Late-phase hard X-ray emission from flares The prototype event (right): March 30, 1969 (Frost & Dennis, 1971), a very bright over-the-limb event with a gradual late-phase hard component H.S. Hudson and Säm Krucker UC Berkeley impulsive phase gradual burst ~20 min We can now study such events against the disk via RHESSI hard X-ray imaging spectroscopy

Properties: limb-occulted era Hard spectrum, sometimes approaching  = 2 Gradual time profile, especially at a limb- occulted view direction Low peak microwave frequency More properties: HXR imaging era More variability in events seen on the disk, including bursts Softer spectra in events seen on the disk Footpoint sources dominate (Qiu et al., 2004)

The January 2005 events The major events in January 2005 were dominated by the coronal phase Of the 5 X-class flares, the four with clear late-phase RHESSI signatures all had type IV radio emission See Saldanha et al., SH23A-0312 as well as the several graphics on the Jan event in this poster; see also Hurford et al. SH23A-0313 on the Jan. 20 event Listen to SH21A-01 (Krucker et al.) and other papers in the special Jan. 20 session This poster is mainly about Jan. 19, 2005, with general comments about the paradigm

GOES X1.5 flare on Jan 19, 2005 impulsive phase gradual HXR emission during decay RHESSI night RHESSI

X-ray imaging: impulsive phase Two-ribbon flare One HXR footpoint on each ribbon (blue) Thermal source in the corona between the ribbons (red) N15W51

A similar source geometry is observed during the gradual HXR emission: we see a similar footpoint-loop structure (Qiu, Lee, & Gary 2004; Kundu et al. 2004)

Footpoint motion Impulsive phase: Motion rather along the ribbon Northern footpoint: km/s Southern footpoint: complex Later phase: Slow motion Northern footpoint: ~2.5 km/s Southern footpoint: ?

Different directions of motion in impulsive phase and gradual phase Dashed lines give main direction of motion. Impulsive phase: Fast motion along ribbon Gradual phase: Slow motion rather perpendicular to ribbon

X-ray spectrum during decay Very hard spectrum with power law index ~2.2 break at ~40 keV above break: ~2.9 Thermal emission dominates below 15 keV thermal ~16MK background power law fit (non-thermal)

Spectral evolution Impulsive phase: hardest emission during peaks, softer during rise and decay (soft-hard-soft) Decay phase: Spectrum is getting harder in time (soft-hard-harder)

Spectral hardening E break Break energy moves to higher values Spectrum above and below break becomes harder with time

A different solar hard X-ray morphology The extended emission is a new paradigm not following the classic soft-hard-soft “Neupert effect” of the impulsive phase The phenomenon has specific hard X-ray spectral signatures plus radio type IV emission On-the-disk events show footpoint emission and differ otherwise from the classic over-the-limb events such as March 30, 1969

Example of April 21, 2002

Example of January 19, 2005 Note lack of soft-hard-soft component; each flux increase has its own spectral hardening

A different model for stellar hard X-rays? Two solar morphologies The new data clarify the distinction between the usual solar hard X-ray emission morphology (soft-hard- soft acceleration; Neupert effect etc) which dominates during the impulsive phase. We now clearly see a different acceleration morphology in the late phase, as presaged by the over-the-limb events (cf. Cliver et al 1986; Kiplinger 1995, Qiu et al. 2004). This morphology competes with the impulsive-phase morphology and rarely dominates, even in major events, until the late phases of some flares. These two hard X-ray morphologies clearly represent different physical processses, even though some aspects (existence of loops and footpoint sources) are similar. Possible stellar interpretation Stellar X-rays are observed from stars of many spectral types, including the T Tauri stages early in the life of a star. We cannot readily expect to detect a hard X- ray component of the usual impulsive-phase type, based upon scaling to the solar case. However we can speculate that this “new” solar paradigm might be at work in these stars, which have very different physical conditions. If so it might be possible to detect (and interpret) stellar hard X-rays in this manner.

Summary There are great RHESSI observations of late- phase HXR emission The emission is from footpoints, NOT from trapped electrons in corona RHESSI gives detailed observations of spectral hardening The footpoint sources have slow (few km/s) footpoint motions, in different directions compared to the impulsive phase