W. C. Parke

Discovery of a Transient Magnetar: XTE J1810-197

We report the discovery of a new X-ray pulsar, XTE J1810-197, that was serendipitously discovered on 2003 July 15 by the Rossi X- Ray Timing Explorer (RXTE) while observing the soft gamma repeater SGR 1806-20. The pulsar has a 5.54 s spin period, a soft X-ray spectrum (with a photon index of ≈4), and is detectable in earlier RXTE observations back to 2003 January but not before. These show that a transient outburst began between 2002 November 17 and 2003 January 23 and that the sources persistent X-ray flux has been declining since then. The pulsar exhibits a high spin-down rate ≈ 10-11 s s-1 with no evidence of Doppler shifts due to a binary companion. The rapid spin-down rate and slow spin period imply a supercritical characteristic magnetic field B 3 × 1014 G and a young age τ ≤ 7600 yr. Follow-up Chandra observations provided an accurate position of the source. Within its error radius, the 1.5 m Russian-Turkish Optical Telescope found a limiting magnitude RC = 21.5. All such properties are strikingly similar to those of anomalous X-ray pulsars and soft gamma repeaters, providing strong evidence that the source is a new magnetar. However, archival ASCA and ROSAT observations found the source nearly 2 orders of magnitude fainter. This transient behavior and the observed long-term flux variability of the source in absence of an observed SGR-like burst activity make it the first confirmed transient magnetar and suggest that other neutron stars that share the properties of XTE J1810-197 during its inactive phase may be unidentified transient magnetars awaiting detection via a similar activity. This implies a larger population of magnetars than previously surmised and a possible evolutionary connection between magnetars and other neutron star families.

Discovery of Cyclotron Resonance Features in the Soft Gamma Repeater SGR 1806-20

We report evidence of cyclotron resonance features from the Soft Gamma Repeater SGR 1806-20 in outburst, detected with the Rossi X-Ray Timing Explorer in the spectrum of a long, complex precursor that preceded a strong burst. The features consist of a narrow 5.0 keV absorption line with modulation near its second and third harmonics (at 11.2 and 17.5 keV, respectively). The line features are transient and are detected in the harder part of the precursor. The 5.0 keV feature is strong, with an equivalent width of similar to500 eV and a narrow width of less than 0.4 keV. Interpreting the features as electron-cyclotron lines in the context of accretion models leads to a large mass-to-radius ratio (M/R > 0.3 M. km(-1)) that is inconsistent with neutron stars or that requires a low (5-7) x 10(11) G magnetic field that is unlikely for SGRs. The line widths are also narrow compared with those of electron-cyclotron resonances observed so far in X-ray pulsars. In the magnetar picture, the features are plausibly explained as being ion-cyclotron resonances in an ultrastrong magnetic field that have recently been predicted from magnetar candidates. In this view, the 5.0 keV feature is consistent with a proton-cyclotron fundamental whose energy and width are close to model predictions. The line energy would correspond to a surface magnetic field of 1.0 x 10(15) G for SGR 1806-20, in good agreement with that inferred from the spin-down measure in the source.

SPECTRAL LAGS AND THE LAG-LUMINOSITY RELATION: AN INVESTIGATION WITH SWIFT BAT GAMMA-RAY BURSTS

Spectral lag, the time difference between the arrival of high-energy and low-energy photons, is a common feature in gamma-ray bursts (GRBs). Norris et al. reported a correlation between the spectral lag and the isotropic peak luminosity of GRBs based on a limited sample. More recently, a number of authors have provided further support for this correlation using arbitrary energy bands of various instruments. In this paper, we report on a systematic extraction of spectral lags based on the largest Swift sample to date of 31 GRBs with measured redshifts. We extracted the spectral lags for all combinations of the standard Swift hard X-ray energy bands: 15-25 keV, 25-50 keV, 50-100 keV, and 100-200 keV and plotted the time dilation corrected lag as a function of isotropic peak luminosity. The mean value of the correlation coefficient for various channel combinations is -0.68 with a chance probability of {approx}0.7 x 10{sup -3}. In addition, the mean value of the power-law index is 1.4 +- 0.3. Hence, our study lends support to the existence of a lag-luminosity correlation, albeit with large scatter.

New Evidence For Proton Cyclotron Resonance In a Magnetar Strength Field From SGR 1806-20

A great deal of evidence has recently been gathered in favor of the picture that soft gamma repeaters and anomalous X-ray pulsars are powered by ultrastrong magnetic fields (B > 1014 G; i.e., magnetars). Nevertheless, present determination of the magnetic field in such magnetar candidates has been indirect and model-dependent. A key prediction concerning magnetars is the detection of ion-cyclotron resonance features, which would offer a decisive diagnostic of the field strength. Here we present the detection of a 5 keV absorption feature in a variety of bursts from the soft gamma repeater SGR 1806-20, confirming our initial discovery (Ibrahim et al.) and establishing the presence of the feature in the sources burst spectra. The line feature is well explained as proton-cyclotron resonance in an ultrastrong magnetic field, offering a direct measurement of SGR 1806-20s magnetic field (B ≈ 1015 G) and clear evidence of a magnetar. Together with the sources spin-down rate, the feature also provides the first measurement of the gravitational redshift, mass, and radius of a magnetar.

Minimum variability time-scales of long and short GRBs

ABSTRACT We have investigated the variability of a sample of long and short Fermi/GBM Gammaray bursts (GRBs) using a fast wavelet technique to determine the smallest timescales. The results indicate different variability time scales for long and short burstsin the source frame and that variabilities on the order of a few milliseconds are notuncommon. The data also indicate an intriguing relation between the variability scaleand the burst duration.Key words: Gamma-ray bursts 1 INTRODUCTIONThe prompt emission from Gamma-ray Bursts (GRBs)shows very complicated time profiles that hitherto elude asatisfactory explanation. Fenimore & Ramirez-Ruiz (2000)reported a correlation between variability of GRBs and thepeak isotropic luminosity. The existence of the variability-luminosity correlation suggests that the prompt emissionlight curve is embedded with temporal information relatedto the microphysics of GRBs. Several models have been pro-posed to explain the observed temporal variability of GRBlightcurves. Leading models such as the internal shock model(reference) and the photospheric model (reference) link therapid variability directly to the activity of the central en-gine. Others invoke relativistic outflow mechanisms to sug-gest that local turbulence amplified through Lorentz boost-ing leads to causally disconnected regions which in turnact as independent centers for the observed prompt emis-sion. In more recent models, both Morsony et al. (2010)and Zhang & Yan (2011) argue that the temporal variabil-ity may show two different scales depending on the physicalmechanisms generating the prompt emission.In order to further our understanding of the promptemission phase of GRBs and to explicitly test some of thekey ingredients in the various models it is clearly importantto extract the variability for both short and long gamma-ray bursts in a robust and unbiased manner. It is also clear

Probing Curvature Effects in the Fermi GRB 110920

Curvature effects in gamma-ray bursts (GRBs) have long been a source of considerable interest. In a collimated relativistic GRB jet, photons that are off-axis relative to the observer arrive at later times than on-axis photons and are also expected to be spectrally softer. In this work, we invoke a relatively simple kinematic two-shell collision model for a uniform jet profile and compare its predictions to GRB prompt-emission data for observations that have been attributed to curvature effects such as the peak-flux-peak-frequency relation, i.e., the relation between the νF{sub ν} flux and the spectral peak, E{sub pk} in the decay phase of a GRB pulse, and spectral lags. In addition, we explore the behavior of pulse widths with energy. We present the case of the single-pulse Fermi GRB 110920 as a test for the predictions of the model against observations.

The minimum variability time‐scale and its relation to pulse profiles of Fermi GRBs

We present a direct link between the minimum variability time-scales extracted through a wavelet decomposition and the rise times of the shortest pulses extracted via fits of 34 Fermi Gamma-ray Burst Monitor (GBM) Gamma-Ray Burst (GRB) light curves comprised of 379 pulses. Pulses used in this study were fitted with lognormal functions, whereas the wavelet technique used employs a multiresolution analysis that does not rely on identifying distinct pulses. By applying a corrective filter to published data fitted with pulses, we demonstrate agreement between these two independent techniques and offer a method for distinguishing signal from noise.

The Hurst exponent of Fermi gamma-ray bursts

Using a wavelet decomposition technique, we have extracted the Hurst exponent for a sample of 46 long and 22 short Gamma-ray bursts (GRBs) detected by the Gamma-ray Burst Monitor (GBM) aboard the Fermi satellite. This exponent is a scaling parameter that provides a measure of long-range behavior in a time series. The mean Hurst exponent for the short GRBs is significantly smaller than that for the long GRBs. The separation may serve as an unbiased criterion for distinguishing short and long GRBs.

A new frequency–luminosity relation for long gamma-ray bursts?

We have studied power density spectra (PDS) of 206 long Gamma-Ray Bursts (GRBs). We fitted the PDS with a simple power-law and extracted the exponent of the powerlaw (�) and the noise-crossing threshold frequency (fth). We find that the distribution

Radiative Corrections to Elastic Electron-Proton Scattering for Polarized Electrons

We analyze the radiative correction to high energy elastic electron-proton scattering of polarized electrons. We show that if the approximations inherent in the calculations developed by Tsai and given in the work of Mo and Tsai, which have been used in the analysis of almost all experimental data pertain- ing to medium and high energy elastic electron scattering for the past three decades, are maintained, then the same radiative correction applies both in the case of initially polarized and unpolarized electrons. and longitudinal polarizations of the recoil protons were measured in order to obtain the ratio of the protons elastic electromagnetic form factors, GEp /GMp . Given that radiative corrections to elastic electron-proton scattering are generally of the order of 20% - 30% for four-momentum transfer squared in the range considered in these experiments (0.5 to 3.5 (GeV/c) 2 ), the question arises as to whether the same radiative correction used in the case of unpolarized beams and targets can be applied in the case of polarized electron beams when the polarization of the recoil proton is measured. We show here that if the approxi- mations inherent in the calculations developed in (1) and given in (2) are maintained, then the same radiative correction applies both in the case of initially polarized and unpolarized electrons. In Sec. II we present the cross section for the scattering of polarized electrons from unpolarized protons in the absence of radiative corrections. In Sec. III we give each of the matrix elements associated with the radiative correction and discuss the significant approximations that are made in (1) to evaluate their contribution to the cross section. We then show that with these approximations the radiative corrections do not depend on the polarization of either the electron or the proton in the initial or final state.