Aris Karastergiou

Abstract:

We have observed the 5.54 s anomalous X-ray pulsar XTE J1810-197at radio, millimeter, and infrared (IR) wavelengths, with the aim of learning about its broadband spectrum. At the IRAM 30 m telescope, we have detected the magnetar at ν = 88 and 144 GHz, the highest radio-frequency emission ever seen from a pulsar. At 88 GHz we detected numerous individual pulses, with typical widths ∼2 ms and peak flux densities up to 45 Jy. Together with nearly contemporaneous observations with the Parkes, Nançay, and Green Bank telescopes, we find that in late 2006 July the spectral index of the pulsar was -0.5 ≲ α ≲ 0 (with flux density Sν ∝ να) over the range 1.4-144 GHz. Nine dualfrequency Very Large Array and Australia Telescope Compact Array observations in 2006 May-September are consistent with this finding, while showing variability of α with time. We infer from the IRAM observations that XTE J1810-197 remains highly linearly polarized at millimeter wavelengths. Also, toward this pulsar, the transition frequency between strong and weak scattering in the interstellar medium may be near 50 GHz. At Gemini, we detected the pulsar at 2,2 μm in 2006 September, at the faintest level yet observed, Ks = 21.89 ± 0.15. We have also analyzed four archival IR Very Large Telescope observations (two unpublished), finding that the brightness fluctuated within a factor of 2-3 over a span of 3 years, unlike the monotonie decay of the X-ray flux. Thus, there is no correlation between IR and X-ray flux, and it remains uncertain whether there is any correlation between IR and radio flux. © 2007. The American Astronomical Society. All rights reserved.

Abstract:

We have conducted observations of 22 pulsars at frequencies of 0.7, 1.4 and 3.1 GHz and present their polarization profiles. The observations were carried out for two main purposes. First we compare the orientation of the spin and velocity vectors to verify the proposed alignment of these vectors by Johnston et al. (2005). We find, for the 14 pulsars for which we were able to determine both vectors, that 7 are plausibly aligned, a fraction which is lower than, but consistent with, earlier measurements. Secondly, we use profiles obtained simultaneously at widely spaced frequencies to compute the radio emission heights. We find, similar to other workers in the field, that radiation from the centre of the profile originates from lower in the magnetosphere than the radiation from the outer parts of the profile.

Abstract:

Motivated by recent results on the location of the radio emission in pulsar magnetospheres, we have developed a model which can account for the large diversity found in the average profile shapes of pulsars. At the centre of our model lies the idea that radio emission at a particular frequency arises from a wide range of altitudes above the surface of the star and that it is confined to a region close to the last open field lines. We assert that the radial height range over which emission occurs is responsible for the complex average pulse shapes rather than the transverse (longitudinal) range proposed in most current models. By implementing an abrupt change in the height range to discriminate between young, short-period, highly-energetic pulsars and their older counterparts, we obtain the observed transition between the simple and complex average pulse profiles observed in each group respectively. Monte Carlo simulations are used to demonstrate the match of our model to real observations.

Abstract:

We have observed the 5.54s anomalous X-ray pulsar XTE J1810-197 at radio, millimeter, and infrared (IR) wavelengths, with the aim of learning about its broad-band spectrum. At the IRAM 30m telescope, we have detected the magnetar at 88 and 144GHz, the highest radio-frequency emission ever seen from a pulsar. At 88GHz we detected numerous individual pulses, with typical widths ~2ms and peak flux densities up to 45Jy. Together with nearly contemporaneous observations with the Parkes, Nancay, and Green Bank telescopes, we find that in late 2006 July the spectral index of the pulsar was -0.5

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