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The Astrophysical Journal Letters, 784:L19 (5pp), 2014 April 1 

© 2014. The American Astronomical Society. All rights reserved. Printed in the U.S.A. 


doi:10.1088/2041-8205/784/2/L19 


X-RAY SPECTRAL COMPONENTS OBSERVED IN THE AFTERGLOW OF GRB 130925A 

Eric C. Bellm 1 , Nicolas M. Barriere 2 , Varun Bhalerao 3 , Steven E. Boggs 2 , S. Bradley Cenko 4 , Finn E. Christensen 5 , 
William W. Craig 2 ’ 6 , Karl Forster 1 , Chris L. Fryer 7 , Charles J. Hailey 8 , Fiona A. Harrison 1 , Assaf Horesh 9 , 
Chryssa Kouveliotou 10 , Kristin K. Madsen 1 , Jon M. Miller 11 , Eran O. Ofek 9 , Daniel A. Perley 1 , Vikram R. Rana 1 , 
Stephen P. Reynolds 12 , Daniel Stern 13 , John A. Tomsick 2 , and William W. Zhang 4 

1 Cahill Center for Astronomy and Astrophysics, California Institute of Technology, Pasadena, CA 91125, USA; ebellm@caltech.edu 
2 Space Sciences Laboratory, University of California, Berkeley, CA 94720, USA 
3 Inter-University Center for Astronomy and Astrophysics, Post Bag 4, Ganeshkhind, Pune 41 1007, India 
4 NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA 
5 DTU Space — National Space Institute, Technical University of Denmark, Elektrovej 327, DK-2800 Lyngby, Denmark 

6 Lawrence Livermore National Laboratory, Livermore, CA 94550, USA 

7 CCS-2, Los Alamos National Laboratory, Los Alamos, NM 87545, USA 

8 Columbia Astrophysics Laboratory, Columbia University, New York, NY 10027, USA 
9 Benoziyo Center for Astrophysics, Weizmann Institute of Science, 76100 Rehovot, Israel 
10 Astrophysics Office/ZP12, NASA Marshall Space Flight Center, Huntsville, AL 35812, USA 
11 Department of Astronomy, The University of Michigan, 500 Church Street, Ann Arbor, MI 48109, USA 
12 Physics Department, NC State University, Raleigh, NC 27695, USA 
13 Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91109, USA 
Received 2014 January 17; accepted 2014 February 26; published 2014 March 11 

ABSTRACT 

We have identified spectral features in the late-time X-ray afterglow of the unusually long, slow-decaying 
GRB 130925 A using NuSTAR , Swift /X- Ray Telescope, and Chandra. A spectral component in addition to an 
absorbed power law is required at >4 o significance, and its spectral shape varies between two observation epochs 
at 2 x 10 5 and 10 6 s after the burst. Several models can fit this additional component, each with very different 
physical implications. A broad, resolved Gaussian absorption feature of several keV width improves the fit, but it 
is poorly constrained in the second epoch. An additive blackbody or second power-law component provide better 
fits. Both are challenging to interpret: the blackbody radius is near the scale of a compact remnant (10 8 cm), while 
the second power-law component requires an unobserved high-energy cutoff in order to be consistent with the 
non-detection by Fermi /Large Area Telescope. 

Key word: gamma-ray burst: individual (GRB 130925 A) 

Online-only material: color figures 


1. INTRODUCTION 

Recent work has identified several “ultra-long” gamma-ray 
bursts (GRBs) with properties distinct from normal long GRBs 

(Levan et al. 2014, and references therein). These events have 

initial bursting phases lasting thousands of seconds in gamma- 
rays and show long-lived, highly variable X-ray afterglows. It 

is currently unclear whether these bursts are simply extreme 
examples of the long GRB class, as suggested by Zhang et al. 

(20 1 3); if they are related to the even longer candidate relativistic 
tidal disruption events (TDEs) Swift J1 644+57 (Bloom et al. 

2011; Levan et al. 2011; Burrows et al. 2011) and Swift 
J2058+05 (Cenko et al. 2012); or if they represent a new subclass 

of transient, perhaps with large-radius progenitors (Woosley & 
Heger 2012; Gendre et al. 2013; Nakauchi et al. 2013). 

The bright, nearby GRB 130925 A is similar to previously 
reported ultra-long GRBs and, with the launch of NuSTAR , 
provides an opportunity to observe the X-ray spectrum at high 

sensitivity over a broad energy band. Here we report time- 
varying spectral features in the late-time X-ray afterglow of 

GRB 130925 A that were initially discovered by NuSTAR and 
confirmed in a second epoch by NuSTAR and Chandra. Our 

detections are at higher energies and significantly later times 
than previously reported afterglow features. 

Before the era of routine afterglow observations with Swift/ 
X-Ray Telescope (XRT), several authors claimed detection of 
lines in GRB X-ray afterglows on top of otherwise smooth 


power-law (PL) spectra (e.g., Piro et al. 2000; Amati et al. 
2000; Reeves et al. 2002). Most reports were of emission lines 
at relatively low signal-to-noise ratio (S/N), and there was 
substantial controversy over the methods used to assess line 
significance (Protassov et al. 2002; Sako et al. 2005). Since 
the advent of Swift , no firm afterglow line detections have been 
reported despite its greater sensitivity and systematic follow-up, 
calling previous reports into question (for a review, see Hurkett 
et al. 2008). 

However, statistically significant blackbody components have 
been reported in the early-time ( t < 10 3 s) afterglow spectra 
of several bursts observed by Swift-XKT (Starling et al. 2012 
and references therein). The inferred rest- frame temperatures 
are typically a few tenths of a keV, the inferred radii are 
~10 12 cm, and the blackbody component provides 10%-50% of 
the 0.3-10 keV flux. The first detections were in low-luminosity, 
supernova (SN)-associated GRBs, leading to suggestions that 
the emission was due to shock breakout from the SN (e.g., 
Campana et al. 2006). Systematic searches have found thermal 
components in early afterglows of classical GRBs as well 
(Sparre & Starling 2012; Friis & Watson 2013), giving credence 
to alternative interpretations including late-time emission from 
a prompt photosphere (Friis & Watson 2013) or emission from 
a cocoon around the jet (Suzuki & Shigeyama 2013; Nakauchi 
et al. 2013). 

Of particular relevance are reports of additional components 
in the afterglows of other ultra-long GRBs. The “Christmas 


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The Astrophysical Journal Letters, 784:L19 (5pp), 2014 April 1 



t (days) 

Figure 1 . Swift-X RT lightcurve for GRB 130925 A (black) plotted over the XRT 
lightcurves of other afterglows. The NuSTAR (N) and Chandra (C) observation 
times are marked. 

(A color version of this figure is available in the online journal.) 

Day Burst” GRB 10 1225 A showed evidence of two separate 
blackbody components, a 1 keV X-ray blackbody with radius 
2 x 10 11 cm observed 6 ks after the burst, and a UVOIR 
blackbody with radius 2-7 x 10 14 cm which cooled over 18 days 
(Thone et al. 2011). In GRB 11 1209 A, Stratta et al. (2013) 
reported the XMM detection of a second, hard PL component 
(r ~ 0) during the steep decay phase ~70 ks after the burst. 

2. OBSERVATIONS 

GRB 130925 A produced several emission episodes triggering 
Swift/ Burst Alert Telescope (BAT), Fermz’/ Gamma-ray Burst 
Monitor, and MAXI. Swift-BXI triggered on GRB 130925 A at 
T 0 = 2013-09-25 04:11:24 UT (Lien et al. 2013). Fermi-G BM 
triggered on a precursor episode about 15 minutes before the 
Swift trigger (Fitzpatrick & The Fermi GBM Team 2013), and 
MAXI triggered on an emission episode nearly 4 ks after the 
initial Swift trigger (Suzuki et al. 2013). The final BAT detection 
of the emission occurred during a flare observed by XRT, at 
7 q + 7.1 ks (Markwardt et al. 2013). Despite an automated 
repointing, Fermi/ Large Area Telescope (LAT) did not detect 
any emission (Kocevski et al. 2013). Both the INTEGRAL- SPI 
Anti-Coincidence Shield and Konus-WIND detected gamma- 
rays from the burst over a total interval of nearly 5 ks (Savchenko 
et al. 2013; Golenetskii et al. 2013). 

Swift-X RT observed large, repeated flares from the burst 
(Evans et al. 2013; Figure 1). The extraordinary length of the 
bursting phase led Burrows et al. (2013) to suggest similarity to 
the proposed jetted TDE Swift J 1644+5 7, although Golenetskii 
et al. (2013) argued that some previous ultra-long events thought 
to be GRBs had been observed with similar total duration. 

Starting around 2 x 10 4 s after the Swift trigger, the X-ray 
afterglow entered a steady decay phase without new flares 
(Figure 1). The observed decline is similar to other GRB 
afterglows and differs markedly from the weeks of flaring 
observed for Swift J 1644+57. 

In contrast, the source was faint at optical-NIR wavelengths. 
Rapid followup observations found a NIR-bright ( K = 18, 
r' > 22 mag AB) source near the X-ray position (Sudilovsky 
et al. 2013a). Spectroscopy of the host galaxy provided a redshift 


Bellm et al. 

of z = 0.347 (Vreeswijk et al. 2013; Sudilovsky et al. 2013b). 
Late-time Hubble Space Telescope imaging showed that the 
event took place in the plane of a disrupted host galaxy but offset 
O'.' 12 (600 pc in projection) from the galaxy nucleus (Tanvir 
et al. 2013). This offset disfavors a TDE origin for this event, 
although the authors noted that a galaxy merger could produce 
a supermassive black hole offset from the light centroid. 

NuSTAR (Harrison et al. 2013) provides unprecedented 
X-ray sensitivity above 10 keV thanks to the combination of 
its multilayer-coated focusing optics and CdZnTe detectors. 
NuSTAR observed GRB 130925 A during the decay phase be- 
ginning 1.8 days after the Swift trigger (Figure 1). The total 
on- source observation time in the first epoch was 39.2 ks. Our 
initial analysis showed that an absorption feature was needed 
to fit the NuSTAR data (Bellm et al. 2013). We triggered two 
additional NuSTAR observations of 88.2 and 90.7 ks integration 
time; these occurred at 8.8 and 11.3 days after the Swift trig- 
ger. We also obtained a 44.3 ks Director’s Discretionary Time 
observation with Chandra ACIS-S beginning 11.0 days after the 
Swift trigger. 

3. DATA REDUCTION 

We processed the NuSTAR data with HEASOFT 6.14 and the 
NuSTAR Data Analysis Software v. 1.2.0 using CALDB version 
20130509. We extracted source counts from circular regions 
with 40"radius from both NuSTAR modules. We identified back- 
ground regions of 125"radius on the same NuSTAR detectors as 
the source. Since the second and third NuSTAR observations and 
the Chandra observation are nearly contiguous in time and the 
source is only slowly varying, we analyzed these data together 
and refer to them hereafter as the second epoch. We combined 
the NuSTAR data from the second and third observations and 
from both modules into a single spectrum to maximize the S /N. 

We also downloaded and reduced the 13.0 ks of Swift-XKY 
photon-counting-mode data contemporaneous with the first 
NuSTAR epoch (obsid 0057 1 830006) using standard procedures 
in HEASOFT 6.14. 

We processed the Chandra data using standard procedures 
with CIAO v4.5. The data were obtained using 1/4 window 
readout to reduce pileup; we verified that the effect of pileup on 
our spectra is negligible and ignore it in further analysis. 

We rebinned all of the data to >20 counts per bin and fit 
the data using ISIS vl. 6.2-19. We also required the NuSTAR 
bins to have S/N of >4.5, as above ~15 keV the background 
dominates. We minimized x 2 in our fits to the data and use the 
covariance matrix in our significance calculations in Section 4. 
We used fit energy bands of 3-30 keV (NuSTAR), 0.3-10 keV 
(Swift-XKT), and 0.2-10 keV (Chandra). All errors are 90% 
confidence limit (C.L.), and we have used a cosmology with 
h = 0.704, Q m = 0.273, & A = 0.727 (Komatsu et al. 2011). 

4. SPECTRAL MODELING 
4.1. Single Power Law 

GRB X-ray afterglow spectra are usually well-fit by absorbed 
PL models. We froze a Galactic Ah component of 1.7 x 
10 20 cm -2 (Kalberla et al. 2005; Evans et al. 2013) and allowed 
a varying Ah component at the reported redshift of z = 0.347. 

A PL fit to the first-epoch NuSTAR data shows a clear deficit in 
the residuals in the 5-6 keV region (Figure 2). A joint PL fit in- 
cluding the Swift-XKT data improves the parameter constraints, 
particularly for Ah, but the residual structure remains. The good- 
ness of fit is poor, with x 2 = 1.6 (Table 1). A PL fit to the 


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Bellm et al. 



Energy [keV] 

Figure 2. Spectral fits to the first-epoch NuSTAR and Swift data. The top panel 
shows the count spectra and PL model fit. The lower panels show the residuals 
for the PL, gabs x PL, BB + PL, and PL + PL fits. Data are colored blue ( NuSTAR 
module A), navy blue (module B), and green (Swift-X RT). 

(A color version of this figure is available in the online journal.) 

Chandra data and a joint NuSTAR-Chandra PL fit also show 
residual structure (Figure 3) and poor goodness of fit, with 
Xy = 2.2. Additional components (Sections 4. 2-4. 5) improve 
these fits. 

4.2. Absorption Feature 

Multiplying by a Gaussian absorber (gabs x PL) in the first 
epoch markedly improves the fit residuals relative to a PL 
fit (Figure 2). The centroid of the Gaussian absorber is at 
5.9+° 0 4 3 keV and o = 0.9+ q 3 keV, both in the observer frame. 
The Swift data show similar residual structure, and in a joint fit 
the Gaussian absorber gives a similar centroid (6.0 + q 5 3 keV) but 
greater width (1.8 +q 7 keV; Table 1). In the joint fit, x 2 improves 
to 1.1 from 1.6 for three additional parameters. 

In the second epoch, a Gaussian absorber again improves the 
fit relative to a PL (x 2 = 1-2 from 2.2), but the parameters 
are poorly constrained. The joint NuSTAR and Chandra fit 
provides only an upper limit (4.1 keV) on the line centroid. 
This value is inconsistent with that of the first epoch, and the 
required line width is substantially larger (cr = 5.2 + 2, q keV, 
Figure 4). The large shift in the line centroid is difficult to 
explain with absorption by a single species. If the large linewidth 
is interpreted as turbulent velocity broadening, this implies 
relativistic velocities >0. lc that increase from the first epoch to 
the second, an unlikely scenario. 

4.3. Bremsstrahlung 

We obtained good fits (x 2 ~ LI) with an absorbed 
bremsstrahlung plus PL model (Bremss + PL). The component 
is well-constrained in both epochs, with best-fit temperatures of 
1.3=b0.2 and 0.83^; 1 1 2 l keV in the comoving frame. The fit emis- 
sion measures are 1.1 + q 3 x 10 69 cm -3 and 2.3+ q 9 6 x 10 68 cm -3 . 
These extreme emission measures, if produced by a 
constant-density medium, would require densities of order 
10 10 (/?/10 16 cm) _3/2 cm -3 . However, a circumstellar medium 



Energy [keV] 


Figure 3. Spectral fits to the second-epoch NuSTAR and Chandra data. Panels 
are as in Figure 2. The Chandra data are red, and data from the combined 
NuSTAR modules are navy blue. 

(A color version of this figure is available in the online journal.) 

this dense would be optically thick to electron scattering, 
violating the assumptions of the optically thin bremsstrahlung 
model. The emitting region would be optically thin only if the 
radius of the region were > 10 2 ° cm, much larger than typical af- 
terglow radii. More complex density profiles would require even 
higher densities at some locations. Thus while the addition of an 
optically thin bremsstrahlung spectral component improves the 
fit to the data, we are unable to construct a self-consistent phys- 
ical interpretation for it. This problem persists even if instead 
we require a higher temperature for the bremsstrahlung com- 
ponent in order to fit the high-energy excess. The fit is worse 
(X 2 increases by 5.9 in both epochs) and provides only a lower 
limit on the temperature ( kT >25 keV in the comoving frame). 
The emission region must still be larger than 10 18 cm to be 
optically thin. 

Motivated by the presence of possible additional residual 
structure in the Chandra data in the 1-3 keV range, we 
attempted to fit mekal and apec plasma emission models 
to the second-epoch data. With standard abundances, these 
models fit metallicity values of zero, reproducing the unphysical 
Bremss + PL model. Even with highly variable abundances, 
single-temperature plasmas did not provide clear improvements 
in the fit. 

4.4. Blackbody 

We also fit a blackbody plus PL model (BB + PL). The x 2 
surface shows two minima for the blackbody temperature in 
both epochs, one near 5 keV and the second near 0.5 keV. In the 
first epoch the higher temperature is preferred (xj 2 w = 115.3 
versus Xhi g h = 103.1 for 90 degrees of freedom (dof)), while 
in the second epoch the goodness of fit is closer to equivalent 
(Xio W = 156.2 versus x^gh = 157.7 for 130 dof). We argue that 
the higher-temperature blackbody fit is more plausible due to 
its relative consistency with the component observed in the first 
epoch and with theoretical expectations (Section 5). 

The blackbody components provide 11% (29%) of the total 
0.3-30 keV flux in the first (second) epoch. The implied 


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Bellm et al. 


Table 1 

Best-fit Parameters of Spectral Models 


Parameter 


NuSTAR and Swift Epoch 1 



NuSTAR and Chandra Epoch 2 



PL 

gabs x PL 

BB + PL 

PL + PL 

PL 

gabs x PL 

BB + PL 

PL + PL 

Ah ( 10 22 cm -2 ) 

9 cc+0.24 
z.jj— 023 

971 +0.41 
' —0.30 

a 07+O.35 
— 0.34 

7 7 <+0.40 
■ 3 - 3J -0.37 

1.98=1=0.14 

7 07+0.31 
^ -G.439 

2 - 74 -0 2 20 

7 02 +0 - 29 
~ ,,u — 0.27 

r 

3.33=1=0.13 

7 70+0.17 

3 - 96 -0 2 23 

4 02 +0 - 33 
4 - UZ — 3.47 

3.06=1=0.11 

7 Q5+0.55 
:, - yj -0.87 

3 - 86 -o 20 9 

4 77+0.40 
—0.12 

r 2 

Eo (keV) 


c og+0.53 
~’-° y -o .33 


1 7Q+0.65 
1.Z5— 073 


<4.09 


1 65 +0 - 26 
A-OJ — 029 

a (keV) 


1 7S+ 1 - 97 
A ’ /J -0.70 




5-!5^ 



r(E = Eo) 

kT (comoving frame, keV) 


0 72+0.12 
7 —0.18 

5-58^‘w 



2-8-ifs 

4 02 +0 - 73 

^• u -0.56 


X 2 /v 

146.8/92 

98.1/89 

103.1/90 

105.9/90 

288.8/132 

158.9/129 

157.7/130 

161.0/130 

P x (X>X \v) 

2.5E-4 

0.23 

0.16 

0.12 

1.0E-13 

0.04 

0.05 

0.03 


Note. Errors are 90% C.L. 



Energy [keV] 


Figure 4. Unfolded, unabsorbed model spectra for the gabs x PL, BB + PL, and 
PL + PL fits in keV 2 cm -2 s -1 keV -1 . Fits to the first (second) epoch are plotted 
with solid (dashed) lines. For the BB + PL and PL + PL fits, the total model is 
plotted in black, the PL component in orange, and the blackbody or second PL 
component in pink. 

(A color version of this figure is available in the online journal.) 

radii for a spherical emission region are small and consistent 
with constant size: 1 . 1 1° 0 5 8 x 10 8 cm and 1.5+° 0 5 6 x 10 8 cm. 
(The radii for the disfavored low-temperature blackbodies are 
larger but also relatively compact. However, they imply a 
physically unlikely contraction of the emitting region from 
(3.2 ± 0.8) x 10 10 cm to (1.7 ± 0.4) x 10 10 cm.) 

While blackbody components have been reported in other 
GRB afterglow spectra, none have been observed at such late 
times, with such high temperatures, or with such small radii. At 
1-10 days after the burst, the blackbody radius inferred from 
GRB 10 1225 A was over 10 14 cm and could be explained by 
the jet interaction with the circumstellar medium (Thone et al. 
2011). The inferred radius of 10 8 cm for GRB 130925 A is much 
harder to explain with a jet interaction model. This size scale 
is instead on par with the radius of the fallback accretion disks 
expected in stellar collapse (Fryer 2009). 


If we assume we are observing this disk, the fit tempera- 
ture can place constraints on the progenitor by constraining the 
conditions in the disk. The luminosity of an accretion disk is 
roughly equal to the potential energy released in the accretion. 
If we consider material at radius r, the luminosity (L) is given 
by L — GM m mdr/r 2 where m is the accretion rate and dr 
denotes a small annulus of material at radius r (integrating over 
dr would produce the total luminosity). The blackbody emis- 
sion for such an annulus is L — o AT 4 = olnrdr, where 
a is the Stefan-Boltzmann constant and T is the blackbody 
temperature. If we know the temperature, we can then derive 
the accretion rate m — (27rr 3 oT 4 )/(GM B n). For our observed 
temperatures of 4-5.6 keV, the corresponding accretion rate is 
10 -9 -10 -1 ° Mq s -1 . Fallback 10 5 -10 6 s after an SN or GRB 
explosion has been calculated for a range of progenitors and 
explosion energies (MacFadyen et al. 2001; Wong et al. 2014). 
Fallback at late times follows a simple PL (Chevalier 1989) 
and depends on the progenitor and the explosion energy of 
the SN associated with the GRB. Most fallback calculations 
(MacFadyen et al. 2001; Wong et al. 2014) predict fallback 
rates of 10 _7 -10 _1 ° M© s -1 at 10 5 -10 6 s for SN explosions of 
1-3 x 10 51 erg. 

Our accretion rates imply a luminosity near 10 5 times the 
Eddington limit for a stellar mass black hole. Although such 
extreme super-Eddington emission rates have been invoked 
from fallback (Dexter & Kasen 2013), the exact nature of such 
transient accretion is not well known. Steady- state solutions 
of disk accretion find that maintaining emission rates even an 
order of magnitude above Eddington is difficult (Jaroszynski 
et al. 1980). Whether such steady-state limits apply in transient 
situations like our fallback disk remains to be seen (Abramowicz 
2005). Thus without a full model of these transient events, we 
are not able to establish a self-consistent explanation for the 
blackbody emission. 

4.5. Hard Power Law 

Finally, we considered a two PL model (PL + PL) like that 
reported for GRB 111209A (Stratta et al. 2013). This model is 
a slightly worse fit in both epochs than the BB + PL model for 
the same number of free parameters (Table 1). 

Stratta et al. (2013) interpret the very hard (r ~ 0) second 
PL component they report for GRB 111209A at 70 ks after 
the burst as the tail of the hard PL emission sometimes ob- 
served by Fermi-LAI (e.g., Zhang et al. 2011 and references 
therein). This component is detected in the late prompt and 


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The Astrophysical Journal Letters, 784:L19 (5pp), 2014 April 1 

early afterglow phases and decays according to a PL; its physi- 
cal origin remains uncertain. The non-detection by LAT of both 
GRBs complicates this interpretation. An extrapolation of our 
epoch 1 PL flux to the 0.1-10 GeV band gives a photon flux of 
3 x 10 -6 photons cm -2 s _1 , a value higher than the upper limit 
of 7 x 10 7 photons cm 2 s 1 reported by Kocevski et al. (2013) 
in the first 2 ks after the burst, when the afterglow — and thus 
presumably the hard component — was much brighter. The prob- 
lem is even more severe for the component reported by Stratta 
et al. (2013): its higher flux and much harder spectral index ex- 
trapolate to a 0.1-10 GeV photon flux of 1.5 photons cm -2 s _1 , 
an extremely high value sufficient to trigger the LAT. We ex- 
amined the late-time LAT data for both bursts and confirm no 
excess emission. Consistency with the nondetection by LAT in 
both cases thus requires a cutoff above the NuSTAR and XMM 
bandpasses but below the LAT bandpass at 30 MeV. This phe- 
nomenological model is plausible, but the connection of these 
components to the early-time hard PL components detected by 
LAT in other GRBs therefore remains speculative. 

4.6. Component Significance 

We verified the significance of the additional spectral compo- 
nents using Monte Carlo simulations according to the method 
of posterior predictive p- values (Protassov et al. 2002). We ini- 
tialized each fit by stepping the additional feature through a 
grid in energy and finding the largest relative improvement in 
X 2 (cf. Hurkett et al. 2008). This procedure accounts for the 
“look-elsewhere” effect of multiple trials, as we have no a priori 
expectation of the observed line energy or component temper- 
ature. In none of our 10 4 simulated realizations of a null PL 
model did fits with alternative models (gabs x PL, BB + PL, or 
PL + PL) produce improvements in x 2 as large as observed in 
the real data. This implies that the spectral features are signif- 
icant at >3.9 a in both epochs: the x 2 improvement for each 
model fit is extremely unlikely to be due to chance if the true 
underlying model were simply an absorbed PL. 

5. CONCLUSION 

Our late-time afterglow observations of GRB 130925 A 
require an additional spectral component at high significance. 
Several alternative models provide acceptable fits to the data. 
These spectral features are detected more than 1 Ms after the 
burst, much later than any components previously reported in 
X-ray afterglows, probing a largely unexplored phase of after- 
glow evolution. Several unique features of GRB 130925 A make 
it possible to detect these late-time features for the first time. 
The unusually bright afterglow enables high-quality spectral 
fits, and NuSTAR has excellent sensitivity at the relevant ener- 
gies and can constrain the continuum above 10 keV. Moreover, 
the primary PL is unusually soft, so the high-energy compo- 
nent is not swamped. It is not yet clear whether this emission 
is related to progenitor physics unique to this unusual, ultra- 
long burst; NuSTAR observations of the bright “canonical” long 
GRB 130427 A were consistent with emission by a single spec- 
tral component (Kouveliotou et al. 2013). Future observations 
of bright afterglows will be needed to determine the prevalence 
of these late-time spectral components and identify the relevant 
emission mechanism. 

This work was supported under NASA contract No. 
NNG08FD60C and uses data from the NuSTAR mission, a 
project led by the California Institute of Technology, managed 


Bellm et al. 

by the Jet Propulsion Laboratory, and funded by the National 
Aeronautics and Space Administration. We thank the NuSTAR 
Operations team for executing the target of opportunity ob- 
servations. This research has used the NuSTAR Data Anal- 
ysis Software (NuSTARDAS) jointly developed by the ASI 
Science Data Center (ASDC, Italy) and the California Institute 
of Technology (USA). These results are based in part on ob- 
servations made by the Chandra X-ray Observatory. We thank 
the Chandra director for granting discretionary time and the 
Chandra team for prompt execution of the observations. 
Facilities: NuSTAR , Swift , Chandra 

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