The X-ray-rich GRB030528 was detected by the HETE satellite and its
localization was rapidly disseminated. However, early optical observations
failed to detect a counterpart source. In a 2-epoch ToO observation with
Chandra, we discovered a fading X-ray source likely counterpart to GRB030528.
The source brightness was typical of X-ray afterglows observed at similar
epochs. Other observers detected an IR source at a location consistent with the
X-ray source. The X-ray spectrum is not consistent with a large absorbing
column.Comment: 4 pages, 1 figures, to appear in Santa Fe GRB Conference Proceedings,
  200

Butler, N.

Dullighan, A.

Ford, P.

Hurley, K.

Jernigan, J.

Lamb, D.

Ricker, G.

Vanderspek, R.

English

arXiv

N. Butler

Crossref

Chandra Observations of the Optically Dark GRB 030528

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Chandra Observations of the Optically Dark
GRB 030528
N. Butler∗, A. Dullighan∗, P. Ford∗, G. Ricker∗, R. Vanderspek∗, K.
Hurley†, J. Jernigan† and D. Lamb∗∗
∗Center for Space Research, Massachusetts Institute of Technology, MA
†Space Sciences Laboratory, Berkeley, CA
∗∗Department of Astronomy, University of Chicago, IL
Abstract. The X-ray-rich GRB 030528 was detected by the HETE satelliteand its localization was
rapidly disseminated. However, early optical observations failed to detect a counterpart source. In
a 2-epoch ToO observation with Chandra, we discovered a fading X-ray source likely counterpart
to GRB 030528. The source brightness was typical of X-ray afterglows observed at similar epochs.
Other observers detected an IR source at a location consistet with the X-ray source. The X-ray
spectrum is not consistent with a large absorbing column.
OBSERVATIONS
The X-ray-rich (i.e. for the fluenceS, log[SX(2− 30 kev)/Sγ(30−400 kev)] > −0.5)
GRB 030528 was detected by theHETE satellite [1] with a 2′ radius (90% confidence)
SXC localization. The initial SXC error region was later revised [17] after the discovery
of an unaccounted-for systematic effect, resulting in a shift in position center and
an expansion of the error region to 2.5′ radius. Early R-band observations reaching
R ≈ 18.7 roughly 140 minutes after the burst [2] and unfiltered observations reaching
20.5 magnitude roughly 14 hours after the burst [16] failed to etect a counterpart.
On 3 June, theChandra Observatory targeted the field of GRB 030528 as part of
a series of GTO target-of-opportunity observations focusing on optically-dark GRBs
discovered byHETE. The 25 ksec observation spanned the interval 12:22-20:08 UT,
5.97 - 6.29 days after the burst. The revised SXC error regionfrom Villasenor et al. [17]
was completely contained within the field-of-view of theChandra ACIS-S3 chip. From
9 June 8:14 UT to 9 June 14:19 UT, 11.8 to 12.1 days post-burst,Chandra again targeted
the field of GRB 030528 for a 20 ksec second epoch (E2) observation with ACIS-S3.
CHANDRA E1 SOURCES
As reported in Butler et al. [4], 4 candidate sources were detct d within the revised SXC
error region. Seven additional non-stellar point sources wre detected within the entire
ACIS-S3 field-of-view. Positions and other data for these sources are shown in Table
1. None of the sources were anomalously bright relative to objects in Chandra deep
field observations [see, e.g., 15]. We had performed deep observations with Magellan
prior to theChandra observation, but none of theChandra sources were in our field of
view. However, near-IR observations of a portion of the SXC error region containing
two of the E1Chandra sources revealed a fading Ks-band source [10]. Between 0.7 and
3.6 days after the burst a fade by 0.9 magnitudes was observedfor a source spatially
coincident with the brightestChandra source. After the E1Chandra observation, deep
observations in the radio (6.8 days after the burst) [9] and in I-band (8.7 days after the
burst, I>21.5) [13] failed to detect a counterpart source.
TABLE 1. Four point sources are detected in the 0.5-8.0 keV band in theChan-
dra E1 observation lying within the revised SXC error region. Eight additional,
non-stellar point sources are detected in the ACIS-S3 field-of-view. From the E1
net counts, we calculate E290%, the 90% confidence region for the expected net
counts in E2, following Kraft et al. [12]. The columns labeled “∆C” and “PC” are
explained in Section . Small values ofPC indicate sources likely to have faded
between E1 and E2. Source #17 was situated on a chip gap in E2.
Epoch 1 Epoch 2
# Chandra Name Net (Bg) E290% Net (Bg) ∆C P
(%)
C
1 CXOU J170400.3-223710 39.5 (1.5) 24.1,41.1 8.5 (2.5) 6.970.01
4 CXOU J170348.4-223826 30.1 (2.9) 16.4,30.2 20.3 (2.7) 0.01 37.7
9 CXOU J170400.1-223548 10.8 (2.2) 4.1,12.6 5.4 (3.6) 0.17 28.8
10 CXOU J170354.0-223654 9.2 (2.8) 3.4,12.6 8.3 (2.7) 0.00 44.7
...
2 CXOU J170358.7-224237 30.6 (3.4) 14.1,28.0 23.9 (2.1) 0.00 36.1
3 CXOU J170355.7-223503 23.1 (2.9) 11.8,24.6 15.1 (3.9) 0.03 2.3
5 CXOU J170342.8-223548 23.7 (4.3) 12.4,26.0 10.7 (6.3) 0.85 9.8
8 CXOU J170403.9-223543 12.7 (2.3) 5.7,16.0 4.4 (2.6) 0.94 8.9
14 CXOU J170341.4-223646 6.1 (4.9) 1.1,9.0 9.7 (5.3) 0.00 48.8
15 CXOU J170411.2-224032 11.0 (4.0) 4.3,14.4 5.5 (3.5) 0.2024.1
17 CXOU J170345.8-224133 10.5 (4.5) 1.1,4.0 ...
AFTERGLOW CONFIRMED IN E2
Table 1 shows the number of counts detected in E1, along with the 90% confidence
interval for E2 based on the E1 values. The E2 observations were reported in Butler et
al. [5]. We have used a circular extraction region for each source, with radius set to 2
times the 95% encircled energy radiusr. This varies over the chip and is approximated
via r = 2.05−0.55∗d+0.18∗d2 arcsec, withd measured in arcminutes from the center
of the ACIS-S3 chip. We use an annular background region ten times larger than the
signal region, centered on and surrounding the signal region. The exposure is calculated
separately for each source extraction region in each epoch.
GRB X-ray afterglows typically fade in brightness with timeast−1.3, with t measured
from the GRB [7]. Assuming no spectral evolution, this implies a count-rate fade factor
of approximately 2.5 between E1 and E2. We can test whether the data for each source
prefers a fade versus a constant count rate by fitting the datafor each source first with
a single-rate model (Model A), then fitting the data with a model allowing the E2 rate
to be lower than the E1 rate (Model B). We do the fits by maximizing the logarithm of
the Poisson likelihood (i.e. the Cash [6] statisticC). We then simulate 104 data sets for
each source using the count rate determined from Model A, andwe count the number
of these which yield a larger∆C than the observed value when fit with Model B. These
fractions are expressed as probabilities (PC) in Table 1.
Of the four sources in the revised SXC error region (#’s 1,4,9,10), source #1 has
faded far below the 90% confidence range established in E1. The significance of the
fade is approximately 3σ , and we estimate the temporal index (assuming a power-law
fade) to beα = 2.0±0.8. This is somewhat steeper than the typicalt−1.3 fade for X-ray
afterglows [7], though it is characteristic of afterglows which have undergone a so-called
“jet-break” [8]. None of the otherChandra sources were observed to fade at a high level
of significance, and source #1 (also a fading IR source as discussed above) is extremely
likely to be counterpart to GRB 030528.
COLUMN DENSITY CONSTRAINTS
We reduce the spectral data using the standard CIAO1 processing tools. We use “con-
tamarf”2 to correct for the quantum efficiency degradation due to contamination in the
ACIS chips, important for energies below∼ 1 keV. There are not enough source counts
for detailed spectral fitting with theChandra data. However, we can use the instrumental
response determined in the steps above in combination with the to al number of detected
source and background counts in the 0.5-8 keV band (Table 1) and the number of source
plus background counts detected in the 0.5-0.6 keV band (2 counts) to constrain the
model column density. Assuming a power-law spectrum, Figure 1 shows how the two
low-energy counts become increasingly improbable as the column density increases.
Except for the case of high redshift (z > 1), Figure 1 implies a column density∼< 10
22
cm−2. This implies an extinction in R-band ofA ∼< 3 mag. At high redshift (z ∼ 3), the
column is likely
∼
< 1023 cm−2.
CONCLUSIONS
An X-ray observation∼6 days after GRB 030528 detected the afterglow at a flux level
(1.4× 10−14 erg cm−2 s−1) typical for GRB X-ray afterglows [see, 7] at that epoch.
Here we have assumed a typical power-law spectrum with photon indexΓ = 1.9 and
the GalacticNH = 1.6×1021 cm−2. A second epoch observations decisively revealed
that source #1 had faded, establishing securely that this was the counterpart X-ray
afterglow to GRB 030528. The X-ray spectrum appears to implya fairly low column
density, which in turn implies a fairly low amount of reddening in the source frame.
Thus, although the detection of a near-IR counterpart with no detection of an optical
counterpart for this burst perhaps points toward dust extinctio , we find no supporting
1 http://cxc.harvard.edu/ciao/
2 http://space.mit.edu/CXC/analysis/ACIS_Contam/script.html
 0.0001
 0.001
 0.01
 0.1
 1
1021 1022 1023
P
 (
≥ 
2 
ct
s 
w
ith
 E
<
0.
6 
ke
V
)
NH (z=0)
Γ =1.4
Γ =1.9
Γ =2.4
 0.001
 0.01
 0.1
 1
1021 1022 1023
P
 (
≥ 
2 
ct
s 
w
ith
 E
<
0.
6 
ke
V
)
NH
Γ =1.9
z =1.0
z =2.0
z =3.0
FIGURE 1. The probability is very low that a power-law spectrum with the indices shown (left
plot) andNH ∼> 10
22 cm−2 could have yielded two source counts below 0.6 keV, as observed.
The possibility that the counts could have come from the background is accounted for, using
an annular background region approximately 100 times larger than the source region and sur-
rounding the source region. The expected number of background counts in the 3.3′′radius source
extraction region is 0.02 counts. Because the host redshifti unknown, any local contribution in
excess of the Galactic column (right plot) may be less well constrained. Forz = 1, it is likely
that the local column is
∼
< 1022 cm−2.
evidence in the X-ray data. The publication of additional photometric data in various
passbands for this burst, if available, would help to constrain any possible extinction by
dust in the GRB host galaxy. We will perhaps learn that the aftrglow to GRB 030528
was intrinsically faint rather than heavily extincted, as appears to be common in may
GRBs [see, e.g., 3, 14].
REFERENCES
1. Atteia, J-L., et al. 2003,GCN, 2256
2. Ayani, K., & Yamaoka, H. 2003,GCN, 2257
3. Berger, E., et al. 2002,ApJ, 581, 981
4. Butler, N. R., et al. 2003a,GCN, 2269
5. Butler, N. R., et al. 2003b,GCN, 2279
6. Cash, W. 1979,ApJ, 228, 939
7. Costa, E., et al. 1999,A&AS, 138, 425
8. Frail, D. A., et al. 2001,ApJ, 562, L55
9. Frail, D. A., & Berger, E., et al. 2003,GCN, 2270
10. Greiner, J., Rau, A., & Klose, S. 2003,GCN, 2271
11. Henden, A. 2003,GCN, 2267
12. Kraft, R., Burrows, D., & Nousek, J. 1999,ApJ, 374, 344
13. Mirabel, N., & Halpern, J. 2003,GCN, 2273
14. Ricker, G. R., et al. 2003,in these proceedings
15. Rosati, P., et al. 2002,ApJ, 566, 667
16. Valentini, G., et al. 2003,GCN, 2258
17. Villasenor, J., et al. 2003,GCN, 2261


Chandra Observations of the Optically Dark GRB030528

Chandra Observations of the Optically Dark GRB030528

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