# Gamma-ray Bursts: 50 Years and Counting!
## 5 The HETE-2 Era (2000-2004)  
After the first High Energy Transient Explorer 1 (HETE-1) satellite was lost during its launch on 4 November 1996, a second mission was designed, and HETE-2 was then successfully launched on 9 October 2000. HETE-2 was a NASA science mission with international collaboration (mainly with Japan and France), with the main objective of making multiwavelength observations of GRB counterparts for the first time [59]. The satellite carried on board three main instruments: the French Gamma Telescope (FREGATE), the Wide-Field X-Ray Monitor (WXM), and the Soft X-ray Camera (SXC). FREGATE was composed of wide-field (\(\sim\)\(\pi\) FoV) gamma-ray spectrometers sensitive between 6 and 400 keV, designed to conduct spectroscopy of GRBs thanks to its high spectral and timing resolutions. The WXM consisted of a pair of orthogonal, one-dimensional X-ray detectors sensitive in an energy range of 2 to 25 keV, allowing excellent localizations of \(\sim\)10 arcminutes. The SXC consisted of two orthogonal sets of one-dimensional coded-aperture X-ray imagers sensitive in the 0.5-14 keV energy range. These instruments had an identical field of view of 1.5 steradians and were able to communicate with each other in order to coordinate observations.  
On 29 March 2003, HETE observed three different GRBs. The first of them, GRB030329, was fundamental to our understanding of GRBs by definitively confirming the association between long GRBs and supernovae [60; 61]. GRB030329's distance from Earth allowed its afterglow to be studied extensively. On 6 April 2003, a spectroscopic analysis of the burst's optical afterglow revealed discernible peaks at approximately 570 nm and 470 nm. This spectral profile was effectively reproduced through a composite model, integrating a power-law distribution with the spectral characteristics derived from SN1998 bw. Noteworthy was the sustained evolution of supernova-like features during the subsequent weeks following the initial burst. Optical observations conducted at the Kitt Peak National Observatory indicated a luminosity in the burst's optical afterglow that surpassed predictions based on a power-law decay. This observed departure from the expected decay pattern could be rationalized by postulating the influence of additional luminosity originating from a concurrent supernova event (later named SN2003 dh) [62].  
A further fundamental contribution of HETE was the discovery of X-ray Flashes (XRFs). XRFs are intense and transient cosmic events characterized by a rapid release of X-ray radiation. Although very similar to GRBs, the distinction between them lies in the energy distribution of the emitted radiation. Originally defined operationally as those X-ray signals detected by BeppoSAX's WFCs (energies between 2 and 25 keV), which were not triggered and not detected by the GRBM (in the energy range 40-700 keV) [63], they are now considered a subclass of gamma-ray bursts (GRBs), sharing similarities with traditional GRBs but with a spectral peak in the X-ray range. Even though the nature of XRFs is largely unknown, the most credited model posits that XRFs could be given by off-axis GRB emission: it is thus postulated that gamma rays are indeed emitted, but their trajectories are directed away from our instruments [64]. Consequently, the initially observable phenomenon is confined to lower-energy X-rays emitted in a divergent beam, exhibiting greater dispersion compared to the more narrowly focused gamma-ray beam.  
This era can be summarized as follows:  
* The common spectral properties of SN1998 bw and SN2003 dh provided a \"smoking gun\" for their common origin, proving once and for all the association between long-duration GRBs and supernovae.
* Discovery of X-ray Flashes (XRFs), a subpopulation of GRBs.
* Together with BeppoSAX, HETE-2 enabled accurate localization (of the order of arcminutes) that kick-started the era of multiwavelength observations of GRBs before the advent of _Swift_.