# Presentation: 115242

## Short transients detected in WFC (2–25 keV) with little/no signal in GRBM (40–700 keV) and no BATSE (>20 keV) trigger

17 events detected in 3.8 years of observations (cf. 36 GRBs)

- Short transients detected in WFC (2–25 keV) with little/no signal in GRBM (40–700 keV) and no BATSE (>20 keV) trigger
- 17 events detected in 3.8 years of observations (cf. 36 GRBs)

- Use BATSE continuous data (>20 keV) to search for (*untriggered*) -ray emission
- Characterize -ray properties and wide-band spectra
- Compare to the observed properties of GRBs

**X-ray Flashes and Gamma-ray Bursts**

**R. M. Kippen **(LANL)**, P. M. Woods **(USRA/NSSTC)**,**
**J. Heise **(SRON)**, J. J. M. in ’t Zand **(SRON)**, **
**R. D. Preece **(UAH/NSSTC), and **M. S. Briggs **(UAH/NSSTC)

## 10 unocculted events observed with BATSE
9 events detected (<300 keV) by combining detectors

~Isotropic sky Dist.
Qualitatively similar to weak GRBs

- 9 events detected (<300 keV) by combining detectors
- ~Isotropic sky Dist.
- Qualitatively similar to weak GRBs

- Search for -ray Emission

## Standard Burst Parameters

- Apply Std. BATSE burst analysis to flashes
- Compare to BATSE GRB catalog (1973 bursts w/flux, flu, dur)
- Flashes consistent with weak, long-duration GRBs
- How do detailed spectra compare?

## WFC/BATSE Joint Spectral Analysis

- Time averaged flash spectra
- Good agreement between WFC & BATSE around 20 keV
- 8 events have significant curvature (compared to single power law fit)
*E*peak and ** are well constrained
- Band/Comp models generally consistent

## Flashes vs. Bright GRBs

- Compare to Preece *et al.* results for 156 bright GRB (BATSE-only)
- Also compare 18 WFC-selected GRBs (BATSE-only spectra)
- WFC-selected GRBs consistent with bright GRBs
- Flashes have consistent ** & **
*E*peak significantly different (lower)

## Slide 6

## Flashes vs. Dim GRBs

- Compare “Comptonized” joint fit results to Mallozzi *et al.* [2000] _BATSE_ GRB spectral catalog (1023 bursts)
** consistent with long GRBs
*E*peak *roughly* follows hardness intensity trend of long GRBs

## Flashes vs. Dimmest GRBs

- 400 bursts per year (full-sky) with similar hardness, duration, and intensity = 40% of all _long_ bursts
- Consistent with WFC detection rate of FXTs = 32% of all SAX bursts

- Flashes vs. *Dimmest* GRBs

- Compare to BATSE 5B catalog Briggs *et al.* [2002] (2704 triggers)
- Characterize spectra with broad-band hardness ratios
- Flashes similar in hardness to bursts of same intensity

## Conclusions

- X-ray flashes similar to GRBs except that they emit most power in X-rays
- Joint WFC+BATSE spectral fits reveal that flashes have significantly curved spectra — similar to GRBs
- Spectra of X-ray flashes are roughly consistent with the long GRB *E*peak vs. intensity trend
- Could represent a large extension of the full GRB population

- _Caveats:_
- Small sample size
- Subject to selection and analysis biases
- _Future:_
- Afterglow counterparts (so far elusive) will conclusively show how flashes are related to GRB
- BeppoSAX & HETE continue to routinely observe flashes, but lack sensitive -ray data
- Swift (5–150 keV) should provide a large sample for wide-band spectral comparisons

## BATSE Search Results (20–300 keV)

- 10 unocculted events with BATSE continuous data
- 9 detections (> 5 sigma in select, summed detectors)
- No detections above 300 keV

## Spectral Evolution

## Spectral Evolution