Gamma-Ray Burst Searches with Virgo: current analyses & future prospects

1. Gamma-Ray Burst Searches with Virgo: current analyses & future prospects Alessandra Corsi

2. Formation of a GRB could begin either with the merger of two compact objects or with the collapse of a massive star • As a result, an energy as high as E~1054 ergs can be released in a compact volume of space (~106 cm) • A fireball of e+/e- pairs, photons and baryons is formed and expands converting thermal energy in bulk kinetic energy carried by the baryons originally present in the explosion site (Mb) • When the thermal motion becomes sub-relativistic, the bulk Lorentz factor saturates to  =E/Mbc2 Fireball Model • If  not uniform, faster shells collide with slower ones, and internal shocks form • Part of the kinetic energy is converted in internal prompt emission • The fireball expands and collects ISM • It starts to be decelarated and an external shock forms • The inital evolution is inverted: bulk kinetic energy is converted into internal energy across the shock front • The accelerated electrons radiate via synchrotron emissionafterglow emission ~1016 cm ~1013 cm

3. The fireball model E.g. afterglow phase: emission processes, circum-burst medium (density and structure) How to derive clues on the nature of the progenitor? n < 1 cm-3 and uniform medium d I will consider the link with the GW domain n~102-104 cm-3 and evidence for a stellar wind n  r-2 ~1016 cm ~1013 cm

4. GW emission from GRB progenitors Long GRB:no in-spiral phase, only merger and ring-down Short GRB:we expect the GW signal to be emitted in 3 phases: in-spiral, merger and ring-down

5. Long GRBs: the progenitor • Collapsar model (e.g. Woosley 1998): • “GRB as the birth cry of a BH”: when the collapse of the iron core of a rotating massive progenitor proceeds directly to a BH formation, the stellar mantle falls into the newly formed BH and angular momentum slows the collapse along the equator, ultimately forming an accretion disk that, within a few seconds, launches particle jets along the rotation axis powering a GRB” • “The jets pass through the outer shells of the star and, combined with the vigorous winds of newly forged radioactive metals blowing off the disk inside, give rise to the supernova event” • Collisions among shells of the jet moving at different velocities, far from the explosion and moving close to light speed, create the GRB, which can only be seen if the jet points toward us

6. Short GRBs: the progenitor • General picture: Merger events of NS+NS or BH+NS systems widely favored: • Seems unlikely that typical energies of short GRBs set free during the dynamical merging; the following accretion phase in a postmerger system consisting of a central BH and a surrounding torus is much more promising • BH-torus system geometry: relatively baryon-poor regions along the rotational axis  thermal energy release preferentially above the BH poles via e.g.  anti-annihilation can lead to collimated, highly relativistic jets of baryonic matter if thermal energy deposition rate per unit solid angle sufficiently large. • -rays produced in internal shocks when blobs of ultra-relativistic matter in the jet collide with each other. When the jet hits the ISM, the afterglow is produced

7. GRB: progenitor models and time duration Both progenitor types result in the formation of a few solar mass BH, surrounded by a torus whose accretion can provide a sudden release of energy, sufficient to power a burst. But different natural timescales imply different burst durations: • LONG: death of massive stars  free-fall time of the material falling on the disk form outside, tff ≈ 30s(M/10M⊙) −1/2 (R/1010cm)3/2 • SHORT: coalescing compact objects  duration set by the viscous timescale of the gas accreting onto the newly-formed BH (short due to the small scale of the system)

8. GRBs: EM signal and GW GW emission EM signal Progenitor models Source position, (distance), trigger time: on-source time window Detecting GW + From the GW signal: hints on the masses involved in the process Total energy output: hints on the disk mass Nature of the progenitor

9. Choice for an on-source time window: relevant timescales • The emitted GWs are in the frequency band accessible to VIRGO & LIGO only for the last few minutes of in-spiral (e.g. Cutler & Flanagan 1994); • Simulated launch + evolution of relativistic jets driven by thermal energy deposition (e.g.  anti- annihilation) near BH accretion torus systems: after 100 ms of constant energy supply (i.e. tacc~100 ms), the fireball is at d ~3x109 cm. Standard fireball model: GRB produced at d >1013 cm  another 400 ms after the central engine supply shut off are needed to reach typical internal shock distances (Aloy et al. 2004); • But in long GRBs: delay between GWs and em signal dominated by the time necessary for the jet to push through the stellar envelope: can be of the order of 100s (Meszaros)

10. Connection between disk mass & energy output Linking E,isoof a GRB to the energy output from the central engine and the mass Macc accreted on the BH (during the phase when neutrino emission from the accreted torus is sufficiently powerful to drive the jets): chain of efficiency parameters corresponding to the different steps of physical processes between the energy release near the BH and the -ray emission at distances of ~ 1013cm: Disk mass: depends on themass ratio q  enters in the in-spiral GW signal (chirp mass) Directly measured: GRB prompt emission E,iso=f1 f2 f3 f-1 f4 Maccc2 Efficiency at which accreted rest mass energy can be converted to neutrino emission Fraction of ultra-relativistic jet energy emitted in -rays via dissipative shocks when optically thin conditions are reached:broad-band afterglow modeling + prompt emission f=2jet/4 =1-cosjet Fraction of the sky covered by the two polar jets with semi-opening angles jetand solid angles jet:afterglow light curve Conversion efficiency of  anti- to e+e- pairs Fraction of e+ - -photon fireball energy which drives the ultra-relativistic outflow with  >100

11. Torus formation in NS mergers Oechslin & Janka 2006: • q<1: less massive but slightly larger star tidally disrupted and deformed into an enlongated primary spiral arm which is mostly accreted onto the more massive companion. Its tail, however, contributes a major fraction to the subsequently forming thick disk/torus around a highly deformed and oscillating central remnant. • q~1 and ~ equally sized stars: both stars tidally stretched but not disrupted and directly plunge together into a deformed merger remnant. The disk mass is around 0.05 M for q=1 and 0.26 M for q=0.55.

12. Estimating the strains of GWs from GRB progenitor candidates M=(m1, m2,)3/5 (m1, + m2,)-1/5 =(m1, ) q-2/5/(q+1)1/5M=m1+m2 =m1/(1+q) • In-spiral:hc(f) ~1.4x10-21(d /10 Mpc)-1M 5/6 (f /100 Hz)-1/6 f  fi~ 1000 [M/2.8M ]-1 Hz • Merger:hc~ 2.7x10-22 (d /10 Mpc)-1 (4 /M)F-1/2(a) (m /0.05)1/2 fi f  fq~ 32 kHz F(a) (M/M)-1 Em= m (4/M)2 M c2 F(a)= 1- 0.63 (1-a)3/10 • Ring-down:slowly damped mode, l=m=2, peaked at fq and width f: f ~ fq/Q(a), where Q(a)=2(1-a)-9/20 hc(fq) ~2.0x10-21 (d /10 Mpc) -1 (/M ) [Q /14 F]1/2 (r /0.01)1/2 Short GRBs, optimal filtering can be applied Detectability Optimal filtering (in-spiral & ring down): 2opt=4  [hc(f) (f Sh(f))-0.5] 2 d(lnf)   (d /10 Mpc)-1M 5/6  5 Short & Long GRBs, order of magnitude estimate of hc Kobayashi & Meszaros 2003 Short & Long GRBs, but frequency too high Typical values a ~ 0.98 m~ 0.05 r~ 0.01 Kobayashi & Meszaros 2003

13. Advanced LIGO Advanced LIGO Kobayashi & Meszaros 2003 Advanced LIGO Fryer et al. 1999, Belczynski et al. 2002

14. GRB 980425 d=40 Mpc GRB 980425 d=40 Mpc GW signal amplitude and VIRGO sensitivity curve [f Sh(f)]1/2 ns-ns m1= m2 =1.4 M - - - - in-spiral -  -  -merger bh-ns m1=12 M m2 =1.4 M - - - in-spiral -  -merger ring-down Virgo Virgo Advanced Virgo Advanced Virgo 62 Mpc 53 Mpc 280 Mpc 220 Mpc 2300 Mpc 1100 Mpc bh-he m1=3 M m2 =0.4 M - - - - merger ring-down Collapsar m1= m2 =1 M - - - - merger Virgo Advanced Virgo Virgo Advanced Virgo 23 Mpc 27 Mpc 62 Mpc 110 Mpc 95 Mpc 490 Mpc

15. GRB analysis with Virgo On behalf of Virgo collaboration Virgo-Rome group: Fulvio Ricci & EGO: Elena Cuoco Vesf-EGO: fellowship support

16. Virgo C7 run & GRB 050915a GRB 050915a: good position in one of the data stretch of the run, z unknown, T90=53 s (15-350 keV), long GRB  we expect the progenitor to be a collapsar and search for a burst type event in VIRGO data Virgo sensitivity during C7 run and nominal in black; LIGO Hanford during S2 (coincidence analysis with GRB 030329) in red (2 km) and blue (4 km).

17. Definition of on-source and bkg region http://gcn.gsfc.nasa.gov/swift2005 grbs.html. No events with SNR>10 GPS 810818575 • Source region: a time window 180 s long, 2 min before the EM trigger and 1 min after. Covers most astrophysical predictions, trigger uncertainty and accounts for the favored ordering where GW precede the GRB • bkg region: a single lock stretch of data, 16500s long around the source region, used to estimate the statistical properties of the bkg and the pipeline efficiency • On both the on-source and bkg region, we run the “Wavelet Detection Filter” (WDF) by Elena Cuoco (VIR-NOT-EGO-1390-305 & VIR-NOT-EGO-1390-110), selecting all events with SNR > 4. • On-source and off-source distrib. are statistically compatible  UL analysis

18. Pipeline efficiency evaluation on bkg region: e.g. sine-gaussian signals Q=2psf0f0=characteristic frequency EGW=(hSGrss)2 c3dL222f02/[5G(1 + z)] If the GRB was optimally oriented, the UL would be a factor of ((F+)2+(Fx)2)-1~7 lower. At nominal sensitivity, the noise strain @200 Hz is a factor of 15 lower. Under these optimistic assumptions, our strain UL implies an energy UL of ~0.2 M at the distance of GRB 980425 (40 Mpc). Kobayashi and Meszaros 2003: in the case of a merger of two blobs of 1 M each, assuming that 5% of the mass energy goes in GWs, one expects EGW~0.1M.

19. Prospects WDF and population studies: • Some changes to the wavelet bases used for the filter in order to apply the search also to the in-spiral phase of short GRBs (Elena Cuoco) • Need to set-up a Virgo procedure for population studies: while the GW signals from individual GRBs may be too weak to be detected directly, the small correlations they induce in the data near the GRB trigger time may still be detectable by statistical comparison to data from times not associated with a GRB Joint LIGO-Virgo searches within the External Trigger group: • Virgo sensitivity during VSR1 improved nearly of a factor of 10 @ 200 Hz with respect to C7. LIGO-Virgo GRB searches for data collected during this period (S5-VSR1) are planned (External Trigger group activities). 50 GRB triggers received during the joint LIGO-Virgo data-taking period (May – Oct. 2007). A coincident analysis will allow us to quantify how much we gain by using more than one detector • Work in progress: definition of a common set of simulated signals to estimate pipelines efficiency and compare LIGO-Vigo results (LIGO effort) • Currently, starting to test the kind of improvement we may have on Virgo ULs by using coincidences with LIGO detectors (e.g. GRB 050520B and GRB 070729)

20. E.g.: GRB 070729 Starting to analyze 3 hrs of data around the GRB trigger time (59 windows, 180s long). A preliminary check for the maximum SNR of the Virgo triggers that survive in a 4 detector coincidence search (+/- 20 ms) gives a maximum SNR of 3.5 (to compare with SNR max = 9 during C7 run) in the 3 hrs of data. SNR distribution of triggers found in one of the 59 windows (180s) considered for bkg studies in coincidence with GRB 070729

21. … waiting for the plus and advanced configurations Virgo /LIGO (red) Virgo+/eLIGO (green, 2008-09)‏ AdvVirgo/LIGO (blue -2013-14)‏ 10 • BNS range: 121 Mpc • BBH range: 856 Mpc • 1 kHz sens.: 6 10-24

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