D. Marsden

Nature versus Nurture: The Origin of Soft Gamma-Ray Repeaters and Anomalous X-Ray Pulsars

Soft gamma-ray repeaters (SGRs) and anomalous X-ray pulsars (AXPs) are young and radio-quiet X-ray pulsars that have been rapidly spun-down to slow spin periods clustered in the range 5-12 s. Most of these unusual pulsars also appear to be associated with supernova shell remnants (SNRs) with typical ages less than 30 kyr. By examining the sizes of these remnants versus their ages, we demonstrate that the interstellar media that surrounded the SGR and AXP progenitors and their SNRs were unusually dense compared to the environments around most young radio pulsars and SNRs. We explore the implications of this evidence on magnetar and propeller-based models for the rapid spin-down of SGRs and AXPs. We find that evidence of dense environments is not consistent with the magnetar model unless a causal link can be shown between the development of magnetars and the external interstellar medium. Propeller-driven spin-down by fossil accretion disks for SGRs and AXPs appears to be consistent with dense environments since the environment can facilitate the formation of such a disk. This may occur in two ways: (1) formation of a pushback disk from the innermost ejecta pushed back by prompt reverse shocks from supernova remnant interactions with massive progenitor wind material stalled in dense surrounding gas or (2) acquisition of disks by a high-velocity neutron stars, which may be able to capture sufficient amounts of comoving outflowing ejecta slowed by the prompt reverse shocks in dense environments.

HST observations of SGR~0526-66: new contraints on accretion and magnetar models

Soft γ-ray repeaters (SGRs) are among the most enigmatic sources known today. Exhibiting huge X-ray and γ-ray bursts and flares, as well as soft quiescent X-ray emission, their energy source remains a mystery. Just as mysterious are the anomalous X-ray pulsars (AXPs), which share many of the same characteristics. Thanks to recent Chandra X-Ray Observatory observations, SGR 0526-66, the first SGR, now appears to be a transition object bridging the two classes, and therefore observations of it have implications for both SGRs and AXPs. The two most popular current models for their persistent emission are accretion of a fossil disk and decay of an enormous (~1015 G) magnetic field in a magnetar. We show how deep optical observations of SGR 0526-66, the only SGR with small enough optical extinction for meaningful observations, show no evidence of an optical counterpart. These observation place strong new constraints on both accretion disk and magnetar models and suggest that the spectral energy distribution may peak in the hard UV. Almost all accretion disks are excluded by the optical data, and a magnetar would require a ~1015-1016 G field.

Correlations between Spectral Properties and Spin-down Rate in Soft Gamma-Ray Repeaters and Anomalous X-Ray Pulsars

Anomalous X-ray pulsars (AXPs) and soft gamma-ray repeaters (SGRs) are X-ray sources with unusual properties distinguishing them from both rotation-powered and most accretion-powered pulsars. Using archival ASCA data over the energy range 0.5-10.0 keV, we have studied the spectra of the persistent emission from these sources and their variation with spin-down rate. Using a single-power-law spectral model, we find that the overall hardness of the spectra increase with increasing spin-down rate, and therefore the spectral and spin-down mechanism are inextricably linked in these objects. In terms of the two-component blackbody plus power law spectral model, this correlation is seen as an increasing hardness of the high-energy component with increasing spin-down rate, with the temperature of the low-energy blackbody component remaining essentially constant. Also, for the two component spectral model, the ratio of the 2-10 keV power-law and bolometric blackbody luminosities gradually increases with the spin-down rate. We discuss these results in terms of the various theoretical models for SGRs and AXPs.

Does Pulsar B1757–24 Have a Fallback Disk?

Radio pulsars are thought to spin down primarily as a result of torque from magnetic dipole radiation (MDR) emitted by the time-varying stellar magnetic field as the star rotates. This assumption yields a characteristic age for a pulsar, which has generally been assumed to be comparable to the actual age. Recent observational limits on the proper motion of pulsar B1757-24, however, revealed that the actual age (>39 kyr) of this pulsar is much greater than its MDR characteristic age (16 kyr), calling into question the assumption of pure MDR spin-down for this and other pulsars. To explore the possible cause of this discrepancy, we consider a scenario in which the pulsar acquired an accretion disk from supernova ejecta and the subsequent spin-down occurred under the combined action of MDR and accretion torques. A simplified model of the accretion torque involving a constant mass inflow rate at the pulsar magnetosphere can explain the age and period derivative of the pulsar for reasonable values of the pulsar magnetic field and inflow rate. We discuss testable predictions of this model.

Solar Activity and Cloud Opacity Variations: A Modulated Cosmic Ray Ionization Model

Abstract The observed correlation between global low cloud amount and the flux of high energy cosmic rays supports the idea that ionization plays a crucial role in tropospheric cloud formation. This idea is explored quantitatively with a simple model linking the concentration of cloud condensation nuclei to the varying ionization rate due to cosmic rays. Among the predictions of the model is a variation in global cloud optical thickness, or opacity, with cosmic ray rate. Using the International Satellite Cloud Climatology Project (ISCCP) database (1983–99), a search was conducted for variations in the yearly mean visible cloud opacity and visible cloud amount due to cosmic rays. After separating out temporal variations in the data due to the Mount Pinatubo eruption and El Nino–Southern Oscillation, systematic variations in opacity and cloud amount due to cosmic rays were identified. It was found that the fractional amplitude of the opacity variations due to cosmic rays increases with cloud altitude, becom...

Environmental influences in SGRs and AXPs

Soft gamma-ray repeaters (SGRs) and anomalous x-ray pulsars (AXPs) are young (<100 kyr), radio-quiet, x-ray pulsars which have been rapidly spun-down to slow spin periods clustered at 5–12 s. Nearly all of these unusual pulsars also appear to be associated with supernova shell remnants (SNRs) with typical ages <20 kyr. If the unusual properties of SGRs and AXPs are due to an innate feature, such as a super-strong magnetic field, then the pre-supernova environments of SGRs and AXPs should be typical of neutron star progenitors. This is not the case, however, as we demonstrate that the interstellar media which surrounded the SGR and AXP progenitors and their SNRs were unusually dense compared to the environments around most young radio pulsars and SNRs. Thus, if these SNR associations are real, the SGRs and AXPs can not be “magnetars,” and we suggest instead that the environments surrounding SGRs and AXPs play a controlling role in their development.