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16 yr OF RXTE MONITORING OF FIVE ANOMALOUS X-RAY PULSARS

We present a summary of the long-term evolution of various properties of the five non-transient anomalous X-ray pulsars (AXPs) 1E 1841–045, RXS J170849.0–400910, 1E 2259+586, 4U 0142+61, and 1E 1048.1–5937, regularly monitored with RXTE from 1996 to 2012. We focus on three properties of these sources: the evolution of the timing, pulsed flux, and pulse profile. We report several new timing anomalies and radiative events, including a putative anti-glitch seen in 1E 2259+586 in 2009, and a second epoch of very large spin-down rate fluctuations in 1E 1048.1–5937 following a large flux outburst. We compile the properties of the 11 glitches and 4 glitch candidates observed from these 5 AXPs between 1996 and 2012. Overall, these monitoring observations reveal several apparent patterns in the behavior of this sample of AXPs: large radiative changes in AXPs (including long-lived flux enhancements, short bursts, and pulse profile changes) are rare, occurring typically only every few years per source; large radiative changes are almost always accompanied by some form of timing anomaly, usually a spin-up glitch; only 20%-30% of timing anomalies are accompanied by any form of radiative change. We find that AXP radiative behavior at the times of radiatively loud glitches is sufficiently similar to suggest common physical origins. The similarity in glitch properties when comparing radiatively loud and radiatively silent glitches in AXPs suggests a common physical origin in the stellar interior. Finally, the overall similarity of AXP and radio pulsar glitches suggests a common physical origin for both phenomena.

Detailed high-energy characteristics of AXP 4U 0142+61 - Multi-year observations with INTEGRAL, RXTE, XMM-Newton, and ASCA

4U 0142+61 is one of the Anomalous X-ray Pulsars exhibiting hard X-ray emission above 10 keV discovered with INTEGRAL. In this paper we present detailed spectral and temporal characteristics both in the hard X-ray (> 10 keV) and soft X-ray (<10 keV) domains obtained using data from INTEGRAL, XMM-Newton, ASCA and RXTE. Accumulating data collected over four years with the imager IBIS-ISGRI aboard INTEGRAL, the time-averaged total spectrum shows a power-law like shape with photon index Gamma = 0.93 +/- 0.06. 4U 0142+61 is detected up to 229 keV and the flux between 20 keV and 229 keV is (15.01 +/- 0.82) x 10(-11) erg cm(-2) s(-1), which exceeds the energy flux in the 2-10 keV band by a factor of similar to 2.3. Using simultaneously collected data with the spectrometer SPI of INTEGRAL the combined total spectrum yields the first evidence for a spectral break above 100 keV. Assuming a logparabolic function for the spectral shape above 20 keV the peak energy of 4U 0142+61 is 228(-41)(+65) keV. There is no evidence for significant long-term time variability of the total emission from 4U 0142+61. Both the total flux and the spectral index are stable within the 17% level (1 sigma). Pulsed emission is measured with ISGRI up to 160 keV. The 20-160 keV profile shows a broad double-peaked pulse with a 6.2 sigma detection significance. The total pulsed spectrum can be described with a very hard power-law shape with a photon index G = 0.40 +/- 0.15 and a 20-150 keV flux of (2.68 +/- 1.34) x 10(-11) erg cm(-2) s(-1). To perform accurate phase-resolved spectroscopy over the total X-ray window, we produced pulse profiles in absolute phase for INTEGRAL-ISGRI, RXTE-PCA, XMM-Newton-PN and ASCA-GIS. The two known pulses in all soft X-ray profiles below 10 keV are located in the same phases. Three XMM-Newton observations in 2003-2004 show statistically identical profiles. However, we find a significant profile morphology change between an ASCA-GIS observation in 1999 following a possible glitch of 4U 0142+61. This change can be accounted for by differences in relative strengths and spectral shapes (0.8-10 keV) of the two pulses. The principle peak in the INTEGRAL pulse profile above 20 keV is located at the same phase as one of the pulses detected below 10 keV. The second pulse detected with INTEGRAL is slightly shifted with respect to the second peak observed in the soft X-ray band. We performed consistent phase-resolved spectroscopy over the total high-energy band and identify at least three genuinely different pulse components with different spectra. The high level of consistency between the detailed results from the four missions is indicative of a remarkably stable geometry underlying the emission scenario. Finally, we discuss the derived detailed characteristics of the high-energy emission of 4U 0142+61 in relation to three models for the non-thermal hard X-ray emission.

X-RAY AND NEAR-IR VARIABILITY OF THE ANOMALOUS X-RAY PULSAR 1E 1048.1 5937: FROM QUIESCENCE BACK TO ACTIVITY

We report on new and archival X-ray and near-IR observations of the anomalous X-ray pulsar 1E 1048.1–5937 performed between 2001 and 2007 with the Rossi X-Ray Telescope Explorer (RXTE), the Chandra X-Ray Observatory, the Swift Gamma-Ray Burst Explorer, the Hubble Space Telescope (HST), and the Very Large Telescope. Monitoring with RXTE revealed that following its ~2001-2004 active period, 1E 1048.1–5937 entered a phase of timing stability; at the same time, simultaneous observations with Chandra and HST in 2006 showed that its X-ray and near-IR radiative properties, all variable prior to 2005, stabilized. Specifically, the 2006 X-ray spectrum is consistent with a two-component blackbody plus power law, with an average -->kt = 0.52 keV and -->Γ = 2.8, at a mean flux level of ~ -->6.5 × 10−12 erg cm−2 s−1 ( -->2–10 keV). The near-IR counterpart in 2005-2006 was detected at -->H ~ 22.7 mag and -->Ks ~ 21.0 mag, considerably fainter than previously measured. In 2007 March, this newfound quiescence was interrupted by sudden X-ray flux, spectral, and pulse morphology changes, simultaneous with a large glitch and near-IR enhancement. Our RXTE observations revealed a factor of ~3 increase in pulsed flux (2-10 keV), while observations with Chandra and Swift saw the total X-ray flux increase much more than the pulsed flux, reaching a peak value of >7 times the quiescent value (2-10 keV). We find a strong anticorrelation between X-ray flux and pulsed fraction, and a correlation between X-ray spectral hardness and flux. Simultaneously with the radiative and timing changes, we observed the X-ray pulse profile change significantly from nearly sinusoidal to having multiple peaks. We compare these remarkable events with other magnetar outbursts and discuss implications in the context of AXP emission models.

The 2006-2007 Active Phase Of Anomalous X-Ray Pulsar 4U 0142+61: Radiative and Timing Changes, Bursts, and Burst Spectral Features

After at least six years of quiescence, anomalous X-ray pulsar (AXP) 4U 0142+61 entered an active phase in 2006 March that lasted several months and included six X-ray bursts as well as many changes in the persistent X-ray emission. The bursts, the first seen from this AXP in >11 yr of Rossi X-Ray Timing Explorer monitoring, all occurred in the interval between 2006 April 6 and 2007 February 7. The burst durations ranged from (0.4-1.8) × 103 s. The first five burst spectra are well modeled by blackbodies, with temperatures kT ~ 2-9 keV. However, the sixth burst had a complicated spectrum that is well characterized by a blackbody plus two emission features whose amplitude varied throughout the burst. The most prominent feature was at 14.0 keV. Upon entry into the active phase, the pulsar showed a significant change in pulse morphology and a likely timing glitch. The glitch had a total frequency jump of (1.9 ± 0.4) × 10–7 Hz, which recovered with a decay time of 17 ± 2 days by more than the initial jump, implying a net spin-down of the pulsar. Within the framework of the magnetar model, the net spin-down of the star could be explained by regions of the superfluid that rotate slower than the rest. The bursts, flux enhancements, and pulse morphology changes can be explained as arising from crustal deformations due to stresses imposed by the highly twisted internal magnetic field. However, unlike other AXP outbursts, we cannot account for a major twist being implanted in the magnetosphere.

10 Years of RXTE Monitoring of the Anomalous X-Ray Pulsar 4U 0142+61: Long-Term Variability

We report on 10 yr of monitoring of the 8.7-s Anomalous X-ray Pulsar 4U 0142+61 using the Rossi X-Ray Timing Explorer (RXTE). This pulsar exhibited stable rotation from 2000 until February 2006: the RMS phase residual for a spin-down model which includes ν,\(\dot{\nu}\), and\(\ddot{\nu}\)is 2.3%. We report a possible phase-coherent timing solution valid over a 10-yr span extending back to March 1996. A glitch may have occurred between 1998 and 2000, but it is not required by the existing data. We also report that the source’s pulse profile has been evolving in the past 6 years, such that the dip of emission between its two peaks has been getting shallower since 2000, almost as if the profile is recovering to its pre-2000 morphology, in which there was no clear distinction between the peaks. These profile variations are seen in the 2–4 keV band but not in 6–8 keV. Finally, we present the pulsed flux time series of the source in 2–10 keV. There is evidence of a slow but steady increase in the source’s pulsed flux since 2000. The pulsed flux variability and the narrow-band pulse profile changes present interesting challenges to aspects of the magnetar model.

The Long-Term Radiative Evolution of Anomalous X-Ray Pulsar 1E 2259+586 After Its 2002 Outburst

We present an analysis of five XMM-Newton observations of the anomalous X-ray pulsar (AXP) 1E 2259+586 taken in 2004 and 2005 during its relaxation following its 2002 outburst. We compare these data with those of five previous XMM-Newton observations taken in 2002 and 2003, and find the observed flux decay is well described by a power law of index –0.69 ± 0.03. As of mid-2005, the source may still have been brighter than preoutburst, and was certainly hotter. We find a strong correlation between hardness and flux, as seen in other AXPs. We discuss the implications of these results for the magnetar model.

CHANDRA AND RXTE OBSERVATIONS OF 1E 1547.0 - 5408: COMPARING THE 2008 AND 2009 OUTBURSTS

We present results from Chandra X-ray Observatory and Rossi X-ray Timing Explorer (RXTE) observations of the magnetar 1E 1547.0–5408 (SGR J1550–5418) following the sources outbursts in 2008 October and 2009 January. During the time span of the Chandra observations, which covers days 4 through 23 and days 2 through 16 after the 2008 and 2009 events, respectively, the source spectral shape remained stable in the Chandra band, while the pulsars spin-down rate in the same span in 2008 increased by a factor of 2.2 as measured by RXTE. This suggests decoupling between the sources spin-down and radiative changes, hence between the spin-down-inferred magnetic field strength and that inferred spectrally. The lack of spectral variation during flux decay is surprising for models of magnetar outbursts. We also found a strong anti-correlation between the phase-averaged flux and the pulsed fraction in the 2008 and 2009 Chandra data, but not in the pre-2008 measurements. We discuss these results in the context of the magnetar model.

Derivation of the Semi-circle Law from the Law of Corresponding States

We show that, for the transition between any two quantum Hall states, the semi-circle law and the existence of a duality symmetry follow solely from the consistency of the law of corresponding states with the two-dimensional scaling flow. This puts these two effects on a sound theoretical footing, implying that both should hold exactly at zero temperature, independently of the details of the microscopic electron dynamics. This derivation also shows how the experimental evidence favours taking the two-dimensional flow seriously for the whole transition, and not just near the critical points.

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.

ON THE EXTENDED EMISSION AROUND THE ANOMALOUS X-RAY PULSAR 1E 1547.0-5408

We present an analysis of the extended emission around the anomalous X-ray pulsar 1E 1547.0-5408 using four XMM-Newton observations taken with the source in varying states of outburst as well as in quiescence. We find that the extended emission flux is highly variable and strongly correlated with the flux of the magnetar. Based on this result, as well as on spectral and energetic considerations, we conclude that the extended emission is dominated by a dust-scattering halo and not a pulsar wind nebula (PWN), as has been previously argued. We obtain an upper limit on the 2-10 keV flux of a possible PWN of 4.7 Multiplication-Sign 10{sup -14} erg s{sup -1} cm{sup -2}, three times less than the previously claimed value, implying an efficiency for conversion of spin-down energy into nebular luminosity of <9 Multiplication-Sign 10{sup -4} (assuming a distance of 4 kpc). We do, however, find strong evidence for X-ray emission from the supernova remnant shell surrounding the pulsar, as previously reported.