What made GRBs 060505 and 060614?
WhatmadeGRBs 060505 and 060614?
Páll Jakobsson a, Johan P. U. Fynbo b
aCentre for Astrophysics Research, University of Hertfordshire, College Lane,
Hatfield, Herts, AL10 9AB, UK
bDark Cosmology Centre, Niels Bohr Institute, University of Copenhagen, Juliane
Maries Vej 30, 2100 Copenhagen, Denmark
Abstract
Recent observations of two nearby SN-less long-duration gamma-ray bursts (GRBs),
which share no obvious characteristics in their prompt emission, suggest a new phe-
nomenological type of massive stellar death. Here we briefly review the observational
properties of these bursts and their proposed hosts, and discuss whether a new GRB
classification scheme is needed.
Key words: Gamma rays: bursts, Supernovae: general
PACS: 95.85.Kr, 97.60.Bw, 98.70.Rz
1 Introduction
A broad-lined and luminous type
Ic core-collapse supernova (SN) is
predicted to accompany every long-
duration gamma-ray burst (GRB)
in the standard collapsar model
(Woosley, 1993). Although this as-
sociation had been confirmed in ob-
servations of several nearby GRBs
(e.g. Hjorth et al., 2003), a new
controversy commenced when no
SN emission accompanied GRBs
060505 (z = 0.09, duration ∼4 s)
and 060614 (z = 0.13, duration
∼100 s) down to limits fainter than
any known type Ic SN and hundreds
of times fainter than the archetypal
SN1998bw (Della Valle et al., 2006;
Fynbo et al., 2006; Gal-Yam et al.,
2006). The upper panels of Fig.1 il-
lustrate how easily such SNe would
have been detected in the case of
GRB060505.
An important clue to the origin and
progenitors of these bursts, is the
nature of the host galaxies. The
GRB060505 host is a spiral galaxy,
atypical for long-duration bursts but
not unheard of (GRB980425: Fynbo
et al., 2000; GRB990705: Le Floc’h
et al., 2002; GRB020819: Jakobs-
son et al., 2005). The burst occurred
inside a compact star-forming H II
region in one of the spiral arms, and
a spatially resolved spectroscopy
(lower panel of Fig.1) revealed that
the properties of the GRB site are
Preprint submitted to New Astronomy Reviews 4 December 2018
http://arxiv.org/abs/0704.1421v1
Fig. 1. (a) The field (20′′× 20′′) of GRB060505 as observed from the VLT in the
R-band on 22 May 2006. The arrow marks the position where the optical afterglow
was detected in earlier imaging. (b) As the image would have looked had a SN like
1998bw been present in the data. The strict upper limits strongly exclude the bright
SNe 1998bw and 2006aj that were associated with long GRBs. (c) Similar to (b),
but with a very faint Ic SN, such as 2002ap, added. (d) The 2-D optical spectrum
obtained with VLT/FORS2. The slit covered the centre of the host galaxy and the
location of GRB060505. As seen in the spectrum, this site is a bright star-forming
region in the host galaxy suggesting that the progenitor was a massive star.
similar to those found for other long-
duration GRBs with a high specific
star formation rate (SSFR) and low
metallicity (Thöne et al., 2007). The
GRB060614 host is significantly
fainter (one of the least luminous
GRB host ever detected) with a
moderate SSFR.
2 Discussion
2.1 High extinction?
Could the emission from an associ-
ated SN be completely obscured by
dust along the line-of-sight? The lev-
els of Galactic extinction are very low
in both directions. Host extinction of
more than a magnitude is also un-
likely in either case since the host
galaxy spectra display no reddening
as derived from the Balmer line ra-
tios. In addition, the GRB060614 af-
terglow is clearly detected in the UV
(Holland, 2006).
2.2 Wrong redshifts?
Another option is that the proposed
host galaxies are chance encounters
along the line-of-sight (Cobb et al.,
2006; Schaefer & Xiao, 2006), and
the real GRB redshifts are much
higher (rendering a SN too faint to
be observed). However, a few ob-
servational facts argue against this
scenario. In the case of GRB060614:
(i) the UV detection places an up-
per limit of around 1.1 on the red-
shift; (ii) no absorption components
in the optical afterglow spectrum
(Fugazza et al., 2006), as expected
for a low redshift, but not for a high-
z burst with a foreground galaxy;
(iii) very deep HST images of the
field should have revealed the “true
host” at z . 1.1, but none was
seen (Gal-Yam et al., 2006). For
GRB060505 it is extremely unlikely
that the afterglow accidentally su-
perposed right on top of a small star-
forming region within a foreground
spiral galaxy.
2.3 No SNe: a problem?
The host galaxies and the GRB loca-
tion within them strongly suggest an
association with star formation, and
hence a massive stellar origin. It is
important to realize that the lack of a
strong SN emission was actually pre-
dicted as a variant of the original col-
lapsar model, e.g. collapse of a mas-
sive star with an explosion energy so
small that most of the 56Ni falls back
into the black hole (e.g. Heger et al.,
2003; Fryer et al., 2006). In another
variant of the collapsar model, pro-
genitor stars with relatively low an-
gular momentum could also produce
SN-less GRBs (MacFadyen, 2003).
We should also remember that
the duration distributions of short
and long GRBs overlap. In fact,
the GRB060505 duration of 4 s
is near the ∼5 s duration which
Donaghy et al. (2006) find as the
point of roughly equal probability of
a given burst lying in either the short
or long class. It has been suggested
that the physical mechanism for this
burst is the same as for short bursts,
i.e. a merger of compact objects
(Ofek et al., 2007), although the pro-
genitor time delay of only . 7Myr is
on the borderline for allowed values
(Thöne et al., 2007). However, such
short time delays have been proposed
via newly recognized formation chan-
nels, which lead to the formation
of tighter double compact objects
with short lifetimes and therefore
possible prompt merger within hosts
(Belczynski, 2007). Whether such
channels require a low metallicity as
found for GRB060505 (Thöne et al.,
2007) remains to be explored.
2.4 Classification problem?
With the added complication that
the ∼100 s long GRB060614 is lo-
cated among the short bursts in the
lag-luminosity plot, it has been ar-
gued that a new GRB classification
scheme is required (Gehrels et al.,
2006). We do not think this is the
case, as the current GRB classifi-
cation is operationally well defined.
Rather that new observations are
warning us not necessarily to expect
a very simple mapping between the
duration of the GRB and the nature
of the progenitor: long bursts (>2 s)
synonymous with massive stars and
short bursts (<2 s) synonymous with
compact object mergers.
Others want to abandon the long-
short paradigm altogether due to
these “oddball” bursts, and invent
a new terminology: Type I and II
bursts similar to the SN classifica-
tion scheme (Zhang et al., 2007). In
this scheme, eight different proper-
ties have to be considered for each
burst/host. However, this scheme
can be ambiguous (e.g. GRB060505)
and is not operational, i.e. involves
observables that are not available for
most bursts (associated SN). Using
proposed hosts (i.e. a nearby bright
galaxy) to make a distinction be-
tween the two burst populations can
also be risky (e.g. GRB060912A:
Levan et al., 2007). In addition, one
might envisage a Type III category
consisting of the new type of bursts
(massive white dwarf/neutron star
merger) suggested by King et al.
(2007). These could produce long
bursts definitely without an accom-
panying SN and have a strong corre-
lation with star formation. However,
rare members of the class need not
be near star-forming regions, and
could have any type of host galaxy.
It is clear that the two SN-less long
bursts from last summer have raised
a few warning flags, i.e. how we think
about the long/short dichotomy. At
this point in time, we only recom-
mend that people keep an open mind.
Acknowledgements
We thank all the co-authors of the
Fynbo et al. (2006) paper. PJ ac-
knowledges support by a Marie
Curie Intra-European Fellowship
within the 6th European Community
Framework Program under contract
number MEIF-CT-2006-042001.
References
Belczynski, K., 2007. NewAR (this is-
sue).
Cobb, B. E., et al., 2006. ApJ 651,
Della Valle, M., et al., 2006. Nature
444, 1050.
Donaghy, T. Q., et al., 2006. ApJ,
submitted (astro-ph/0605570).
Fryer, C. L., et al., 2006. ApJ 650,
1028.
Fugazza, D., et al., 2006. GCN 5271.
Fynbo, J. P. U., et al., 2000. ApJ 542,
Fynbo, J. P. U., et al., 2006. Nature
444, 1047.
Gal-Yam,A., et al., 2006. Nature 444,
1053.
Gehrels, N., et al., 2006. Nature 444,
1044.
Heger, A., et al., 2003. ApJ 591, 288.
Hjorth, J., et al., 2003. Nature 423,
Holland, S. T., 2006. GCN 5255.
Jakobsson, P., et al., 2005. ApJ 629,
King, A., et al., 2007. MNRAS 374,
Levan, A. J., et al., 2007. MNRAS,
submitted.
MacFadyen, A. I., 2003. In AIP Conf.
Proc. 662, ed. G. R. Ricker & R. K.
Vanderspek, 202.
Ofek, E. O., et al., 2007. ApJ, sub-
mitted (astro-ph/0703192).
Schaefer, B. E., & Xiao,
L., 2006. ApJ, submitted
(astro-ph/0608441).
http://arxiv.org/abs/astro-ph/0605570
http://arxiv.org/abs/astro-ph/0703192
http://arxiv.org/abs/astro-ph/0608441
Thöne, C. C., et al., 2007. ApJ, sub-
mitted (astro-ph/0703407).
Woosley, S. E., 1993. ApJ 405, 273.
Zhang, B., et al. 2007. ApJ 655, L25.
http://arxiv.org/abs/astro-ph/0703407
	Introduction
	Discussion
	High extinction?
	Wrong redshifts?
	No SNe: a problem?
	Classification problem?
	Acknowledgements
