arXiv subjects

J. M. Hartman

Publications and source records attributed to J. M. Hartman.

8 recordsLinked to original sources

arXiv2018

Design and characterization of the Large-Aperture Experiment to Detect the Dark Age (LEDA) radiometer systems

The Large-Aperture Experiment to Detect the Dark Age (LEDA) was designed to detect the predicted O(100)mK sky-averaged absorption of the Cosmic Microwave Background by Hydrogen in the neutral pre- and intergalactic medium just after the cosmological Dark Age. The spectral signature would be associated with emergence of a diffuse Ly$α$ background from starlight during 'Cosmic Dawn'. Recently, Bowman et al. (2018) have reported detection of this predicted absorption feature, with an unexpectedly large amplitude of 530 mK, centered at 78 MHz. Verification of this result by an independent experiment, such as LEDA, is pressing. In this paper, we detail design and characterization of the LEDA radiometer systems, and a first-generation pipeline that instantiates a signal path model. Sited at the Owens Valley Radio Observatory Long Wavelength Array, LEDA systems include the station correlator, five well-separated redundant dual polarization radiometers and backend electronics. The radiometers deliver a 30-85MHz band (16<z<34) and operate as part of the larger interferometric array, for purposes ultimately of in situ calibration. Here, we report on the LEDA system design, calibration approach, and progress in characterization as of January 2016. The LEDA systems are currently being modified to improve performance near 78 MHz in order to verify the purported absorption feature.

astro-ph.IM↗

arXiv2015

Monitoring the Sky with the Prototype All-Sky Imager on the LWA1

We present a description of the Prototype All-Sky Imager (PASI), a backend correlator and imager of the first station of the Long Wavelength Array (LWA1). PASI cross-correlates a live stream of 260 dual-polarization dipole antennas of the LWA1, creates all-sky images, and uploads them to the LWA-TV website in near real-time. PASI has recorded over 13,000 hours of all-sky images at frequencies between 10 and 88 MHz creating opportunities for new research and discoveries. We also report rate density and pulse energy density limits on transients at 38, 52, and 74 MHz, for pulse widths of 5 s. We limit transients at those frequencies with pulse energy densities of $>2.7\times 10^{-23}$, $>1.1\times 10^{-23}$, and $>2.8\times 10^{-23}$ J m$^{-2}$ Hz$^{-1}$ to have rate densities $<1.2\times10^{-4}$, $<5.6\times10^{-4}$, and $<7.2\times10^{-4}$ yr$^{-1}$ deg$^{-2}$

astro-ph.IM↗

arXiv2014

Detection of Radio Emission from Fireballs

We present the findings from the Prototype All-Sky Imager (PASI), a backend correlator of the first station of the Long Wavelength Array (LWA1), which has recorded over 11,000 hours of all-sky images at frequencies between 25 and 75 MHz. In a search of this data for radio transients, we have found 49 long (10s of seconds) duration transients. Ten of these transients correlate both spatially and temporally with large meteors (fireballs), and their signatures suggest that fireballs emit a previously undiscovered low frequency, non-thermal pulse. This emission provides a new probe into the physics of meteors and identifies a new form of naturally occurring radio transient foreground.

astro-ph.EP↗

arXiv2014

Limits on GRB Prompt Radio Emission Using the LWA1

As a backend to the first station of the Long Wavelength Array (LWA1) the Prototype All Sky Imager (PASI) has been imaging the sky $>$ -26$^{\circ}$ declination during 34 Gamma Ray Bursts (GRBs) between January 2012 and May 2013. Using this data we were able to put the most stringent limits to date on prompt low frequency emission from GRBs. While our limits depend on the zenith angle of the observed GRB, we estimate a 1$σ$ RMS sensitivity of 68, 65 and 70 Jy for 5 second integrations at 37.9, 52.0, and 74.0 MHz at zenith. These limits are relevant for pulses $\geq$ 5 s and are limited by dispersion smearing. For pulses of length 5 s we are limited to dispersion measures ($DM$s) $\leq$ 220, 570, and 1,600 pc cm$^{-3}$ for the frequencies above. For pulses lasting longer than 5s, the $DM$ limits increase linearly with the duration of the pulse. We also report two interesting transients, which are, as of yet, of unknown origin, and are not coincident with any known GRBs. For general transients, we give rate density limits of $\leq$ $7.5\times10^{-3}$, $2.9\times10^{-2}$, and $1.4\times10^{-2}$ yr$^{-1}$ deg$^{-2}$ with pulse energy densities $>1.3\times 10^{-22}$, $1.1\times 10^{-22}$, and $1.4\times 10^{-22}$ J m$^{-2}$ Hz$^{-1}$ and pulse widths of 5 s at the frequencies given above.

astro-ph.HE↗

arXiv2012

Accelerated Orbital Expansion And Secular Spin Down of the Accreting Millisecond Pulsar SAX J1808.4-3658

The accreting millisecond pulsar SAX J1808.4-3658 has shown a peculiar orbital evolution in the past with an orbital expansion much faster than expected from standard binary evolutionary scenarios. Previous limits on the pulsar spin frequency derivative during transient accretion outbursts were smaller than predicted by standard magnetic accretion torque theory, while the spin evolution between outbursts was consistent with magnetic dipole spin-down. In this paper we present the results of a coherent timing analysis of the 2011 outburst observed by the Rossi X-ray Timing Explorer and extend our previous long-term measurements of the orbital and spin evolution over a baseline of thirteen years. We find that the expansion of the 2 hr orbit is accelerating at a rate 1.6E-20 s/s^2 and we interpret this as the effect of short-term angular momentum exchange between the mass donor and the orbit. The gravitational quadrupole coupling due to variations in the oblateness of the companion can be a viable mechanism for explaining the observations. No significant spin frequency derivatives are detected during the 2011 outburst (<4E-13 Hz/s) and the long term spin down remains stable over thirteen years with a rate of approximately -1E-15 Hz/s.

astro-ph.HE↗

arXiv2009

A decade of timing an accretion-powered millisecond pulsar: The continuing spin down and orbital evolution of SAX J1808.4-3658

The Rossi X-ray Timing Explorer has observed five outbursts from the transient 2.5 ms accretion-powered pulsar SAX J1808.4-3658 during 1998-2008. We present a pulse timing study of the most recent outburst and compare it with the previous timing solutions. The spin frequency of the source continues to decrease at a rate of (-5.5+/-1.2)x10^-18 Hz/s, which is consistent with the previously determined spin derivative. The spin-down occurs mostly during quiescence, and it is most likely due to the magnetic dipole torque from a B = 1.5x10^8 G dipolar field at the neutron star surface. We also find that the 2 hr binary orbital period is increasing at a rate of (3.80+/-0.06)x10^-12 s/s, also consistent with previous measurements. It remains uncertain whether this orbital change reflects secular evolution or short-term variability.

astro-ph.HE↗

arXiv2007

Discovery of the accretion-powered millisecond pulsar SWIFT J1756.9-2508 with a low-mass companion

We report on the discovery by the Swift Gamma-Ray Burst Explorer of the eighth known transient accretion-powered millisecond pulsar, SWIFT J1756.9-2508, as part of routine observations with the Swift Burst Alert Telescope hard X-ray transient monitor. The pulsar was subsequently observed by both the X-Ray Telescope on Swift and the Rossi X-Ray Timing Explorer Proportional Counter Array. It has a spin frequency of 182 Hz (5.5 ms) and an orbital period of 54.7 minutes. The minimum companion mass is between 0.0067 and 0.0086 solar masses, depending on the mass of the neutron star, and the upper limit on the mass is 0.030 solar masses (95% confidence level). Such a low mass is inconsistent with brown dwarf models, and comparison with white dwarf models suggests that the companion is a He-dominated donor whose thermal cooling has been at least modestly slowed by irradiation from the accretion flux. No X-ray bursts, dips, eclipses or quasi-periodic oscillations were detected. The current outburst lasted approximately 13 days and no earlier outbursts were found in archival data.

astro-ph↗

arXiv1999

Anisotropic Condensation of Helium in Nanotube Bundles

Helium atoms are strongly attracted to the interstitial channels within a bundle of carbon nanotubes. The strong corrugation of the axial potential within a channel can produce a lattice gas system where the weak mutual attraction between atoms in neighboring channels of a bundle induces condensation into a remarkably anisotropic phase with very low binding energy. We estimate the binding energy and critical temperature for 4He in this novel quasi-one-dimensional condensed state. At low temperatures, the specific heat of the adsorbate phase (fewer than 2% of the total number of atoms) greatly exceeds that of the host material.

cond-mat.stat-mech↗