Is the late near-infrared bump in short-hard GRB 130603B due to the Li-Paczynski kilonova?
Zhi-Ping Jin, Dong Xu, Yi-Zhong Fan, Xue-Feng Wu, Da-Ming Wei
aa r X i v : . [ a s t r o - ph . H E ] O c t Draft version October 2, 2018
Preprint typeset using L A TEX style emulateapj v. 5/2/11
IS THE LATE NEAR-INFRARED BUMP IN SHORT-HARD GRB 130603B DUE TO THE LI-PACZYNSKIKILONOVA?
Zhi-Ping Jin , Dong Xu , Yi-Zhong Fan , Xue-Feng Wu , and Da-Ming Wei Key Laboratory of Dark Matter and Space Astronomy, Purple Mountain Observatory, Chinese Academy of Science, Nanjing, 210008,China Dark Cosmology Centre, Niels Bohr Institute, University of Copenhagen, Juliane Maries Vej 30, 2100 Copenhagen, Denmark Chinese Center for Antarctic Astronomy, Purple Mountain Observatory, Chinese Academy of Sciences, Nanjing 210008, China
Draft version October 2, 2018
ABSTRACTShort-hard gamma-ray bursts (GRBs) are widely believed to be produced by the merger of twobinary compact objects, specifically by two neutron stars or by a neutron star orbiting a black hole.According to the Li-Paczynski kilonova model, the merger would launch sub-relativistic ejecta and anear-infrared/optical transient would then occur, lasting up to days, which is powered by the radioac-tive decay of heavy elements synthesized in the ejecta. The detection of a late bump using the
HubbleSpace Telescope ( HST ) in the near-infrared afterglow light curve of the short-hard GRB 130603B isindeed consistent with such a model. However, as shown in this Letter, the limited
HST near-infraredlightcurve behavior can also be interpreted as the synchrotron radiation of the external shock drivenby a wide mildly relativistic outflow. In such a scenario, the radio emission is expected to peak witha flux of ∼ µ Jy, which is detectable for current radio arrays. Hence, the radio afterglow datacan provide complementary evidence on the nature of the bump in GRB 130603B. It is worth notingthat good spectroscopy during the bump phase in short-hard bursts can test validity of either modelabove, analogous to spectroscopy of broad-lined Type Ic supernova in long-soft GRBs.
Subject headings:
Gamma rays: general – radiation mechanisms: non-thermal INTRODUCTION
GRB 130603B triggered the Burst Alert Tele-scope (BAT) on board the
Swift satellite at 15:49:14UT on 2013 June 3 (Melandri et al. 2013). Ithad a T duration of 0 . ± .
02 s in the 15-350keV band (Barthelmy et al. 2013) and the BAT lightcurve reveals no trace of extended emission at the ∼ .
005 counts det − s − level (Norris et al. 2013).The spectral lag analysis reveals no significant de-lay of the high and low energy photons (Norris et al.2013). All these facts together render GRB 130603Ba prototypical short-hard gamma-ray burst (GRB;de Ugarte Postigo et al. 2013; Bromberg et al. 2013).GRB 130603B is the first short GRB with absorp-tion spectroscopy (de Ugarte Postigo et al. 2013). Theother remarkable discovery made in GRB 130603B is aninfrared bump appearing at t ∼ [email protected] (YZF) speculated (Narayan et al. 2001). In view of the fun-damental importance of such a kind of interpretation,it is necessary to check whether other possibilities ex-ist, and if they do, how these possibilities can be furtherconstrained. That is the main purpose of this letter. THE DIFFICULTY OF INTERPRETING THE LATEINFRARED BUMP AS THE REGULAR AFTERGLOW
Tanvir et al. (2013) suggested that there are tworeasons against the regular afterglow origin of the lateinfrared bump of GRB 130603B. One is that the opticalafterglow lightcurve of GRB 130603B drops with timemore quickly than t − for t >
10 hours after the trig-ger of the burst. The near-infrared flux, on the otherhand, is in excess of the same extrapolated power law(see Figure 2 of Tanvir et al. 2013). The other is thesignificant color evolution of the transient, defined as thedifference between the magnitudes in each filter, whichevolves from R − H ≈ . ± .
15 mag at about 14hr to greater than R − H ≈ . ∼ . − ν − . between the optical and X-ray bands as that expected in the standard afterglowmodel (Piran 1999), the near-infrared to X-ray spectralenergy distribution (SED) of GRB 130603B 8.5 hr af-ter the burst onset can be nicely fitted by an extinc-tion of A V = 0 .
86 mag and a Small Magellanic Cloud(SMC) extinction law. The SED is well fitted with aspectral break at ν break ≈ Hz and the lower and -9 -8 -7 -6 -5 -4 F l u x den s i t y ( Jy ) Frequency (Hz)
Fig. 1.—
SED fit to the afterglow of GRB 130603B. The red solidline is the 0.6 day intrinsic broken power-law spectra with indexes0.65 and 1.15 and the break frequency 6.0 × Hz. The dashedline is the extinct spectrum with A v =0.9 for the host galaxy (withSMC extinction law) and the Galactic A v =0.06. The optical dataand 3 σ − curves/00557310/. high energy spectral indexes are α O = − . ± .
09 and α X = − . ± .
11, respectively. Using the optical af-terglow data at t ∼ . ν break shifts to ∼ × Hz (see Fig-ure 1). The optical and X-ray spectra suggest that thebreak frequency is the so-called cooling frequency ν c inthe fireball afterglow model (Piran 1999). For the burstborn in stellar wind, ν c ∝ t / , i.e., the later the obser-vation, the higher the cooling frequency. The SEDs at t ∼ .
35 day and 0 . ν c ∝ t − / for the burst born in the ISM-likemedium. Hence, ν c ∼ ( t/ .
35 day) − / ∼ × Hz at t ∼ ν c ∝ t . All these facts together rule out the presence ofa significant color evolution of the infrared/optical after-glow emission in the time interval of 0 . − t ∼ Hubble Space Telescope ( HST ) data is very rare andother scenarios should also been investigated. THE SECOND-COMPONENT JET MODEL FOR THEINFRARED BUMP OF GRB 130603B?
Two component jet model has been adopted to inter-pret some peculiar afterglow emission of both long andshort GRBs (for the former, see, e.g., Berger et al. 2003;Huang et al. 2004; Racusin et al. 2008; for the latter,see Jin et al. 2007). In such a model, the narrow en-ergetic core produce prompt γ − ray emission and thenthe early bright afterglow emission while the much widerbut less energetic ejecta component will emerge at a latetime, depending on its bulk Lorentz factor. The infrared bump likely peaks at t ∼ ∼ E / , w , ( t/ − / n − / , (1)where E k , w is the kinetic energy of the mildly relativisticoutflow component and n is the number density of thecircum-burst medium (for simplicity, below we just dis-cuss the ISM-like medium that is favored by the SEDs).Note that here and throughout the text the convenience Q x = Q/ x has been adopted except for specific nota-tions.We also point out that a mildly relativistic outflowcomponent is not unexpected. For example, in both thedouble neutron star merger scenario the neutron star-black hole merger scenario, a wide but mildly relativis-tic outflow surrounding the ultra-relativistic GRB ejectamay be formed as a result of the interaction of the outflowwith the surrounding material (e.g., Aloy et al. 2005).After the merger of the double neutron stars, a supra-massive/stable magnetar rather than a black hole maybe formed (e.g., Gao & Fan 2006; Zhang 2013; Giaco-mazzo & Perna 2013). The wind of the magnetar thatpossibly suffers from significant kinetic energy loss viagravitational wave radiation (Fan et al. 2013) may beable to accelerate the material ejected from the doubleneutron star merger to a mildly relativistic velocity aswell (Fan & Xu 2006; Gao et al. 2013).The cooling Lorentz factor of the external for-ward shock electrons can be estimated as ν c ≈ Hz E − / , ǫ − / , − n − ( t/ − / (1 + z ) − / (Piran1999), where ǫ B is the fraction of shock energy given tothe magnetic field and z = 0 .
356 is the redshift of GRB130603B (de Ugarte Postigo et al. 2013). For the nar-row and wider ejecta components, the number density ofthe medium should be the same and the initial kineticenergy is expected to be different and usually we have E k , n > E k , w . As mentioned above, for the narrow ejectacomponent ν c , n ∼ Hz at t ∼ .
35 day, hence ν c , w ∼ Hz ( E k , n E k , w ) / ( ǫ B , n ǫ B , w ) / ( t .
35 day ) − / . (2)To interpret the identified softness of near-infrared bump(i.e., ∆( R − H ) ≈ . ± .
15 mag), the syn-chrotron radiation spectrum of the second-componentejecta should be softer than that of the early ( t ∼ . ν − . ± . . The requiredpower-law distribution index of the electrons acceleratedby the wide-component ejecta is p w ∼ . ± . ν c , w < ν F606W at t ≥ p n ∼ . ǫ B , w ≥ . ǫ B , n ( E k , n E k , w ) / . (3)It is unclear why the narrow and wide outflow compo-nents have different ǫ B (possibly also ǫ e and/or p ). How-ever, we note that the best-fitted microphysical param-eters of GRBs differ from burst to burst (Panaitescu &Kumar 2001) and no universal values have been obtained.Moreover, in the modeling of the afterglow emission ofsome GRBs within the two-component jet scenario, thebest-fitted microphysical parameters are found to be dif-ferent for the narrow and wide components (e.g., Jin etal. 2007; Racusin et al. 2008). Hence, we suggest thatthe request ǫ B , w > ǫ B , n is reasonable and possible.Simultaneous with the ∼ . µ Jy infrared emission,the radio radiation flux is expected to be F ν radio ≥ . µ Jy ( ν radio . × Hz ) − ( p w − / ∼ µ Jy , (4)for p w ∼ . ν radio ∼ Hz. This is be-cause both the typical synchrotron radiation frequency ν m , w ≈ × Hz E / , w , ǫ / , w , − ǫ , w , − ( t/ − / and the synchrotron self-absorption frequency ν a , w ≈ . × Hz E . , w , ǫ . , w , − n . ǫ . , w , − ( t/ − . arebelow ∼ Hz. Note that even for p w ∼ .
3, we have F ν radio ∼ µ Jy. Such a flux is bright enough to bereliably detected by some radio arrays (for example, theKarl G. Jansky Very Large Array) in performance. Thenon-detection will in turn impose a tight constraint onthe external forward shock radiation origin of the infraredbump.To better show our idea, we calculate the flux numer-ically. The code used here has been developed in Fan &Piran (2006) and Zhang et al. (2006). The dynamicalevolution of the outflow is calculated using the formulaein Huang et al. (2000), which can be used to describe thedynamical evolution of the outflow for both the relativis-tic and non-relativistic phases. The energy distributionof the shock-accelerated electrons is calculated by solv-ing the continuity equation with the power-law sourcefunction Q = Kγ − p w e , normalized by a local injectionrate (Moderski et al. 2000). The cooling of the electronsdue to both synchrotron and inverse Compton has beentaken into account.In Figure 2, we have presented one numerical exam-ple which can fit the limited HST data of GRB 130603B.The physical parameters adopted in the fit are as fol-lows: ǫ e , w = 0 . ǫ B , w = 0 . p w = 2 . n = 1 . − , E k , w = 4 × erg, the initial Lorentz factor of the out-flow Γ = 3 .
0, and the half-opening angle is assumed tobe θ j = 1 .
0. As one can see both the temporal and spec-tral properties of the infrared bump of GRB 130603B canbe reproduced. The radio afterglow emission is so bright( ∼ µ Jy) that can be well detected by Karl G. Jan-sky Very Large Array-like telescopes. The non-detectionwould impose a tight constraint on the mildly relativisticoutflow model. DISCUSSION
Our understanding of short GRBs, a kind of γ − ray outbursts with a duration less than two sec-onds (Kouveliotou et al. 1993), has been revolution-ized due to the successful performance of the Swift satel-lite. For a good fraction of short GRBs, the binary-neutron-star or the black hole-neutron star merger model(Eichler et al. 1989; Narayan et al. 1992) has been sup-ported by their host galaxy properties and by the non-association with bright supernovae. A smoking-gun sig-nature, i.e., a supernova-like infrared/optical transientpowered by the radioactive decay of heavy elementssynthesized in the ejecta launched by either the neu-tron star binary merger or the neutron star-black holemerger (i.e., the Li-Paczynski kilonova), however, hasnot yet been unambiguously identified. The best can-didate of such a smoking-gun signature is likely the in- -3 -2 -1 F l u x ( Jy ) t (s) 5.8 GHz 1.6 m 0.6 m Fig. 2.—
Multi-wavelength afterglow emission of a wide mildlyrelativistic outflow component, which is consistent with the
HST data (extinction corrected) of the near-infrared bump of GRB130603B (Tanvir et al. 2013). frared bump detected at t ∼ HST near-infrared data is verylimited and can be interpreted as the synchrotron radi-ation of the external shock driven by a wide mildly rel-ativistic outflow. Interestingly, a wide mildly relativisticoutflow associated with the ultra-relativistic GRB ejectahas been “observed” in the numerical simulation (e.g.,Aloy et al. 2005) or “predicted” in the magnetar centralengine model of short GRBs (e.g., Fan & Xu 2006; Gaoet al. 2013). In the mildly relativistic outflow model, theradio emission is expected to peak at t ∼ s (see Fig.2)with a flux ∼ µ Jy, which is detectable for some ra-dio arrays in performance. While the synchrotron radioradiation powered by the kilonova outflow is expectedto peak at t ∼ × ( V nova / . c ) − / E / , n − / s,where E nova ( V nova ) is the kinetic energy (velocity) ofthe kilonova outflow and c is the speed of light. Hence,the radio afterglow data can provide complementary ev-idence on the nature of the near-infrared bump in GRB130603B or similar events in the future. It is worth not-ing that good spectroscopy during the bump phase inshort-hard bursts can test validity of either model above,analogous to spectroscopy of broad-lined Type Ic super-nova in long-soft GRBs.If the mildly relativistic outflow model has been con-firmed by (future) observations, the near-infrared bumplike that detected in GRB 130603B is still valuablefor those interested in searching for the electromagneticcounterparts of the merger of two neutron stars or a neu-tron star and a black hole since such a kind of signalis expected to be almost isotropic due to its low bulkLorentz factor. As shown in the numerical simulation(Aloy et al. 2005), the mildly relativistic outflow maybe common, thus the observational prospect can not beignored. ACKNOWLEDGMENTS