Gamma ray bursts (GRBs) are known to come in two duration classes, separated
at ~2 s. Long bursts originate from star forming regions in galaxies, have
accompanying supernovae (SNe) when near enough to observe and are likely caused
by massive-star collapsars. Recent observations show that short bursts
originate in regions within their host galaxies with lower star formation rates
consistent with binary neutron star (NS) or NS - black hole (BH) mergers.
Moreover, although their hosts are predominantly nearby galaxies, no SNe have
been so far associated with short GRBs. We report here on the bright, nearby
GRB 060614 that does not fit in either class. Its ~102 s duration groups it
with long GRBs, while its temporal lag and peak luminosity fall entirely within
the short GRB subclass. Moreover, very deep optical observations exclude an
accompanying supernova, similar to short GRBs. This combination of a long
duration event without accompanying SN poses a challenge to both a collapsar
and merging NS interpretation and opens the door on a new GRB classification
scheme that straddles both long and short bursts.Comment: 13 pages, 2 figures, accepted in Natur

Barthelmy, S. D.

Burrows, D. N.

Gehrels, N.

Granot, J.

Kaneko, Y.

Kouveliotou, C.

Mangano, V.

Markwardt, C. B.

Meszaros, P.

Nakar, E.

Norris, J. P.

Nousek, J. A.

O'Brien, P. T.

Page, M.

Palmer, D. M.

Parsons, A. M.

Roming, P. W. A.

Sakamoto, T.

Sarazin, C. L.

Schady, P.

Stamatikos, M.

Woosley, S. E.

English

arXiv

Gamma ray bursts (GFU3s) are known to come in two duration classes, separated at approx.2 s. Long bursts originate from star forming regions in galaxies, have accompanying supernovae (SNe) when near enough to observe and are likely caused by massive-star collapsars. Recent observations show that short bursts originate in regions within their host galaxies with lower star formation rates, consistent with binary neutron star (NS) or NS - black hole (BH) mergers. Moreover, although their hosts are predominantly nearby galaxies, no SNe have been so far associated with short GRBs. We report here on the bright, nearby GRB 060614 that does not fit in either class. Its approx.102 s duration groups it with long GRBs, while its temporal lag and peak luminosity fall entirely within the short GRB subclass. Moreover, very deep optical observations exclude an accompanying supernova, similar to short GRBs. This combination of a long duration event without accompanying SN poses a challenge to both a collapsar and merging NS interpretation and opens the door on a new GRB classification scheme that straddles both long and short bursts

NASA Technical Reports Server

Source of Acquisition 1 NASA G O ~ M  Space night center i 1 
i 
Swift detects a remarkable gamma-ray burst, GRB 060514, that introduces a new 
classification scheme 
N. Gehrels', J.P. Norris', V. Mangano2, S.D. Barthelmy', D.N. Burrows3, J. Granot4 , Y. 
Kanelto', C. Kouveliotod, C.B. Mark~ardt ' .~ ,  P. MCszAr~s~~~,  E. Nakarg, J.A. Noused, 
P.T. O'Brien'', M. Page", D.M. Palmer12, A.M. Parsons', P.W.A. Roming3, T. 
Sal tam~to '~ '~,  C.L. Sarazin14, P. S~hady".~,  M. Stamatik~s'.'~, & S.E. Woosley" 
NASA/Goddard Space Flight Center, Greenbelt, MD 20771, USA f 
7 -INAF -- Osservatorio Astronomico di Brera, Via Bianchi 46, 23807 Merate, Italy 
'Department of Astronomy & Astrophysics, Pennsylvania State Universiw, PA I 6802, 
USA 
'KIPAC, Stanford University, P. 0. Box 20450, MS 29, Stanford, CA 94309, USA 
j USRA, NSSTC, VP-62, 320 Sparkman Drive, Huntsville, Alabama 35805, USA 
6NASA/Marshall Space Flight Center, NSSTC, VP-62, 320 Sparkman Drive, Huntsville, 
Alabama 35805, USA 
Department of Astronomy, University of Maryland, College Park, MD 20742, USA 7 
'Department of Physics, Pennsylvania State University, PA 16802, USA 
https://ntrs.nasa.gov/search.jsp?R=20070016685 2019-08-30T00:50:44+00:00Z
'Theoretical Astrophysics, California Institute of Technology, MS 130-33, Pasadena, 
CA 91125, USA 
"Department of Physics and Astronomy, University of Leicester, Leicester LE1 7RH, 
UK 
Mullard Space Science Laboratory, Universiy College London, Dovking, RH5 6NT, 11 
UK 
I2Los Alamos National Laboratory, Los Alamos, NM 87545, USA 
I3Oak Ridge Associated Universities, P. 0. Box 11 7, Oak Ridge Tennessee 3 7831 -01 I 7, 
USA 
Department of Astronomy, University of Virginia, Charlottesville, VA 22904-4325, 14 
USA 
Department of Astronomy and Astrophysics, University of California at Santa Cruz, 1.5 
Sanla Cruz, CA 95064, USA 
Gamma ray bursts (GFU3s) are known to come in two duration classes', separated 
at -2 s. Long bursts originate from star forming regions in galaxies2, have 
accompanying supernovae (SNe) when near enough to observe and are likely 
caused by massive-star collapsars3. Recent  observation^^-'^ show that short bursts 
originate in regions within their host galaxies with lower star formation rates, 
consistent with binary neutron star (NS) or NS - black hole (BH) 
Moreover, although their hosts are predominantly nearby galaxies, no SNe have 
3 
been so far  associated with short GRBs. We report here on the bright, nearby 
GRB 060614 that does not fit in either class. Its -102 s duration groups it with 
long GRBs, while its temporal lag and peak luminosity fall entirely within the 
short GRB subclass. Moreover, very deep optical observations exclude an 
accompanying  upe ern ova'^-*^, similar to short GRBs. This combination of a long 
duration event without accompanying SN poses a challenge to both a collapsar and 
merging NS interpretation and opens the door on a new GRB classification scheme 
that straddles both long and short bursts. 
The Burst Alert Telescope (BAT) onboard the Swift satelliteI6 detected GRB 060614 on 
2006 June 14 at 12:43:48 UT. Based on the initial positions from the on-board X-ray 
and UV/optical telescopes (XRT, UVOT), the burst was subsequently located by 
ground- and space-based telescopes at the outskirts of a relatively nearby faint dwarf 
galaxy at a redshift of z=O. 12517. We find the s u g g e ~ t i o n ' ~ ~ ~ ~  of a chance alignment 
between a background GRB and foreground galaxy at z=O.125 to not be credible; the 
chance probability of the observed 0.5" offset between the GRB and the ~ 0 . 1 2 5  galaxy 
to be by chance is only 2 ~ l O - ~ .  Also, fits to the combined UVOT and XRT spectra give 
2 4 . 3  at the 99.99% confidence level, excluding the suggestedI8 location at e1 .4 .  (See 
Gal-Yam et al.14 for additional evidence against a chance alignment.) 
The event duration, T90 = 102 s (15-350 keV; where T90 is the time during which 90% of 
the event photons were collected), clearly places GRB 060614 in the long burst 
category'. As the event was also at close proximity, it became a prime candidate for a 
SN search in its light curve, as indeed had been found in four individual cases in the 
past2'. The three accompanying  paper^'^‘'' report very tight upper limits -- 100 times 
lower than previous detections -- in these searches. Thus far, strict SN limits had been 
found only for the GRBs of the short variety. The question then is: why was there no 
supernova associated with the long GRB 0606 14? 
Interestingly, a close examination of the BAT light curve of GRB 060614 (Figure 1) 
reveals a first short, hard-spectrum episode of emission (lasting 5 s) followed by an 
extended and somewhat softer episode (lasting -100 s). The total energy content of the 
second episode is five times that of the first [fluence of (1.69*0.02)~1 O-' erg cme2 and 
(3.3*0.1)~10-~ erg cm-2, respectively, in the 15-350 keV band]. The recent discovery of 
a similar two-component emission structure in several Swift and HETE-2 short GRBs 
prompted Norris and Bonnel12' to search the BATSE GRB database for similar events. 
They found 3 bursts with a bright softer emission component, spectrally similar to that 
of GRB 060614 (corresponding to roughly -1% of the number of short BATSE GIBS). 
Weaker soft components are found in 4 out of 16 Swift and HETE-2 short bursts and 11 
out of 130 Konus short G R B s ~ ~ ,  a significant -10%-25% of their short GRB databases. 
In fact, it may be that the majority of short bursts have such soft components; currently, 
only 2 Swift short GRBs (051221A 03 13) were bright enough to allow low fractional 
limits to be set. It is interesting to note that regardless of their duration, almost all of 
these events with softer components have a common appearance, namely a 5 - 10 s gap 
between the first and second episodes and a humped shape of the second component. 
5 
Another method o distinguish categories of GRBs is 3 compute the zmporal lags 
between their light curve features in different energy  band^^'^^'^^. We introduce a new 
variant on this approach by including short bursts in a plot of peak luminosity (Lpeak) 
versus lag (tias); the redshift measurements from the past year for a number of short 
Swifi GRBs makes the comparison possible for the first time. There is an anti- 
correlation between lag (tlas) and peak luminosity (Lpeak) for long bursts as shown in 
Figure 2. In contrast, short bursts have small tlag and small Lpeak and occupy a separate 
area of parameter space fiom long bursts. The lag for GRB 060614 for the first 5 s is 3 
f 6 ms. Thus, in spite of its long duration, GRB 060614 falls in the same region of the 
lag-luminosity plot as short bursts. Moreover, we were able for the first time to 
accurately calculate a lag for the softer second episode due to its spiky light curve. We 
find tlas=;*9 ms, consistent with the short-hard episode, and thus indicating a similar 
origin. The physical cause of lags in GRBs is not yet well understood, but it has been 
suggested23 that bursts with smaller lags have more relativistic outflows. 
From the above, it is difficult to determine unambiguously which category GRB060614 
falls into. It is a long event by the traditional definition, but it lacks an associated SN as 
observed in other nearby long GRBs, and it shows morphological similarities with Swift 
short bursts. Further, the complex structure of the second component and similar lag 
with the first component indicates that the extended episode is due to the long-lived 
activity ojthe burst central engine rather than due to the onset of the afterglow 
emission. However, its spectral lag and low luminosity make it distinct from other long 
GRBs and place it together with short burts. Perhaps the odd characteristics of GRB 
060614 point to a new subclass with a different physical origin identified by a third 
GRB property, namely spectral lag. However, we first must ask what it would take to 
fit such a class of events into either the collapsar or merger models. 
Interestingly, short-hard bursts with continuing activity resembling long-soft ones are a 
prediction of some massive star models25, favoured when the pre-explosive mass loss is 
unusually high and the jet energy low. The lack of a SN requires that significant 
production of j6Ni is avoided during the star collapse. This might be possible if the disk 
wind is weak and if 56Ni that had been produced initially fell back in a weak supernova 
explosion. Nevertheless, producing less than lom3 Mo of '%i is a challenge for the 
collapsar Another alternative is a failed SN; the small observed lag may imply 
a higher velocity outflow with insufficient energy deposited in the star for it to explode. 
In the NS merger model the duration is set by the lifetime of the accretion disk around 
the newly born BH and is expected27 to be -0.1 s. The accretion during a BH-NS 
merger might be longer if, as suggested by simulations2', a significant amount of mass 
is ejected to a bound orbit and falls back on the BH on time scales larger than 1 s. In 
short, GRB 0606 14 may be either just another odd event in the motley-crew collection 
of GRBs, or, more likely, one of the first established members of an emerging new class 
of events that include both short and/or long GRBs. 
Reprints and permission information is available at 
npg.nature.com/reprintsandpermission. The authors declare no competing financial 
interests. Correspondence and requests for materials should be addressed to N. Gehrels 
(gehrels@milkyway .gsfc.nasa.gov). 
7 
1. Kouveliotou, C. et al. Identification of two classes of gamma-ray bursts. Astrophys. J .  
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3. Woosley, S.E. Gamma-ray bursts from stellar mass accretion disks around black 
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the physics of cosmic explosions. Nature this issue (astro-ph 0608257) (2006). 
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due to a hypernova. Nature this issue (astro-ph 0608322) (2006). 
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17. Price, P.A., Berger, E. & Fox, D.B. GRB 060614: redshift. GCNCircuZar 5275 
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Gamma-Ray Bursts Is Required. (astro-ph 0608441) (2006). 
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superposition. (astro-ph 0608678). 
20. Kanelto, Y., et al. Prompt and afterglow emission properties of gamma-ray bursts 
with spectroscopically identified supernovae. (astro-ph 06071 lo), Astrophys. J . ,  in press 
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21. Norris, J.P., & Bonnell, J.T. Short gamma-ray bursts with extended emission. 
Astrophys. J. 643,266 (2006). 
22. Mazets, E.P., et al. Konus catolog of short GRBs. 
http://www.ioffe.ru/LEA/shortGRBs/Catalog/ (astro-ph 02092 1 9) (2002). 
9 
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ray bursts: GRBs 010326B, 040802,05121 1 and 060121 (astro-ph 0605570) (2006) 
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for a soft underlying component. Astrophys. J .  617, 1251-1257 (2004). 
Figure 1 The light curve of GRB 060614 as observed with the BAT. The mask- 
tagged BAT light curve is shown in 4 energy bands in the top panels, with a sum in the 
bottom panel. The outtake panel gives an expanded view of the first episode. There is a 
hint of a 9 s periodicity between 7 and 50 s, but we find it to be not statistically 
significant. The spectrum is well fitted by a power-law in both intervals with a photon 
index of 1.63k0.07 in the first 5 s and a softer inndex of 2.02*0.04 in the following 120 
s. The burst was also detected with Ko~us -WIND~~ with Epeak = 302 (-83, +214) lteV in 
the first 5 s. The total energy radiated assuming isotropic emission, EISO, in the 1 keV - 
10 MeV range in the GRB restframe for the first 5 s is 1 .8x1050 erg. We note that the 
Epeak-Eiso value for the short episode falls far off of the long-burst Amati relationship. 
The spacecraft repointed XRT and UVOT at the burst in 90 s. The X-ray emission (0.3- 
10 keV) was initially among the brightest to date at 5 ~ 1 0 ' ~  erg cm s at 100 s. The 
afterglow light curves suggest a jet break at -1.5 days, implying a narrow jet occupying 
~ 1 0 ~ ~  of the total solid angle and suggesting an energy of erg in the relativistic jet, 
roughly comparable to that inferred for Swift short GRBs. 
-2 - I  
11 
Figure 2 Spectral lag as a function of peak luminosity showing GRB 060614 in the 
region of short GRBs. The lags and peak luminosities are corrected to the source 
frame of the GRB. The lags are defined as the time difference between light curve 
structures in the 50-100 keV and 15-25 keV channels. The energy correction (K- 
correction) to the source fi-ame for the lag is approximately ( l + ~ ) ' . ~ ~ ,  making the total 
energy correction plus time dilation correction (1 +z)-'-~~. The data points labelled as 
long bursts are from Swijt/BAT, with the exception of GRB 030528 which is a very 
long-lagged HETE-2 burst. The dashed line illustrates the lag-luminosity correlation for 
long bursts. Outliers are bursts with unusual properties: GRE3 060729 with extremely 
long-lived afterglow and 05 10 16B with extremely soft prompt emission. The blue data 
points for short bursts are from SwijUBAT. The peak luminosity was calculated for the 
short bursts and GRB 060614 using 64 rns binning. In green are the 4 nearby long 
GRBs with associated SNe. The three of the four SNe-associated underluminous 
nearby GRBs (980425,031203 and 060218) fall below the long-burst correlation, while 
the only SN-associated GRB with normal luminosity (030329) falls near the line giving 
further evidence of it being a normal GRB (the only normal GRB with a firm SN 
association). None of the SN-associated GRBs falls near the short grouping. Note that 
our detailed calculation of the lag of GRB 030329 gives a larger value here than 
previously found3' in a higher energy band. 
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13 
Figure 2 


SWIFT Detects a remarkable Gamma-ray Burst, GRB 060514, that introduces a New Classification Scheme

Swift detects a remarkable gamma-ray burst, GRB 060614, that introduces
  a new classification scheme

Swift detects a remarkable gamma-ray burst, GRB 060614, that introduces a new classification scheme

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