The variable radio-to-X-ray spectrum of the magnetar XTE J1810-197

TL;DR

This study presents multi-wavelength observations of the magnetar XTE J1810-197, revealing its broad-band spectrum, high millimeter-wave polarization, and flux variability across IR, radio, and X-ray bands, with implications for interstellar scattering.

Contribution

First detection of the magnetar at 88 and 144GHz, establishing its broad-band spectrum and polarization properties, and analyzing flux variability over several years.

Findings

Detected the magnetar at 88 and 144GHz, the highest radio frequencies observed from a pulsar.

Found the spectral index varies over time between -0.5 and 0 across 1.4-144GHz.

Observed IR flux fluctuations without correlation to X-ray flux over three years.

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<alpha<0 over the range 1.4-144GHz. Nine dual-frequency Very Large Array and Australia Telescope Compact Array observations in 2006 May-September are consistent with this finding, while showing variability of alpha 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 50GHz. At Gemini, we detected the pulsar at 2.2um in 2006 September, at the faintest level yet observed, K_s=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 We have also analyzed four archival IR Very Large Telescope observations (two unpublished), finding that the brightness fluctuated within a factor of 2-3 over a span of 3 years, unlike the monotonic decay of the X-ray flux.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.