Extinction Columns and Intrinsic X‐Ray Spectra of the Anomalous X‐Ray Pulsars

Abstract

The X-ray spectra of anomalous X-ray pulsars have long been fit by smooth, empirical models such as the sum of a blackbody plus a power law. These reproduce the $0.5Y10 keV range well, but fail at lower and higher energies, grossly overpredicting the optical and underpredicting the hard X-ray emission. A poorly constrained source of uncertainty in determining the true, intrinsic spectra, in particular at lower energies, is the amount of interstellar extinction. In previous studies, extinction column densities with small statistical errors were derived as part of the fits of the spectra to simple continuum models. Different choices of model, however, each produced statistically acceptable fits but a wide range of columns. Here we attempt to measure the interstellar extinction in a modelindependent way, using individual absorption edges of the elements O, Fe, Ne, Mg, and Si in X-ray grating spectra taken with XMM-Newton. We find that our inferred equivalent hydrogen column density N H for 4U 0142+61 is a factor of 1.4 lower than the typically quoted value from blackbody plus power-law fits, and is now consistent with estimates based on the dust scattering halo and visual extinction. For three other sources, we find column densities consistent with earlier estimates. We use our measurements to recover the intrinsic spectra of the AXPs empirically, without making assumptions on what the intrinsic spectral shapes ought to be. We find that the power-law components that dominate at higher energies do not extend below the thermal peak.

X-ray intensity-hardness correlation and deep IR observations of the anomalous X-ray pulsar 1RXS J170849-400910

“…1 we present the K s band field around the Chandra 0.8 position of 1RXS J170849-400910 (Israel et al 2004). Besides sources A and B proposed by Israel et al (2003b), Safi-Harb and West (2005) and Durant and van Kerkwijk (2006), as the possible IR counterparts to this AXP, many further faint sources (e.g. Star C and Star D) were detected strongly consistent with the Chandra uncertainty region.…”

“…Note that the 2.5σ variability recently reported for source B by Durant and van Kerkwijk (2006) does not seem to be a conclusive word on the IR counterpart of this source, especially considering that source B has IR magnitudes much brighter than all other AXPs.…”

“…We marked all the faint sources being consistent at 3σ with the AXP position. Note that our astrometry (following Israel et al 2003b) is slightly different from the one reported by Durant and van Kerkwijk (2006), this is due to a different catalogue used for the astrometry and thus the H − Ks colour could not be determined for these other faint objects. Nonetheless, we note that the Ks magnitudes of e.g.…”

“…Searches for optical/IR counterparts ruled out the presence of a massive companion (Israel et al 1999). Very recently, two different objects were proposed by different groups, as being 1RXS J170849-400910 IR counterpart, and there still is an open debate on which one is the AXP counterpart (Israel et al 2003b;Safi-Harb and West 2005;Durant and van Kerkwijk 2006). A diffuse (∼8 ) radio emission at 1.4 GHz was recently reported, possibly associated with the supernova remnant G346.5-0.1 (Gaensler et al 2000).…”

10 years of RXTE monitoring of anomalous X-ray pulsar 4U 0142+61: long-term variability

“…However, in other instances, the two fluxes were not correlated, as for 1E 1048−5937 , and Durant, van Kerkwijk, & Hulleman 2004. In 4U 0142+61, reported variations in the IR are not seen contemporaneously in our X-ray data, and are on time scales much shorter than that of the X-ray variation reported here Durant & van Kerkwijk 2006). Wang et al (2006) observed mid-IR emission from 4U 0142+61 which they argue is associated with a passive fall-back disk irradiated by the central X-ray pulsar.…”