Comments on ``Are Swift Gamma-Ray Bursts consistent with the Ghirlanda
  relation?", by Campana et al.(astro--ph/0703676)
7 Comments on “Are Swift Gamma-Ray Bursts consistent with
the Ghirlanda relation?”, by Campana et al.(astro–ph/0703676)
G. Ghirlanda a, L. Navaa b, G. Ghisellini a, C. Firmania c
aOsservatorio Astronomico di Brera, via Bianchi 46, I–20387, Merate, Italy
bUniv. degli Studi dell’Insubria, via Valleggio 11, I–22100, Como, Italy
c Instituto de Astronomı́a, U.N.A.M., A.P. 70-264, 04510, México, D.F., México
ABSTRACT
In their recent paper, Campana et al. (2007) found that 5 bursts, among those detected
by Swift, are outliers with respect to the Epeak–Eγ (“Ghirlanda”) correlation. We instead
argue that they are not.
1. Introduction
Campana et al. (2007, C07 hereafter) investi-
gate the Epeak–Eγ (so called “Ghirlanda”) cor-
relation, including all GRBs detected by Swift
for which we know the redshift, the peak energy
Epeak and we have information on the presence of
the jet break, necessary to estimate the jet open-
ing angle, and therefore to calculate the collima-
tion corrected bolometric energy, Eγ . In a similar
study performed by us (Ghirlanda et al. 2007,
G07 hereafter), we concluded that there was no
new outlier with respect to the Epeak–Eγ correla-
tion (besides GRB 980425 and GRB 031203, but
see Ghisellini et al. 2006), while C07 claim that
there are 5 Swift bursts which do not obey the
correlation. The sample of GRBs studied by C07
and G07 is the same. In the following we give
arguments contrasting the claim of C07.
2. GRB 060526
This burst is the second most important out-
lier (in term of contribution to the χ2) presented
by C07. Both C07 and G07 used the same
source of data: Schaefer (2007) for the fluence
and Epeak, and Dai et al. (2006) for tjet. Using
the listed bolometric fluence one obtains Eγ,iso =
2.53 × 1052 erg. We recomputed the bolometric
fluence from the spectral parameters reported by
Schaefer (2007), obtaining Eγ,iso = 2.58 × 10
erg, which is the value we used. Instead C07
list an isotropic energy Eγ,iso = 1.07
+0.16
−0.14 × 10
erg. We remind that the isotropic energy is found
through
Eγ,iso = Sbol
4πd2L
(1 + z)
where Sbol is the bolometric fluence and the
(1+z) term accounts for the cosmological time di-
lation. Neglecting the (1+ z) term, and using the
bolometric fluence Sbol = 1.17×10
−7, as listed by
Schaefer (2007), one obtains Eγ,iso = 1.07× 10
erg, which is the value reported in C07. We there-
fore suggest that C07, for this burst, missed the
(1 + z) = 4.21 term when calculating Eγ,iso. The
Eγ value used by C07 is therefore larger than the
value found by G07 because of the larger Eγ,iso
(tjet is the same).
A separate problem concerns the values of
Epeak and bolometric fluence for this burst re-
ported by Schaefer (2007). In fact, this burst
showed two main peaks in BAT, separated by
∼200 seconds, with the second peak having
http://arxiv.org/abs/0704.0234v1
http://arxiv.org/abs/astro--ph/0703676
a slightly larger fluence than the first, with
a softer spectrum. The spectral behaviour
of this burst is thus complex, and the value
of Epeak = 25 ± 5 keV reported by Schaefer
(with a fluence corresponding to the first peak
only) may be controversial. For this reason
we have analyzed the available Swift data for
this burst. Our results and the consequences
for the Ghirlanda correlation can be found at:
www.brera.inaf/utenti/gabriele/060526/060526.html
3. GRB 050922C and GRB 060206
These two bursts lack optical data at times late
enough to encompass the jet break time predicted
by the Ghirlanda relation. The fact that there is
indeed an early break in the optical does not guar-
antee that this is a jet break, since we now know
that there is the possibility of multiple breaks in
the optical. In these cases only a lower limit on
the break time can be taken, corresponding to the
latest optical observations, as discussed in G07.
4. GRB 050401 and GRB 050416A
Several authors published a partial coverage of
the optical afterglow of these two bursts, but none
of them discussed the results which can be ob-
tained by collecting all the available data (at least
in one band). Therefore, the claim that in these
GRBs there is no apparent break refers to the par-
tial coverage presented in each paper. Because of
that, in G07 we constructed the light curves with
data from different sources.
In GRB 050401 the result of the fitting is some-
what dependent from the (yet unknown) assumed
magnitude of the host galaxy, which can con-
tribute to the late photometric points. Further-
more, there is a large uncertainty in the normal-
isation of the De Pasquale et al. (2006) points,
because they used a different reference star for
their differential photometry. What we plotted
in Fig. 1 of G07 assumes the maximum possible
displacement (–0.5 mag): assuming a lower one
would inevitably worsen a single power law decay
For GRB 050416A, it is true that Soderberg et
al. (2006) stated that a single power law decay
plus a 1998bw–like supernova light curve can fit
the data, but also in this paper there is no com-
plete collection of points coming from the avail-
able literature. Anyway, SN 2006aj associated
with GRB 060218 is by far the best studied at
early times, so using this as a template should
give a more reliable result. In this case the pres-
ence of a break in the optical light curve is clearly
required.
Given all the above, we think that in these two
GRBs there exists a margin of subjectivity for
judging the presence or not of a possible jet break
(this margin is however small for GRB 050416A).
But just because of this, it is not appropriate to
declare that they are outliers, and treat them as
such in the fits. At the very least, one should
consider them having a lower limit in Eγ corre-
sponding to the jet break time we have derived.
5. Additional comments
The pre–Swift data plotted in the figures of C07
are the values of Eγ calculated taking Eγ,iso from
Firmani et al. (2006) and the jet angles from
Nava et al. (2006), who reported slightly differ-
ent values of Eγ,iso. Since the derived jet angle
depends upon Eγ,iso, this procedure is not cor-
rect.
When calculating the χ2
value for the bursts in
the sample of Nava et al. (2006), C07 find agree-
ment in the case of an homogeneous circumburst
medium, and a larger χ2
in the case of a wind
profile. We instead confirm the original value re-
ported in Nava et al. (2006).
We note that the χ2
values given in Table 2
of C07 for the “Swift data achromatic breaks”
and “Swift data pure breaks” cases, do not cor-
respond to the values given in the text.
GRB 061121 is plotted as a lower limit in Eγ ,
and lies to the left of the Ghirlanda correlation. It
should not be included in the fit as instead done
in C07.
A symmetric error on a linear quantity trans-
forms into an asymmetric error in the logarithm.
We believe that C07 underestimated the error on
Eγ,iso due to the systematic choice of the smallest
error in the logarithmic quantity. In G07, instead,
we propagated the errors in the logarithmic space.
Finally, in Fig. 2 of C07 (wind case) there is an
additional pre–Swift burst which is not present
in Fig. 1.
6. Conclusions
We would like to stress that we are not willing
to defend the Epeak–Eγ correlation to death. As
any other scientific result, it must be the object
of severe scrutiny from the scientific community.
This is even more true since its potential cosmo-
logical use makes this correlation very important
[as well as the related, model independent and
assumption free, Liang & Zhang (2005) correla-
tion]. Furthermore, its existence can flag some
crucial property of GRB physics which are not yet
fully understood (but some attempts have already
been done, see Thompson 2006 and Thompson,
Meszaros & Rees 2007). Therefore to demon-
strate that this correlation is the result of some
selection effects (or not), or that its dispersion is
much larger than what it is now (or not), or that
there are outliers (or not) is a mandatory task,
that must be pursued carefully.
REFERENCES
1. Campana, S., Guidorzi, C., Tagliaferri,
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2. Dai et al., 2007, ApJ submitt.
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3. De Pasquale, M., Beardmore, A.P.,
Barthelmy, S.D., et al., 2006, MNRAS,
365, 1031
4. Ghirlanda, G., Nava, L., Ghisellini G.
& Firmani C., 2007, A&A, in press
(astro–ph/0702352) (G07)
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Bosnjak, Z., Tavecchio, F., & Firmani C.,
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7. Schaefer, B.E. 2007, astro-ph/0612285
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al., 2006, ApJ subm. (astro–ph/0607511)
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http://arxiv.org/abs/astro--ph/0703676
http://arxiv.org/abs/astro-ph/0609269
http://arxiv.org/abs/astro--ph/0702352
http://arxiv.org/abs/astro-ph/0612285
http://arxiv.org/abs/astro--ph/0607511
http://arxiv.org/abs/astro--ph/0608282
	Introduction
	GRB 060526
	GRB 050922C and GRB 060206
	GRB 050401 and GRB 050416A
	Additional comments
	Conclusions
