ray flux increase (pulsed and phase-averaged) by more than an order of magnitude having two distinct components. The first component is linked to the burst activity and decays within ~2 days, during which the energy spectrum is considerably harder than during the quiescent state of the source. The second component decays over the year following the glitch according to a power law in time with an exponent -0.22 ? 0.01. The pulsed fraction decreased initially to ~15% rms but recovered rapidly to the preoutburst level of ~23% within the first 3 days. The pulse profile changed significantly during the outburst and recovered almost fully within 2 months of the outburst. A glitch of size ??max/? = (4.24 ? 0.11) ? 10-6 was observed in 1E 2259+586, which preceded the observed burst activity. The glitch could not be well fitted with a simple partial exponential recovery. An exponential rise of ~20% of the frequency jump with a timescale of ~14 days results in a significantly better fit to the data; however, contamination from a systematic drift in the phase of the pulse profile cannot be excluded. A fraction of the glitch (~19%) was recovered in a quasi-exponential manner having a recovery timescale of ~16 days. The long-term postglitch spin-down rate decreased in magnitude relative to the preglitch value. The changes in the source properties of 1E 2259+586 during its 2002 outburst are shown to be qualitatively similar to changes seen during or following burst activity in two SGRs, thus further solidifying the common nature of SGRs and AXPs as magnetars. The changes in persistent emission properties of 1E 2259+586 suggest that the star underwent a plastic deformation of the crust that simultaneously impacted the superfluid interior (crustal and possibly core superfluid) and the magnetosphere. Finally, the changes in persistent emission properties coincident with burst activity in 1E 2259+586 enabled us to infer previous burst-active episodes from this and other AXPs. The nondetection of these outbursts by all-sky gamma-ray instruments suggests that the number of active magnetar candidates in our Galaxy is larger than previously thought.

How common are magnetars? the consequences of magnetic-field decay

Ultramagnetized neutron stars, or magnetars, have been invoked to explain several astrophysical phenomena. We examine how the magnetic field of a magnetar will decay over time and how this decay affects the cooling of the object. We find that for sufficiently strong nascent fields, field decay alters significantly the cooling evolution relative to similarly magnetized neutron stars with constant fields. As a result, old magnetars can be expected to be bright in the soft X-ray band. The soft X-ray source RX J0720.4-3125 may well be the nearest such old magnetar.

Structure of merger remnants. III: Phase-space constraints

This paper explores the evolution of the coarsely grained phase-space density in mergers and in galaxy formation. In particular, numerical simulations are used to determine the properties of remnants produced by major mergers between equal-mass galaxies. Contrary to some existing claims, remnants of mergers between stellar disks are found to lack sufficient material at high phase-space densities to be identified as elliptical galaxies. We quantify this effect by computing the cumulative coarsely grained phase-space distribution, s(f), for the remnants and compare it to that derived from simple models of the mass profiles of ellipticals

THE CENTRAL X-RAY POINT SOURCE IN CASSIOPEIA A

The spectacular ii —rst light ˇˇ observation by the Chandra X-Ray Observatory revealed an X-ray point source near the center of the 300 yr old Cas A supernova remnant. We present an analysis of the public X-ray spectral and timing data. No coherent pulsations were detected in the Chandra/HRC data. The 3 p upper limit on the pulsed fraction is less than 35% for P ( 20 ms. The Chandra/ACIS spectrum of the point source may be —tted with an ideal blackbody (kT \ 0.5 keV) or with blackbody models modi—ed by the presence of a neutron star atmosphere (kT \ 0.25¨0.35 keV), but the temperature is higher and the inferred emitting area lower than expected for a 300 yr old neutron star according to standard cooling models. The spectrum may also be —tted with a power-law model (photon index ! \ 2.8¨3.6). Both the spectral properties and the timing limits of the point source are inconsistent with a young Crab-like pulsar but are quite similar to the properties of the anomalous X-ray pulsars. The spectral parameters are also very similar to those of the other radio-quiet X-ray point sources in the supernova remnants Pup A, RCW 103, and PKS 1209(52. Current limits on an optical counterpart for the Cas A point source rule out models that invoke fallback accretion onto a compact object if fallback disk properties are similar to those in quiescent low-mass X-ray binaries. However, the optical limits are mar- ginally consistent with plausible alternative assumptions for a fallback disk. In this case, accreting neutron star models can explain the X-ray data, but an accreting black hole model is not promising. Subject headings: accretion, accretion disksblack hole physicsstars: neutron ¨ supernovae: individual (Cassiopeia A) ¨ supernova remnantsX-rays: stars

Autofib Redshift Survey — II. Evolution of the galaxy luminosity function by spectral type

We determine the evolution of the galaxy luminosity function (LF) as a function of spectral type using the Autofib redshift survey, a compendium of over 1700 galaxy redshifts in various magnitude-limited samples spanning b_J=11.5-24.0. To carry out this analysis we have developed a cross-correlation technique which classifies faint galaxy spectra into one of six types based on local galaxy templates. Tests and simulations show that this technique yields classifications correct to within one type for more than 90% of the galaxies in our sample. We have also developed extensions of the step-wise maximum likelihood method and the STY parametric method for estimating LFs which are applicable to recovering an evolving LF from multiple samples. We find that: (i) The spectra and LF of E/S0 galaxies show no appreciable evolution out to at least z ~ 0.5. (ii) Early-type spirals show modest evolution, characterised by a gradual steepening of the faint end of their LF with redshift. (iii) Out to z ~ 0.5, the overall evolution of the galaxy population is dominated by changes seen in late-type spirals. The characteristic luminosity (L^*) of these galaxies appears to brighten with redshift and there are signs of strong density evolution (a rapid increase in \phi^*). These effects appear to be luminosity dependent so that the LF steepens at higher redshift. These trends are accompanied by a steep increase in the median [OII] equivalent width, implying a rapid increase in the star-formation rate with redshift at fixed luminosity---a given star-formation rate is found at higher redshift in galaxies of higher luminosity. We find that these conclusions are robust with respect to spectral classification errors and the luminosity function estimator.

Using long‐term transit timing to detect terrestrial planets

ABSTRACT We propose that the presence of additional planets in extrasolar planetary systemscan be detected by long-term transit timing studies. If a transiting planet is on aneccentric orbit then the presence of another planet causes a secular advance of thetransiting planet’s pericenter over and above the effect of general relativity. Althoughthis secular effect is impractical to detect over a small number of orbits, it causes long-term differences in when future transits occur, much like the long-term decay observedin pulsars. Measuring this transit-timing delay would thus allow the detection of eitherone or more additional planets in the system or the first measurements of non-zerooblateness (J 2 ) of the central stars.Keywords: planetarysystems–celestial mechanics–gravitation–extrasolarplanets– stellar oblateness 1 INTRODUCTIONThe study of long-term orbital precession was one of thetriumphs of celestial mechanics, when the planetary theo-ries of Laplace and Lagrange showed that essentially all theknown long-term precessions of the planetary orbits couldbe explained by their mutual gravitational interaction. Theperturbation caused by the small planetary masses ‘breaks’the perfect central force character of the Sun’s gravitationalfield, causing the planetary orbital nodes to regress and theirperihelia to slowly advance, with typical periods of 10

Thermonuclear Stability of Material Accreting onto a Neutron Star

We present a global linear stability analysis of nuclear fuel accumulating on the surface of an accreting neutron star, and we identify the conditions under which thermonuclear bursts are triggered. The analysis reproduces all the recognized regimes of hydrogen and helium bursts and in addition shows that at high accretion rates, near the limit of stable burning, there is a regime of delayed mixed bursts that is distinct from the more usual prompt mixed bursts. In delayed mixed bursts, a large fraction of the fuel is burned stably before the burst is triggered. Bursts thus have longer recurrence times but at the same time have somewhat smaller fluences. Therefore, the parameter ?, which measures the ratio of the energy released via accretion to that generated through nuclear reactions in the burst, is up to an order of magnitude larger than for prompt bursts. This increase in ? near the threshold of stable burning has been seen in observations. We explore a wide range of mass accretion rates, neutron star radii, and core temperatures and calculate a variety of burst properties. From a preliminary comparison with data, we suggest that bursting neutron stars may have hot cores, with Tcore 107.5 K, consistent with interior cooling via the modified Urca or similar low-efficiency process, rather than Tcore ~ 107 K, as expected for the direct Urca process. There is also an indication that neutron star radii are somewhat small, 10 km. Both of these conclusions need to be confirmed by comparing more careful calculations with better data.

Birefringence and dichroism of the QED vacuum

We use an analytic form for the Heisenberg - Euler Lagrangian to calculate the birefringent and dichroic properties of the vacuum for arbitrarily strong wrenchless fields.

Powering Anomalous X-Ray Pulsars by Neutron Star Cooling

Using recently calculated analytic models for the thermal structure of ultramagnetized neutron stars, we estimate the thermal fluxes from young (t~1000 yr) ultramagnetized (B~1015 G) cooling neutron stars. We find that the pulsed X-ray emission from objects such as 1E 1841-045 and 1E 2259+586, as well as many soft-gamma repeaters, can be explained by photon cooling if the neutron star possesses a thin insulating envelope of matter of low atomic weight at densities ρ<107-108 g cm−3. The total mass of this insulating layer is M~10−11-10−8 M☉.