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XMM-Newton view of Swift J1834.9-0846 and its Magnetar Wind Nebula

20 Sep 2012-The Astrophysical Journal (American Astronomical Society)-Vol. 757, Iss: 1, pp 39

AbstractWe 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.

Topics: Energy source (60%), Soft gamma repeater (57%), Magnetar (56%), Point source (53%), Luminosity (51%)

Summary (2 min read)

1. INTRODUCTION

  • 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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The Astrophysical Journal, 757:39 (10pp), 2012 September 20 doi:10.1088/0004-637X/757/1/39
C
2012. The American Astronomical Society. All rights reserved. Printed in the U.S.A.
XMM-NEWTON VIEW OF SWIFT J1834.90846 AND ITS MAGNETAR WIND NEBULA
G. Younes
1,2
, C. Kouveliotou
2,3
, O. Kargaltsev
4
, G. G. Pavlov
5,6
,E.G
¨
o
ˇ
g
¨
s
7
, and S. Wachter
8
1
Universities Space Research Association, 6767 Old Madison Pike NW, Suite 450, Huntsville, AL 35806, USA
2
NSSTC, 320 Sparkman Drive, Huntsville, AL 35805, USA
3
Astrophysics Office, ZP12, NASA/Marshall Space Flight Center, Huntsville, AL 35812, USA
4
Department of Astronomy, University of Florida, Bryant Space Science Center, Gainesville, FL 32611, USA
5
Department of Astronomy and Astrophysics, Pennsylvania State University, 525 Davey Lab, University Park, PA 16802, USA
6
St.-Petersburg State Polytechnical University, Polytekhnicheskaya ul. 29, 195251 St.-Petersburg, Russia
7
Faculty of Engineering and Natural Sciences, Sabancı University, Orhanlı-Tuzla,
˙
Istanbul 34956, Turkey
8
Infrared Processing and Analysis Center, California Institute of Technology, Pasadena, CA 91125, USA
Received 2012 April 12; accepted 2012 July 25; published 2012 September 4
ABSTRACT
We report on the analysis of two XMM-Newton observations of the recently discovered soft gamma repeater Swift
J1834.90846, 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
/
˙
E
rot
0.7. The
higher efficiency points to a different energy source for the MWN of Swift J1834.90846, most likely bursting
activity of the magnetar, powered by its high magnetic field, B = 1.4 × 10
14
G.
Key words: stars: neutron X-rays: individual (Swift J1834.90846) X-rays: ISM
1. INTRODUCTION
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 10
14
–10
15
G).
Their existence was predicted theoretically in 1992 (Duncan
& Thompson 1992; Paczynski 1992), but was only confirmed
in 1998 with RXTE observations (Kouveliotou et al. 1998,
1999; for detailed magnetar reviews please refer to Woods &
Thompson 2006; Mereghetti 2008). SGRs and AXPs share many
characteristics such as long spin periods (2–12 s) and large
spin-down rates that imply very high surface dipole magnetic
fields of 10
14
–10
15
G. They are all persistent X-ray emitters
with luminosities significantly larger than those expected from
rotational energy losses; instead the magnetar X-ray emission
is attributed to the decay of their powerful magnetic fields and
sub-surface heating (Thompson & Duncan 1996). Magnetars
enter active episodes during which they emit short (0.1 s) bursts
of hard X-/soft γ -rays with luminosities ranging from 10
37
to
10
41
erg s
1
; very rarely, they emit giant flares (GFs) that last
several minutes with luminosities 10
46
erg s
1
. The typical
magnetar bursts are attributed to neutron star crust quakes caused
by the evolving magnetic field under its surface (Thompson &
Duncan 1995).
An interesting question in the magnetar field is their evolu-
tionary link, if any, to their less magnetically powerful counter-
parts, rotation-powered pulsars (RPPs). The latter sources are
known to produce particle outflows, often resulting in spec-
tacular pulsar wind nebulae (PWNe; see Kargaltsev & Pavlov
2008 for a review) of which the Crab is the most famous exam-
ple (Weisskopf et al. 2000). The PWN X-ray emission is due
to synchrotron radiation from the shocked relativistic outflow
of electrons and positrons produced by the pulsar. Magnetars
are also expected to produce particle outflows, either steady
or released during outbursts accompanying bright bursts or GFs
(Thompson & Blaes 1998; Harding et al. 1999; Tong et al. 2012).
The GF of 2004 December 27 from SGR J180620 released
at least 4 × 10
43
erg of energy in the form of magnetic fields
and relativistic particles (Gaensler et al. 2005). Given the strong
magnetic fields associated with this class of neutron stars, the
idea, therefore, of a magnetar wind nebula (MWN) seems very
plausible.
Only a few claims have been made so far for the detection
of a nebula around a magnetar. The first one was the radio
nebula around SGR J180620 (Kulkarni et al. 1994), which
was shown later to be enshrouding a luminous blue variable
star, unrelated to the SGR (Hurley et al. 1999). Elongated
and expanding radio emission was unambiguously identified
following the GF of SGR J180620 (Gaensler et al. 2005;
Gelfand et al. 2005), most likely associated with jets produced
by the flare. A variable radio source indicating particle outflow
was also seen after the GF of SGR 1900+14 (Frail et al. 1999).
Recently, Rea et al. (2009b), Safi-Harb & Kumar (2008;see
also Gonzalez & Safi-Harb 2003), and Vink & Bamba (2009)
reported the discovery of unusual extended emission around
three high B-field sources, a rotating radio transient, RRAT
J18191458, a high-B pulsar PSR J11196127, and a magnetar
1E 1547.05408 (SGR J15505418), respectively. The latter
case was shown to be a halo on the basis of correlated flux
variations in the extended emission and the magnetar (Olausen
et al. 2011). In summary, to date there is no unambiguous
evidence for the existence of a PWN/MWN around a magnetar.
Confirmed detections of MWNe would reconcile observations
1

The Astrophysical Journal, 757:39 (10pp), 2012 September 20 Younes et al.
with theoretical predictions of their existence and would shed
light on the nature of magnetar outflows and the environmental
properties of magnetars.
Swift J1834.90846 is the last in a long line of magnetar
discoveries during the last three years, owing to the synergy
between NASAs three observatories, Swift, RXTE, and Fermi.
It was discovered on 2011 August 7, when it triggered the Swift/
Burst Alert Telescope and the Fermi/Gamma-ray Burst Monitor
with a soft, short burst (D’Elia et al. 2011; Guiriec et al. 2011).
The magnetar nature of Swift J1834.90846 was established
with RXTE/PCA and Chandra target of opportunity (TOO)
observations, which revealed a coherent X-ray pulsation at a spin
period P = 2.482295 s (G
¨
o
ˇ
g
¨
s & Kouveliotou 2011;G
¨
o
ˇ
g
¨
s
et al. 2011b), and a spin-down rate ˙ν =−1.3(2)× 10
12
Hz s
1
(Kuiper & Hermsen 2011), implying a dipole surface magnetic
field B = 1.4 × 10
14
G, and a spin-down age and energy loss
rate τ = 4.9 kyr and
˙
E
rot
= 2.1 × 10
34
erg s
1
, r espectively.
Kargaltsev et al. (2012, K+12 hereinafter) studied the spatial,
temporal, and spectral properties of Swift J1834.90846 using
the available Swift, RXTE, and Chandra post-outburst observa-
tions, and one Chandra pre-outburst observation taken in 2009
June. The persistent X-ray light curve of the source, spanning
48 days after the first burst, showed that the 2–10 keV flux de-
cayed steadily as a power law (PL) with index α = 0.53 ± 0.07
(F t
α
). The source spectrum (2–10 keV) was well fit with
either an absorbed PL with a photon index Γ 3.5±0.5oranab-
sorbed blackbody (BB) with a temperature kT = 1.1 ± 0.1keV,
and an emitting area radius of 0.26 km (assuming a source dis-
tance of 4 kpc, see below). The hydrogen column density was
of the order of 10
23
cm
2
, depending on the model spectrum.
Finally, K+12 reported the presence of an extended emission
up to a radius of 10

from the center of the source, most likely
a dust scattering halo, considering the large absorption toward
the source position. However, an even more extended emis-
sion, with radius >30

, was detected in the 2009 pre-outburst
Chandra observation. The asymmetrical shape of this emis-
sion, northeast–southwest of the source, poses a challenge to
the dust scattering halo interpretation, especially since this ex-
tended component was detected while the point source was not
seen down to a limit of 10
15
erg cm
2
s
1
.
Here, we report the analysis of two XMM-Newton observa-
tions of Swift J1834.90846, taken in 2005 September and 2011
September (one month after the source outburst), with emphasis
on the analysis of the environment around the source. Section 2
describes the observations and data reduction techniques. We
present our results of the spatial, timing, and spectral analysis in
Section 3. We discuss the spectral and temporal results of Swift
J1834.90846 and the implication of our extended emission
analysis in the context of MWN in Section 4. Given a plausible
association between Swift J1834.90846 and the SNR W41,
we will assume that both are at the same distance (4 kpc; Tian
et al. 2007; Leahy & Tian 2008; K+12) throughout the paper.
2. XMM-NEWTON OBSERVATIONS
AND DATA REDUCTION
The field of the newly discovered magnetar, Swift
J1834.90846, was observed twice with XMM-Newton.The
first observation (ObsID 0302560301, obs. 1 hereafter; PI: Gerd
Puehlhofer), taken in 2005 September for an exposure time of
about 20 ks, was intended to image the HESS J1834087 field in
which Swift J1834.90846 lies. During this observation, Swift
J1834.90846 was 2
off-axis from the nominal on-axis posi-
tion, which is small enough not to cause substantial vignetting.
The EPIC-PN and MOS detectors were operating in Prime Full
Frame mode using the medium filter. Data from all three EPIC
instruments were analyzed in the past (EPIC-PN, Tian et al.
2007; EPIC-MOS, Mukherjee et al. 2009). We re-analyzed this
observation to look for an extended emission at the position of
Swift J1834.90846.
The second XMM-Newton observation (ObsID 0679380201,
obs. 2 hereafter) was a TOO (PI: Norbert Schartel) taken on
2011 September 17 for an exposure of about 24 ks, with
Swift J1834.90846 being at the aim point of the three EPIC
detectors. The EPIC-PN detector was operating in Prime Full
Frame mode using the medium filter. The EPIC-MOS detectors,
on the other hand, were operating in Small Window mode.
The two observations were reduced and analyzed in a ho-
mogeneous manner using the Science Analysis System (SAS)
version 11.0.0 and FTOOLS version 6.11.1. Data were selected
using event patterns 0–4 and 0–12 for PN and MOS, respectively,
during only good X-ray events (“FLAG==0”). We excluded in-
tervals of enhanced particle background during obs. 1, resulting
in an effective exposure time of 14 ks in the MOS cameras.
Response matrices were generated using the task rmfgen. These
responses were spatially averaged using a point-spread func-
tion (PSF) model for point-like sources and a flat uniform flux
distribution for extended sources.
Background events for point-like sources were extracted from
a source-free region with the same size as the source on the s ame
CCD. We followed the same procedure for the background
extraction of extended sources since they only cover a small
region in the sky with a size of 2
–3
(see Section 3.1).
For point-like sources, the background spectrum was directly
subtracted from the source spectrum. Such a method corrects
for both the instrumental and the cosmic X-ray background
simultaneously. Since our extended sources are not very large
(see Section 3.1) one can expect that same method would work
reasonably well for their spectra. However, to ensure that the
background contribution is accurately accounted for, we also
tried a more rigorous background-estimate procedure, where
we first modeled the background spectrum and then included
the background contribution as an additional model component
while fitting the source spectrum.
We used the Extended Science Analysis Software (ESAS)
package
9
for the purpose of background modeling. First, the
instrumental background is extracted from the CCDs where
our extended emission lies, using the filter-wheel closed data,
i.e., derived from observations where the filter wheel is in
the closed position. We correct both the background and the
source spectra for the instrumental background. Then, we
fit the resulting background spectrum with a combination of
two thermal components and an absorbed PL. We froze the
temperature of one of the thermal components to 0.1 keV
assuming emission from the local hot bubble. The temperature
of the second thermal model, which represents the emission
from the interstellar/intergalactic medium, was left free to
vary (Snowden et al. 2004, 2008). The absorption in the PL
was frozen to the Galactic value toward Swift J1834.90846,
N
H
= 1.63 × 10
22
cm
2
, and the photon index of the PL
was frozen to 1.5 assuming unresolved active galactic nucleus
(AGN) contribution (e.g., distant quasars and/or nearby low-
luminosity AGN; Porquet et al. 2004; Sazonov et al. 2008;
Younes et al. 2011). We also added a Gaussian emission line
with a centroid energy of 1.5 keV to model the instrumental
9
http://xmm.esac.esa.int/sas/current/doc/esas/index.html
2

The Astrophysical Journal, 757:39 (10pp), 2012 September 20 Younes et al.
Tab le 1
Spectral Model Parameters, Fluxes, and Luminosities of Swift J1834.90846 and its Surrounding Medium
Source Model N
H
Γ kT N
a
or R
b
χ
2
ν
/dof F
210 keV
Absorbed L
c
2
10keV
(10
22
cm
2
)(keV) (10
12
erg cm
2
s
1
)(10
34
erg s
1
)
Swift J1834.90846 (post-outburst) PL 24 ± 14.2± 0.1 ... 5.67
+0.02
0.01
1.15/232 1.25
+0.02
0.03
1.6
+0.2
0.1
Swift J1834.90846 (post-outburst) BB 12.9 ± 0.6 ... 0.96 ± 0.02 0.24 ± 0.02 1.04/232 1.19
+0.03
0.04
0.16 ± 0.01
Swift J1834.90846 (pre-outburst)
d
PL 24(fixed) 4.2(fixed) ... ... ... 0.04 0.07
Region A (post-outburst) PL 25
+6
5
4.5
+0.7
0.6
... 1.48 ± 0.02 0.9/57 0.19 ± 0.02 0.3
+0.5
0.2
Region A (post-outburst)
e
PL 31
+10
9
5.0
+1.0
0.9
... 3.20
+0.02
0.01
0.9/57 0.16 ± 0.02 0.4
+0.5
0.2
Region A (pre-outburst) PL 13
+8
6
1.7
+1.4
1.1
... 0.005
+0.007
0.003
1.3/80.12
+0.06
0.05
0.04
+0.02
0.01
Region B (post-outburst) PL 15 ± 53.4
+1.0
0.9
... 0.3 ± 0.1 1.0/23 0.35 ± 0.06 0.21
+0.15
0.06
Region B (post-outburst)
f
PL 17 ± 43.2
+0.7
0.6
... 0.2 ± 0.1 0.9/46 0.35 ± 0.04 0.21
+0.10
0.06
Region B (pre-outburst) PL 16(fixed) 3.5 ± 0.6 ... 0.2
+0.2
0.1
1.7/19 0.15
+0.06
0.05
0.10
+0.04
0.03
Notes.
a
PL normalization in units of 10
2
photons cm
2
s
1
keV
1
at 1 keV.
b
BB radius, in units of km.
c
2–10 keV power-law luminosity or bolometric BB luminosity (πR
2
σT
4
), assuming a source distance of 4 kpc (Tian et al. 2007).
d
Fluxes and luminosities converted from the count rate in Section 3.1 using PIMMS, assuming the corresponding spectral parameters.
e
Spectral results including the possible contribution from region B (see Section 3.3).
f
Spectral parameters derived using a modeled background as described in Section 2.
EPIC-PN Al Kα line. The model fit to the background spectrum
was good, with χ
2
ν
= 1.3 for 42 degrees of freedom (dof).
The temperature of the thermal component is kT 1.0keV,a
reasonable value for the intergalactic medium X-ray emission.
Finally, we fit the extended emission spectra with an absorbed
PL, including the background best-fit model.
The best-fit parameters to our 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). Hence, in the following the background
for extended sources was estimated directly, as usually done for
point-like sources.
3. RESULTS
3.1. Spatial Analysis
The X-ray images (1.5–8 keV) of Swift J1834.90846 are
shown in Figure 1 for obs. 1 (MOS1+2 cameras, lower panel)
and obs. 2 (PN camera, upper and middle panels).
10
The
middle and lower panels are smoothed with a Gaussian of
FWHM 20

to accentuate the extended emission.
We extracted the radial profile from a set of circular annuli
centered at the position of Swift J1834.90846 using the
MOS1+2 and PN cameras for obs. 1 and obs. 2, respectively
(Figure 2). These radial profiles were then fit by re-normalizing
an XMM-Newton PSF template (to have similar number of
counts at the core) and adding a constant background (dot-
dashed line). 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 rms values of the
PSF fit to our radial profiles are 0.10 and 0.35 for obs. 1 and
obs. 2, respectively, indicating that a PSF alone is not sufficient
to explain the observed source radial profiles, and that an excess
emission is present. Indeed, extended emission is clearly visible
in both observations, starting at around 15

and 25

for obs. 1
and obs. 2, respectively. The extent of this emission is larger
10
During obs. 1 Swift J1834.90846 lies on a CCD gap in the PN camera and
these data were not used; obs. 2 used MOS cameras in Small Window mode.
and more obvious in obs. 2, stretching out to r 150

.The
emission in obs. 1 is detected up to r 70

.
It is clear from Figure 1 (middle panel) that the extended
emission around Swift J1834.90846 becomes asymmetrical
in shape at r 50

. We quantified the asymmetrical shape of
this extended emission in obs. 2 (which has better statistics
than obs. 1), by collapsing the counts in the X (east–west)
and Y (south–north) directions, in a rectangular region of
222 × 91 pixels around the SGR, excluding any point sources
in the field. Since our source lies very close to the PN CCD gap,
we used an exposure-map-corrected image for this analysis to
correct for these CCD gaps, which also corrects for bad pixels.
The background level, shown as a black solid line in Figure 3,
is the mean value of the total counts in two regions taken
at rectangular areas away from the source in both directions.
The profile is centered at the SGR central pixel, with the
dotted lines representing the 25

point-like source emission,
i.e., the SGR, and the dashed lines showing the extent of the
extended emission. It is clear from both panels of Figure 3 that
the extended emission is asymmetrical. In the X-direction, the
emission extends up to 165

to the right of the source, but only
90

to the left. In the Y-direction, the emission extends up to
125

below the source center and only up to 45

above it.
Finally, we detect in obs. 1 a weak excess emission consistent
with a point source at the position of Swift J1834.90846.
Since the emission around Swift J1834.90846 shows an excess
over the PSF fit starting at 18

(see Figure 2), we estimate the
count rate in a 18

radius circle centered on the source. We
find a rate of 0.0028 ± 0.0006 counts s
1
, which represents
a detection at the 4.6σ level. We also detect asymmetrical
emission west–southwest of the SGR, consistent with the
shape and direction of the post-outburst asymmetrical emission
discussed above.
We summarize our spatial analysis results in Figure 1.In
the post-outburst observation (upper and middle panels), the
smallest green circle with a 25

radius represents the Swift
J1834.90846 point-source emission (taking into account the
PSF). The green annulus with inner and outer radii of 25

and 50

, respectively (region A hereinafter), represents the
3

The Astrophysical Journal, 757:39 (10pp), 2012 September 20 Younes et al.
Figure 1. Post-outburst XMM-Newton EPIC-PN observation of Swift
J1834.90846 in 2011 (obs. 2, upper and middle panels) and pre-outburst
2005 EPIC MOS1+MOS2 observation (obs. 1, bottom panel). The middle and
bottom images are Gaussian smoothed with an FWHM of 5.0 pixels (20

).
The smallest green circle with a 25

radius represents the Swift J1834.90846
point-source emission. The annulus with 25

r 50

represents the symmet-
rical extended emission around the point source (region A). The ellipse of major
(minor) axis of 145

(95

) encloses the asymmetrical extended emission around
Swift J1834.90846 (region B). Other sources in the field are labeled. North is
up and west is right.
symmetrical extended emission, most likely a dust scattering
halo ( see Section 3.3), similar to the one seen in the Chandra
post-outburst observation (K+12). Beyond r 50

from the
center of Swift J1834.90846, the asymmetrical extended
emission is mostly seen toward the west–southwest of the SGR
(middle panel); we approximate this region with an ellipse of
major (minor) axis of 145

(95

) (region B hereinafter). Similar
asymmetrical emission is seen in the pre-outburst XMM-Newton
observation with some hints of weak excess emission at the
position of the SGR (lower panel). A similar extended emission
has been reported for the Chandra pre-outburst observations,
when the source was in quiescence (K+12). The asymmetrical
shape argues against a dust scattering halo origin, and its small
size with the lack of any radio counterpart makes a supernova
remnant (SNR) explanation questionable. A third option is,
therefore, a wind nebula powered by the magnetar. We will
discuss these possibilities in Section 4.
3.2. Timing Analysis
For our timing analysis, which was only performed for obs. 2,
we first converted the arrival times of all 2900 events within the
25

source photon extraction region to the arrival times at the
solar system barycenter. We then employed a Z
2
1
test (Buccheri
et al. 1983) to search for pulsed signal from the source. We
detect the pulsed signal very clearly (with a Z
2
1
peak of about
750) at a frequency of 0.4028466(5) Hz. Note that the measured
pulse frequency of Swift J1834.90846 is consistent within
uncertainties with the spin ephemeris reported by K+12.
We 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. We find that the pulse fraction shows a hint of
energy dependence: it is (57 ± 13)% in the 2–5 keV band and (70
± 17)% in 5–10 keV. The pulsed fraction i n the 2–10 keV band
is (60 ± 15)%. 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.
We also searched for pulse profile evolution in time by splitting
the effective duration of the XMM-Newton pointing into three
parts and generating the pulse profile in each segment in the
2–10 keV range. We find no significant variation of pulse shape
throughout the observation as well as between the XMM-Newton
and Chandra observations.
3.3. Spectral Analysis
3.3.1. Post-outburst Observation
Based on our radial profile analysis of obs. 2, we extracted
the spectra of Swift J1834.90846 in a circular region with a
radius of 25

from the PN camera and with a radius of 20

from
the MOS1/MOS2 cameras (extended emission started at 20

from the center of the SGR in the MOS cameras), collecting
2900 and 1020 counts, respectively. Background events were
extracted from source-free circles with the same radii as for
the source and on the same CCD, resulting in 56 and 32, PN
and MOS1/MOS2 background counts, respectively. The spectra
were then grouped to have a minimum of 25 counts per bin.
Finally, we made sure that the point-source spectrum was not
affected by pile-up using the XMM-Newton SAS task epatplot.
Table 1 includes the results of our spectral analysis of the point
source and both extended regions (see below).
We fit the point-source (Swift J1834.90846) spectrum with
an absorbed PL and with an absorbed BB model. The latter gave
a better fit, with a reduced χ
2
of 1.04 for 232 dof, corresponding
to an improvement of 26 in χ
2
for the same number of dof. From
the BB fit, we estimate the emitting area radius to be R = (0.24±
0.02)d
4
km, where d
4
= d/4 kpc, consistent with the value
derived from the Chandra data taken 1 month earlier. Table 1
gives the PL and BB best-fit parameters, and the absorbed fluxes
and luminosities. Figure 5 upper (lower) panel shows the best-
fit PL (BB) model and the residuals in terms of sigma. In each
panel of Figure 5, the upper (black dots) fits are the EPIC-PN
data and the two lower fits (blue diamonds and red stars) are
the MOS1 and MOS2 data. We note here that the fluxes and
luminosities of Swift J1834.90846 are half the values derived
4

The Astrophysical Journal, 757:39 (10pp), 2012 September 20 Younes et al.
2 5 10 20 25 50 100
0.1
1
10
Radius (arcsec)
Counts s
−1
arcsec
−2
2 5 10 20 100
10
−2
10
−1
Radius (arcsec)
Counts s
−1
arcsec
−2
Figure 2. Radial profile of the X-ray emission (1.5–8 keV) of Swift J1834.90846 using the XMM-Newton data from the post-outburst obs. 2 (PN, left panel) and the
pre-outburst obs. 1 (MOS1+2, right panel). The black solid line represents the best-fit PSF for each camera. Extended emission is clearly seen beyond 20

and 15

in obs. 2 and obs. 1, respectively.
−200 −150 −100 −50 0.0 50 100 150 200
0
100
200
300
400
500
600
700
800
900
Total counts in Y−pixel column
Distance from center of SGR (arcsec)
164
−88
100 200 300 400 500 600 700 800 900 1000
−150
−100
−50
0
50
100
150
200
Distance from center of SGR (arcsec)
Total counts in X−pixel column
125
−45
Figure 3. Left column: projected total counts of a pixel column in the Y-direction (north–south, left panel) and the X-direction (east–west, right panel) in a rectangular
region around Swift J1834.90846. The dotted lines delimit a 25

circular region around the SGR. The dashed lines represent the extent of the asymmetrical extended
emission.
Figure 4. Pulse profiles of the persistent X-ray emission of Swift J1834.90846,
accumulated between 2–5, 5–10, and 2–10 keV from top to bottom.
from the Chandra data almost a month earlier (K+12). Finally,
we note that a more complex, two-component model, typically
used to fit magnetar X-ray spectra, is not required by the data.
We then binned the spectra of the point source to the PN
spectral resolution and searched for potential line-like fea-
tures in the time-integrated and the time-resolved spectra. The
time-integrated spectrum revealed two possible lines (absorp-
tion and emission) between 3 and 5 keV. To investigate t he lines,
we first added a Gaussian emission profile with a best-fit energy
of 3.7 keV, which reduced χ
2
by 8, for 3 dof. The addition of
an absorption line with a best-fit energy of 4.2 keV resulted
in an equal improvement. Adding both lines together does not
improve the spectral fit further. We then performed Monte Carlo
simulations (MCSs) to rigorously assess the significance of these
spectral features. We took the best-fit absorbed PL model as our
null hypothesis. We simulated 1000 spectra based on this model
with the XSPEC fakeit command, and fitted each spectrum with
the null hypothesis model. We then added an absorption line to
the model (gabs in XSPEC) and re-fit the spectrum. For each
simulated spectrum, we recorded the Δχ
2
between the null hy-
pothesis PL model and the PL + absorption feature model, and
compared the values to the real Δχ
2
. This procedure resulted in
an absorption line significance at only the 90% confidence level.
Including an emission line at 3.7 keV, instead of an absorption
line, gave the same level of significance. We note that this sig-
nificance level is insensitive to the null hypothesis model since
an absorbed BB gave similar results (95% confidence level). We
conclude that the lines are not significant in the time-integrated
spectrum of Swift J1834.90846.
Next, we performed both time-resolved and phase-resolved
spectroscopy to investigate whether there are specific intervals
(phases) where the lines are more prevalent. For the former case,
we split the 24 ks observation into four equal segments and
fit each of the four spectra with an absorbed PL model. We find
that the source spectrum is constant throughout the observation.
Only in segment two (6.7513.50 ks) did we see evidence for
the presence of an emission line at 3.8 keV (Figure 6,first
5

Figures (11)
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Abstract: It is proposed that the main observational signature of magnetars, high-field neutron stars, is gamma-ray bursts powered by their vast reservoirs of magnetic energy. If they acquire large recoils, most magnetars are unbound from the Galaxy or reside in an extended, weakly bound Galactic corona. There is evidence that the soft gamma repeaters are young magnetars. It is argued that a convective dynamo can also generate a very strong dipole field after the merger of a neutron star binary, but only if the merged star survives for as long as about 10-100 ms. Several mechanisms which could impart a large recoil to these stars at birth, sufficient to escape from the Galactic disk, are discussed.

2,304 citations




Journal ArticleDOI
Abstract: We calculate the quiescent X-ray, neutrino, and Alfven wave emission from a neutron star with a very strong magnetic field, Bdipole ~ 1014 − 1015 G and Binterior ~ (5–10) × 1015 G. These results are compared with observations of quiescent emission from the soft gamma repeaters and from a small class of anomalous X-ray pulsars that we have previously identified with such objects. The magnetic field, rather than rotation, provides the main source of free energy, and the decaying field is capable of powering the quiescent X-ray emission and particle emission observed from these sources. New features that are not present in the decay of the weaker fields associated with ordinary radio pulsars include fracturing of the neutron star crust, strong heating of its core, and effective suppression of thermal conduction perpendicular to the magnetic field. As the magnetic field is forced through the crust by diffusive motions in the core, multiple small-scale fractures are excited, as well as a few large fractures that can power soft gamma repeater bursts. The decay rate of the core field is a very strong function of temperature and therefore of the magnetic flux density. The strongest prediction of the model is that these sources will show no optical emissions associated with X-ray heating of an accretion disk.

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Journal ArticleDOI
21 May 1998-Nature
Abstract: Soft gamma-ray repeaters (SGRs) emit multiple, brief (approximately O.1 s) intense outbursts of low-energy gamma-rays. They are extremely rare; three are known in our galaxy and one in the Large Magellanic Cloud. Two SGRs are associated with young supernova remnants (SNRs), and therefore most probably with neutron stars, but it remains a puzzle why SGRs are so different from 'normal' radio pulsars. Here we report the discovery of pulsations in the persistent X-ray flux of SGR1806-20, with a period of 7.47 s and a spindown rate of 2.6 x 10(exp -3) s/yr. We argue that the spindown is due to magnetic dipole emission and find that the pulsar age and (dipolar) magnetic field strength are approximately 1500 years and 8 x 10(exp 14) gauss, respectively. Our observations demonstrate the existence of 'magnetars', neutron stars with magnetic fields about 100 times stronger than those of radio pulsars, and support earlier suggestions that SGR bursts are caused by neutron-star 'crust-quakes' produced by magnetic stresses. The 'magnetar' birth rate is about one per millenium, a substantial fraction of that of radio pulsars. Thus our results may explain why some SNRs have no radio pulsars.

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