ORPHAN AFTERGLOWS: THE OFF- AXIS VIEW OF GAMMA RAY BURSTS

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1 ORPHAN AFTERGLOWS: THE OFF- AXIS VIEW OF GAMMA RAY BURSTS Giancarlo Ghirlanda INAF- Osservatorio Astronomico di Brera (Milano- Italy) D Burlon, G Ghisellini, R Salvaterra, M. G Bernardini, S Campana, S Covino, P D'Avanzo, V D'Elia, A Melandri, T Murphy, L Nava, S. D Vergani, G Tagliaferri Gamma Ray Bursts: jeaed/relaevisec transients Orphan alerglows (OA) the parent populaeon of GRBs, characterisecs (Eme/flux space) Detectability prospects & ideneficaeon issues Conclusions Bologna IRA 20/10/2015

2 Standard model of GRBs Central engine: BH+Disk V 2 V 1 Interstellar medium n~ cm -3 Matter outflow is relativistic V 1 <V 2 Γ>>100 Γ High variability γ-ray photons ms- h duraeon Relativistic (internal) shocks à PROMPT Relativistic (external) shock à AFTERGLOW Smooth decay X-ray, optical, radio emission months-years Time

3 OrientaEon & relaevisec beaming GRB + A'erglow ϑ view > ϑ jet GRB ORPHAN + A'erglow Γ=1/ϑ view During alerglow Γ(t)~t - 3/2 Start to see the emission when: BEAMING = VIEWING ANGLE N orph 2 2 jet N GRB ϑ jet = 5 for each GRB à ~ 260 Orphans

4 ORPHAN GRBs 1. Gamma- Ray DARK parent populaeon of GRBs 2. DominaEng in number > X each GRB 3. Unavoidable if: GRB are relaevisec & jeaed The more off axis (a) the later the emission peaks and (b) the dimmer the flux at peak

5 Orphan alerglows searches as transients in (blind) surveys Band Ref Results X- ray Grindlay 1999; Greiner et al candidates à flare stars OpEcal/NIR Rau et al (12 deg 2, R=23) 4 candidates à flare stars Radio Malacrino et al (490 deg 2, r=22.5) CFTHLS Levinson et al (1.4 GHz NVSS vs FIRST, 6000 deg 2 ) 3 candidates à flare stars 9 candidates à 5 false posieve, 2 non transients (Gal- Yam et al. 2006) No conclusive deteceon so far

6 How many Orphan GRBs in radio surveys? Frail et al Rossi et al Past studies [e.g. Frail+12, Totani+02, Rossi+08] simplifying assumpeons 1) Jet (e.g. unique value rather than distribueon 2) ExtrapolaEon of bright GRB properees 3) No realisec cosmological effects 4) No full jet dynamics included 5) No radiaeve evolueon (and not complete emission mechansims, e.g. no SSC or IC)

7 PSYCHE Predict the emission of the ENTIRE GRB populaeon (GRB+Orphans) 1. Obs rate of GRBs (SwiL, Fermi, Batse) 2. Fluence distribueons 3. Ep- Eiso correlaeon (rest frame) 4. Ep,obs- fluence plane PopulaEon SYnthesis Code and 1. Luminosity Fct + formaeon rate(z) 2. Distrib (log- normal) Γ 0 and ϑ jet 3. Randomly oriented in the sky Macro- physical param z, E k, 0, jet, view 5. OpEcal 6. X- ray 7. Radio Hydrodynamic Emission model Micro physical param n, e, B, p, k A. Canova (Louvre) Ghirlanda+2012, MNRAS; Ghirlanda+2013, MNRAS; Ghirlanda+2014,PASA; Ghirlanda+2015, A&A

8 PSYCHE Orphan GRB populaeon: DIM and SLOW transients Ghirlanda et al. 2014, PASA

9 PSYCHE Orphan GRBs RADIO: Ghirlanda et al.,2014 Ghirlanda et al PASA Simulated Orphan ALerglow populaeon consistent with current radio upper limits (from surveys) and past esemates limited to the bright end of the flux distribueon (e.g. Frail et al. 2012, Panaitescu et al. 2002; Rossi et al. 2008)

10 Orphan ALerglows Emescales T peak T(>S lim ) T = 1 view jet T =1 1yr 3yr T dyn Average physical duraeon (T dyn ) = 1yr Same order of Eme of peak (with respect to T0, i.e. unknown) Survey sensievity à bias duraeon towards lower values Ts << T dyn T peak

11 PSYCHE Radio surveys ~150 MHz (LOFAR, MWA) Ghirlanda+2014 Orphan GRBs: predicted deteceon rates Telescope name ν S lim Rate [GHz] [mjy] [deg 2 yr 1 ] ASKAP MeerKAT/Ph MeerKAT/Ph SKA/Ph SKA/Ph2 1.4(8.4) ( ) WSRT/AperTIF EVLA LOFAR MWA GMRT GMRT Figure 5. Density contours (1,2 and 3σ as labelled) representing the distribution of the flux (at 8.4GHz) of the OA population versus the time when their light curve peaks. The 3σ upper limits (years after the explosion) at 8.4 GHz. S06 and B13 port indeed upper limits on the late time radio flux the monitored sources with the exception of SN2001 (see S06), and SN2003gk (see B13), which are in detected. Nonetheless, further monitoring of these events in the radio band and through VLBI obse tions, excluded that they produced a relativistic We show in Fig.?? the upper limits on the 8.4 G flux of the SN Ib/c observed by S06 and B13 which all Rate consistent for with a given the density sensievity contour of the distri tion of the simulated population of OA. The 1, 2, limit at 5σ significance 3σ contours represent the boundary containing resp tively 68.2%, 95.4%, and 99.7% of the points distri tion in the plane F peak t peak. 6 Summary and Discussion Number of OA [yr - 1 ] ~ Rate x Survey Coverage x Ts 1.4 GHz VAST- Wide VAST- Deep- SF SKA- 1 SURV Orphan afterglows are GRBs whose emission is tectable only during the afterglow phase (at o cal/nir and radio frequencies). Their prompt γ emission is unobservable because the viewing angle θ is larger than the jet opening angle θ jet (off axis GRB In these events the afterglow emission becomes obs able when the bulk Lorentz factor, which is decreas during the afterglow phase, becomes Γ 1/θ view.a this time, which represents the peak of the OA l curve, the emission is similar to that for an obser within the jet opening angle. OA make up a majority of the population of GR However, none have1-2 beenyr - 1 observed so far, do to t lack of a prompt emission 0.5 yr trigger. - 1 Their detection possible as transients in deep/wide field surveys. H ever, so far no detection 380 ofyr OA - 1 has been confirmed searches in archival optical/radio observations. In c rent and future surveys OA might represent a consid able fraction of detected transients. In this paper we have used the results of a populat synthesis code for GRBs (G13) that simulates the LOFAR/MWA 1500 deg 0.12 mjy (rms) No Orphan 10 4 deg 0.5 mjy 30 deg mjy 3x10 3 deg 0.9 μjy 3 GHz VLASS- AS deg 2 (2 ep) 0.12 mjy 20 yr - 1

12 Short Gamma Ray bursts (ß à GW) Current prediceons for short are obtained by rescaling long GRBs by factors PSYCHE extension to short GRBs in progress more esemates to come soon. (GG+16, Salafia+2016) Metzger+2015

13 CompeEng transients ExtragalacEc synchrotron radio transients Metzger et al Four different source classes within two decades of duraeon and four orders of magnitude in luminosity K. Mooley (PhD Thesis)

14 ClassificaEon problem Time domain: follow up could help (but not alone) to classify Metzger+2015 Spectral characterizaeon: 1) OpEcal spectra (features) 2) Broad band SED à mm and radio peculiariees 3) If distance is known (e.g. spectro- z) à energeec argument Ghirlanda+2015

15 Conclusions Orphan ALerglows (OA): GRBs without Gamma- ray detected emission Natural consequence of GRBs being jeaed- relaevisiec sources Outnumber GRBs (i.e. OA = x GRBs) GRBs and OA form a conenuus populaeon (same progenitors, physics) only an orientaeon effect. PSYCHE à characterizing OA properees: Sub- mjy slow (several months years transients) OA deteceon in the radio: VAST- W (few) VLASS (>few) à SKA SURV (hundreds) Combined temporal and spectral studies (e.g. SED) to idenefy them among compeeng extragalacec transients