[Loops] New paper on evidence of hot plasma in non-flaring active region loops

[Reale Fabio]

Dear all,
    please find on Astro-PH at the following link:

http://arxiv.org/abs/0904.0878

a new paper that supports the detection of widespread hot plasma in 
non-flaring active regions from Hinode/XRT observations (see the 
abstract below). The paper has just been accepted for publication on 
ApJ. We have devoted an Appendix to the issue of filter calibration.
Thank you for your attention
Best regards
Fabio Reale

Evidence of widespread hot plasma in a non-flaring coronal active region 
from Hinode/XRT
Authors:   Fabio Reale, Paola Testa, James A. Klimchuk, Susanna Parenti

    Abstract: Nanoflares, short and intense heat pulses within spatially
    unresolved magnetic strands, are now considered a leading candidate
    to solve the coronal heating problem. However, the frequent
    occurrence of nanoflares requires that flare-hot plasma be present
    in the corona at all times. Its detection has proved elusive until
    now, in part because the intensities are predicted to be very faint.
    Here we report on the analysis of an active region observed with
    five filters by Hinode/XRT in November 2006. We have used the filter
    ratio method to derive maps of temperature and emission measure both
    in soft and hard ratios. These maps are approximate in that the
    plasma is assumed to be isothermal along each line-of-sight.
    Nonetheless, the hardest available ratio reveals the clear presence
    of plasma around 10 MK. To obtain more detailed information about
    the plasma properties, we have performed Monte Carlo simulations
    assuming a variety of non-isothermal emission measure distributions
    along the lines-of-sight. We find that the observed filter ratios
    imply bi-modal distributions consisting of a strong cool (log T ~
    6.3-6.5) component and a weaker (few percent) and hotter (6.6 < log
    T < 7.2) component. The data are consistent with bi-modal
    distributions along all lines of sight, i.e., throughout the active
    region. We also find that the isothermal temperature inferred from a
    filter ratio depends sensitively on the precise temperature of the
    cool component. A slight shift of this component can cause the hot
    component to be obscured in a hard ratio measurement. Consequently,
    temperature maps made in hard and soft ratios tend to be
    anti-correlated. We conclude that this observation supports the
    presence of widespread nanoflaring activity in the active region.