arXiv subjects

David L. Kaplan

Publications and source records attributed to David L. Kaplan.

At least 199 records · Page 11Linked to original sources

arXiv2014

Serendipitous discovery of a dying Giant Radio Galaxy associated with NGC 1534, using the Murchison Widefield Array

Recent observations with the Murchison Widefield Array at 185~MHz have serendipitously unveiled a heretofore unknown giant and relatively nearby ($z = 0.0178$) radio galaxy associated with NGC\,1534. The diffuse emission presented here is the first indication that NGC\,1534 is one of a rare class of objects (along with NGC\,5128 and NGC\,612) in which a galaxy with a prominent dust lane hosts radio emission on scales of $\sim$700\,kpc. We present details of the radio emission along with a detailed comparison with other radio galaxies with disks. NGC1534 is the lowest surface brightness radio galaxy known with an estimated scaled 1.4-GHz surface brightness of just 0.2\,mJy\,arcmin$^{-2}$. The radio lobes have one of the steepest spectral indices yet observed: $α=-2.1\pm0.1$, and the core to lobe luminosity ratio is $<0.1$\%. We estimate the space density of this low brightness (dying) phase of radio galaxy evolution as $7\times10^{-7}$\,Mpc$^{-3}$ and argue that normal AGN cannot spend more than 6\% of their lifetime in this phase if they all go through the same cycle.

astro-ph.GA↗

arXiv2014

Limits on low frequency radio emission from southern exoplanets with the Murchison Widefield Array

We present the results of a survey for low frequency radio emission from 17 known exoplanetary systems with the Murchison Widefield Array. This sample includes 13 systems that have not previously been targeted with radio observations. We detected no radio emission at 154 MHz, and put 3 sigma upper limits in the range 15.2-112.5 mJy on this emission. We also searched for circularly polarised emission and made no detections, obtaining 3 sigma upper limits in the range 3.4-49.9 mJy. These are comparable with the best low frequency radio limits in the existing literature and translate to luminosity limits of between 1.2 x 10^14 W and 1.4 x 10^17 W if the emission is assumed to be 100% circularly polarised. These are the first results from a larger program to systematically search for exoplanetary emission with the MWA.

astro-ph.EP↗

arXiv2014

Study of Redshifted HI from the Epoch of Reionization with Drift scan

The detection of the Epoch of Reionization (EoR) in the redshifted 21-cm line is a challenging task. Here we formulate the detection of the EoR signal using the drift scan strategy. This method potentially has better instrumental stability as compared to the case where a single patch of sky is tracked. We demonstrate that the correlation time between measured visibilities could extend up to 1-2 hr for an interferometer array such as the Murchison Widefield Array (MWA), which has a wide primary beam. We estimate the EoR power based on cross-correlation of visibilities across time and show that the drift scan strategy is capable of the detection of the EoR signal with comparable/better signal-to-noise as compared to the tracking case. We also estimate the visibility correlation for a set of bright point sources and argue that the statistical inhomogeneity of bright point sources might allow their separation from the EoR signal.

astro-ph.IM↗

arXiv2014

Identification of the Optical Counterpart of Fermi Black Widow Millisecond Pulsar PSR J1544+4937

We report the optical identification of the companion to the {\it Fermi} black widow millisecond pulsar PSR J1544+4937. We find a highly variable source on Keck LRIS images at the nominal pulsar position, with 2 magnitude variations over orbital period in the B, g, R, and I bands. The nearly achromatic light curves are difficult to explain with a simply irradiated hemisphere model, and suggest that the optical emission is dominated by a nearly isothermal hot patch on the surface of the companion facing the pulsar. We roughly constrain the distance to PSR J1544+4937 to be between 2 and 5 kpc. A more reliable distance measurement is needed in order to constrain the composition of the companion.

astro-ph.HE↗

arXiv2014

A 1.05 $M_\odot$ Companion to PSR J2222-0137: The Coolest Known White Dwarf?

The recycled pulsar PSR J2222-0137 is one of the closest known neutron stars, with a parallax distance of $267_{-0.9}^{+1.2}\,$pc and an edge-on orbit. We measure the Shapiro delay in the system through pulsar timing with the Green Bank Telescope, deriving a low pulsar mass ($1.20\pm0.14$ $M_\odot$) and a high companion mass ($1.05\pm0.06$ $M_\odot$) consistent with either a low-mass neutron star or a high-mass white dwarf. We can largely reject the neutron star hypothesis on the basis of the system's extremely low eccentricity (3e-4) - too low to have been the product of two supernovae under normal circumstances. However, despite deep optical and near-infrared searches with SOAR and the Keck telescopes we have not discovered the optical counterpart of the system. This is consistent with the white dwarf hypothesis only if the effective temperature is <3000 K, a limit that is robust to distance, mass, and atmosphere uncertainties. This would make the companion to PSR J2222-0137 one of the coolest white dwarfs ever observed. For the implied age to be consistent with the age of the Milky Way requires the white dwarf to have already crystallized and entered the faster Debye-cooling regime.

astro-ph.SR↗

arXiv2014

Spectroscopy of the inner companion of the pulsar PSR J0337+1715

The hierarchical triple system PSR J0337+1715 offers an unprecedented laboratory to study secular evolution of interacting systems and to explore the complicated mass-transfer history that forms millisecond pulsars and helium-core white dwarfs. The latter in particular, however, requires knowledge of the properties of the individual components of the system. Here we present precise optical spectroscopy of the inner companion in the PSR J0337+1715 system. We confirm it as a hot, low-gravity DA white dwarf with Teff=15,800+/-100 K and log(g)=5.82+/-0.05. We also measure an inner mass ratio of 0.1364+/-0.0015, entirely consistent with that inferred from pulsar timing, and a systemic radial velocity of 29.7+/-0.3 km/s. Combined with the mass (0.19751 Msun) determined from pulsar timing, our measurement of the surface gravity implies a radius of 0.091+/-0.005 Rsun; combined further with the effective temperature and extinction, the photometry implies a distance of 1300+/-80 pc. The high temperature of the companion is somewhat puzzling: with current models, it likely requires a recent period of unstable hydrogen burning, and suggests a surprisingly short lifetime for objects at this phase in their evolution. We discuss the implications of these measurements in the context of understanding the PSR J0337+1715 system, as well as of low-mass white dwarfs in general.

astro-ph.SR↗

arXiv2013

Properties of the Eclipsing Double-White Dwarf Binary NLTT 11748

We present high-quality ULTRACAM photometry of the eclipsing detached double-white dwarf binary NLTT 11748. This system consists of a carbon/oxygen white dwarf and an extremely-low mass (< 0.2 Msun) helium-core white dwarf in a 5.6 hr orbit. To date such extremely-low mass WDs, which can have thin, stably-burning outer layers, have been modeled via poorly-constrained atmosphere and cooling calculations where uncertainties in the detailed structure can strongly influence the eventual fates of these systems when mass-transfer begins. With precise (individual precision ~1%) high-cadence (~2 s) multi-color photometry of multiple primary and secondary eclipses spanning >1.5 yr, we constrain the masses and radii of both objects in the NLTT 11748 system to a statistical uncertainty of a few percent. However, we find that overall uncertainty in the thickness of the envelope of the secondary carbon/oxygen white dwarf leads to a larger (~13%) systematic uncertainty in the primary He WD's mass. Over the full range of possible envelope thicknesses we find that our primary mass (0.136-0.162 Msun) and surface gravity (log(g)=6.32-6.38; radii are 0.0423-0.0433 Rsun) constraints do not agree with previous spectroscopic determinations. We use precise eclipse timing to detect the Romer delay at 7 sigma significance, providing an additional weak constraint on the masses and limiting the eccentricity to e*cos(omega)= -4e-5 +/- 5e-5. Finally, we use multi-color data to constrain the secondary's effective temperature (7600+/-120 K) and cooling age (1.6-1.7 Gyr).

astro-ph.SR↗

arXiv2013

Neuronal Growth as Diffusion in an Effective Potential

Current understanding of neuronal growth is mostly qualitative, as the staggering number of physical and chemical guidance cues involved prohibit a fully quantitative description of axonal dynamics. We report on a general approach that describes axonal growth in vitro, on poly-D-lysine coated glass substrates, as diffusion in an effective external potential, representing the collective contribution of all causal influences on the growth cone. We use this approach to obtain effective growth rules that reveal an emergent regulatory mechanism for axonal pathfinding on these substrates.

physics.bio-ph↗

arXiv2013

Temperature response of the neuronal cytoskeleton mapped via atomic force and fluorescence microscopy

Neuronal cells change their growth properties in response to external physical stimuli such as variations in external temperature, stiffness of the growth substrate, or topographical guidance cues. Detailed knowledge of the mechanisms that control these biomechanical responses is necessary for understanding the basic principles that underlie neuronal growth and regeneration. Here, we present elasticity maps of living cortical neurons (embryonic rat) as a function of temperature, and correlate these maps to the locations of internal structural components of the cytoskeleton. Neurons display a significant increase in the average elastic modulus upon a decrease in ambient temperature from 37°C to 25°C. We demonstrate that the dominant mechanism by which the elasticity of the neurons changes in response to temperature is the stiffening of the actin components of the cytoskeleton induced by myosin II. We also report a reversible shift in the location and composition of the high-stiffness areas of the neuron cytoskeleton with temperature. At 37°C the areas of the cell displaying high elastic modulus overlap with the tubulin-dense regions, while at 25°C these high-stiffness areas correspond to the actin-dense regions of the cytoskeleton. These results demonstrate the importance of considering temperature effects when investigating cytoskeletal dynamics in cells.

physics.bio-ph↗

arXiv2013

A study of fundamental limitations to statistical detection of redshifted HI from the epoch of reionization

In this paper we explore for the first time the relative magnitudes of three fundamental sources of uncertainty, namely, foreground contamination, thermal noise and sample variance in detecting the HI power spectrum from the Epoch of Reionization (EoR). We derive limits on the sensitivity of a Fourier synthesis telescope to detect EoR based on its array configuration and a statistical representation of images made by the instrument. We use the Murchison Widefield Array (MWA) configuration for our studies. Using a unified framework for estimating signal and noise components in the HI power spectrum, we derive an expression for and estimate the contamination from extragalactic point-like sources in three-dimensional k-space. Sensitivity for EoR HI power spectrum detection is estimated for different observing modes with MWA. With 1000 hours of observing on a single field using the 128-tile MWA, EoR detection is feasible (S/N > 1 for $k\lesssim 0.8$ Mpc$^{-1}$). Bandpass shaping and refinements to the EoR window are found to be effective in containing foreground contamination, which makes the instrument tolerant to imaging errors. We find that for a given observing time, observing many independent fields of view does not offer an advantage over a single field observation when thermal noise dominates over other uncertainties in the derived power spectrum.

astro-ph.CO↗

arXiv2013

Serendipitous Discovery of An Infrared Bow Shock Near PSR J1549-4848 with Spitzer

We report on the discovery of an infrared cometary nebula around PSR J1549$-$4848 in our Spitzer survey of a few middle-aged radio pulsars. Following the discovery, multi-wavelength imaging and spectroscopic observations of the nebula were carried out. We detected the nebula in Spitzer IRAC 8.0, MIPS 24 and 70 $μ$m imaging and in Spitzer IRS 7.5--14.4 $μ$m spectroscopic observations, and also in the WISE all-sky survey at 12 and 22 $μ$m.These data were analyzed in detail, and we find that the nebula can be described with a standard bow-shock shape, and that its spectrum contains polycyclic aromatic hydrocarbon and H$_2$ emission features. However, it is not certain which object drives the nebula. We analyze the field stars and conclude that none of them can be the associated object because stars with a strong wind or mass ejection that usually produce bow shocks are much brighter than the field stars. The pulsar is approximately 15\arcsec\ away from the region in which the associated object is expected to be located. In order to resolve the discrepancy, we suggest that a highly collimated wind could be emitted from the pulsar and produce the bow shock. X-ray imaging to detect the interaction of the wind with the ambient medium and high-spatial resolution radio imaging to determine the proper motion of the pulsar should be carried out, which will help verify the association of the pulsar with the bow shock nebula.

astro-ph.HE↗

arXiv2013

Elasticity Maps of Living Neurons Measured by Combined Fluorescence and Atomic Force Microscopy

Detailed knowledge of mechanical parameters such as cell elasticity, stiffness of the growth substrate, or traction stresses generated during axonal extensions is essential for understanding the mechanisms that control neuronal growth. Here we combine Atomic Force Microscopy based force spectroscopy with Fluorescence Microscopy to produce systematic, high-resolution elasticity maps for three different types of live neuronal cells: cortical (embryonic rat), embryonic chick dorsal root ganglion, and P-19 (mouse embryonic carcinoma stem cells) neurons. We measure how the stiffness of neurons changes both during neurite outgrowth and upon disruption of microtubules of the cell. We find reversible local stiffening of the cell during growth, and show that the increase in local elastic modulus is primarily due to the formation of microtubules. We also report that cortical and P-19 neurons have similar elasticity maps, with elastic moduli in the range 0.1-2 kPa, with typical average values of 0.4 kPa (P-19) and 0.2 kPa (cortical). In contrast, DRG neurons are stiffer than P-19 and cortical cells, yielding elastic moduli in the range 0.1-8 kPa, with typical average values of 0.9 kPa. Finally, we report no measurable influence of substrate protein coating on cell body elasticity for the three types of neurons.

physics.bio-ph↗

arXiv2013

The hunt for new pulsars with the Green Bank Telescope

The Green Bank Telescope (GBT) is the largest fully steerable radio telescope in the world and is one of our greatest tools for discovering and studying radio pulsars. Over the last decade, the GBT has successfully found over 100 new pulsars through large-area surveys. Here I discuss the two most recent---the GBT 350 MHz Drift-scan survey and the Green Bank North Celestial Cap survey. The primary science goal of both surveys is to find interesting individual pulsars, including young pulsars, rotating radio transients, exotic binary systems, and especially bright millisecond pulsars (MSPs) suitable for inclusion in Pulsar Timing Arrays, which are trying to directly detect gravitational waves. These two surveys have combined to discover 85 pulsars to date, among which are 14 MSPs and many unique and fascinating systems. I present highlights from these surveys and discuss future plans. I also discuss recent results from targeted GBT pulsar searches of globular clusters and Fermi sources.

astro-ph.HE↗

arXiv2013

Science with the Murchison Widefield Array

Significant new opportunities for astrophysics and cosmology have been identified at low radio frequencies. The Murchison Widefield Array is the first telescope in the Southern Hemisphere designed specifically to explore the low-frequency astronomical sky between 80 and 300 MHz with arcminute angular resolution and high survey efficiency. The telescope will enable new advances along four key science themes, including searching for redshifted 21 cm emission from the epoch of reionisation in the early Universe; Galactic and extragalactic all-sky southern hemisphere surveys; time-domain astrophysics; and solar, heliospheric, and ionospheric science and space weather. The Murchison Widefield Array is located in Western Australia at the site of the planned Square Kilometre Array (SKA) low-band telescope and is the only low-frequency SKA precursor facility. In this paper, we review the performance properties of the Murchison Widefield Array and describe its primary scientific objectives.

astro-ph.IM↗

arXiv2013

A Metal-Rich Low-Gravity Companion to a Massive Millisecond Pulsar

Most millisecond pulsars with low-mass companions are in systems with either helium-core white dwarfs or non-degenerate ("black widow" or "redback") stars. A candidate counterpart to PSR J1816+4510 was identified by Kaplan et al. (2012) whose properties were suggestive of both types of companions although identical to neither. We have assembled optical spectroscopy of the candidate companion and confirm that it is part of the binary system with a radial velocity amplitude of 343+/-7 km/s, implying a high pulsar mass, Mpsr*sin^3i=1.84+/-0.11 Msun, and a companion mass Mc*sin^3i=0.192+/-0.012 Msun, where i is the inclination of the orbit. The companion appears similar to proto-white dwarfs/sdB stars, with a gravity log(g)=4.9+/-0.3, and effective temperature 16000+/-500 K. The strongest lines in the spectrum are from hydrogen, but numerous lines from helium, calcium, silicon, and magnesium are present as well, with implied abundances of roughly ten times solar (relative to hydrogen). As such, while from the spectrum the companion to PSR J1816+4510 is superficially most similar to a low-mass white dwarf, it has much lower gravity, is substantially larger, and shows substantial metals. Furthermore, it is able to produce ionized gas eclipses, which had previously been seen only for low-mass, non-degenerate companions in redback or black widow systems. We discuss the companion in relation to other sources, but find we understand neither its nature nor its origins. Thus, the system is interesting for understanding unusual stellar products of binary evolution, as well as, independent of its nature, for determining neutron-star masses.

astro-ph.SR↗

arXiv2012

Continuous Gravitational Waves from Isolated Galactic Neutron Stars in the Advanced Detector Era

We consider a simulated population of isolated Galactic neutron stars. The rotational frequency of each neutron star evolves through a combination of electromagnetic and gravitational wave emission. The magnetic field strength dictates the dipolar emission, and the ellipticity (a measure of a neutron star's deformation) dictates the gravitational wave emission. Through both analytic and numerical means, we assess the detectability of the Galactic neutron star population and bound the magnetic field strength and ellipticity parameter space of Galactic neutron stars with or without a direct gravitational wave detection. While our simulated population is primitive, this work establishes a framework by which future efforts can be conducted.

gr-qc↗

arXiv2012

Comparing H-alpha and HI Surveys as Means to a Complete Local Galaxy Catalog in the Advanced LIGO/Virgo Era

Identifying the electromagnetic counterparts of gravitational wave (GW) sources detected by upcoming networks of advanced ground-based interferometers will be challenging due in part to the large number of unrelated astrophysical transients within the ~10-100 square degree sky localizations. A potential way to greatly reduce the number of such false positives is to limit detailed follow-up to only those candidates near galaxies within the GW sensitivity range of ~200 Mpc for binary neutron star mergers. Such a strategy is currently hindered by the fact that galaxy catalogs are grossly incomplete within this volume. Here we compare two methods for completing the local galaxy catalog: (1) a narrow-band H-alpha imaging survey; and (2) an HI emission line radio survey. Using H-alpha fluxes, stellar masses (M_star), and star formation rates (SFR) from galaxies in the Sloan Digital Sky Survey (SDSS), combined with HI data from the GALEX Arecibo SDSS Survey and the Herschel Reference Survey, we estimate that a H-alpha survey with a luminosity sensitivity of L_H-alpha = 1e40 erg/s at 200 Mpc could achieve a completeness of f_SFR ~ 75% with respect to total SFR, but only f_Mstar ~ 33% with respect to stellar mass (due to lack of sensitivity to early-type galaxies). These numbers are significantly lower than those achieved by an idealized spectroscopic survey due to the loss of H-alpha flux resulting from resolving out nearby galaxies and the inability to correct for the underlying stellar continuum. An HI survey with sensitivity similar to the proposed WALLABY survey on ASKAP could achieve f_SFR ~ 80% and f_Mstar ~ 50%, somewhat higher than that of the H-alpha survey. Finally, both H-alpha and HI surveys should achieve > 50% completeness with respect to the host galaxies of short duration gamma-ray bursts, which may trace the population of binary neutron star mergers.

astro-ph.CO↗

arXiv2012

Orbital Evolution of Compact White Dwarf Binaries

The new-found prevalence of extremely low mass (ELM, Mhe<0.2 Msun) helium white dwarfs (WDs) in tight binaries with more massive WDs has raised our interest in understanding the nature of their mass transfer. Possessing small (Menv~1e-3 Msun) but thick hydrogen envelopes, these objects have larger radii than cold WDs and so initiate mass transfer of H-rich material at orbital periods of 6-10 minutes. Building on the original work of D'Antona et al., we confirm the 1e6 yr period of continued inspiral with mass transfer of H-rich matter and highlight that the inspiraling direct-impact double WD binary HM Cancri likely has an ELM WD donor. The ELM WDs have less of a radius expansion under mass loss, thus enabling a larger range of donor masses that can stably transfer matter and become a He mass transferring AM CVn binary. Even once in the long-lived AM CVn mass transferring stage, these He WDs have larger radii due to their higher entropy from the prolonged H burning stage.

astro-ph.SR↗