PoS(GRB 2012)104 http://pos.sissa.it/ ce. chady, V. Sudilovsky on, the duration of the central- er that the afterglow emission vari- e discuss the signatures of late-time e possibility that the central-engine res of the long-lasting central-engine ares and the recent studies on pre- and es obtained by the GROND instrument. e, Sacramento, CA 95841, USA rlisle, PA 17013, USA hagen, 20100, Copenhagen, ., in the temporal and spectral variability 78 Tautenburg, Germany enza 3, 20126, Milano, Italy senbachstraße, 85748 Garching, ive Commons Attribution-NonCommercial-ShareAlike Licen ∗ [email protected] Speaker. engine activity has been usually consideredability to time-scales. be The discussion much on short the nature of X-ray fl of the optical-NIR afterglow. We will discuss these signatu In the standard view describing the nature of long GRB emissi post-cursor emission observed in the gammacan rays be unveiled characterised th by a much longercentral-engine activity. In activity this from work a w different perspective, i.e activity unveiled thanks to the rich multicolour light curv ∗ Copyright owned by the author(s) under the terms of the Creat c M. Nardini Università degli studi di Milano-Bicocca, Piazza della Sci Max-Planck-Institut für Extraterrestrische Physik, Gies Germany S. Klose, D.A. Kann, A. Nicuesa Guelbenzu,Thüringer A. Landessternwarte Tautenburg, Rossi Sternwarte 5, 077 Gamma-Ray Bursts 2012 Conference -GRB2012, May 07-11, 2012 Munich, Germany Unveiling long lasting central-engine activity with Optical-NIR afterglows E-mail: J. Greiner, R. Filgas, J. Elliott, F. Olivares E., A. Rau, P. S T. Krühler Dark Cosmology Centre, Niels Bohr Institute, Univ. of Copen A. Updike Department of Physics and Astronomy, Dickinson College, Ca Denmark P. Afonso American River College, Physics Dpt., 4700 College Oak Driv

PoS(GRB 2012)104 ) al- s 20 ks K , ∼ temporal H M. Nardini , 1 J 4 . , ′ 0 z early afterglow , ± ′ i 2 , ′ . r 8 , d-based robotic tele- ′ − g = Silla observatory [6] is able rise a remarkable case of optical- ert that the extreme behaviour α es of prominent rebrightening erglow, the optical emission has Fig. 2). Its GROND light curve n than what expected in the pre- zen seconds after the trigger and n GRB 100621A. This GRB rep- a-Ray burst Optical Near-infrared framework of different theoretical teep he fast GROND response we have veils that deviations from a simple our since the observed light curves 7 bands (i.e, rightens by about 1.1 magnitudes in ibed by the forward-shock emission tail the temporal and spectral evolu- 508 [16][13][9]. Nowadays, thanks ent. The complete light curve results by the sum of a smoothly-connected ver, already in these ys from some hours up to weeks after iour. Some GRBs showed more com- w model as shown in the left panel of red. After the bump a shallow decay s intense wiggles, lasting until d discussion on the light-curve fitting). imple power-law evolution and sudden unprecedented resolution. Here we will ing starting 3.5 ks after the trigger and al-NIR afterglows for every single portion of the multi-power-law fitting β 2 − t α − ν ∝ ) t , ν ( F X-Ray Telescope (XRT) afterglow localisation, rapid groun Swift Five years after the GROND first light we are now allowed to ass In [11] we discussed the broad-band evolution of GRB 081029, During the first decade after the discovery of the first GRB aft Here we adopt the standard notation 1 index for the rising portionphase starts, of characterised the by the triple presence power-law of is some much requi les observed in GRB 081029 is notobserved unique. with In GROND. Fig. In 2 this weresents section show the 4 we most exampl focus extreme in example particularbeen observed able o so to far. observe the Thanks early to rise t of a first afterglow compon when an achromatic steepening occurs (see [11] for a detaile lowing the study the optical-NIR spectral evolutionexploit with an this multi-colour capability oftion GROND to of study some in events de showingmodels. prominent rebrightenings in the 2. Examples of optical rebrightenings observed with GROND 2.1 Light curves NIR rebrightening observed by GRONDis (see characterised upper left by panel an in extremelypeaking sharp at intense 5.9 re-brighten ks. Duringall this 7 time optical-NIR the bands. observed light Thebroken curve light power-law b and curve a is smoothly-connected well tripleFig. represented power-la 1. In order to account for the fast rebrightening a very s been considered to be characterisedwere by usually well quite fitted a with single smooth orthe behavi broken trigger power-law deca [7]. This smoothin evolution a can standard be external-shock very scenarioyears, well (see some descr events e.g. did not [14]). followplex this light Howe simple, curves general characterised behav by clearlate-time deviations optical from rebrightenings a like s into the the fast case of GRBscopes can 970 start to observegive the a much GRB denser optical photometric afterglowpower-law coverage a evolution than few in before. do the This optical un vious bands years. are much Among more these commo ground-basedDetector instrument, (GROND) mounted the on Gamm the 2.2to m simultaneously MPG/ESO observe telescope the at GRB La optical-NIR afterglow in Unveiling long lasting central-engine activity with Optic 1. Introduction PoS(GRB 2012)104 M. Nardini band GROND J . This is true in β nel) and ptical rebrightening studies ties other than the prominent ultaneously with the optical evolution. The rebrightening is 100316B shows a very smooth, in other cases with late optical n in the right panel of Fig. 1. In highlighted. and SED for each single telescope racterised by sharp sudden changes NIR spectral index. In all reported the spectral index ic break. These features are however one observed for GRB 081029, with by a spectrum that is redder than the elled as a superposition of two separate 100621A is at present the only case in steep 1 ed in GRBs 081029 and 100621A) de- evolution. This is what we show in the matic. With GROND we can now for α istribution (SED) from non-simultaneous wed by a smooth decay phase superposed β ility to follow the whole temporal evolution al-NIR afterglows 3 band GROND light curve of the afterglow of GRB 081029 (left pa ′ r 14. The two light curves plotted in Fig. 1 show other similari ≈− In [11], we found that no X-ray rebrightening is observed sim The great improvement we can report here with respect to the o rise light curve of thecomponents afterglow with of the value GRB of 100621A the second (right component panel) rise index mod from a sum of 2this smoothly-connected case, triple the power-laws sudden as rebrightening show is even steeper than the α Figure 1: Unveiling long lasting central-engine activity with Optic particular when the light-curvethe evolution first is time strongly follow chro theexposure, SED tracking by the extracting light a curve simultaneous with 7-b a corresponding plots reported in Fig.always 2. accompanied All by 4 a simultaneous GRBscases, change show moreover, a of the the strongly new optical- chromatic risingpre-bump component phase. is characterised Aflattenings similar like GRB spectral 061126 change [4][10] and was GRB observed 071003 [15][3]. also optical rebrightening. In both cases thewith steep several rise achromatic is wiggles follo and endingnot with an observed achromat in all thesteeper bursts (with in respect our to sample. thecay shallow after For post-rise the example, decay rebrightening GRB observ (see Fig.which 2). we Unfortunately have GRB a GROND detection of the first afterglow rise. 2.2 Spectral evolution already discussed in the literature is relatedalso to the from possib the spectral point ofin view. the light-curve Since evolution, these to extract GRBs a are Spectralphotometric cha Energy detections D can can result in a wrong estimate of PoS(GRB 2012)104 opt β M. Nardini for the other GRBs in 2 s ng an optical-NIR rebright- bands in GRB 081029). ′ r s last issue will be discussed and ′ ure of these “bumpy afterglows” g ows” in our sample are: i) the pres- ome ks after the trigger, ii) a sudden of X-ray flux around the optical peak e not corrected for either Galactic and host obtained from the fitting of the simultaneous ossible host galaxy dust absorption. (affecting opt al-NIR afterglows β 4 Upper panels: GROND optical-NIR light curves of 4 GRBs showi http://www.swift.ac.uk/xrt_curves/ The observational features characterising “bumpy aftergl 2 ening. Lower panels: evolution of the spectral index galaxy dust absorption nor for redshifted Lyman absorption Figure 2: observations in the 7 GROND bands. Light curve magnitudes ar Unveiling long lasting central-engine activity with Optic values are obtained taking into account of the Galactic and p bump of GRB 081029. When comparing GROND with XRT light curve our updated sample, we confirmeven this if finding a to hint be oftime) a simultaneous can general activity be feat claimed (i.e., for a bothin small GRB a 100814A excess forthcoming and paper GRB in 100621A. preparation. Thi 3. Discussion ence of an extremely fast and bright optical rebrightening s PoS(GRB 2012)104 . c ν ese > X ν M. Nardini n l to reproduce all the features le to GRB 100814A and GRB nset of the afterglow produced e claimed this possibility but no econd component is related to a e. Weaker or softer post-cursors sion of the faster (and narrower) ood agreement with the predicted sum of two separate components: els are able to account for all the htening [5], however, no signature ed if the starting time of the second late prompt [3][10] have been pro- hange observed during the rebright- changes in the observed light curve framework of the standard external- tion boundary of the stellar wind of ilable and no flare is observed before table to account for several complex he rising component is disentangled e. For example, the first model that well-sampled long GRBs. In the first, is effect should be prominent for fre- tion of the central-engine is therefore k. According to the late prompt sce- mpt scenario) is not simultaneous with bsence of a simultaneous bump in the re complete discussion on the case of ion related to a late-time activity of the r the encounter of the blast wave with a ve been proposed in the literature. Un- imes of hundreds of seconds are already ated to the sudden increase of the exter- erial which failed to reach the escape ve- ignature in other bands. Several examples ncies al-NIR afterglows 5 10 cannot be produced according to the standard version of th − < rise α , and this could explain the absence of an X-ray signature whe 3 c ν < ν < m ν See e.g. [14] for a definition of these characteristic freque As discussed above, the most commonly invoked scenarios fai 3 Unveiling long lasting central-engine activity with Optic colour evolution during such a rebrightening, and iii) the a X-ray light curve. These featuresshock cannot afterglow scenario. be explained This in modeloptical the had afterglow been light already curves unsui andfortunately, several modifications none ha of the3 most common commonly features used characterising alternative theis mod events usually described invoked to abov explainnal optical medium rebrightenings density is [8] rel thatthe is progenitor expected and to the occur surroundingquencies at interstellar the medium. transi Th However, even sharp discontinuities in the densitywind-termination profile shock o can only produce[12]. smooth and Other diluted models like theposed in two-component order jet to [1][2] explain the and complexa the chromatic flattening evolution of or a bumpby in the the slower light (and curvejet wider) can is jet be already appears observed decreasing when ifnario fast the the the to afterglow o observed due emis broad-band the lightthe early-time standard curve jet forward-shock is brea afterglow composed emission and ofcentral-engine a the sustained radiat by the accretion oflocity fall-back of mat the progenitor star.from the In pre-bump one both and models, thisening. fact the allows However nature the a sudden steep of colour t c models. Moreover, for the two-componentclosure jet, relations. we also find no g 4. Conclusion characterising these fast rebrighteningsGRB (see 081029 [11] that can for be a generalised).component This mo (either problem in can the be two-component solv jet orthe in the GRB late trigger pro but delayedreactivation by of the a central-engine we few can hundred try to seconds. identify a If s this s of a pre-/post-cursor prompt emission activity with delay t known and are sometimes accompaniedof by late a late gamma optical emission rebrig iscould observed be in visible the as GRBs X-ray presentedearly flares abov XRT (like coverage in was GRB available. 071003).100621A for Now, In which we dense [11] can early-time w coverage extend inthe our X-rays optical is samp ava bump starts. A clear signature of a late reactiva PoS(GRB 2012)104 , 279 3: , 253 , 1997, 415 , 1998, , 2006, , eneous M. 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Nakar, et al., we can therefore constrain this modeldescription with of an the additional expected fixe spectral evolution above X-rays. References model with the observed colour evolution and with the absenc Unveiling long lasting central-engine activity with Optic not a general feature of this class of events. Recently, [17] between shells emitted by the central-engine at delayed tim [13] Sokolov, V.V. et al., [14] A. Panaitescu, & P. Kumar, can produce sharp optical rebrighteningslike like GRB 100621A the we ones can we infer ar the [15] Perley D. A. et al., [16] M. Vietri, [17] A. Vlasis, et al.,