Investigating the nature of the INTEGRAL gamma-ray bursts and sub-threshold triggers with Swift follow-up
Götz, D.; Mereghetti, S.; Gehrels, N.; O'Brien, P. T.; Bird, A. J.; Maccarone, T.; Rowlinson, A.; Wiersema, K.; Osborne, J. P.; Starling, R. L. C.; Tanvir, N. R.; Higgins, A. B.
United Kingdom, France, Italy, United States, Netherlands
Abstract
We explore the potential of the INTErnational Gamma-Ray Astrophysics Laboratory (INTEGRAL) to improve our understanding of the low-fluence regime for explosive transients, such as Gamma-ray Bursts (GRBs). We probe the nature of the so-called 'WEAK' INTEGRAL triggers, when the gamma-ray instruments record intensity spikes that are below the usual STRONG significance thresholds. In a targeted Swift follow-up campaign, we observed 15 WEAK triggers. We find six of these can be classified as GRBs. This includes GRB 150305A, a GRB discovered from our campaign alone. We also identified a source coincident with one trigger, IGRW 151019, as a candidate active galactic nucleus. We show that real events such as GRBs exist within the INTEGRAL Burst Alert System (IBAS) WEAK trigger population. A comparison of the fluence distributions of the full INTEGRAL IBAS and Swift-BAT GRB samples showed that the two are similar. We also find correlations between the prompt gamma-ray and X-ray properties of the two samples, supporting previous investigations. We find that both satellites reach similar, low fluence levels regularly, although Swift is more sensitive to short, low-fluence GRBs.