XMM-Newton view of Swift J1834.9-0846 and its Magnetar Wind Nebula
Abstract: We report on the analysis of two XMM-Newton observations of the recently discovered soft gamma repeater Swift J1834.9–0846, taken in 2005 September and one month after the source went into outburst on 2011 August 7. We performed timing and spectral analyses on the point source as well as on the extended emission. We find that the source period is consistent with an extrapolation of the Chandra ephemeris reported earlier and the spectral properties remained constant. The source luminosity decreased to a level of 1.6 × 10^(34) erg s^(–1) following a decay trend of ∝ t^(–0.5). Our spatial analysis of the source environment revealed the presence of two extended emission regions around the source. The first (region A) is a symmetric ring around the point source, starting at 25" and extending to ~50". We argue that region A is a dust scattering halo. The second (region B) has an asymmetrical shape extending between 50" and 150", and is detected both in the pre- and post-outburst data. We argue that this region is a possible magnetar wind nebula (MWN). The X-ray efficiency of the MWN with respect to the rotation energy loss is substantially higher than those of rotation-powered pulsars: η_X ≡ L_(MWN,0.5-8 keV)/Ė_rot ≈ 0.7. The higher efficiency points to a different energy source for the MWN of Swift J1834.9–0846, most likely bursting activity of the magnetar, powered by its high magnetic field, B = 1.4 × 10^(14) G.
Summary (2 min read)
- Soft gamma repeaters (SGRs) and anomalous X-ray pulsars (AXPs) are two empirical classes of objects widely accepted to comprise the magnetar population, i.e., isolated neutron stars with ultra-strong magnetic fields (B 1014–1015 G).
- Magnetars enter active episodes during which they emit short (0.1 s) bursts of hard X-/soft γ -rays with luminosities ranging from 1037 to 1041 erg s−1; very rarely, they emit giant flares (GFs) that last several minutes with luminosities 1046 erg s−1.
- The EPIC-PN and MOS detectors were operating in Prime Full Frame mode using the medium filter.
- The best-fit parameters to their extended sources spectra using the two background-estimation methods, i.e., directly or through modeling, were in very good agreement within the error bars at the 1σ level (Table 1).
3.1. Spatial Analysis
- The middle and lower panels are smoothed with a Gaussian of FWHM 20′′ to accentuate the extended emission.
- This PSF template, given as an XMM-Newton calibration file (XRT3_XPSF_0013.CCF), is the best-fit King function (King 1966) to the radial profile of many bright point sources observed with the EPIC cameras.
- The authors quantified the asymmetrical shape of this extended emission in obs.
3.2. Timing Analysis
- For their timing analysis, which was only performed for obs.
- The authors then employed a Z21 test (Buccheri et al. 1983) to search for pulsed signal from the source.
- The authors then investigated the energy and time dependence of the pulse profiles.
- Figure 4 shows the background subtracted pulse profiles in the 2–5, 5–10, and 2–10 keV, respectively, from top to bottom panels.
- This value is marginally lower than the value of 85% ± 10% obtained from the Chandra observation (K+12), indicating a decline in pulse fraction in over about one month.
3.3.1. Post-outburst Observation
- Based on their radial profile analysis of obs.
- For each simulated spectrum, the authors recorded the Δχ2 between the null hypothesis PL model and the PL + absorption feature model, and compared the values to the real Δχ2.
- The significance is too low to claim a firm line detection; more sensitive observations during a new source burst active episode could provide better statistics.
- All fit parameters and absorbed fluxes and luminosities are given in Table 1.
3.3.2. Pre-outburst Observation
- The authors collected 45 counts from the 18′′ radius circle around Swift J1834.9−0846 as shown in the lower panel of Figure 1, not enough for a proper spectral analysis.
- Next, the authors collected ∼100 counts from region A and binned the spectrum at 15 counts bin−1.
- These results are also discussed in Section 4.
4.2. A Halo around Swift J1834.9−0846: Region A
- The spectrum and flux of the symmetrical extended emission (region A) fits well a dust scattering halo interpretation.
- Since the scattering cross section of the dust particles is proportional to E−2, a halo is expected to have a softer spectrum than the illuminating source, i.e., Swift J1834.9−0846.
- This trend is evident from Figure 9, which shows the flux evolution of region A and Swift J1834.9−0846, between the post-outburst Chandra (K+12) and XMM-Newton observations .
4.3. Asymmetrical Extended Emission (Region B): An MWN?
- PWNe are often observed around these pulsars and are believed to be the synchrotron radiation of the shocked wind (see Kaspi et al.
- Therefore, not only the unrealistically high “efficiency” ηX ∼ 0.7, but also the large size support the hypothesis that the observed asymmetrical nebula (region B) could not be produced by the magnetar in quiescence via rotation-powered wind.
- When a magnetar is in an active state, the pressure of its wind (ejected particles and magnetic fields) is much higher than that in quiescence.
- Finally, the authors would like to discuss some other possibilities for the origin of the extended X-ray emission around Swift J1834.9−0846.
- To test this hypothesis, the authors extracted the spectrum of region A+region B during obs.
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