Paul M. Woods

Magnetar-like X-ray bursts from an anomalous X-ray pulsar.

Anomalous X-ray pulsars (AXPs) are a class of rare X-ray emitting pulsars whose energy source has been perplexing for some 20 years. Unlike other X-ray emitting pulsars, AXPs cannot be powered by rotational energy or by accretion of matter from a binary companion star, hence the designation ‘anomalous’. Many of the rotational and radiative properties of the AXPs are strikingly similar to those of another class of exotic objects, the soft-γ-ray repeaters (SGRs). But the defining property of the SGRs—their low-energy-γ-ray and X-ray bursts—has not hitherto been observed for AXPs. Soft-γ-ray repeaters are thought to be ‘magnetars’, which are young neutron stars whose emission is powered by the decay of an ultra-high magnetic field; the suggestion that AXPs might also be magnetars has been controversial. Here we report two X-ray bursts, with properties similar to those of SGRs, from the direction of the anomalous X-ray pulsar 1E1048.1 - 5937. These events imply a close relationship (perhaps evolutionary) between AXPs and SGRs, with both being magnetars.

Magnetar-like Emission from the Young Pulsar in Kes 75

We report the detection of magnetar-like x-ray bursts from the young pulsar PSR J1846–0258, at the center of the supernova remnant Kes 75. This pulsar, long thought to be exclusively rotation-powered, has an inferred surface dipolar magnetic field of 4.9 × 1013 gauss, which is higher than those of the vast majority of rotation-powered pulsars, but lower than those of the approximately 12 previously identified magnetars. The bursts were accompanied by a sudden flux increase and an unprecedented change in timing behavior. These phenomena lower the magnetic and rotational thresholds associated with magnetar-like behavior and suggest that in neutron stars there exists a continuum of magnetic activity that increases with inferred magnetic field strength.

Statistical properties of SGR 1900+14 bursts

We study the statistics of soft gamma repeater (SGR) bursts using a database of 187 events detected with BATSE and 837 events detected with the Rossi X-Ray Timing Explorer Proportional Counter Array; all events are from SGR 1900+14 during its 1998-1999 active phase. We find that the fluence or energy distribution of bursts is consistent with a power law of index 1.66, over 4 orders of magnitude. This scale-free distribution resembles the Gutenberg-Richter law for earthquakes and gives evidence for self-organized criticality in SGRs. The distribution of time intervals between successive bursts from SGR 1900+14 is consistent with a lognormal distribution. There is no correlation between burst intensity and the waiting times till the next burst, but there is some evidence for a correlation between burst intensity and the time elapsed since the previous burst. We also find a correlation between the duration and the energy of the bursts, but with significant scatter. In all these statistical properties, SGR bursts resemble earthquakes and solar flares more closely than they resemble any known accretion-powered or nuclear-powered phenomena. Thus, our analysis lends support to the hypothesis that the energy source for SGR bursts is internal to the neutron star and plausibly magnetic.

The first outburst of the new magnetar candidate SGR 0501+4516

We report here on the outburst onset and evolution of the new soft gamma-ray repeater SGR 0501+4516. We monitored the new SGR with XMM- Newton starting on 2008 August 23, 1 day after the source became burst active, and continuing with four more observations in the following month, with the last one on 2008 September 30. Combining the data with the Swift X-ray telescope (Swift-XRT) and Suzaku data, we modelled the outburst decay over a 3-month period, and we found that the source flux decreased exponentially with a time-scale of t(c) = 23.8 d. In the first XMM-Newton observation, a large number of short X-ray bursts were observed, the rate of which decayed drastically in the following observations. We found large changes in the spectral and timing behaviour of the source during the first month of the outburst decay, with softening emission as the flux decayed, and the non-thermal soft X-ray spectral component fading faster than the thermal one. Almost simultaneously to our second and fourth XMM-Newton observations (on 2008 August 29 and September 2), we observed the source in the hard X-ray range with INTEGRAL, which clearly detected the source up to similar to 100 keV in the first pointing, while giving only upper limits during the second pointing, discovering a variable hard X-ray component fading in less than 10 days after the bursting activation. We performed a phase-coherent X-ray timing analysis over about 160 days starting with the burst activation and found evidence of a strong second derivative period component [(sic) = -1.6(4) x 10(-19) s s(-2)]. Thanks to the phase connection, we were able to study the phase-resolved spectral evolution of SGR 0501+ 4516 in great detail. We also report on the ROSAT quiescent source data, taken back in 1992 when the source exhibits a flux similar to 80 times lower than that measured during the outburst, and a rather soft, thermal spectrum.

SGR J1550-5418 Bursts Detected with the Fermi Gamma-Ray Burst Monitor during Its Most Prolific Activity

We have performed detailed temporal and time-integrated spectral analysis of 286 bursts from SGR J1550−5418 detected with the Fermi Gamma-ray Burst Monitor (GBM) in 2009 January, resulting in the largest uniform sample of temporal and spectral properties of SGR J1550−5418 bursts. We have used the combination of broadband and high time-resolution data provided with GBM to perform statistical studies for the source properties. We determine the durations, emission times, duty cycles, and rise times for all bursts, and find that they are typical of SGR bursts. We explore various models in our spectral analysis, and conclude that the spectra of SGR J1550−5418 bursts in the 8–200 keV band are equally well described by optically thin thermal bremsstrahlung (OTTB), a power law (PL) with an exponential cutoff (Comptonized model), and two blackbody (BB) functions (BB+BB). In the spectral fits with the Comptonized model, we find a mean PL index of −0.92, close to the OTTB index of −1. We show that there is an anti-correlation between the Comptonized Epeak and the burst fluence and average flux. For the BB+BB fits, we find that the fluences and emission areas of the two BB functions are correlated. The low-temperature BB has an emission area comparable to the neutron star surface area, independent of the temperature, while the hightemperature BB has a much smaller area and shows an anti-correlation between emission area and temperature. We compare the properties of these bursts with bursts observed from other SGR sources during extreme activations, and discuss the implications of our results in the context of magnetar burst models.

DISCOVERY OF A NEW SOFT GAMMA REPEATER, SGR J1833???0832

On 2010 March 19, the Swift/Burst Alert Telescope triggered on a short burst with temporal and spectral characteristics similar to those of soft gamma repeater (SGR) bursts. The source location, however, did not coincide with any known SGR. Subsequent observations of the source error box with the Swift/X-Ray Telescope and the Rossi X-ray Timing Explorer led to the discovery of a new X-ray source with a spin period of 7.56 s, confirming SGR J1833-0832 as a new magnetar. Based on our detailed temporal and spectral analyses, we show that the new SGR is rapidly spinning down (4 x 10(-12) s s(-1)) and find an inferred dipole magnetic field of 1.8 x 10(14) G. We also show that the X-ray flux of SGR J1833-0832 remained constant for approximately 20 days following the burst and then started to decline. We derived an accurate location of the source with the Chandra X-ray Observatory and we searched for a counterpart in deep optical and infrared observations of SGR J1833-0832, and for radio pulsed emission with the Westerbork Radio Synthesis Telescope. Finally, we compare the spectral and temporal properties of the source to other magnetar candidates.

Timing Noise in SGR 1806–20

We have phase-connected a sequence of Rossi X-Ray Timing Explorer Proportional Counter Array observations of SGR 1806-20 covering 178 days. We find that a simple secular spin-down model does not adequately fit the data. The period derivative varies gradually during the observations between 8.1x10-11 and 11.7x10-11 s s(-1) (at its highest, approximately 40% larger than the long-term trend), while the average burst rate as seen with the Burst and Transient Source Experiment drops throughout the time interval. The phase residuals give no compelling evidence for periodicity, but more closely resemble timing noise as seen in radio pulsars. The magnitude of the timing noise, however, is large relative to the noise level typically found in radio pulsars (Delta8=4.8; frequency derivative average power approximately 7x10-20 cycles(2) s(-3)). Combining these results with the noise levels measured for some anomalous X-ray pulsars, we find that all magnetar candidates have Delta(8) values larger than those expected from a simple extrapolation of the correlation found in radio pulsars. We find that the timing noise in SGR 1806-20 is greater than or equal to the levels found in some accreting systems (e.g., Vela X-1, 4U 1538-52, and 4U 1626-67), but the spin-down of SGR 1806-20 has thus far maintained coherence over 6 yr. Alternatively, an orbital model with a period Porb=733 days provides a statistically acceptable fit to the data. If the phase residuals are created by Doppler shifts from a gravitationally bound companion, then the allowed parameter space for the mass function (small) and orbital separation (large) rule out the possibility of accretion from the companion sufficient to power the persistent emission from the SGR.

The timing evolution of 4U 1630-47 during its 1998 outburst

We report on the evolution of the timing of 4U 1630(47 during its 1998 outburst using data obtained with the Rossi X-ray T iming Explorer (RXT E). The count rate and position in hardness inten- sity, color-color diagrams, and simple spectral —ts are used to track the concurrent spectral changes. The source showed seven distinct types of timing behavior, most of which show diUerences with the canonical black hole spectral/timing states. In marked contrast to previous outbursts, we —nd quasi-periodic oscil- lation (QPO) signals during nearly all stages of the outburst with frequencies between 0.06 and 14 Hz and a remarkable variety of other characteristics. In particular, we —nd large (up to 23% rms) amplitude QPOs on the early rise. Later, slow 0.1 Hz semiregular short (D5 s), 9%¨16% deep dips dominate the light curve. At this time there are two QPOs, one stable near 13.5 Hz and the other whose frequency drops from the 6¨8 Hz range to D4.5 Hz during the dips. BeppoSAX observations during the very late declining phase show 4U 1630(47 in a low state. Subject headings: accretion, accretion disksbinaries: closestars: individual (4U 1630(47) ¨ X-rays: stars 0.03 and 0.3 Hz, and the energy spectrum is well —t with a power law with index of ! \( 1.5 to (2.5 up to at least 200 keV (Gilfanov et al. 1994). In the high state (HS), the PDS shows weak (few percent rms) power-law noise. The energy spectrum is dominated by a soft component that can be satisfactorily —t by a multitemperature disk blackbody. The —tted temperature (kT D 1 keV) and innermost radius are comparable to values expected for a stellar-mass black hole. In the very high state (VHS), the soft component increases in —ux by a further factor of 2¨8 over that in the high state. The power-law component strengthens and is visible in the energy spectra. The power density spectrum shows a variable broadband component and 3¨10 Hz quasi-periodic oscillation (QPO). The noise component is either band-limited with a break between 1 and 20 Hz or a power law similar to that in the high state. Occasionally at times between an HS and an LS, an intermediate state (IS) is observed, e.g., GX 339(4

LONG-TERM X-RAY CHANGES IN THE EMISSION FROM THE ANOMALOUS X-RAY PULSAR 4U 0142+61

We present results obtained from X-ray observations of the anomalous X-ray pulsar (AXP) 4U?0142+61 taken between 2000 and 2008 using XMM-Newton, Chandra, and Swift. These observations coincide with periods of long-term changes and burst epochs previously reported using the Rossi X-ray Timing Explorer (RXTE). In observations taken before 2006, we find that the pulse profile became more sinusoidal and the pulsed fraction increased with time. These results confirm those derived using RXTE and expand the observed evolution to energies below 2?keV. The total flux in the 0.5-10?keV band determined with XMM-Newton is observed to be nearly constant in observations taken before 2006, while an increase of ~10% is seen afterwards and coincides with the burst activity detected from the source in 2006-2007. After these bursts, the evolution toward more sinusoidal pulse profiles ceased while the flux and pulsed fraction returned to pre-bursts levels. No evidence for large-scale, long-term changes in the emission as a result of the bursts is seen. We also report on observations taken with the Gemini telescope after two bursts which show source magnitudes consistent with previous measurements. Our results demonstrate the wide range of X-ray variability characteristics seen in AXPs and we discuss them in light of current emission models for these sources.

THE 2001 APRIL BURST ACTIVATION OF SGR 1900+14: X-RAY AFTERGLOW EMISSION

After nearly two years of quiescence, the soft gamma-ray repeater SGR 1900+14 again became burst-active on 2001 April 18, when it emitted a large flare, preceded by few weak and soft short bursts. After having detected the X- and gamma-prompt emission of the flare, BeppoSAX pointed its narrow field X-ray telescopes to the source in less than 8 hr. In this paper we present an analysis of the data from this and from a subsequent BeppoSAX observation, as well as from a set of RXTE observations. Our data show the detection of an X-ray afterglow from the source, most likely related to the large hard X-ray flare. In fact, the persistent flux from the source, in 2-10 keV, was initially found at a level ~5 times higher than the usual value. Assuming an underlying persistent (constant) emission, the decay of the excess flux can be reasonably well described by a t-0.9 law. A temporal feature (a ~ day long bump) is observed in the decay light curve approximately 1 day after the burst onset. This feature is unprecedented in SGR afterglows. We discuss our results in the context of previous observations of this source and derive implications for the physics of these objects.