Gamma-Ray Bursts Review of the GRB phenomenon Outline of the observational facts and empirical phenomenological relations Outline of models Markus Garczarczyk Max Planck Institute – May 31st 2004

Gamma Ray Burst? Gamma-Ray Bursts (GRB) are electromagnetic signals in the gamma-ray band (in the spectral domain), with short durations (in the temporal domain) They are unusual in having most of their electromagnetic output in gamma-rays, typically lasting tens of seconds Discovered serendipitously in the late 1960s

Compton Gamma-Ray Observatory CGRO was launched in 1991 –Burst and Transient Experiment (BATSE) provided evidence for an isotropic spatial distribution  significant support to a cosmological origin interpretation –Energetic Gamma Ray Experiment Telescope (EGRET) detected bursts in the hard gamma-ray band

GRB Coordination Network Started in 1993 (up to now ~100 recipients around the world) Located at Goddard Space Flight Centre in Greenbelt MD Collecting RA, Dec locations of GRBs from other satellites and distributing the alerts to the clients t < 1s for Burst Monitor System Receiving and distributing messages about the follow up observations on various GRBs to the clients

GRB Satellites SatelliteStartEnd?InstrumentEnergy rangeGRB/yAccuracyDelay HETE ?FREGATE4 – 400 keV50N/A WXM1 – 10 keV2030’8s SXC0.5 – 2 keV1030’8s INTEGRAL ?IBIS15 keV – 10 MeV122’15s SPI20 keV – 8 MeV2030’15s SWIFT ?BAT10 – 150 keV3005’8s XRT0.3 – 10 keV3002.5’’90s UVOT170 – 650 nm3000.5’’90s AGILE ?GRID30 MeV – 30 GeV2020’60s S-AGILE10 – 40 keV203’> 1h GLAST ?LAT10 MeV – 100 GeV20010’30s GBM5 keV – 30 keV  5s ECLAIRS2007?2010?LAXT3 – 50 keV2005’’5s SXC0.5 – 14 keV2005’’5s WFOC500 – 700 nm2005’’5s

Observation Facts Prompt emission –Temporal properties –Spectral properties –HE photons –Polarization properties Afterglow –Global properties –X-ray –Optical –Radio –Taxonomy

Prompt Emission: Temporal Properties Durations: span 5 orders of magnitude, i.e. from  s to  10 3 s Bi-modal duration distribution : ~20s for long burst and ~0.2s for short bursts Lightcurves: very irregular Widths of individual pulses (δt) vary in a wide range shortest: 6ms GRB longest: 2000s GRB991208

Consider isotropically emitting source at a distance D Scale of the emission area At high photon energy population, production of e + e - pairs is likely. f p is fraction of photon pairs that satisfy The average optical depth for this process is The Compactness Problem Gamma-rays should be attenuated in the source before travelling through the universe and reaching the earth  relativistic bulk motion

Prompt Emission: Spectral Properties The continuum spectrum is non-thermal  “Band-function”: smoothly-joining broken power law -Low energy photon spectral index (α) -High energy photon spectral index (β) -Transition energy (E 0 ) Peak of the energy spectrum for β  -2 (E p ) Examples of GRB prompt emission spectrum Out of 156 bright BATSE burst with 5500 total spectra

Prompt Emission: HE Photons Dozens of BATSE GRBs has been detected at higher energies Detections are consistent with a Band spectrum extended to high energies without further breaks -GeV emission detected in GRB lasting 1.5h -Evidence of TeV excess reported by MILAGRITO for GRB970417

Prompt Emission: Polarisation Properties RHESSI (Ramaty High Energy Spectroscopic Imager) found that the prompt emission of the GRB was strongly polarized. This supports the scenario that the gamma-rays are generated by synchrotron emission. It rule out thermal emission or energy loss by relativistic electrons in intense radiation fields Polarisation degree 80%±20%

Afterglows Rest frame densities of particles and photons small  no two-body encounter Synchrotron emission as major radiation mechanism Detected in X-ray, optical/infrared and radio bands Lightcurves generally displays power-law behaviour self absorption νaνa νmνm synchrotron absorption FmFm cooling νcνc radioX-rayoptical

Afterglows: Global Properties Not all bursts have afterglows detected in all of the three main bands X-ray afterglows are the most commonly detected BeepoSAX: 60% with X-ray afterglows are detected also in optical band  40% “dark bursts” HETE: 10% of “dark bursts” Radio afterglows are detected in 50% of all GRB afterglows Every GRB with an afterglow detection has an underlying host galaxy  normal, faint, star forming galaxies

Afterglows: Global Properties GRBs are at cosmological distances Some GRBs are associated with SN explosions May 2004: 33 redshift measurements z = … 4.5 GRB / SN1997cy GRB / SN1997ef GRB / SN1998bw GRB / SN2003dh

Afterglows: Taxonomy According to the afterglow data, GRB can be further classified into several sub-types: a)Optically dark bursts: A fraction of GRBs with precise localizations do not have bright enough optical afterglows to be detectable: Dust extinction High redshift Intrinsically faint nature b)Fast-fading GRBs: Several bursts show a steep afterglow decay (α~ -2) in their early phase. They do not fit into the standard scenario and may constitute a peculiar class of GRBs (e.g. GRB980519, GRB980326)

Standard Energy Release Long GRBs have a standard energy reservoir: Corrections for the fact that the bursts are beamed to a small part of the sky  jets For 24 GRBs with known z and measured E ,iso the corrected E ,iso Θ j 2 is constant ~10 51 eV

Central Engine Requirements Energy: accelerate approximately solar masses to relativistic velocities, Γ > 100 Beaming: Most are beamed with opening angles 0.02 to 0.2 radians Long and Short Bursts: same mechanism? Rates: 1 per 10 7 year per galaxy, about 1/1000 the rate of supernovae, equivalent to about one per day Time Scales: Variability ~1 ms, duration on the order of 50 s  cannot be produced from single explosion Possible SN Association: Some GRBs seem to be associated with SN GRB shows SN spectrum, SN2003dh Iron Lines: Observed in some X-ray afterglows. Large amount of iron near central engine necessary Star Formation Association: GRBs seem to be prevalent in star forming regions Distribution: Within galaxies, don’t seem to be outside of galaxies

Two Basic Scenarios 1.Hypernova (Collapsar) – rare types of supernovae Hundreds of SN in the universe every day SN may sometimes emit jets of material, leading to a GRB 2.Merger of two neutron stars or neutron star and a black hole Forming black hole surrounded by a massive disk Model works only for short bursts and hard bursts earth We see them only if the jet is pointing to the earth

GRB Generic Model I.Hidden central inner engine which produces a relativistic outflow of energy –NS-NS, NS-BH, Collapsar, Hypernova… II.Energy transport from the engine to an outer region –Kinetic energy flux by relativistic particles is easiest III.Conversion of energy to the observed prompt radiation, i.e. the burst –Kinetic energy is converted to thermal energy in shocks, then radiated away as gamma-rays. Fireball model: internal and external shocks. –Beams of blobs of matter interacting with the SN shell. Cannonball model IV.Later, there is a conversion of the remaining energy into radiation, i.e. the afterglow a)Inner engine of GRB shines for long time, produces both the pre-cursor as well as the afterglow b)Slowing down of relativistic shell by the ISM, i.e. the external shock

Fireball Model Large concentration of electromagnetic radiation in small region of space Fireball: thermal plasma of photons, electrons and positrons Synchrotron emission from acceleration of e - in relativistic shocks external shocks source internal shocks GRB are generated when slower shells with faster shells collide γ-ray ISM radio X-ray optical Afterglows are generated when propagating shells interact with the ISM medium Sari and Piran 1997 NS & NS merger Hypernova & Collapsar

Cannonball Model Core-collapse SN  all GRBs are associated with SN Half of the GRBs with known z are too far to see their associated SN Cannonballs: –Beams of blobs of matter Γ ≈ 1000 –“cannonball” crosses SN shell  heat up by the collision  re-emit their own radiation  boost the light of the shell –1 GRB pulse per cannonball Afterglows are generated when the boosted SN shells decelerate through synchrotron radiation with the ISM source γ-ray

Emission Mechanisms – Review Synchrotron emission Synchrotron self-absorption –Irrelevant during the GRB itself –More important in late times, i.e. afterglow Synchrotron self-compton –Large fraction of low energy radiation will be up-scattered by IC Can have photon energies in the GeV end TeV range Fermi acceleration –Proton acceleration in the internal shocks can generate UHECR ~10 20 eV* *Eli Waxman, astro-ph/

HE Photons from GRBs? r0r0 r ph rcrc r is rsrs r dec electron self-IC cross-IC between the shocks electron self-IC proton synchrotron photo-meson cascade proton synchrotron + photo-meson cascade pn inelastic collisions in the early phase baryonic photosphere component

Broad Range in the Astrophysics 1.Stellar context: GRBs are related to the deaths of massive stars GRB study is closely related to the fields of stellar structure and evolution, supernovae and supernovae remnants 2.Galactic context: GRB afterglow lightcurves and spectral features probe the properties of the ambient interstellar medium or the prestellar wind 3.Cosmological context: detecting high-z GRBs allow to see into deeper and earlier epochs of the universe 4.High Energy Astrophysics: among many other models, a GRB origin of the UHECRs is a leading model for the so-called “bottom-up models” Cosmic ray photons are believed to be accelerated in GRB shocks. Potential high energy neutrino source for large area neutrino telescopes 5.Gravity: several GRB progenitor scenarios are believed to generate gravitational wave signals

Conclusions Because of slowness in slewing instruments and/or bad weather in ground based instruments GRB observations started hours after the burst trigger (t obs < 10min for GRB990123, GRB021004, GRB and GRB030418) The situation will change in October 2004 with the launch of the SWIFT satellite Nature of short, hard bursts is still unsolved In the GeV-TeV energy range only modest, low significance detections have been reported (EGRET:, MILAGRITO: GRB970417a; GRAND: GRB971110) With the fastest slewing time of all IACTs (~20s) MAGIC has a big potential to detect gamma-rays from GRB explosions when they are is still ongoing The MAGIC GRB Alert System is already working…