A&A 427, 815-823 (2004)
DOI: 10.1051/0004-6361:20041545
A. Rau 1 - J. Greiner 1 - S. Klose 2 - M. Salvato1 - J. M. Castro Cerón3 - D. H. Hartmann4 - A. Fruchter 3 - A. Levan3 - N. R. Tanvir5 - J. Gorosabel3,6 - J. Hjorth7 - A. Zeh2 - A. Küpcü Yoldas1 - J. P. Beaulieu8 - J. Donatowicz9 - C. Vinter7 - A. J. Castro-Tirado6 - J. P. U. Fynbo7,10 - D. A. Kann2 - C. Kouveliotou 11 - N. Masetti12 - P. Møller13 - E. Palazzi 12 - E. Pian 12,14 - J. Rhoads4 - R. A. M. J. Wijers 15 - E. P. J. van den Heuvel 15
1 - Max-Planck-Institut für extraterrestrische Physik,
Giessenbachstrasse, 85748 Garching, Germany
2 - Thüringer
Landessternwarte Tautenburg, 07778 Tautenburg, Germany
3 - Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD
21218, USA
4 - Clemson University, Department of Physics and Astronomy, Clemson,
SC 29634-0978, USA
5 - Department of Physical Sciences, Univ. of Hertfordshire, College
Lane, Hatfield Herts, AL10 9AB, UK
6 - Instituto de Astrofísica de Andalucía (IAA-CSIC),
Apartado de Correos, 3.004, 18.080 Granada, Spain
7 - Niels Bohr Institute, Astronomical Observatory, University of
Copenhagen, Juliane Maries Vej 30, 2100 Copenhagen, Denmark
8 - Institut d'Astrophysique CNRS, 98bis boulevard Arago, 75014 Paris, France
9 - Technical University of Vienna, Dept. of Computing, Wiedner Hauptstrasse 10, Vienna, Austria
10 - Department of Physics and Astronomy, University of Aarhus, Ny Munkegade, 8000 Aarhus C, Denmark
11 - NSSTC, SD-50, 320 Sparkman Drive, Huntsville, AL 35805, USA
12 - IASF/CNR, Sezione di Bologna, Via Gobetti 101, 40129 Bologna, Italy
13 - European Southern Observatory, Karl Schwarzschild-Strasse 2,
85748 Garching, Germany
14 - INAF, Osservatorio Astronomico di Trieste, Via Tiepolo 11, 34131 Trieste, Italy
15 - Astronomical Institute "Anton Pannekoek'', 1098 SJ Amsterdam, The Netherlands
Received 28 June 2004 / Accepted 8 August 2004
Abstract
The rapid dissemination of an arcmin-sized HETE-2
localization of the long-duration X-ray flash GRB 030528 led to a
ground-based multi-observatory follow-up campaign. We report the
discovery of the near-IR afterglow, and also describe the detection of
the underlying host galaxy in the optical and near-IR bands. The
afterglow is classified as "optically dark'' as it was not detected
in the optical band. The K-band photometry presented here suggests
that the lack of optical detection was simply the result of
observational limitations (lack of rapid and deep observations plus
high foreground extinction). Simple power law fits to the afterglow in
the K-band suggest a typically decay with a slope of
.
The properties of the host are consistent with the idea
that GRB hosts are star forming blue galaxies. The redshift of
GRB 030528 can not be determined accurately, but the data favour
redshifts less than unity. In addition, we present an optical and
near-IR analysis of the X-ray source CXOU J170354.0-223654 from the
vicinity of GRB 030528.
Key words: gamma rays: bursts
Optical and near-IR afterglows play a crucial role in the
understanding of the phenomenon of long duration Gamma-ray bursts
(GRB). While prompt -ray emission has been known since 1973
(Klebesadel et al. 1973), a major breakthrough in GRB research came
with the discoveries of the first X-ray afterglow (Costa et al. 1997)
and optical transient (van Paradijs et al. 1997). Firm evidence for the
cosmological origin of GRBs was first obtained with the determination
of the redshift of z=0.835 for GRB 970508 from absorption lines in
the optical afterglow (Metzger et al. 1997). To date, afterglows for 75 well localized long duration GRBs have been detected and 36 redshifts
from emission lines in the underlying host galaxy and/or absorption
features in the optical afterglow were determined (see J. Greiner's
web page
). For
nearly all well localized bursts an X-ray afterglow was found whenever
X-ray observations were performed, but only 53 bursts were also
detected in the optical and/or near-IR band. One day after the GRB,
optical transients exhibit R-band magnitudes that are typically in
the range of
19-22 and
-band magnitudes of 16-19. Optical/near-IR afterglow light curves can be characterized by
a power law in time,
,
with
(van Paradijs et al. 2000). For the remaining 23 GRBs with X-ray and/or
radio afterglow no optical and/or near-IR transient could be
detected. For this group of bursts the term "dark burst'' was
introduced. GRBs detected in the near-IR but lacking an optical
afterglow constitute a sub-group, and can be labeled "optically dark
bursts''.
In many cases observational limitations can account for the non-detection in the optical or near-IR-band. A slow reaction time, a location in a crowded field, possibly high Galactic foreground extinction, or unfavorable observing conditions, like bright moon and twilight, can explain non-detections of the counterparts. HETE-2 revealed that rapid and accurate localizations of the prompt emission in nearly all cases lead to the detection of an optical transient (Lamb et al. 2004). However, this does not provide a valid explanation for all dark bursts (Klose et al. 2003). In some cases, e.g. GRB 970828 (Groot et al. 1998; Djorgovski et al. 2001) and GRB 990506 (Taylor et al. 2000) even rapid (less than half a day after the GRB) and deep (R >23) observations did not reveal an afterglow, despite a clearly fading source in the X-ray and/or radio band.
There are many reasons for the non-detection of the optical transients of bursts with known X-ray or radio afterglows (e.g., Fynbo et al. 2001a; Lazzati et al. 2002). In addition to the observational biases mentioned above, the existence of "dark bursts'' may reflect a broad distribution of physical parameters of the GRB itself or of its environment, as in the case of GRB 990506 (see Taylor et al. 2000). The rapidly decaying radio afterglow of this burst together with the non-detection in the optical could be due to an extremely low-density medium surrounding the GRB.
Since the spectroscopic confirmation of SN2003dh underlying the
afterglow of GRB 030329 (Hjorth et al. 2003; Stanek et al. 2003) it is
now widely believed that long-duration GRBs are associated with the
death of massive stars (e.g., Heger et al. 2003). Because of the
short lifetime of these progenitors of 106 years, they do
not propagate far from their birth place in star forming
regions. Consequently, the optical and near-IR emission could suffer
from significant attenuation in the dusty medium. The X-ray and radio
afterglow emission may still be observable. Whether a burst is
"dark'' or has a detectable optical/near-IR transient would
therefore depend on the conditions of the ISM in the vicinity of the GRB. However, it is conceivable that dust destruction by the prompt
emission and early afterglow phase alters the circumstances (Galama &
Wijers 2001; Galama et al. 2003).
Another possibility to explain "dark bursts'' is to place them at
high redshift (Lamb & Reichart 2000). The observed redshift
distribution of GRBs is very broad and currently ranges between
z=0.0085 to z=4.5 with a broad peak around (e.g. Jakobsson et al. 2004). GRBs at still higher redshifts are
expected based on the association with massive stars discussed
above. However, the sensitivity of stellar mass loss to metalicity
combined with the requirement that jets must successfully emerge from
the stellar envelope suggests that single, massive stars in the early
universe may not result in observable GRBs (e.g., Heger et al. 2003).
Alternative scenarios, some perhaps including binary stars, may very
well produce GRBs at redshifts above z=6, where the Lyman alpha
absorption edge will be shifted through the optical into the near-IR
band. The resulting Lyman alpha suppression could then easily account
for the lack of optical detections. On the other hand, observations
show that the high-z explanation for "darkness'' can not apply in
all cases. For example, GRB 970828 and GRB 000210
revealed underlying host galaxies at positions coincident with those
of the X-ray and radio afterglows (Djorgovski et al. 2001; Piro et al. 2002) for which spectra indicate redshifts of z=0.958 and z=0.8463, respectively.
Here we report on the discovery of the near-IR afterglow of the optically dark GRB 030528 and it's underlying host galaxy. After describing the prompt emission properties and afterglow searches by other teams (Sect. 2), we present our optical and near-IR observations (Sect. 3) and their reduction (Sect. 4). We show the properties of the near-IR afterglow and of the host galaxy (Sect. 5) and discuss the results in the context of dark bursts (Sect. 6).
On May 28, 2003 the HETE-2 French Gamma-ray Telescope (FREGATE)
and the Wide-field X-ray Monitor (WXM) triggered on a long-duration
Gamma-ray burst (HETE trigger #2724) at 13:03 UTC. The event was
moderately bright with a
fluence of
erg cm-2 and a peak flux on a one second
time scale of
erg cm-2 s-1 in the
30-400 keV band (Atteia et al. 2003a). The burst duration (given
as T90, which is the time over which a burst emits from 5% of
its total measured counts to 95%) was
T90=49.1 s (30-400 keV)
and the high energy spectrum peaked at 32 keV. The burst is
classified as an X-ray flash according to the fluence ratio
.
The properties of XRFs, X-ray
rich bursts and GRBs apparently form a continuum (Lamb et al. 2004). At
107 min after the onset of the burst a confidence circle with 2
radius centered at RA(J2000
,
Dec(J2000)=-22$^$38
59
derived from the HETE-2 Soft
X-ray Camera (SXC) was released to the community. Figure 1
shows a
-band finding chart centered on this initial error
circle.
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Figure 1:
15 min exposure ![]() |
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Despite several rapid response observations, no optical afterglow was
found in this crowded field in the Galactic Plane (
,
). Table 1 provides a list of upper
limits. Later, a correction of the initial HETE-2 error
circle had to be applied. The radius of the confidence circle increased
to 2
5 and the centroid was displaced by 1
3. This
modification was announced on May 31, 2.4 days after the burst
(Villasenor et al. 2003; Fig. 1).
Table 1: Time after the burst, filters and limiting magnitudes of published optical/near-IR observations for the afterglow of GRB 030528.
A 26.1 ks Chandra observation performed on June 3
(5.97-6.29 days after the burst) of the revised HETE-2
confidence circle revealed several X-ray sources in the 0.5-8 keV
band (Butler et al. 2003a). Two of these,
J170400.3-223710 and
CXOU J170354.0-223654 were located inside our field of
view. Following these detections, we inspected earlier multi-epoch
near-IR SofI images and found that one source, coincident with C 1,
exhibited significant fading (Greiner et al. 2003a) which made this
source the most likely afterglow candidate. A second Chandra
observation (20 ks exposure on June 9) showed that only one of the
X-ray sources, C 1, was in fact fading (Butler et al. 2003b). This
observation confirmed the identification of C 1 as the afterglow of GRB 030528. The X-ray observations are described by Butler et al. (2004). C 1 is inside the initial HETE-2 error circle. Therefore,
the optical non-detections mentioned above, are not a result of it's
later revision. Follow-up observations in the radio (Frail & Berger
2003) did not detect C 1, or any other source inside the revised error
circle.
Shortly (0.6684 days) after the initial 2
confidence circle was
released, ToO imaging with the 3.58 m ESO-New Technology Telescope
(NTT) equipped with the Son of ISAAC (SofI) infrared spectrograph and
imaging camera at La Silla/Chile were initiated
(Table 2). SofI is equipped with a
HgCdTe
Hawaii array with 18.5
m pixel size and a plate scale of 0
29 per pixel. It has a field of view of 5
5. During the
first two nights (
0.7 and
1.7 days after the burst)
imaging in J, H and
was performed. During the fourth night
(
3.6 days post-burst) only
-band imaging was carried out. The
seeing conditions during the observations of this very crowded field
were good for the first and third epoch (
0
8) but less
favorable for the second epoch (1
1-1
6). All of the above
imaging was centered on the initial HETE-2 error circle. Due to
the later increase and shift of the confidence circle and the 5
5 field of view of NTT-SofI, the observations do not cover the entire
revised error circle (see Fig. 1).
At 14.9 days after the burst, one K-band observation was
performed with the 3.8 m United Kingdom Infra-Red Telescope
Fast-Track Imager (UKIRT UFTI) on Mauna Kea under good seeing
conditions (0.6
). UFTI consists of a
HgCdTe Rockwell
array with 18.5
m pixels and a plate scale of 0
09 per pixel, giving a field of view of
.
Nearly-Mould I-band photometry was obtained with the Mosaic2 imager
at the 4 m Blanco Telescope at the Cerro Tololo Inter-American
Observatory (CTIO) 6.6 and 32.6 days after the burst. The Mosaic2
consists of eight
SITe CCDs with a pixel size of 15
m. The plate scale of 0
27 per pixel at the 4 m
Blanco-Telescope produces a field of view of
.
In addition, late time -band imaging was performed with the
Infrared Spectrometer And Array Camera (ISAAC) at the 8.2 m ESO Very
Large Telescope (VLT) Antu in Paranal/Chile 111, 121, 124, and 125 days after the burst. ISAAC is equipped with a
pixel Rockwell Hawaii HgCdTe array with a 18.5
m pixel size. The
plate scale of 0
147 per pixel provides a
field of view.
Further, late epoch V and R-band observations were obtained with
the Danish Faint Object Spectrograph and Camera (DFOSC) at the Danish
1.54 m Telescope at La Silla/Chile 381-386 days after the
burst. DFOSC consists of a
EEV/MAT CCD with a pixel
size of 15
m and a plate scale of 0
39 per pixel. As the
instument optics does not utilise the full chip area, only
pixels are illuminated, giving a field of view of
.
These and the above mentioned observations
are summarized in Table 2.
Table 2:
Observation log.
stands for mid-observation time after
the burst. Magnitudes and flux densities are corrected for Galactic
foreground extinction. (1): All ISAAC
observations combined.
Table 3:
Stars used for the flux calibration of the imaging data for
C 1 (Fig. 2). I-band magnitudes are from field
photometry provided by Henden (2003) and J, H and -band
magnitudes are taken from the 2MASS All-Sky Point Source
Catalog. (1): no I-band magnitudes available, (2): no
uncertainties provided by 2MASS.
The NTT-SofI and VLT-ISAAC near-infrared images were reduced using
ESO's Eclipse package (Devillard 1997). The reduction of the
Blanco-Mosaic2 data was performed with bbpipe, a script based on
the IRAF/MSCRED, the UKIRT-UFTI observation was reduced using
ORACdr and the DFOSC data were reduced with IRAF. Astrometry was performed using IRAF/IMCOORDS and the
coordinates of stars in the field provided by the 2MASS All-Sky Point
Source
Catalog. For
the photometry we used IRAF/DAOPHOT. To account for the
distortions in the SofI images caused by the position of C 1 at the
edge of the field of view (see Fig. 1), we used stars
contained in the 2MASS Catalog from the vicinity of the source for the
photometric calibrations of the
and
fields. The
stars are listed in Table 3. We only used the stars
for which magnitude uncertainties in the relevant bands were
provided. The Mosaic2 I-band images were calibrated using the USNOFS
field photometry of Henden (2003), in particular the three stars B, C
& G (Fig. 2). The photometric measurements are
partly hampered by the combination of instrumental distortions and the
high density of sources in the field. The crowdedness of the field
also affected the set of comparison stars. Only source F
(Fig. 2) is sufficiently isolated to provided high
quality calibration. The photometry for A, B, C, D, E and G is less
accurate in comparison to F, but not by much. In any case all
individual uncertainties are taken into account in the error
analysis. The DFOSC V and R-band calibration was performed using
observations of the standard star G153-41 (Landolt 1992).
![]() |
Figure 2:
Combined late time VLT/ISAAC ![]() ![]() ![]() ![]() |
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To establish the proper photometric zero-points for the different
instruments used in this study we cross-checked stellar colors (of
non-saturated stars in the field) against a set of theoretical colors
along the main sequence. We utilized synthetic stellar spectra from
the library of Pickles (1998) and convolved those with the filter
transmission curves and efficiencies of SofI, ISAAC, Mosaic2 and DFOSC. A good match to within a zero-point accuracy of 0.05 mag
is obtained in all bands.
The 2MASS catalog provides the standard J, H & -band
magnitudes. In addition to these bands we also present observations
obtained in
and K. The
-band filter has a width of 0.16
m and is narrower than J-band filter (0.29
m) and the K-band filter is broader than the
-band filter with a width
of 0.35
m (instead of 0.27
m). Furthermore, the
-band
is centered at 2.16
m while the K-band is centered at
2.20
m. However, since
and K have respectively higher
and lower transmission than the J and
,
the net effect is that
(
).
All magnitudes are corrected for Galactic foreground extinction
according to the prescription given by Schlegel et al. (1998). For the
coordinates of the afterglow of GRB 030528 we find
E(B-V)=0.60,
AK=0.22 mag, AH=0.35 mag, AJ=0.54 mag, AI=1.17 mag,
AR=1.61 mag and
mag.
We now describe our results on the near-IR afterglow and on the optical/near-IR observations of the underlying host galaxy of GRB 030528 and discuss host properties in terms of population synthesis models. We apply the same methodology to the source C 10, which is unrelated to the GRB, but defer the description to Appendix A.
C 1 is the brightest X-ray source in the initial Chandra field at
a flux level of
erg cm-2 s-1 at 0.5-8 keV (Butler et al. 2004). This value was calculated assuming a
power law spectrum with a slope of
and taking into
account Galactic foreground extinction due to a neutral hydrogen
column density of
cm-2. At the X-ray
position of C 1 a faint object in our SofI J, H &
-band
observations
0.7 days post-burst is apparent. The source is
near the detection limits in J and H, but significantly detected
in
.
The magnitudes, corrected for foreground extinction in the
Galaxy, are
,
and
(see also Table 2).
Seeing conditions during the second night ( days
post-burst) only allow us to derive brightness limits for C 1 in the
J and H band but we were able to detect the source at
.
The J and H-band data are insufficient to test
variability, and the two
-band measurements are formally
consistent with a constant source. However, on June 1 (3.6 days
post-burst) fading became apparent. At that time the source had
declined to
,
corresponding to a change by roughly
1 mag within three days. Figure 3 shows the
light curve of the source in
together with all near-IR
observations presented here and the near-IR upper limits published in
the GRB Coordinates Network (GCN)
.
![]() |
Figure 3:
Foreground extinction corrected magnitudes in I (marked
by a cross), J/![]() ![]() ![]() ![]() ![]() |
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In contrast to the K-band variability, no fading is observed in the
I-band observations taken 6.6 and 32.6 days after the burst. The
source is persistent at a brightness of
,
which we
interpret as the I-band magnitude of the host galaxy. Similarly,
comparing the late time (>100 days post-burst) ISAAC
-band
observations with the SofI J-band data from the first night
(
days post-burst), the source also remains constant within
the uncertainties of the measurements. Thus, the decay of the near-IR
afterglow is only detected in the
-band.
In order to compare the afterglow decay in the near-IR with that in
the X-ray band, we estimate the power law slope, ,
from the
few
-band data shown in Fig. 3. Obviously,
the uncertainties in the photometry and the poor sampling of the light
curve do not allow us to derive an accurate description of the
afterglow behavior. A major source of uncertainty is introduced by the
fact that we do not know when a jet break may have occurred. If the
break occurred before our first
-band observation at t=0.7 days
the subsequent decay slope may have been close to
.
If the
afterglow is best described by a single power law, the shallowest
slope could be around
.
In that case, the afterglow
contribution to the K-band flux at 14.9 days post-burst is not
negligible. Slopes much steeper than
can be imagined
upon arbitrarily placing the break time close to t=3 days. This is
likely to be the case considering typical break times of
days. Therefore, it appears reasonable that the
post-break near-IR slope falls in the range of
.
The
significant uncertainties in the slopes leave the possibility that the
near-IR and X-ray (
;
Butler et al. 2004) decays are
parallel.
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Figure 4:
116 min exposure K-band image taken with the 3.8 m
UKIRT equipped with the UFTI on June 12, 14.9 days after the prompt
emission. North is up and East to the left. The potential host
galaxy (RA(2000
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The K-band image from an UKIRT/UFTI observation 14.9 days after the
burst shows that the image of the source is extended relative to the
point spread function of the field in East-West direction
(
)
(Fig. 4). This
elongation is consistently seen in the late time ISAAC
-band
images. The center of the source is located at RA(2000
,
Dec(2000
s. The positional coincidence with the
afterglow suggests that this is the underlying host galaxy of GRB 030528. We cannot exclude a residual point-like contribution to the
total flux, including the afterglow and possibly an additional
supernova (Zeh et al. 2004). A free fit to the K-band afterglow
light curve gives a host magnitude of
.
Assuming that
after about 10 days all fluxes at shorter wavelengths (V - J) are
exclusively due to the host galaxy, we find a host magnitude of
,
an average I-band magnitude of
,
and
.
In the H-band we use the
early SofI measurement of
as an upper limit to the
host brightness.
![]() |
Figure 5:
Results of the photometric redshift fit of C 1 with
template spectra following Bender et al. (2001). Upper panel: the
probability density of the redshift of the object for late type
galaxies. The probability for the redshift drops above ![]() |
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Figures 5 and 6 show the
resulting redshift probability density function together with the
photometric measurements of the galaxy and two fitting galaxy template
spectra. Late type star forming galaxies lead to significantly better
matches to the observations (reduced )
in contrast to
ellipticals (reduced
). The lack of U-band
observations for GRB 030528 and the poor quality of the photometry
reduces the power of the method and results in a relatively
unconstrained redshift range. For late type galaxies redshifts beyond
of z=4 appeared to be ruled out, while for ellipticals redshifts
should not exceed z=0.2. For completeness we also considered
stellar spectra, and find that all available templates produce fits
worse than those for elliptical galaxies (reduced
;
not plotted here).
![]() |
Figure 6: Same as Fig. 5 for early type galaxies. |
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The discovery of the near-IR afterglow and optical/near-IR host galaxy of GRB 030528 presented here, again demonstrates the importance of rapid, deep multi-wavelength follow-up observations. In this particular case rapid and deep near-IR observations were obtained, but the afterglow would probably not have been identified without the Chandra observations in the X-ray band. Guided by the X-ray data, we discovered the afterglow in a crowded field at a position significantly affected by image distortions. However, it took additional X-ray observations to confirm the near-IR-candidate. The chain of events for GRB 030528 emphasizes that observational programs directed at the identification and study of GRB afterglows depend critically on at least three ingredients; rapid response, deep imaging, and multi wavelength coverage. Lacking any one of these, GRB 030528 would have most likely been labeled as a "dark burst''. The fact that it was caught in the near-IR still attaches the label "optically dark'' GRB, and one wonders if this is an instrumental effect or intrinsic.
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Figure 7:
Published K-band magnitudes of all GRB afterglows observed
by May 2004 (dots; references listed in Appendix B) in
comparison to the faint K-band afterglow of GRB 030528 (filled
squares). Data are not corrected for Galactic extinction. The
straight lines indicate a decay with a slope
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A further challenge to the task of finding afterglows is the competition in brightness between the afterglow and the host galaxy. Except for extremely early observations the afterglow flux may be comparable or significantly less than the integrated flux from a normal galaxy. This obviously poses a challenge as the signal-to-noise ratio is essential for any detection algorithm.
Considering the luminosity functions for the host galaxies and the
afterglows one potentially encounters a further challenge. A bright
afterglow against the backdrop of a faint host is probably easy to
identify, while a faint afterglow from a bright host could more easily
escape detection. Figure 7 demonstrates that the K-band
afterglow of GRB 030528 was indeed very faint when compared to all
K-band detections of afterglows reported in the literature so
far. Only the K-band afterglow of GRB 971214 was fainter at the time
of its discovery (Ramaprakash et al. 1998). On the other hand,
Fig. 7 also shows that most K-band afterglows detected
by mid 2004 occupy a region, which spans over only 3 magnitudes. Assuming that
measured at t=14.9 days is in
fact the magnitude of the host (an underlying supernova contribution
might introduce a small upwards correction) this galaxy would be among
the brightest GRB hosts in the
-band to date.
Based on the few photometric data points for the afterglow, some of
which only provide upper limits due to the contribution of the host,
we derive a rough lower limit on the spectral slope, .
Such
a spectral slope would not be atypical. Also, the temporal decay is in
the range of the standard value of
(van Paradijs et al. 2000).
The near-IR afterglow was significantly detected only in the
-band. Does the non-detection in the J and H-bands require
large intrinsic extinction? Both, the J-band and the H-band
addresses the above question to some extent. Assuming a typical
afterglow law for the spectral energy distribution of
with
(0.4) we estimate
foreground extinction corrected J and H-band magnitudes at
t=0.6 days post burst. The expected J-band magnitude of
(
20.6) is consistent with the observed value of
.
As the host seems to dominate the J-band emission
at that time, no constraint on the intrinsic extinction of the
afterglow can be set. However, the expected H-band magnitude of
(
19.6) is significantly brighter than the observed
value of
and thus indicative of some additional
intrinsic extinction. Nevertheless, the data only allow us to derive a
rough estimate for the observer frame extinction of
.
From
estimates of the effective neutral hydrogen column density in the
X-ray band (Butler et al. 2004) it is clear that intrinsic extinction
in the host galaxy may account for at most a few magnitudes in the
R-band. These two approaches yield comparable values, but the
uncertainties are large in either case.
The
mag color of the host seems to be consistent
with the colors in the sample of GRB host galaxies detected in the
K-band (Le Floc'h et al. 2003). Our spectral template fitting (see
Sect. 5) is consistent with the idea that GRB host galaxies are
actively star forming blue galaxies as emphasized by Le Floc'h et al. (2003). It is perhaps fair to assume that the host of GRB 030528 is
similar to the host sample discussed by these authors. Le Floc'h et al. find that GRB hosts appear to be sub-luminous at approximately 8% of L* in a Schechter distribution function. If, for simplicity, we
assume that the host of GRB 030528 has an absolute brightness of
exactly this value (corresponding
MK =- 22.25) the K-band
magnitude derived from the fit to the afterglow light curve of
implies a redshift of
(for currently
accepted cosmological parameters). Applying the pseudo redshift
indicator of Atteia (2003b) to the HETE-2 data gives z'=0.36,
which is close to the above value. The assumed absolute magnitude is
uncertain by about
2 mag and pseudo redshifts are also uncertain
to within a factor two to three. It is thus clear that the redshift of
the host of GRB 030528 is by no means established. However, it seems
reasonable to interpret the observations to imply redshifts of the
order of unity or less. At z=0.4 the angular extent of 1
5 of
the host (see Fig. 4) implies a linear dimension of
8.7 kpc (assuming standard cosmology). This would suggest the
host to be a blue compact star forming galaxy.
In summary, we have demonstrated that observations in the near-IR hold the promise to detect afterglows that escape in the optical band because of possible reddening. Despite the success of the discovery of the afterglow of GRB 030528 coverage was insufficient to establish a well sampled light curve and to derive an accurate redshift. However, our detection of the host galaxy provides indirect evidence for a low redshift. Despite a significant allocation of observing time at large aperture telescopes, the sampling of this afterglow fell short of optimal coverage and depth. The well recognized shortage of global resources is likely to present a major hurdle to afterglow programs in the Swift-era when the burst detection rate is expected to increase dramatically.
Acknowledgements
This work is primarily based on observations collected at the European Southern Observatory, Chile, under the GRACE proposal 71.D-0355 (PI: E.v.d.Heuvel) with additional data obtained at the Cerro Tololo Inter-American Observatory and the United Kingdom Infra-Red Telescope. We are highly indebted to the ESO staff, in particular C. Cid, C. Foellmi, P. Gandhi, S. Hubrig, R. Johnson, R. Mendez, J. Pritchard, L. Vanzi & J. Willis for the prompt execution of the observing requests and all additional effort related to that. We thank the anonymous referee for insightful and helpful comments and P. Reig for his attempt to observe the host with the 1.3 m telescope at Skinakas Observatory. This publication makes use of data products from the Two Micron All Sky Survey, which is a joint project of the University of Massachusetts and the Infrared Processing and Analysis Center/California Institute of Technology, funded by the National Aeronautics and Space Administration and the National Science Foundation. This research has made use of data obtained from the HETE science team via the website http://space.mit.edu/HETE/Bursts/Data. HETE is an international mission of the NASA Explorer program, run by the Massachusetts Institute of Technology.
We here summarize our results on the optical and near-IR observations
of
J170354.0-223654. At the X-ray position a source is
detected in the SofI J, H and
as well as in the Mosaic
I-band and DFOSC V and R-band images
(Fig. A.1, RA(2000)=17:03:54.0,
Dec(2000)=-22:36:53). C 10 is outside the field of view of the ISAAC
and UFTI observations. Table A.1 lists the brightness and
flux density values for all observations with C 10 in the field of
view. The source shows no significant variation in any band. Likewise,
the two Chandra observation also indicate that C 10 is constant
in the X-ray band. This makes an association with GRB 030528 very
unlikely.
The counterpart of C 10 appears point like in the optical/near-IR
images (Fig. A.1). This allows both, a stellar and a
Galactic nature of the object. In case of a star, the distance of the
source in the Galaxy and thus the degree of extinction is
unknown. Exploring the range from a nearby location (quasi
unextinguished) to the opposite side of the Milky Way (foreground
extinction assumed as given above) we derive ranges for the near-IR
colors of
J - K=0.2-0.6 mag and
I - K=0.6-1.6 mag. Due to the
unknown extinction, the optical colours span an even wider range. The colors
constrain the spectral type to G-K for a main sequence star or
supergiant (Johnson 1966). Figure A.2 shows an
example fit of the spectrum of a G0 star to our photometric data
(lower panel; dotted line). However, the ratio of X-ray to bolometric
flux can be used to test the stellar origin of C 10. From the observed
Chandra counts in the energy range of 0.5-8 keV (a total of 17.5 counts in 45 ks) we derive the X-ray flux as
erg cm-2 s-1(assuming a power law shape with
). The lower value gives
the flux corrected for the hydrogen column in the Galaxy
(
cm-2) while the upper limit results
for an object with
.
Using the colors and bolometric
corrections for G-K main sequence stars given by Johnson (1966)
together with the observed K-band magnitudes and filter width, we
estimate the bolometric flux to be
erg cm-2 s-1. The resulting ratio of log(
exceeds significantly the value
typically observed for G-K stars (-4.5 to -6.5; Pallavicini et al. 1981). Therefore, although the photometric data allow an association
of C 10 with a single star, the X-ray properties make this very
unlikely. Nevertheless, we can not exclude an X-ray binary system in
the Galactic Plane with the X-ray radiation coming from an accretion
disk around a black hole or from a neutron star surface and the
optical/near-IR emission being produced by the accretion disk, outflow
and secondary star.
![]() |
Figure A.1: V-band image of the field around C 10 taken with the D1m54 in June 2004. Coincident with the Chandra position (error circle as given in Butler et al. 2003a) we detect a point like source. North is up and East to the left. |
Table A.1: Brightness and flux densities of C 10 corrected for foreground extinction in the Galaxy (assuming an extragalactic origin).
![]() |
Figure A.2: Same as Fig. 5 for C 10. Upper panel: a global maximum of the probability density is evident at z<0.2. Lower panel: best fitting (late type) galaxy model (solid line) and best fitting (G0) stellar spectrum (dotted line). |
C 10 can also be associated with a galaxy behind the Milky
Way. Similarly to the analysis of C 1, we applied the photometric
redshift technique of Bender et al. (2001). Figure A.2 shows the redshift probability
density function and the best fitting galaxy spectrum. A fit of a late
type galaxy template to the data shows no sufficient match (reduced
). Early type galaxies are excluded by the fit with
a reduced
.
The redshift for the best fitting galaxy
template is z<0.2. Given the point-like appearance of the
source, z=0.1 would require a compact small galaxy. As for a star,
we use the X-ray properties to test for a possible galaxy nature of
C 10. From the best fitting galaxy template spectrum the expected
B-band flux can be estimated. A late type galaxy with the observed
multicolor magnitudes as shown in Fig. A.2, has
erg cm-2 s-1. Using the
extinction corrected X-ray flux given above, we derive
log (
/
FB)=-2.6 consistent with the observations of normal
galaxies (Fabbiano 1989).
The applied photometric redshift method does not allow us to estimate the redshift probability for AGNs. As shown above, templates of stars and normal galaxies have problems to fit the data. The appearance as a point source together with the X-ray properties instead make an association of C 10 with an AGN the most likely solution. Spectroscopic observations are necessary to confirm this result.
We used the following bursts for the compilation of K-band afterglow light curves in Fig.7: GRB 970508 (Chary et al. 1998), GRB 971214 (Gorosabel et al. 1998; Ramaprakash et al. 1998), GRB 980329 (Larkin et al. 1998; Reichart et al. 1999), GRB 980613 (Hjorth et al. 2002), GRB 990123 (Kulkarni et al. 1999; Holland et al. 2004), GRB 991208 (Bloom et al. 1999), GRB 991216 (Vreeswijk et al. 1999; Garnavich et al. 2000; Halpern et al. 2000), GRB 000131 (Andersen et al. 2000), GRB 000301C (Kobayashi et al. 2000; Jensen et al. 2001; Rhoads & Fruchter 2001), GRB 010222 (Masetti et al. 2001), GRB 001011 (Gorosabel et al. 2002), GRB 000926 (Di Paola et al. 2000; Fynbo et al. 2001b), GRB 011121 (Price et al. 2002; Greiner et al. 2003b), GRB 011211 (Jakobsson et al. 2003), GRB 020305 (Burud et al. 2002), GRB 020322 (Mannucci et al. 2002), GRB 020405 (Masetti et al. 2003), GRB 020813 (Covino et al. 2003), GRB 021004 (Di Paola et al. 2002), GRB 021211 (Fox et al. 2003b), GRB 030115 (Kato & Nagata 2003; Dullighan et al. 2004), GRB 030227 (Castro-Tirado et al. 2003), GRB 030323 (Vreeswijk et al. 2004), GRB 030429 (Nishiyama et al. 2003; Jakobsson et al. 2004), XRF 030723 (Fox et al. 2003a; Fynbo et al. 2004), GRB 031203 (Malesani et al. 2004; Prochaska et al. 2004).