Papers by Paola Lucia Testa
The Astrophysical Journal, 2011
... Juan Martínez-Sykora1,2, Bart De Pontieu1, Paola Testa3 and Viggo Hansteen2. 1 Lockheed Marti... more ... Juan Martínez-Sykora1,2, Bart De Pontieu1, Paola Testa3 and Viggo Hansteen2. 1 Lockheed Martin Solar and Astrophysics Laboratory, Palo Alto, CA 94304, USA; jm ... In Section 2, we describe the Bifrost code used for simulating the solar corona and the setup of the simulation. ...
The Astrophysical Journal, 2009
Nanoflares, short and intense heat pulses within spatially unresolved magnetic strands, are now c... more 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 linesof-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 -2isothermal 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.
The Astrophysical Journal, 2012
Determining the temperature distribution of coronal plasmas can provide stringent constraints on ... more Determining the temperature distribution of coronal plasmas can provide stringent constraints on coronal heating. Current observations with the Extreme ultraviolet Imaging Spectrograph (EIS) on board Hinode and the Atmospheric Imaging Assembly (AIA) on board the Solar Dynamics Observatory provide diagnostics of the emission measure distribution (EMD) of the coronal plasma. Here we test the reliability of temperature diagnostics using three-dimensional radiative MHD simulations. We produce synthetic observables from the models and apply the Monte Carlo Markov chain EMD diagnostic. By comparing the derived EMDs with the "true" distributions from the model, we assess the limitations of the diagnostics as a function of the plasma parameters and the signal-to-noise ratio of the data. We find that EMDs derived from EIS synthetic data reproduce some general characteristics of the true distributions, but usually show differences from the true EMDs that are much larger than the estimated uncertainties suggest, especially when structures with significantly different density overlap along the line of sight. When using AIA synthetic data the derived EMDs reproduce the true EMDs much less accurately, especially for broad EMDs. The differences between the two instruments are due to the: (1) smaller number of constraints provided by AIA data and (2) broad temperature response function of the AIA channels which provide looser constraints to the temperature distribution. Our results suggest that EMDs derived from current observatories may often show significant discrepancies from the true EMDs, rendering their interpretation fraught with uncertainty. These inherent limitations to the method should be carefully considered when using these distributions to constrain coronal heating.
The Astrophysical Journal, 2004
An analysis of Lyman series lines arising from hydrogen-like oxygen and neon ions in the coronae ... more An analysis of Lyman series lines arising from hydrogen-like oxygen and neon ions in the coronae of the active RS CVn-type binaries II Peg and IM Peg, observed using the Chandra High Resolution Transmission Grating Spectrograph, shows significant decrements in the Lyα/Lyβ ratios as compared with theoretical predictions and with the same ratios observed in similar active binaries. We interpret these decrements in terms of resonance scattering of line photons out of the line-of-sight; these observations present the first strong evidence for this effect in active stellar coronae. The net line photon loss implies a non-uniform and asymmetric surface distribution of emitting structures on these stars. Escape probability arguments, together with the observed line ratios and estimates of the emitting plasma density, imply typical line-of-sight sizes of the coronal structures that dominate the X-ray emission of 10 10 cm at temperatures of 3 × 10 6 K and of 10 8 cm at 10 7 K. These sizes are an order of magnitude larger than predicted by simple quasi-static coronal loops models, but are still very small compared to the several 10 11 cm radii of the underlying stars.
The Astrophysical Journal, 2006
Massive stars rarely show intrinsic X-ray variability. The only O-stars credited to be intrinsica... more Massive stars rarely show intrinsic X-ray variability. The only O-stars credited to be intrinsically variable are θ1 Ori C due to effects from magnetic confinement of its wind, and θ2 Ori A suspected of similar activity. Early Chandra observations have shown that the most massive star system in the Orion Trapezium Cluster, θ2 Ori A, shows rapid variability on time scales of hours. We determine X-ray fluxes from observations with Chandra and find that the star shows very strong variability over the last 5 years. We observed a second large outburst of the X-ray source in November 2004 with the high resolution transmission grating spectrometer on-board Chandra and compare the emissivity and line properties in states of low and high flux. In the low state X-ray emissivities indicate temperatures well above 25 MK. In the high state we fi nd an extended emissivity distribution with high emissivities in the range from 3 MK to over 100 MK. The outburst event in stellar terms is one of the most powerful ever observed and the most energetic one in the ONC with a lower total energy limit of 1.5×1037 ergs. The line diagnostics show that under the assumption that the He-like ions are photoexcited the line emitting regions in the low states are as close as within 1 - 2 stellar radii from the O- star&#x27;s photosphere, whereas the hard states suggest a distance of 3 - 5 stellar radii. We discuss the results in the context of stellar flares, magnetic confinement, and binary interactions. By matching the dates of all observati ons with the orbital phases of the spectroscopy binary orbit we find that the two outbursts are very close to the peria stron passage of the stars. We argue that the high X- ray states are possibly the result of reconnection events fr om magnetic interactions of the primary and secondary stars of the spectroscopic binary. Effects from wind collis ions seem unlikely for this system. The low state emissivity and R-ratios strengthen the predicament that the X-ray emission is enhanced by magnetic confinement of the primary wind. We also detect Fe fluorescence indicativ e of the existence of substantial amounts of neutral Fe in the vicinity of the X-ray emission.
The Astrophysical Journal, 2006
Short-period binaries represent extreme cases in the generation of stellar coronae via a rotation... more Short-period binaries represent extreme cases in the generation of stellar coronae via a rotational dynamo. Such stars are important for probing the origin and nature of coronae in the regimes of rapid rotation and activity saturation. VW Cep (P = 0.28 d) is a relatively bright, partially eclipsing, and very active object. Light curves made from Chandra/HETGS data show flaring and rotational modulation, but no eclipses. Velocity modulation of emission lines indicates that one component dominates the X-ray emission. The emission measure is highly structured, having three peaks. Helium-like triplet lines give electron densities of about 3 − 18 × 10 10 cm −3 . We conclude that the corona is predominantly on the polar regions of the primary star and compact.
The Astrophysical Journal, 2009
We compare observations of the non-flaring solar corona made simultaneously with Hinode/XRT and w... more We compare observations of the non-flaring solar corona made simultaneously with Hinode/XRT and with RHESSI. The analyzed corona is dominated by a single active region on 12 November 2006. The comparison is made on emission measures. We derive emission measure distributions vs temperature of the entire active region from multifilter XRT data. We check the compatibility with the total emission measure values estimated from the flux measured with RHESSI
The Astrophysical Journal, 2009
... GEMMA DR ATTRILL, ALEXANDER J. ENGELL, MEREDITH J. WILLS-DAVEY, PAOLO GRIGIS, AND PAOLA TESTA... more ... GEMMA DR ATTRILL, ALEXANDER J. ENGELL, MEREDITH J. WILLS-DAVEY, PAOLO GRIGIS, AND PAOLA TESTA Harvard-Smithsonian Center for Astrophysics, 60 ... 2005): in ∼7% of EIT coronal wave events, the bright front appears as a well-defined, sharp feature, known as ...
The Astrophysical Journal, 2007
We have investigated the optical thickness of the coronal plasma through the analysis of high-res... more We have investigated the optical thickness of the coronal plasma through the analysis of high-resolution X-ray spectra of a large sample of active stars observed with the High Energy Transmission Grating Spectrometer on Chandra.
The Astrophysical Journal, 2005
Recent analysis of high resolution Chandra X-ray spectra has shown that the Ne/O abundance ratio ... more Recent analysis of high resolution Chandra X-ray spectra has shown that the Ne/O abundance ratio is remarkably constant in stellar coronae. Based on this result, we point out the utility of the Ne/O ratio as a discriminant for accretionrelated X-rays from T Tauri stars, and for probing the measure of grain-depletion of the accreting material in the inner disk. We apply the Ne/O diagnostic to the classical T Tauri stars BP Tau and TW Hya-the two stars found to date whose X-ray emission appears to originate, at least in part, from accretion activity. We show that TW Hya appears to be accreting material which is significantly depleted in O relative to Ne. In constrast, BP Tau has an Ne/O abundance ratio consistent with that observed for post-T Tauri stars. We interpret this result in terms of the different ages and evolutionary states of the circumstellar disks of these stars. In the young BP Tau disk (age ∼ 0.6 Myr) dust is still present near the disk corotation radius and can be ionized and accreted, re-releasing elements depleted onto grains. In the more evolved TW Hya disk (age ∼ 10 Myr), evidence points to ongoing coagulation of grains into much larger bodies, and possibly planets, that can resist the drag of inward-migrating gas, and accreting gas is consequently depleted of grain-forming elements.
The Astrophysical Journal, 2009
θ 1 Ori E is a young, moderate mass binary system, a rarely observed case of spectral-type G-gian... more θ 1 Ori E is a young, moderate mass binary system, a rarely observed case of spectral-type G-giants of about 3 Solar masses, which are still collapsing towards the main sequence, where they presumably become X-ray faint. We have obtained high resolution X-ray spectra with Chandra and find that the system is very active and similar to coronal sources, having emission typical of magnetically confined plasma: a broad temperature distribution with a hot component and significant high energy continuum; narrow emission lines from H-and He-like ions, as well as a range of Fe ions, and relative luminosity, L x /L bol = 10 −3 , at the saturation limit. Density, while poorly constrained, is consistent with the low density limits, our upper limits being n e < 10 13 cm −3 for Mg xi and n e < 10 12 cm −3 for Ne ix. Coronal elemental abundances are sub-Solar, with Ne being the highest at about 0.4 times Solar. We find a possible trend in Trapezium hot plasmas towards low relative abundances of Fe, O, and Ne, which is hard to explain in terms of the dust depletion scenarios of low-mass young stars.
The Astrophysical Journal, 2007
We analyze a 96 ks Chandra hetgs observation of the single G-type giant HR 9024. The high flux al... more We analyze a 96 ks Chandra hetgs observation of the single G-type giant HR 9024. The high flux allows us to examine spectral line and continuum diagnostics at high temporal resolution, to derive plasma parameters. A time-dependent 1D hydrodynamic model of a loop with half-length L = 5 × 10 11 cm (∼ R ⋆ /2), cross-section radius r = 4.3 × 10 10 cm, with a heat pulse of 15 ks and 2×10 11 erg cm −2 s −1 deposited at the loop footpoints, satisfactorily reproduces the observed evolution of temperature and emission measure, derived from the analysis of the strong continuum emission. For the first time we can compare predictions from the hydrodynamic model with single spectral features, other than with global spectral properties. We find that the model closely matches the observed line emission, especially for the hot (∼ 10 8 K) plasma emission of the Fe xxv complex at ∼ 1.85Å.
The Astrophysical Journal, 2005
This work is prompted by evidence of sharply peaked emission measure distributions in active star... more This work is prompted by evidence of sharply peaked emission measure distributions in active stars and by the claims of isothermal loops in solar coronal observations, at variance with the predictions of hydrostatic loop models with constant cross section and uniform heating. We address the problem with loops heated at the footpoints. Since steady heating does not allow static loop
The Astrophysical Journal, 2008
The observation by the Swift X-ray Telescope of the Fe Kα 1 , α 2 doublet during a large flare on... more The observation by the Swift X-ray Telescope of the Fe Kα 1 , α 2 doublet during a large flare on the RS CVn binary system II Peg represents one of only two firm detections to date of photospheric Fe Kα from a star other than our Sun. We present models of the Fe Kα equivalent widths reported in the literature for the II Peg observations and show that they are most probably due to fluorescence following inner shell photoionisation of quasi-neutral Fe by the flare X-rays. Our models constrain the maximum height of flare the to 0.15 R * assuming solar abundances for the photospheric material, and 0.1 R * and 0.06 R * assuming depleted photospheric abundances ([M/H] = -0.2 and [M/H] = -0.4, respectively). Accounting for an extended loop geometry has the effect of increasing the estimated flare heights by a factor of ∼3. These predictions are consistent with those derived using results of flaring loop models, which are also used to estimate the flaring loop properties and energetics. From loop models we estimate a flare loop height of 0.13 R * , plasma density of ∼ 4×10 12 cm −3 and emitting volume of ∼ 6 × 10 30 cm 3 . Our estimates for the flare dimensions and density allow us to estimate the conductive energy losses to E cond ≤ 2 × 10 36 erg, consistent with upper limits previously obtained in the literature. Finally, we estimate the average energy output of this large flare to be ∼ 10 33 erg sec −1 , or 1/10th of the stellar bolometric luminosity.
The Astrophysical Journal, 2006
Using the Chandra/High Energy Transmission Grating Spectrometer (hereafter HETGS) we have detecte... more Using the Chandra/High Energy Transmission Grating Spectrometer (hereafter HETGS) we have detected Doppler motion of Capella's X-ray emission lines in the 6 -25Å wave-band. The observed motion follows the expected orbital motion of Capella's primary. This finding implies that the primary G8 III star, not the secondary G1 III star in the Hertzsprung gap, has been the dominant source of hot 10 6.8 -10 7 K plasma at least in the last six years. In addition, the results demonstrate the long-term stability of the HETGS and demonstrate small uncertainties of 25 and 33 km s −1 in the velocity determination with the HEG and MEG, respectively.
The Astrophysical Journal, 2006
... Gagné, M., Oksala, ME, Cohen, DH, Tonnesen, SK, ud-Doula, A., Owocki, SP, Townsend, RHD, &... more ... Gagné, M., Oksala, ME, Cohen, DH, Tonnesen, SK, ud-Doula, A., Owocki, SP, Townsend, RHD, & MacFarlane, JJ 2005, ApJ, 628, 986 First ... 2006, MNRAS, 371, 252 First citation in article | CrossRef link | Abstract at Astrophysics Data System; Lucy, LB 1982, ApJ, 255, 286 First ...
The Astrophysical Journal, 2004
We have analyzed high-resolution X-ray spectra of a sample of 22 active stars observed with the H... more We have analyzed high-resolution X-ray spectra of a sample of 22 active stars observed with the High Energy Transmission Grating Spectrometer on Chandra in order to investigate their coronal plasma density. Densities where investigated using the lines of the He-like ions O VII, Mg XI, and Si XIII. Si XIII lines in all stars of the sample are compatible with the low-density limit (i.e. n e 10 13 cm −3 ), casting some doubt on results based on lower resolution EUVE spectra finding densities n e > 10 13 cm −3 . Mg XI lines betray the presence of high plasma densities up to a few 10 12 cm −3 for most of the sources with higher X-ray luminosity ( 10 30 erg/s); stars with higher L X and L X /L bol tend to have higher densities at high temperatures. Ratios of O VII lines yield much lower densities of a few 10 10 cm −3 , indicating that the "hot" and "cool" plasma resides in physically different structures. In the cases of EV Lac, HD 223460, Canopus, µ Vel, TY Pyx, and IM Peg, our results represent the first spectroscopic estimates of coronal density. No trends in density-sensitive line ratios with stellar parameters effective temperature and surface gravity were found, indicating that plasma densities are remarkably similar for stars with pressure scale heights differing by up to 3 orders of magnitude. Our findings imply remarkably compact coronal structures, especially for the hotter (∼ 7 MK) plasma emitting the Mg XI lines characterized by coronal surface filling factor, f MgXI , ranging from 10 −4 to 10 −1 , while we find f OVII values from a few 10 −3 up to ∼ 1 for the cooler (∼ 2 MK) plasma emitting the O VII lines. We find that f OVII approaches unity at the same stellar surface X-ray flux level as solar active regions, suggesting that these stars become completely covered by active regions. At the same surface flux level, f MgXI is seen to increase more sharply with increasing surface flux. These results 1 SAO Predoctoral Fellow 2 Smithsonian Astrophysical Observatory, MS 3,
The Astrophysical Journal, 2012
ABSTRACT
The Astrophysical Journal, 2011
We analyze coordinated Hinode XRT and EIS observations of a non-flaring active region to investig... more We analyze coordinated Hinode XRT and EIS observations of a non-flaring active region to investigate the thermal properties of coronal plasma taking advantage of the complementary diagnostics provided by the two instruments. In particular we want to explore the presence of hot plasma in non-flaring regions. Independent temperature analyses from the XRT multi-filter dataset, and the EIS spectra, including the instrument entire wavelength range, provide a cross-check of the different temperature diagnostics techniques applicable to broad-band and spectral data respectively, and insights into cross-calibration of the two instruments.
The Astrophysical Journal, 2010
We study the spectral and temporal behavior of X-ray flares from the active M-dwarf EV Lac in 200... more We study the spectral and temporal behavior of X-ray flares from the active M-dwarf EV Lac in 200 ks of exposure with the Chandra/HETGS. We derive flare parameters by fitting an empirical function which characterizes the amplitude, shape, and scale. The flares range from very short (< 1 ks) to long (∼ 10 4 s) duration events with a range of shapes and amplitudes for all durations. We extract spectra for composite flares to study their mean evolution and to compare flares of different lengths. Evolution of spectral features in the density-temperature plane shows probable sustained heating. The short flares are significantly hotter than the longer flares. We determined an upper limit to the Fe K fluorescent flux, the best fit value being close to what is expected for compact loops.
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Papers by Paola Lucia Testa