skip to main content

Search for: All records

Creators/Authors contains: "Halpern, J. P."

Note: When clicking on a Digital Object Identifier (DOI) number, you will be taken to an external site maintained by the publisher. Some full text articles may not yet be available without a charge during the embargo (administrative interval).
What is a DOI Number?

Some links on this page may take you to non-federal websites. Their policies may differ from this site.

  1. Abstract The surface temperature distributions of central compact objects (CCOs) are powerful probes of their crustal magnetic field strengths and geometries. Here we model the surface temperature distribution of RX J0822−4300, the CCO in the Puppis A supernova remnant, using 471 ks of XMM-Newton data. We compute the energy-dependent pulse profiles in 16 energy bands, fully including the general relativistic effects of gravitational redshift and light bending, to accurately model the two heated surface regions of different temperatures and areas, in addition to constraining the viewing geometry. This results in precise measurements of the two temperatures: kT warm = ( 1 + z ) × 0.222 − 0.019 + 0.018 keV and kT hot = (1 + z ) × 0.411 ± 0.011 keV. The two heated surface regions are likely located very close to the rotational poles, with the most probable position of the hotter component ≈ 6° from the rotational pole. For the first time, we are able to measure a deviation from a pure antipodal hot-spot geometry, with a longitudinal offset δ γ = 11 .° 7 − 2 .° 5 + 2 .° 6 . The discovery of this asymmetry, along with the factor of ≈2more »temperature difference between the two emitting regions, may indicate that RX J0822−4300 was born with a strong, tangled crustal magnetic field.« less
    Free, publicly-accessible full text available March 1, 2023
  2. Abstract HESS J0632+057 belongs to a rare subclass of binary systems that emit gamma rays above 100 GeV. It stands out for its distinctive high-energy light curve, which features a sharp “primary” peak and broader “secondary” peak. We present the results of contemporaneous observations by NuSTAR and VERITAS during the secondary peak between 2019 December and 2020 February, when the orbital phase ( ϕ ) is between 0.55 and 0.75. NuSTAR detected X-ray spectral evolution, while VERITAS detected TeV emission. We fit a leptonic wind-collision model to the multiwavelength spectra data obtained over the four NuSTAR and VERITAS observations, constraining the pulsar spin-down luminosity and the magnetization parameter at the shock. Despite long-term monitoring of the source from 2019 October to 2020 March, the MDM observatory did not detect significant variation in H α and H β line equivalent widths, an expected signature of Be-disk interaction with the pulsar. Furthermore, fitting folded Swift-XRT light-curve data with an intrabinary shock model constrained the orbital parameters, suggesting two orbital phases (at ϕ D = 0.13 and 0.37), where the pulsar crosses the Be-disk, as well as phases for the periastron ( ϕ 0 = 0.30) and inferior conjunction ( ϕ IFC =more »0.75). The broadband X-ray spectra with Swift-XRT and NuSTAR allowed us to measure a higher neutral hydrogen column density at one of the predicted disk-passing phases.« less
  3. ABSTRACT

    Accurate measurements of the masses of neutron stars are necessary to test binary evolution models, and to constrain the neutron star equation of state. In pulsar binaries with no measurable post-Keplerian parameters, this requires an accurate estimate of the binary system’s inclination and the radial velocity of the companion star by other means than pulsar timing. In this paper, we present the results of a new method for measuring this radial velocity using the binary synthesis code Icarus. This method relies on constructing a model spectrum of a tidally distorted, irradiated star as viewed for a given binary configuration. This method is applied to optical spectra of the newly discovered black widow PSR J1555–2908. By modeling the optical spectroscopy alongside optical photometry, we find that the radial velocity of the companion star is 397 ± 4 km s−1 (errors quoted at 95 per cent confidence interval), as well as a binary inclination of >75°. Combined with γ-ray pulsation timing information, this gives a neutron star mass of 1.67$^{+0.15}_{-0.09}$ M⊙ and a companion mass of 0.060$^{+0.005}_{-0.003}$ M⊙, placing PSR J1555–2908 at the observed upper limit of what is considered a black widow system.

  4. The angular size of a star is a critical factor in determining its basic properties. Direct measurement of stellar angular diameters is difficult: at interstellar distances stars are generally too small to resolve by any individual imaging telescope. This fundamental limitation can be overcome by studying the diffraction pattern in the shadow cast when an asteroid occults a star, but only when the photometric uncertainty is smaller than the noise added by atmospheric scintillation. Atmospheric Cherenkov telescopes used for particle astrophysics observations have not generally been exploited for optical astronomy due to the modest optical quality of the mirror surface. However, their large mirror area makes them well suited for such high-time-resolution precision photometry measurements. Here we report two occultations of stars observed by the Very Energetic Radiation Imaging Telescope Array System (VERITAS) Cherenkov telescopes with millisecond sampling, from which we are able to provide a direct measurement of the occulted stars’ angular diameter at the ≤0.1 mas scale. This is a resolution never achieved before with optical measurements and represents an order of magnitude improvement over the equivalent lunar occultation method. We compare the resulting stellar radius with empirically derived estimates from temperature and brightness measurements, confirming the latter canmore »be biased for stars with ambiguous stellar classifications.« less