Image credit: NASA Alternative Detections of Gamma Ray Bursts René Hudec 2 and Rudolf Slosiar 1 1 Slovak Union of Amateur Astronomers, Bojnice, Slovak Republic 2 Astronomical Institute, Academy of Sciences of the Czech Republic, CZ Ondrejov, Czech Republic

Alternative GRB Detections By Ionospheric Response By Bright Prompt Optical Emission By Promt X-ray Emission

Indirect detection of GRBs by ionospheric response

Introduction We report on the independent and indirect detection of GRBs by their ionospheric response (SID – Sudden Ionospheric Disturbance) observed at VLF (Very Low Frequency). Although few such detections have been already reported in the past, the capability of such alternative and indirect investigations of GRBs still remains to be investigated in more details. We present and discuss the examples of further such VLF/SID detections.

Previous VLF detections of GRBs SGR1806: Detection of a Sudden Ionospheric Disturbance. Campbell et al., GCN 2932, GRB observed as a Sudden Ionospheric Disturbance (SID) P.W. Schnoor GCN 2176, 2003.

Physics behind ionospheric detection I The solar particle stream, solar wind, shapes and controls the Earths‚ magnetic envelope - the magnetosphere- and increases heat in the aurora zones. But not all ionospheric variability is caused by solar or geomagnetic disturbances. The ionosphere is not a constant 'mirror in the sky'. The E layer ( km above ground) and the F1 layer ( km) usually behave in regular, solar-controlled way, but the F2 layer ( km) does not. It is the F2 layer, which has the greatest density of free electrons, and is potentially the most effective reflector of radio waves. (Rishbeth, Nature Vol. 418, 4 July 2002)

Physics behind ionospheric detection II The ionossperic D layer plays in the GRB detections an important role, as the detection of X-ray and gamma-ray triggers is based on the measuremeńt (monitoring) of reflected radio waves from this layer. The ionospheric D layer is not transparent for radio VLF waves (frequencies 3kHz to 30 kHz) and behaves like a mirror. If the transmitter is at large distance (800 to 2000 km) then the radio waves are guided like in a waveguide consisting of the D layer and the earth surface. Any change in the quality of this waveguide results then in the signal change in the SID monitor. The change can be positive but in some cases such as the sudden phase anomaly also negative.

Typical ionospheric behaviour This picture shows the typical behaviour of the ionosphere during one day. Note the different behavior at night with absence of the D layer.

The plot from the SID monitor during the period of enhanced solar activity Demonstration of the possibility and sensitivity of the method. Four Solar Flares (SF) are visible with intensities M6, C6, C2, C4, exactly corresponding to the measurements by the GOES satellite. The peak related to the SF C4 occurs around 15:03UT, which is nearly the detection time of GRB060124A, on Jan 24, 2006, confirming that even at the time of the decay of the D ionospheric layer reliable measurements are feasible. Dec 6, 2006

SGR : A sharp spike, then re- normalization of the ionosphere (Howe, 2004)

The SID receiver and cross loop antenna Instrumentation used for the indirect detection of GRB A. The antenna size is 75 x 75 cm 2-channel receiver used to eliminate false triggers Inexpensive instrumentation suitable for easy duplication for other sites

The detection of GRB A GRB_TIME: 15:54:51.82 UT SID trigger detection 15:56:31 +/- 5sec Swift BAT LCT

The detection of GRB A

For comparison: SID from C2-class solar flare erupted from sunspot 958

GRB080319D – possible detection GRB_TIME: 17:05:09.34 UT SID signal: 17:05:41 +/- 5sec Swift BAT LCT

GRB080320A – possible detection GRB_TIME : 04:37:38.46 UT SID signal : 04:38:59 +/- 5sec. Emitter Tavolara (Sardinia) Emitter Sainte Asise (France) Swift BAT LCT

Speculations In the both recent cases of possible detection of GRB080319D and GRB080320A, some structure appeared afterwards, resembling a propagation ionospheric wave Further observations are necessary to confirm this hypothesis

GRB Induced Propagating Ionospheric Waves Speed: 300 to 1200m/s

Possible GRB080319D Induced Propagating Ionospheric Waves

For illustration: Recent Earthquake May 29, 2008, at 15:46 UT (during yesterday talk of Claudio) Distance Bojnice- Island (epicentre) 2680 km SID disturbance detected with delay of 74 min Hence mean speed of propagation of the ionospheric wave was 603 m/s (In Vulcano, the wave arrived during talk of Anatoly) Ionospheric waves can be triggered both from outside (space radiation) but also from inside (e.g. earthquakes)

Discussion The conditions to detect GRB with SID monitor in VLF The presence of the D layer of the ionosphere The suitable combination of the GRB position (RA, DEC) and time and hence direction and angle of the incoming gamma-ray radiation in relation to the D layer and observing site. The fluence and duration of the GRB. The detection statistics The recent detection rate of GRBs is about 130 in a year. For one observing station, the number of GRBs occuring during the presence of a D layer and in the field of view is about 20 This is ideal number, the real one is less than 10 due to occassional non- availability of transmitters and other technical and observational issues.

Discussion II If the ionospheric detection of GRBs will be definitely confirmed, a dedicated experiment could be considered, namely a dedicated emitter

GRB Detection by Bright OT Emission A small fraction of GRBs is accompanied by bright optical prompt emission (OT) This emission can be as bright as mag 6, and perhaps even brighter Such triggers can be detected in optical light, independently on gamma-ray detection

GRB080319B For ~ 1 minute Brighter in optical light than mag 6 Indication exists that some OT may be as bright as mag 4 at early times Naked eye visibility at z ~ 0.75

Prompt optical emission of GRB Peak 11.5 mag 2 min after GRB, decline 1.4 mag in 2 min Images by WF lens 70 mm aperture at the Czech FRAM RT in Argentina (related to Auger).

Methods to detect bright OTs of GRBs Wide-field monitoring systems, CCD (preferably All-Sky) Wide-field monitoring systems, photographic (secondary use of meteor patrol) Archival Astronomical Plates Always sophisticated s/w needed to find and to verify the triggers Problem: (1) large background (2) typically, we have to look for 1 new object among stars: job for informatics students

Vulcano Workshop CONCAM All Sky Optical Monitoring Lim mag 4….5 i.e. not enough for most scientific goals Price 10x more than the alternative system shown before

Vulcano Workshop Digital CCD all-sky monitoring lim mag ~11 Karlovy Vary Observatory, CZ Peleng 8 mm fish- eye lens (1:3,5-1:16) that provides a 24 mm circular 180° field of view, and a CANON EOS 350D digital CCD camera

10/11/2004Sterrewacht Leiden Lunch Talk29 All Sky CCD Camera, Sonneberg Observatory, 7K x 4K CCD and f/3.5 FE lens. Lim mag 10 in 1 min integration time.

Vulcano Workshop CCD Sky Patrol Test Images Sonneberg lim mag

Vulcano Workshop How the OTs of GRBs should look like? Example of fast OT found - duration less than 5 minutes … and that’s expected Results promising- only 1 OT per plate found (typically)

Vulcano Workshop Optical Transient Analyses - OT in Triangulum, Sonneberg Astrograph Plate, 6 mag above plate limit real object of unknown The searches for analogous OTs are difficult since the plates contains typically – star images OT Comparison plate: no OT Another short OT example, mag < 10

April 27-May 1, Variable Stars Conference ValMez The OTs of (unknown) astrophysical origin have been confirmed also by CCD observations (Brno 60 cm CCD telescope, Filip Hroch) Stars elongated, OT image not elongated – short OT Problem: backround triggers simulating OTs

GRB Detections By Prompt X- ray emission So far most GRB are detected by their gamma-ray emission Most of the GRBs exhibit also X-ray emission Hence the GRBs can be detected also in X- rays (as independent detections) This goal requires sensitive all-sky monitoring Obvious solution is offered by X-ray All-Sky Monitors with Lobster Eye Optics

The front wiew of the mini - lobster module, Schmidt arrangement, based on 100 micron thick plates spaced by 300 microns, 23 x 23 mm each The X-ray measurement at 8 keV in comparison with mathematical simulation measured model LE lens arrangement according to Angel X-ray All Sky Monitor with LE modules

Conclusions The independent and indirect detection of GRBs by their ionospheric response (SID – Sudden Ionospheric Disturbance) observed at VLF (Very Low Frequency) is feasible. We present and discuss examples of such VLF/SID detections of three GRB. The capability of such alternative and indirect investigations of GRBs, as well as the possible contribution to analyses of GRBs still remains to be investigated in more details. The GRBs can be also (independently) detected in optical light and in X-rays, based on suitable optical and X-ray all-sky monitors

The End

GRB080319D – possible detection