Global characteristics of GRBs observed with INTEGRAL and the inferred large population of low-luminosity GRBs

• The g-ray instruments on board INTEGRAL have detected and localised 55 GRBs from launch in October 2002 up to July 2008, including 53 long-duration GRBs (T90 ≳ 2 s) and 2 short-duration GRBs (T90 ≲ 2 s). The spectra of the majority of INTEGRAL GRBs can be well described by single power-laws. In 11 cases, models with curvature, such as the Band model or the power law plus blackbody model, are required to fit the time-averaged burst spectra. INTEGRAL detects proportionally more weak GRBs than Swift because of its higher sensitivity in a smaller field of view. The all-sky rate of GRBs above ∼0.15phcm∼2 s-1 is ∼1400yr-1 in the fully coded field of view of IBIS. Spectral lags i.e. the time delay in the arrival of low-energy g-rays with respect to high-energy g-rays, can be measured for 31 of the GRBs in the sample. Two groups are identified in the spectral lag distribution of INTEGRAL GRBs, one with short lags <0.75 s (between 25-50 keV and 50-300 keV) and one with long lags > 0.75 s. Most of the long-lag GRBs are inferred to have low redshifts because of their tendency to have low peak energies and their faint optical and X-ray afterglows. They are mainly observed in the direction of the supergalactic plane with a quadrupole moment of Q = -0.225 ± 0.090 and hence reflect the local large-scale structure of the Universe. The rate of long-lag GRBs with inferred low luminosity is ∼25% of Type Ib/c SNe. Some of these bursts could be produced by the collapse of a massive star without a SN. Alternatively, they could result from a different progenitor, such as the merger of two white dwarfs or a white dwarf with a neutron star or black hole, possibly in the cluster environment without a host galaxy.

Ämnesord

NATURVETENSKAP  -- Fysik -- Astronomi, astrofysik och kosmologi (hsv//swe)

NATURAL SCIENCES  -- Physical Sciences -- Astronomy, Astrophysics and Cosmology (hsv//eng)

Nyckelord

Cluster environments

Field of views

Large population

Large scale structure of the Universe

Massive stars

Quadrupole moments

Spectral lags

X-ray afterglow

Luminance

Stars

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