The Pulse-Phase–dependent Spectrum of the Anomalous X-Ray Pulsar 1RXS J170849−400910 (original) (raw)
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Unveiling the multi-wavelength phenomenology of Anomalous X-ray Pulsars
2003
During 2002-2003 the number of IR-identified counterparts to the Anomalous X-ray Pulsars (AXPs) has grown to four (4U0142+614, 2E2259+584, 1E 1048-59 and 1RXS J170849-400910) out of the six assessed objects of this class, plus two candidates. More importantly, some new common observational characteristics have been identified, such as the IR variability, the IR flattening in the broad band energy spectrum, the X-ray spectral variability as a function of pulse phase (which are not predicted by the magnetar model), and the SGR-like bursts (which can not be explained in terms of standard accretion models). We present the results obtained from an extensive multi-wavelength observational campaign carried out collecting data from the NTT, CFHT for the optical/IR bands, and XMM, Chandra (plus BeppoSAX archival data) in the X-rays. Based on these results and those reported in the literature, the IR-to-X-ray band emission of AXPs has been compared and studied.
Observations of Anomalous X-ray Pulsars
2001
We review recent results obtained from observations of Anomalous X-ray Pulsars at different wavelengths (X-rays, Optical, IR and Radio) with particular emphasis on results obtained by BeppoSAX. Proposed models for AXPs are briefly presented and critically discussed in the light of these results.
The two-component X-ray spectrum of the 6.4 S pulsar 1E 1048.1-5937
Astronomy and Astrophysics, 1998
The 6.4 s X-ray pulsar 1E 1048.1-5937 was observed by BeppoSAX in 1997 May. This source belongs to the class of "anomalous" pulsars which have pulse periods in range 5-11 s, show no evidence of optical or radio counterparts, and exhibit long-term increases in pulse period. The phase-averaged 0.5-10 keV spectrum can be described by an absorbed power-law and blackbody model. The best-fit photon index is 2.5±0.2 and the blackbody temperature and radius are 0.64±0.01 keV and 0.59 ± 0.02 km (for a distance of 3 kpc), respectively. The detection of blackbody emission from this source strengthens the similarity with two of the more well studied "anomalous" pulsars, 1E 2259+586 and 4U 0142+614. There is no evidence for any phase dependent spectral changes. The pulse period of 6.45026 ± 0.00001 s implies that 1E 1048.1-5937 continues to spin-down, but at a slower rate than obtained from the previous measurements in 1994 and 1996.
Evidence of a Cyclotron Feature in the Spectrum of the Anomalous X-Ray Pulsar 1RXS J170849−400910
The Astrophysical Journal, 2003
We report the results of a long observation of the Anomalous X-ray Pulsar 1RXS J170849−400910 obtained with the BeppoSAX satellite in August 2001. The best fit phase-averaged spectrum was an absorbed power law plus blackbody model, with photon index Γ ∼ 2.4 and a black body temperature of kT bb ∼ 0.4 keV. We confirm the presence of significant spectral variations with the rotational phase of the pulsar. In the spectrum corresponding to the rising part of the pulse we found an absorption-like feature at ∼ 8.1 keV (a significance of 4σ), most likely due to cyclotron resonant scattering. The centroid energy converts to a magnetic field of 9 × 10 11 G and 1.6 × 10 15 G in the case of electrons and protons, respectively. If confirmed, this would be the first detection of a cyclotron feature in the spectrum of an anomalous X-ray pulsar.
The Infrared Counterpart to the Anomalous X-Ray Pulsar 1RXS J170849-400910
The Astrophysical Journal, 2003
We report the discovery of the likely IR counterpart to the Anomalous X-ray Pulsar (AXP) 1RXS J170849−400910, based on the combination of Chandra HRC-I X-ray position, and deep optical/IR observations carried out from ESO telescopes and the Canada France Hawaii Telescope (CFHT) during 1999-2002. Within the narrow uncertainty region we found two relatively faint (K ′ =20.0 and K ′ =17.53) IR objects. Based on their color and position in the J-K ′ versus J-H diagram only the brighter object is consistent with the known IR properties of the counterparts to other AXPs. No variability was detected for this source, similarly to what is observed in the case of 4U 0142+614. Like in other AXPs, we found that the IR flux of 1RXS J170849−400910 is higher then expected for a simple blackbody component extrapolated from the X-ray data. If confirmed, this object would be the fourth IR counterpart to a source of the AXP class, and would make the IR excess a likely new characteristic of AXPs.
Astronomy and Astrophysics, 2005
We report the results of a recent (July 2004) XMM-Newton Target of Opportunity observation of the Anomalous X-ray pulsar 1E 1048.1-5937 together with a detailed re-analysis of previous observations carried out in 2000 and 2003. In July 2004 the source had a 2-10 keV flux of 6.2×10 −12 erg cm −2 s −1 and a pulsed fraction PF =0.68. The comparison of the three data sets shows the presence of an anti-correlation between flux and pulsed fraction, implying that previous estimates of the source energetics based on the assumption of a large and constant pulsed fraction might be significantly underestimated. The source spectrum is well described by a power law plus blackbody model (kT∼0.63 keV, photon index Γ∼2.7-3.5) or, alternatively, by the sum of two blackbodies of which the hotter is Comptonized by relativistic electrons. In this case the temperatures are kT1 ∼0.2-0.3 keV and kT2 ∼0.4-0.5 keV and the emitting area of the cooler component is consistent with the whole neutron star surface. The long term luminosity variation of a factor > ∼ 2 is accompanied by relatively small variations in the spectral shape. Phase resolved spectroscopy indicates a harder spectrum in correspondence of the pulse maximum. No spectral features have been detected with 4σ limits on the equivalent width in the range ∼10-220 eV, depending on line energy and width.
Transient Phenomena in Anomalous X-ray Pulsars
Arxiv preprint arXiv: …, 2007
In 2003 a previously unpulsed Einstein and ROSAT source cataloged as soft and dim (L X of few ×10 33 erg s −1 ) thermal emitting object, namely XTE J1810-197, was identified as the first unambiguous transient Anomalous X-ray Pulsar. Two years later this source was also found to be a bright highly polarized transient radio pulsar, a unique property among both AXPs and radio pulsars. In September 2006 Swift Burst Alert Telescope (BAT) detected an intense burst from the candidate AXP CXOU J164710.2−455216, which entered in an outburst state reaching a peak emission of at least a factor of 300 higher than quiescence. Here, we briefly outline the recent results concerning the outburst phenomena observed in these two AXPs. In particular, XTE J1810-197 has probed to be a unique laboratory to monitor the timing and spectral properties of a cooling/fading AXP, while new important information have been inferred from X-ray and radio band simultaneous observations. CXOU J164710.2−455216 has been monitored in X-rays and radio bands since the very beginning of its outbursting state allowing us to cover the first phases of the outburst and to study the timing and spectral behavior during the first months.
Long-Term X-Ray Changes in the Emission from the Anomalous X-Ray Pulsar 4U 0142+61
The Astrophysical Journal, 2010
We present results obtained from X-ray observations of the anomalous X-ray pulsar (AXP) 4U 0142+61 taken between 2000-2007 using XMM-Newton, Chandra and Swift. In observations taken before 2006, the pulse profile is observed to become more sinusoidal and the pulsed fraction increased with time. These results confirm those derived using the Rossi X-ray Timing Explorer and expand the observed evolution to energies below 2 keV. The XMM-Newton total flux in the 0.5-10 keV band 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 towards more sinusoidal pulse profiles ceased while the pulsed fraction showed a further increase. No evidence for large-scale, long-term changes in the emission as a result of the bursts is seen. The data also suggest a correlation between the flux and hardness of the spectrum, with brighter observations on average having a harder spectrum. As pointed out by other authors, we find that the standard blackbody plus power-law model does not provide the best spectral fit to the emission from 4U 0142+61. We also report on observations taken with the Gemini telescope after two bursts. These observations 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.
Very deep X-ray observations of the anomalous X-ray pulsar 4U 0142+614
Monthly Notices of the Royal Astronomical Society, 2007
We report on two new XMM-Newton observations of the Anomalous X-ray Pulsar (AXP) 4U 0142+614 performed in March and July 2004, collecting the most accurate spectrum for this source. Furthermore, we analyse two short archival observations performed in February 2002 and January 2003 (the latter already reported by in order to study the long term behaviour of this AXP. 4U 0142+614 appears to be relatively steady in flux between 2002 and 2004, and the phase-averaged spectrum does not show any significant variability between the four epochs. We derive the deepest upper limits to date on the presence of lines in 4U 0142+614 spectrum as a function of energy: equivalent width in the 1-3 keV energy range < 4 eV and < 8 eV for narrow and broad lines, respectively. A remarkable energy dependence in both the pulse profile and the pulsed fraction is detected, and consequently pulse-phase spectroscopy shows spectral variability as a function of phase. By making use of XMM-Newton and INTEGRAL data, we successfully model the 1-250 keV spectrum of 4U 0142+614 with three models presented in , namely the canonical absorbed blackbody plus two power-laws, a resonant cyclotron scattering model plus one power-law and two log-parabolic functions.
The Astrophysical Journal, 2002
We report on the detection of variability from the proposed IR counterpart to the Anomalous X-ray Pulsar (AXP) 1E 1048.1−5937 based on Chandra and ESO optical/IR deep observations carried out in [2001][2002]. Within the narrow Chandra uncertainty region for 1E 1048.1−5937 we found only one relatively faint (J=22.1±0.3, J-Ks=2.4) source, while the recently proposed IR counterpart was not detected down to a limiting Ks magnitude of ∼20.7 (3σ confidence level). This implies a remarkable IR brightening of this object, ∆ Ks>1.3, on a timescale of about 50 days. Although our knowledge of the IR properties of AXPs is rather limited (there is only another source, 1E 2259+586, for which IR variability has been detected), the observed IR variability of the proposed counterpart strengthens its association with 1E 1048.1−5937. Our results make the IR (and presumably optical) variability a likely common characteristic of AXPs, and provide new constraints on this class of objects.