Alaa I. Ibrahim

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.

X-Ray Bursts from the Transient Magnetar Candidate XTE J1810–197

We have discovered four X-ray bursts, recorded with the Rossi X-ray Timing Explorer Proportional Counter Array between 2003 September and 2004 April, that we show to originate from the transient magnetar candidate XTE J1810-197. The burst morphologies consist of a short spike or multiple spikes lasting ~1 s each, followed by extended tails of emission where the pulsed flux from XTE J1810-197 is significantly higher. The burst spikes are likely correlated with the pulse maxima, having a chance probability of a random phase distribution of 0.4%. The burst spectra are best fitted to a blackbody with temperatures 4-8 keV, considerably harder than the persistent X-ray emission. During the X-ray tails following these bursts, the temperature rapidly cools as the flux declines, maintaining a constant emitting radius after the initial burst peak. During the brightest X-ray tail, we detect a narrow emission line at 12.6 keV, with an equivalent width of 1.4 keV and a probability of chance occurrence of less than 4 × 10-6. The temporal and spectral characteristics of these bursts closely resemble the bursts seen from 1E 1048.1-5937 and a subset of the bursts detected from 1E 2259+586, thus establishing XTE J1810-197 as a magnetar candidate. The bursts detected from these three objects are sufficiently similar to one another, yet significantly different from those seen from soft gamma repeaters, that they likely represent a new class of bursts from magnetar candidates exclusive (thus far) to the anomalous X-ray pulsar-like sources.

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.

An Unusual Burst from Soft Gamma Repeater SGR 1900+14: Comparisons with Giant Flares and Implications for the Magnetar Model

The soft gamma-ray repeater SGR 1900+14 entered a remarkable phase of activity during the summer of 1998. This activity peaked on 1998 August 27, when a giant periodic γ-ray flare resembling the famous 1979 March 5 event from SGR 0526 - 66 was recorded. Two days later (August 29), a strong, bright burst was detected simultaneously with the Rossi X-Ray Timing Explorer (RXTE) and the Burst and Transient Source Experiment (BATSE). This event reveals several similarities to the giant flares of August 27 and March 5 and shows a number of unique features not previously seen in SGR bursts. Unlike typically short SGR bursts (duration ∼0.1 s), this event exhibits a 3.5 s burst peak that was preceded by an extended (∼1 s) complex precursor, and followed by a long (∼103 s) pulsating tail modulated at the 5.16 s stellar rotation period. Spectral analysis shows a striking distinction between the spectral behavior of the precursor, main peak, and long tail. While the spectrum is uniform during the peak, a significant hard-to-soft spectral evolution is detected in both the precursor and tail emissions. Temporal behavior shows a sharp rise (∼10 ms) at the precursor onset, a rapid cutoff (∼17 ms) at the end of the burst peak, and a gradual decay (∼17 minutes) of the pulsating tail. The tail pulsations show a simple pulse profile that did not evolve with time. The contrasted spectral and temporal signatures of the event suggest that the precursor, main peak, and extended tail are produced by different physical mechanisms, and that the observed tail represents a new emission component from SGRs. We discuss these features and their implications in the context of the magnetar model. The bright 3.5 s component is consistent with a very hot (kT ∼ 1 MeV) trapped fireball, and the precursor with magnetospheric emission in which the radiating particles are heated more continuously. Less than 1% of the fireball energy will be conducted into the exposed surface of the neutron star, thereby dissociating heavy elements and even helium, and inducing rapid transformations between neutrons and protons. The extended afterglow tail of the August 29 burst is consistent with a cooling hot spot of small area (∼13 km2), and indicates that the energy release in an SGR burst is strongly localized.

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.

Discovery of a 6.4 keV Emission Line in a Burst from SGR 1900+14

We present evidence of a 6.4 keV emission line during a burst from the soft gamma repeater SGR 1900+14. The Rossi X-Ray Timing Explorer (RXTE) monitored this source extensively during its outburst in the summer of 1998. A strong burst observed on 1998 August 29 revealed a number of unique properties. The burst exhibits a precursor and is followed by a long (~103 s) tail modulated at the 5.16 s stellar rotation period. The precursor has a duration of ≈0.85 s and shows both significant spectral evolution as well as an emission feature centered near 6.4 keV during the first 0.3 s of the event, when the X-ray spectrum was hardest. The continuum during the burst is well fit with an optically thin thermal bremsstrahlung spectrum with the temperature ranging from ≈40 to 10 keV. The line is strong, with an equivalent width of ~400 eV, and is consistent with Fe Kα fluorescence from relatively cool material. If the rest-frame energy is indeed 6.4 keV, then the lack of an observed redshift indicates that the source is at least ~80 km above the neutron star surface. We discuss the implications of the line detection in the context of models for SGRs.

DISCOVERY OF QUASI-PERIODIC OSCILLATIONS IN THE RECURRENT BURST EMISSION FROM SGR 1806-20

We present evidence for quasi-periodic oscillations (QPOs) in the recurrent outburst emission from the soft gamma repeater SGR 1806–20 using NASAs Rossi X-ray Timing Explorer (RXTE) observations. By searching a sample of 30 bursts for timing signals at the frequencies of the QPOs discovered in the 2004 December 27 giant flare from the source, we find three QPOs at 84, 103, and 648 Hz in three different bursts. The first two QPOs lie within ~1σ from the 92 Hz QPO detected in the giant flare. The third QPO lies within ~9σ from the 625 Hz QPO also detected in the same flare. The detected QPOs are found in bursts with different durations, morphologies, and brightness, and are vindicated by Monte Carlo simulations, which set a lower limit confidence interval ≥4.3σ. We also find evidence for candidate QPOs at higher frequencies in other bursts with lower statistical significance. The fact that we can find evidence for QPOs in the recurrent bursts at frequencies relatively close to those found in the giant flare is intriguing and can offer insight about the origin of the oscillations. We confront our finding against the available theoretical models and discuss the connection between the QPOs we report and those detected in the giant flares. The implications to the neutron star properties are also discussed.

The Discovery of a Transient Magnetar

The newly discovered X‐ray transient XTE J1810‐197 exhibits almost all characteristics of magnetars. It possesses a relatively long spin period of 5.54 s and a rapid spin down rate of ≈ 10−11 s s−1, while showing no evidence for Doppler shifts due to a binary companion. This yields a magnetar‐strength dipole field B = 3 × 1014 G and a young characteristic age τ ⩽ 7600 yr. The spectrum of the source is notably soft (photon index ≈ 4) and optical observations with the 1.5 m Russian‐Turkish Optical Telescope RTT150 revealed a limiting magnitude of Rc = 21.5, both consistent with those of Soft gamma repeaters and anomalous X‐ray pulsars. However, the source shows a significant flux decline for over nine months and is present in archival ASCA and ROSAT observations at nearly two orders of magnitude fainter luminosity. Putting all evidence together shows that we have found the first confirmed transient magnetar. This suggests the presence of other unidentified transient magnetars in a state similar to XTE J1810...

The Pulsed Spectra of Two Extraordinary Pulsars

We report on X‐ray monitoring of two isolated pulsars within the same RXTE field of view. PSR J1811‐1925 in the young supernova remnant G11.2‐0.3 has a nearly sinusoidal pulse profile with a hard pulsed spectrum (photon index Γ ∼ 1.2). The pulsar is a highly efficient (∼ 1% of spin‐down energy) emitter of 2–50 keV pulsed X‐rays despite having a fairly typical B ∼ 2 × 1012 G magnetic field. PSR J1809‐1943/XTE J1810‐197 is a newly discovered slow (P = 5.54 s), apparently isolated X‐ray pulsar which increased in flux by a factor of 100 in 2003 January. Nine months of monitoring observations have shown a decrease in pulsed flux of ∼ 30% without a significant change in its apparently thermal spectrum (kT ∼ 0.7 keV) or pulse profile. During this time, the spin‐down torque has fluctuated by a factor of ∼ 2. Both the torque and the flux have remained steady for the last 3 months, at levels consistent with a magnetar interpretation.