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Fluorescence line diagnostics of the Fe K{\alpha} line at $\sim 6.4$ keV observed in both solar and stellar flares can constrain the latitude and size of the flare loop, even in the absence of imaging observations. However, they are…
Solar flares result in an increase of the solar irradiance at all wavelengths. While the distribution of the flare fluence observed in coronal emission has been widely studied and found to scale as f(E) ~ E^{-\alpha}, with \alpha slightly…
For the first time, we have used RHESSI's spatial and energy resolution to determine the combined chromospheric and coronal density profile of the flaring solar atmosphere in a statistical manner, using a dataset of 838 flares observable in…
Stellar flares affect all atmospheric layers from the photosphere over chromosphere and transition region up into the corona. Simultaneous observations in different spectral bands allow to obtain a comprehensive picture of the environmental…
Fe K$\alpha$ line emission from Galactic center molecular clouds can be produced either via fluorescence after illumination by an X-ray source or by cosmic ray ionization. Unparalleled high-resolution X-ray spectroscopy obtained by…
We report the discovery of a flaring X-ray source with an optical counterpart with Halpha emission and red-excess, in the direction of the SMC. A 100 ksec X-ray observation with Chandra detected a flare lasting 6 ksec in the source CXO…
Aims. The H-alpha emission of solar flare kernels and associated hard X-ray (HXR) emission often show similar time variations but their light curves are shifted in time by energy transfer mechanisms. We searched for fast radiative response…
The X-class solar flare of October 23, 2012, generated continuum photospheric emission and a strong helioseismic wave ("sunquake") that points to an intensive energy release in the dense part of the solar atmosphere. We study properties of…
We present results from two Chandra imaging observations of Ross 154, a nearby flaring M dwarf star. During a 61-ks ACIS-S exposure, a very large flare occurred (the equivalent of a solar X3400 event, with L_X = 1.8x10^30 ergs/s) in which…
Solar white-light flares are characterized by an enhancement in the optical continuum, which are usually large flares (say X- and M-class flares). Here we report a small C2.3 white-light flare (SOL2022-12-20T04:10) observed by the…
The origin of the near-ultraviolet and optical continuum radiation in flares is critical for understanding particle acceleration and impulsive heating in stellar atmospheres. Radiative-hydrodynamic simulations in 1D have shown that high…
Between 2017 and 2024, the Helioseismic and Magnetic Imager (HMI) onboard the Solar Dynamics Observatory has observed numerous white-light solar flares (WLFs). HMI spectro-polarimetric observations of certain WLFs, in particular the X9.3…
Context: Solar flare hard X-rays (HXRs) are thought to be produced by nonthermal coronal electrons stopping in the chromosphere, or remaining trapped in the corona. The collisional thick target model (CTTM) predicts that sources produced by…
X-ray line fluorescence is ubiquitous around powerful accretion sources, namely active galactic nuclei and X-ray binaries. The brightest and best-studied line is the Fe K$\alpha$ line at $\lambda = 1.937$\AA (6.4\,keV). This paper presents…
We present a first emission line analysis of a high resolution X-ray spectrum of the stellar wind of theta 1 Ori C obtained with the High Energy Transmission grating Spectrometer onboard the Chandra X-ray Observatory. The spectra are…
Red-shifted components of chromospheric emission lines in the hard X-ray impulsive phase of solar flares have recently been studied through their 30 s evolution with the high resolution of IRIS. Radiative-hydrodynamic flare models show that…
(Abridged abstract) We present an analysis of the X-ray emission of the rapidly rotating giant star YY Mensae observed by Chandra HETGS and XMM-Newton. Although no obvious flare was detected, the X-ray luminosity changed by a factor of two…
Starting in 2012, we began an unprecedented observational program focused on the supermassive black hole in the center of our Galaxy, Sgr A*, utilizing the High Energy Transmission Gratings Spectrometer (HETGS) instrument on the Chandra…
On two occasions, we obtained nearly simultaneous ~ 4 kilosecond snapshot observations of the Galactic black hole and microquasar XTE J1550-564 with Chandra and RXTE near the peak of its May, 2000 outburst. The low-energy sensitivity of…
The first simultaneous observations of the Fe XVIII 974.86 {\AA} and Fe XX 721.56 {\AA} forbidden lines from the Spectral Imaging of the Coronal Environment (SPICE) spectrograph on Solar Orbiter are presented. The lines were observed from…