Showing papers by "M. H. van Kerkwijk published in 2017"

Hubble Space Telescope Detection of the Millisecond Pulsar J2124-3358 and its Far-ultraviolet Bow Shock Nebula

Abstract: We observed the nearby millisecond pulsar J2124–3358 with the Hubble Space Telescope in broad far-UV (FUV) and optical filters. The pulsar is detected in both bands with fluxes F(1250–2000 A) = (2.5 ± 0.3) × 10−16 erg s−1 cm−2 and F(3800–6000 A) = (6.4 ± 0.4) × 10−17 erg s−1 cm−2, which corresponds to luminosities of ≈5.8 × 1027 and 1.4 × 1027 erg s−1, for d = 410 pc and E(B − V) = 0.03. The optical-FUV spectrum can be described by a power-law model, , with slope α = 0.18–0.48 for a conservative range of color excess, E(B − V) = 0.01–0.08. Since a spectral flux rising with frequency is unusual for pulsar magnetospheric emission in this frequency range, it is possible that the spectrum is predominantly magnetospheric (power law with α < 0) in the optical, while it is dominated by thermal emission from the neutron star surface in the FUV. For a neutron star radius of 12 km, the surface temperature would be between 0.5 × 105 and 2.1 × 105 K for α ranging from −1 to 0, E(B − V) = 0.01–0.08, and d = 340–500 pc. In addition to the pulsar, the FUV images reveal extended emission that is spatially coincident with the known Hα bow shock, making PSR J2124–3358 the second pulsar (after PSR J0437−4715) with a bow shock detected in the FUV.

Spectral sequences of Type Ia supernovae. I. Connecting normal and sub-luminous SN Ia and the presence of unburned carbon

Abstract: Type Ia supernovae are generally agreed to arise from thermonuclear explosions of carbon-oxygen white dwarfs. The actual path to explosion, however, remains elusive, with numerous plausible parent systems and explosion mechanisms suggested. Observationally, type Ia supernovae have multiple subclasses, distinguished by their lightcurves and spectra. This raises the question whether these reflect that multiple mechanisms occur in nature, or instead that explosions have a large but continuous range of physical properties. We revisit the idea that normal and 91bg-like supernovae can be understood as part of a spectral sequence, in which changes in temperature dominate. Specifically, we find that a single ejecta structure is sufficient to provide reasonable fits of both the normal type Ia supernova SN~2011fe and the 91bg-like SN~2005bl, provided that the luminosity and thus temperature of the ejecta are adjusted appropriately. This suggests that the outer layers of the ejecta are similar, thus providing some support of a common explosion mechanism. Our spectral sequence also helps to shed light on the conditions under which carbon can be detected in pre-maximum SN~Ia spectra -- we find that emission from iron can "fill in" the carbon trough in cool SN~Ia. This may indicate that the outer layers of the ejecta of events in which carbon is detected are relatively metal poor compared to events where carbon is not detected.