The Astrophysical Journal, 748:26 (12pp), 2012 March 20 doi:10.1088/0004-637X/748/1/26
C
2012. The American Astronomical Society. All rights reserved. Printed in the U.S.A.
X-RAY OBSERVATIONS OF THE NEW UNUSUAL MAGNETAR SWIFT J1834.9−0846
Oleg Kargaltsev
1
, Chryssa Kouveliotou
2
, George G. Pavlov
3,4
,ErsinG
¨
o
ˇ
g
¨
u¸s
5
,LinLin
5
,
Stefanie Wachter
6
, Roger L. Griffith
6
, Yuki Kaneko
5
, and George Younes
2,7
1
Department of Astronomy, Bryant Space Science Center, University of Florida, Gainesville, FL 32611, USA; oyk100@astro.ufl.edu
2
Science & Technology Office, ZP12, NASA/Marshall Space Flight Center, Huntsville, AL 35812, USA
3
Department of Astronomy and Astrophysics, Pennsylvania State University, 525 Davey Lab, University Park, PA 16802, USA
4
St.-Petersburg State Polytechnical University, Polytekhnicheskaya ul. 29, 195251, St.-Petersburg, Russia
5
Faculty of Engineering and Natural Sciences, Sabancı University, Orhanlı-Tuzla,
˙
Istanbul 34956, Turkey
6
Infrared Processing and Analysis Center, California Institute of Technology, Pasadena, CA 91125, USA
7
Universities Space Research Association, 6767 Old Madison Pike, Suite 450, Huntsville, AL 35806, USA
Received 2011 November 1; accepted 2012 January 9; published 2012 March 1
ABSTRACT
We present X-ray observations of the new transient magnetar Swift J1834.9−0846, discovered with the Swift Burst
Alert Telescope on 2011 August 7. The data were obtained with Swift, Rossi X-ray Timing Explorer (RXTE), CXO,
and XMM-Newton both before and after the outburst. Timing analysis reveals single peak pulsations with a period of
2.4823 s and an unusually high pulsed fraction, 85% ± 10%. Using the RXTE and CXO data, we estimated the period
derivative,
˙
P = 8×10
−12
ss
−1
, and confirmed the high magnetic field of the source, B = 1.4×10
14
G. The decay of
the persistent X-ray flux, spanning 48 days, is consistent with a power law, F ∝ t
−0.5
.IntheCXO/Advanced CCD
Imaging Spectrometer image, we find that the highly absorbed point source is surrounded by extended emission,
which most likely is a dust scattering halo. Swift J1834.9−0846 is located near the center of the radio s upernova
remnant W41 and TeV source HESS J1834−087. An association with W41 would imply a source distance of about
4 kpc; however, any relation to the HESS source remains unclear, given the presence of several other candidate
counterparts for the latter source in the field. Our search for an IR counterpart of Swift J1834.9−0846 revealed no
source down to K
s
∼ 19.5 within the 0.
6 CXO error circle.
Key words: gamma-ray burst: individual (HESS J1834−087) – ISM: individual objects (W41) – stars: neutron –
X-rays: individuals (Swift J1834.9−0846) – X-rays: ISM
Online-only material: color figures
1. INTRODUCTION
The population of magnetars has been growing rapidly in the
last five years, reaching 24 objects as of 2011 August. Origi-
nally comprised of soft gamma repeaters (SGRs) and anomalous
X-ray pulsars (AXPs; Woods & Thompson 2006), the mag-
netar population now includes a few more neutron star (NS)
groups that have been acknowledged as magnetar candidates.
Most of these NSs are slow rotators emitting multiple, very
short (a few times 100 ms) hard X-ray/soft γ -ray bursts. Their
X-ray luminosities are likely powered by the decay of their
high magnetic fields (up to B ∼ 10
15
G), rather than rota-
tional energy losses due to their gradual spin-down (Paczy
´
nski
1992; Duncan & Thompson 1992; Thompson & Duncan 1995,
1996). The current synergy between NASA’s three observatories
(RXTE, Swift, and Fermi) has enabled a much higher rate
of discovery of these objects in the last three years. Dur-
ing 2011 July–August alone, two new candidate magne-
tars were discovered in X-rays, Swift J1822.3−1606 and
Swift J1834.9−0846, when they triggered the Swift/Burst
Alert Telescope (BAT) and the Fermi/Gamma-ray Burst Mon-
itor (GBM). Their timing properties were subsequently es-
tablished with Rossi X-ray Timing Explorer (RXTE) obser-
vations, clinching their magnetar nature. We report here
on the X-ray spectral and temporal properties of the latter
source.
Swift J1834.9−0846 was discovered on 2011 August 7, when
a soft, short burst from the source triggered the BAT at 19:57:46
UT (D’Elia et al. 2011; Halpern 2011); approximately 3.3 hr
later, at 23:16:24.91 UT, another SGR-like burst triggered GBM
from the general direction of the earlier BAT location (Guiriec
et al. 2011). Although the GBM location included a large area
with several magnetar sources, the near time coincidence and the
X-ray properties of these events pointed to a common origin of
a new source (Barthelmy et al. 2011). The source triggered the
BAT again on 2011 August 30 at 23:41:12 UT (Hoversten et al.
2011).
Optical observations of the field ∼16 minutes after the BAT
trigger with the Special Astrophysical Observatory/Big Tele-
scope Alt-azimuth 6 m telescope detected an object at magni-
tude R
c
= 23.44 ±0.34 (Moskvitin et al. 2011). Simultaneous
observations with the 1.5 m Observatorio de Sierra Nevada tele-
scope in the I band did not detect that object to a limit of I = 21.6
(Tello et al. 2011). Archival IR images of the region as part of the
UKIDSS Galactic Plane Survey (Lucas et al. 2008)intheJ, H,
and K bands on 2007 May 10 revealed two sources close to the
Swift/X-ray Telescope (XRT) location of Swift J1834.9−0846
(Levan & Tanvir 2011). None of these objects coincided with
the very precise X-ray position subsequently derived from our
Chandra Target of Opportunity (ToO) observation (G
¨
o
ˇ
g
¨
u¸setal.
2011b).
RXTE/Proportional Counter Array (PCA) observations of the
source on 2011 August 9–10 detected a coherent pulsation at
ν = 0.402853(2) Hz, which corresponded to a spin period
P = 2.482295 s (G
¨
o
ˇ
g
¨
u¸s & Kouveliotou 2011a); this result was
later confirmed with our Chandra ToO observation on 2011
August 22 (G
¨
o
ˇ
g
¨
u¸setal.2011b). Continuous RXTE monitoring
of the source over a time span of two weeks revealed a spin-
down rate ˙ν =−1.3(2) × 10
−12
Hz s
−1
(Kuiper & Hermsen
2011). The corresponding estimate of the surface magnetic
1
The Astrophysical Journal, 748:26 (12pp), 2012 March 20 Kargaltsev et al.
Tab le 1
X-ray Observations of Swift J1834.9−0846
Date ObsID Observatory/Detector Exposure Time Resolution
(Mode) (ks) (s)
2005 Sep 18 0302560301 XMM-Newton EPIC 18.6 0.072
2009 Jun 7 10126 CXO ACIS-S 46.5 3.2
2011 Aug 7 00458907000 Swift/XRT (PC) 1.54 2.5
2011 Aug 7
a
00458907001 Swift/XRT (WT) 0.096 1.8 × 10
−3
2011 Aug 8
a
00458907002 Swift/XRT (WT) 0.129 1.8 × 10
−3
2011 Aug 8 00458907003 Swift/XRT (WT) 1.65 1.8 × 10
−3
2011 Aug 8 00458907004 Swift/XRT (WT) 0.958 1.8 × 10
−3
2011 Aug 9 00458907006 Swift/XRT (WT) 2.67 1.8 × 10
−3
2011 Aug 9 96434-01-01-00 RXTE PCA 3.40 9 × 10
−7
2011 Aug 9 96434-01-02-00 RXTE PCA 9.66 9 × 10
−7
2011 Aug 12 00458907007 Swift/XRT (WT) 5.67 1.8 × 10
−3
2011 Aug 14 00458907008 Swift/XRT (WT) 5.39 1.8 × 10
−3
2011 Aug 14 96434-01-03-00 RXTE PCA 6.78 9 × 10
−7
2011 Aug 18 96434-01-03-01 RXTE PCA 6.75 9 × 10
−7
2011 Aug 18 00458907009 Swift/XRT (WT) 5.73 1.8 × 10
−3
2011 Aug 21 00458907010 Swift/XRT (WT) 2.49 1.8 × 10
−3
2011 Aug 22 14329 CXO ACIS-S 13.0 0.44104
2011 Aug 24 96434-01-04-00 RXTE PCA 6.60 9 × 10
−7
2011 Aug 24
a
00458907011 Swift/XRT (WT) 0.94 1.8 × 10
−3
2011 Aug 27 00458907012 Swift/XRT (WT) 1.95 1.8 × 10
−3
2011 Aug 29 96434-01-05-00 RXTE PCA 6.05 9 × 10
−7
2011 Aug 30 00458907013 Swift/XRT (WT) 2.16 1.8 × 10
−3
2011 Sep 2 96434-01-06-00 RXTE PCA 5.12 9 × 10
−7
2011 Sep 2
a
00458907014 Swift/XRT (PC) 2.06 2.5
2011 Sep 5
a
00458907015 Swift/XRT (PC) 1.72 2.5
2011 Sep 8 96434-01-06-01 RXTE PCA 5.52 9 × 10
−7
2011 Sep 10
a
00458907016 Swift/XRT (PC) 2.01 2.5
2011 Sep 15 00032097001 Swift/XRT (WT) 9.09 1.8 × 10
−3
2011 Sep 18 00032097002 Swift/XRT (WT) 10.45 1.8 × 10
−3
2011 Sep 21 00032097003 Swift/XRT (WT) 7.44 1.8 × 10
−3
2011 Sep 24 00032097004 Swift/XRT (WT) 8.10 1.8 × 10
−3
Notes. Log of all observations used in our analysis.
a
Excluded from the spectral analysis.
field, B = 1.4 × 10
14
G, confirmed the magnetar nature of
Swift J 1834.9−0846.
Swift J1834.9−0846 is located in a field rich in high-
energy sources, which include SNR W41 (Shaver & Goss
1970; Tian et al. 2007), the TeV source HESS J1834−087
(Aharonian et al. 2005), the GeV source 2FGL J1834.3−0848
(Abdo et al. 2011), and the PSR/pulsar wind nebula (PWN) can-
didate XMMU J183435.3−84443/CXOU J183434.9−084443
(Mukherjee et al. 2009; Misanovic et al. 2011). Attempts to
understand the nature and relations between these sources
had already prompted X-ray observations with CXO and
XMM-Newton before the discovery of Swift J1834.9−0846
(Mukherjee et al. 2009; Misanovic et al. 2011). We have trig-
gered additional observations of the region with both CXO and
XMM-Newton. Here we describe the analyses of the RXTE,
Swift, Fermi, and CXO data and compare them to the earlier
observations. The XMM-Newton results will be reported in a
separate paper. Section 2 describes the data sets presented here,
and Section 3 presents the CXO location and discusses possible
optical counterparts. We present the light curve of the persistent
emission in Section 4 and the results of our timing and spectral
analyses in Sections 5 and 6, respectively. Finally, we compare
the properties of Swift J1834.9−0846 with those of other mag-
netars and discuss the possible relation of Swift J1834.9−0846
to other sources in the field in Section 7.
2. X-RAY OBSERVATIONS AND DATA REDUCTION
The field of Swift J1834.9−0846 was observed in X-rays
on 29 occasions with several telescopes; the majority was in
2011, with two earlier observations in 2005 and 2009 (see
Table 1). We have analyzed here 20 Swift
/XRT observations,
8 RXTE/PCA observations, and 1 CXO/Advanced CCD
Imaging Spectrometer (ACIS) observation.
2.1. Swift/XRT Data
Of the 20 Swift/XRT observations listed in Table 1,4were
carried out in the Photon Counting (PC) mode and 16 in
the Window Timing (WT) mode which provides much better
temporal resolution (1.8 ms) at the expense of imaging. We
used the HEASOFT
8
analysis tools to reduce and analyze the
data. We extracted spectra from the Level 2 event data using
the standard grade selection of 0–12 and 0–2 for the PC and
WT mode data, respectively. For the PC mode data, we used an
r = 15
circle as the source region and an annulus with the same
center and inner and outer radii of 30
and 45
as the background
region. For the WT mode data, we extracted the source spectra
using a box centered on the CXO location with a length of 30
aligned to the one-dimensional image. The background spectra
were extracted with a similar size box centered far away from
8
Version 6.10, http://heasarc.gsfc.nasa.gov/docs/software/lheasoft/
2
The Astrophysical Journal, 748:26 (12pp), 2012 March 20 Kargaltsev et al.
Figure 1. Image of CXOU 183452.1−084556 and surrounding emission (0.7–10 keV) obtained with ACIS-S3 on 2011 August 22. The radii of the inner and outer
circles are 2
and 12
, respectively.
(A color version of this figure is available in the online journal.)
the source. We then generated the ancillary response files with
xrtmkarf for each spectrum and regrouped the source spectra
with a minimum of 15 counts bin
−1
. The spectral fitting was
done in XSPEC 12.6.0. Since the source was relatively bright at
the onset of the outburst episode, the first XRT observation in
PC mode (performed during two separate spacecraft orbits) was
split into two parts to uncover early spectral variations. Three
observations in WT mode and three observations in PC mode
were too short to allow determination of spectral parameters.
These observations were, therefore, excluded from our spectral
analysis.
2.2. RXTE/PCA Data
Swift J1834.9−0846 was observed with RXTE in eight
pointings with a total exposure time of about 50 ks spanning
over 30 days (see Table 1). The RXTE data were collected with
the PCA (Jahoda et al. 1996) operating with two out of the five
available proportional counter units in most of the observations.
All data were collected in the GoodXenon mode, where each
photon is time tagged with a minimum time resolution of about
1 μs. We used the PCA data primarily for timing analysis as it is
not an imaging instrument, and the source intensity is relatively
dim compared to the bright background X-ray emission (e.g.,
diffuse Galactic ridge emission and bright point sources in the
1
◦
field of view of RXTE). However, we extracted the pulse
peak spectrum using the longest RXTE pointing to investigate
the source spectral behavior in a joint PCA and CXO analysis
(see Section 6.3).
2.3. CXO Data
We observed Swift J1834.9−0846 on 2011 August 22 with
the CXO ACIS operated in the Timed Exposure mode. The
target was imaged near the aim point on the S3 chip using the
1/8 subarray (8
× 1
field of view). The data of an archival
CXO observation (see Misanovic et al. 2011 for a description)
were also analyzed, taking into account the different angular
resolution and sensitivity. In our analysis, we worked with the
pipeline-produced Level 2 event files (with standard filtering
applied) and utilized CIAO 4.3 with CALDB 4.4.5. The spectral
fitting was done in XSPEC 12.6.0.
3. SOURCE LOCATION AND OPTICAL
COUNTERPART SEARCH
We used the wavdetect CIAO tool to determine the point
sources in our CXO observation. In the vicinity of the
Swift/XRT location we find a point source, which we designate
CXOU J183452.1−084556, centered at R.A. = 18
h
34
m
52.
s
118,
decl. =−08
◦
45
56.
02. We also notice the presence of extended
emission, up to 15
from the point source, with isotropic sur-
face brightness distribution (see Section 6.2.2). The uncertainty
of this position is dominated by the CXO absolute position un-
certainty of 0.
6 (at 90% confidence level).
9
The CXO image of
the vicinity of Swift J1834.9−0846 is shown in Figure 1.
We compared the CXO image to the archival Two Micron
All Sky Survey (2MASS) images of the same region of sky.
We do not detect any near-infrared (NIR) sources within 2
distance from the position of CXOU J183452.1–084556. We
also observed the field of Swift J1834.9−0846 with the Wide
Field Infrared Camera (WIRC; Wilson et al. 2003)onthe5m
Palomar Hale telescope on 2011 August 23. WIRC has a field
of view of 8.
7 × 8.
7 and a pixel scale of 0.2487 arcsec pixel
−1
.
We obtained seven dithered K
s
band images, consisting of four
co-added 30 s exposures taken at each dither position. The
atmospheric conditions were very good, with seeing 1
and
clear skies. The individual frames were reduced in the standard
manner using IRAF, calibrated and mosaicked together. The
9
See http://cxc.harvard.edu/cal/ASPECT/celmon/.
3
The Astrophysical Journal, 748:26 (12pp), 2012 March 20 Kargaltsev et al.
Figure 2. Palomar/WIRC K
s
-band image showing the r = 0.
6 CXO error circle
for CXOU 183452.1−084556. The sources designated as S1 and S2 are the ones
reported by Levan & Tanvir (2011).
Figure 3. Persistent X-ray light curve (2–10 keV) of Swift J1834.9−0846/
CXOU J183452.1−084556 obtained from 48 days monitoring of the source
with Swift/XRT. The dashed line shows the best-fit power-law temporal decay
model (∝ t
−0.53
).
resulting image was astrometrically calibrated using 2MASS.
The astrometric solution carries a formal 1σ error of 0.
1for
the transfer of the 2MASS reference frame to the WIRC image
shown in Figure 2. No sources are detected within the CXO
error circle down to a limiting magnitude of K
s
∼ 19.5(atthe
5σ level). The sources designated as S1 and S2 on the figure are
the ones reported earlier by Levan & Tanvir (2011).
4. PERSISTENT X-RAY LIGHT CURVE OF
SWIFT J 1834.9−0846
Swift J1834.9−0846 was observed on 20 occasions with Swift
after the outburst onset (see Table 1). This coverage allows us
to construct a light curve of the source, which spans 48 days.
In Figure 3, we present the persistent X-ray flux history in
the 2–10 keV range as calculated using the power-law (PL)
spectral model described in Section 6.1. The X-ray light curve
of the source indicates a rapid decay in the very early episode
(1 day), and it is consistent with a steady flux decay over the
Figure 4. Top panel: plot of phase shifts for each RXTE observation of
Swift J1834.9−0846. The solid line is a quadratic trend that fits the time
evolution of the phase shifts. Bottom panel: residuals of the fit.
longer term. A PL fit to the temporal decay trend (i.e., F ∝ t
−α
)
yields a good fit with α = 0.53 ±0.03 and α = 0.53 ±0.07 for
the observed and unabsorbed fluxes, respectively. Notice that
because of the limited spatial resolution, the XRT data include
both the point source and the surrounding extended emission.
As a consequence, the decay trend of the point source cannot be
unambiguously determined from these data.
5. TIMING ANALYSIS
5.1. RXTE
Swift J1834.9−0846 was observed by RXTE on eight oc-
casions with a total exposure time of ∼50 ks, spanning a
time baseline of over 30 days (see Table 1). For our timing
analysis we used data collected in the 2–10 keV range. For each
observation, we first inspected the light curve with 0.03125 s
time resolution and filtered out the times of short spikes and
instrumental artifacts. We then converted the event arrival times
to that of the Solar System Barycenter in Barycentric Dynami-
cal Time using the JPL DE200 ephemeris and the Swift-derived
coordinates of the source.
Next, we employed a Fourier-based pulse profile folding tech-
nique to determine the spin ephemeris of Swift J1834.9−0846.
We first generated a template pulse profile by folding the longest
PCA observation (Observation ID: 96434-01-02-00) at the pulse
frequency determined with a Z
2
1
search (Buccheri et al. 1983).
Then, we generated pulse profiles for all PCA observations as
well as for the CXO pointing, and cross-correlated them with the
template profile to determine the phase shifts with respect to the
template. We obtain t he spin ephemeris of the source by fitting
the phase shifts with a first or a higher order polynomial. We find
that the phase drifts of Swift J1834.9−0846 are best described
with a second-order polynomial (χ
2
= 7.3 for 7 degrees of
freedom, dof) that yields a spin period P = 2.4823018(1) s
and a period derivative
˙
P = 7.96(12) × 10
−12
ss
−1
(epoch: 55783 MJD). In Figure 4, we present the drift of the
pulse phase with respect to the template and the quadratic trend
curve (upper panel), and the fit residuals in cycles (lower panel).
The measured values of P and
˙
P correspond to the follow-
ing spin-down parameters: age τ = P/2
˙
P = 4.9 kyr, power
4
The Astrophysical Journal, 748:26 (12pp), 2012 March 20 Kargaltsev et al.
Figure 5. Dependence of the RXTE pulse profiles on time and energy. The
shaded area in the top left panel corresponds to the phase interval used for
spectral analysis (see Section 6.3).
˙
E = 4π
2
I
˙
PP
−3
= 2.1 × 10
34
erg s
−1
, and magnetic field
B = 3.2 × 10
19
(P
˙
P )
1/2
= 1.4 × 10
14
G.
Finally, Figure 5 shows the pulse profiles obtained from
several RXTE observations folded together using the derived
ephemeris. We note the appearance of additional harmonics in
the low-energy pulse profile of the source (2–5 keV) in RXTE
data taken at later times.
5.2. CXO
We searched for pulsations in the CXO/ACIS data ob-
tained in the 2011 August 22 observation. We used the
733 counts extracted from the r = 1
circle around the
CXOU J183452.1−084556 position in the 2–10 keV band (there
are only four counts below 2 keV, likely from the background).
The time resolution in this observation was 0.44104 s (0.4 s
frame time plus 0.04104 s charge transfer time). The photon
arrival times were transformed to the Solar System Barycenter
using the CIAO axbary tool. The ACIS observation started at
epoch 55795.6489 MJD and continued for T
span
= 13.02 ks.
We calculated the Z
2
1
statistic as a function of trial frequency
with a step of 0.35 μHz (which is about 0.05 T
−1
span
) and found
the maximum Z
2
1
= 467 at ν = 0.4028512 Hz ± 2.0 μHz,
10
implying a very high significance of the pulsed signal. We also
calculated Z
2
n
for n>1 but did not find a strong contribution of
higher harmonics.
Figure 6 (upper panel) shows the pulse profiles with 5 and
10 phase bins. We used these profiles to measure the pulsed
fraction,
11
p = 85% ± 10%. We estimated the uncertainty
of the pulsed fraction using Monte Carlo simulations and
bootstrapping, also accounting for the time resolution and
10
The 1σ uncertainty is calculated as δν = 3
1/2
π
−1
T
−1
span
(Z
2
1,max
)
−1/2
(see
Chang et al. 2012).
11
The pulsed fraction p is defined as the ratio of the number of counts above
the minimum level to the total number of counts.
Figure 6. Top panel: CXO pulse profiles (2–10 keV) with 5 and 10 phase
bins. The shaded regions indicate the peak interval (phases 0.15–0.35) used
for phase-resolved spectroscopy. Bottom panel: CXO pulse profiles (2–10 keV)
with 10 phase bins, averaged over the reference phase.
dead time in the 1/8 subarray mode. We also performed
randomization of the arrival times within the 0.4 s frame time
and re-calculated the pulsed fraction, which remained within
the uncertainty range estimated above.
The pulsed fraction can also be defined as ˜p = [2(Z
2
n
−
2n)/N]
1/2
, where n is the number of harmonics that give a
significant contribution, and N is the number of counts.
12
In our
case, ˜p = 1.13 exceeds 100%, which might be due to dead-
time effects and the relatively large (≈0.18) ratio of the time
resolution to the period. To measure the pulsed fraction more
accurately, the target should be observed with a better time
resolution.
Figure 6 (lower panel) shows a 20-bin pulse profile averaged
over the reference phase.
13
We also produced a pulse profile for
the surrounding extended emission but did not find a statistically
significant pulsed signal.
12
The advantage of this definition is the independence of ˜p of phase binning.
For the case of purely sinusoidal pulsations, ˜p coincides with p (assuming a
very large number of bins and low noise), while it is a factor of
√
2 larger than
the rms measure of variability.
13
This pulse profile was obtained by averaging 100 pulse profiles (20 bins
each) constructed by assigning different phases to the first count and folding
with the SGR period.
5
