Flare-like Events Research Articles

Steadiness of Coronal Heating

The EUI instrument on the Solar Orbiter spacecraft has obtained the most stable, high-resolution images of the solar corona from its orbit with a perihelion near 0.4 au. A sequence of 360 images obtained at 17.1 nm, between 2022 October 25 19:00 and 19:30 UT, is scrutinized. One image pixel corresponds to 148 km at the solar surface. The widely held belief that the outer atmosphere of the Sun is in a continuous state of magnetic turmoil is pitted against the EUI data. The observed plasma variations appear to fall into two classes. By far the dominant behavior is a very low amplitude variation in brightness (1%) in the coronal loops, with larger variations in some footpoint regions. No hints of observable changes in magnetic topology are associated with such small variations. The larger-amplitude, more rapid, rarer, and less well organized changes are associated with flux emergence. It is suggested therefore that while magnetic reconnection drives the latter, most of the active corona is heated with no evidence of a role for large-scale (observable) reconnection. Since most coronal emission-line widths are subsonic, the bulk of coronal heating, if driven by reconnection, can only be of tangentially discontinuous magnetic fields, with angles below about 0.5c S /c A ∼ 0.3β, with β the plasma beta parameter (∼0.01) and c S and c A the sound and Alfvén speeds, respectively. If heated by multiple small flare-like events, then these must be ≲1021 erg, i.e., picoflares. But processes other than reconnection have yet to be ruled out, such as viscous dissipation, which may contribute to the steady heating of coronal loops over active regions.

The multiwavelength afterglow emission of magnetar giant flare-like event GRB 200415A

ABSTRACT Giant flares are the brightest and rarest outbursts from magnetars, with isotropic energies of 1044–1046 erg. GRB 200415A is suggested as a magnetar giant flare from NGC 253. Fermi Large Area Telescope detected the GeV afterglow emission from the flare, which is the first time that the GeV emission is detected from a giant flare. In this paper, we study the multiwavelength afterglow radiation of electrons accelerated by the forward and reverse shocks, produced via interactions between the ejecta and the circumburst medium in the afterglow phase of GRB 200415A. We found that in the GeV band, the forward shock emission is usually higher than the reverse shock emission, and can explain observations well, while in the optical and infrared bands, contributions from the forward and reverse shocks can be comparable for reasonable parameter sets, and the brightness of the forward-shock emission can reach 20 mag (AB). We predict that future telescopes such as Wide Field Survey Telescope, Chinese Space Station Telescope, the Large Synoptic Survey Telescope, and James Webb Space Telescope can detect the optical and infrared afterglow emission from giant flares similar to GRB 200415A. In addition, we consider two cases where the ejecta are dominated by protons or electron–positron pairs. We find that the reverse shock emission is comparable in these two cases for a hard electron spectrum, while for a soft electron spectrum, the reverse shock emission is much weaker in the pair-dominating case..

X-ray observations of two candidate symbiotic binaries in the galactic bulge

ABSTRACT This paper analyses X-ray observations by the Chandra X-ray Observatory of CXOGBS J174614.3−321949 (CXB3) and CXOGBS J173620.2−293338 (CX332), two symbiotic binary star candidates identified by the Galactic Bulge Survey. Using new Chandra observations, we improved their X-ray positional uncertainties to 0.24 and 0.92 arcsec, respectively, confidently associating them with single optical counterparts. In particular, new observations of symbiotic X-ray candidate CX332 further solidify confidence in its coincidence with a carbon star. We demonstrate X-ray variability in both targets with a more recent observation of CX332 showing a decrease in brightness by a factor of 30, while CXB3 observations show it usually in a quiescent state with a factor-of-6 flare-like event in the final observations. In a combined spectral fit for CXB3, we find an NH value of $\sim (2\!-\! 3) \times 10^{22}\, \rm {cm}^{-2}$ with a Γ value of $1.5^{+0.2}_{-0.2}$ for a power-law fit and kT$10.6_{-2.9}^{+5.7}$ keV for an apec fit and an estimated luminosity of ∼8.4 × 1032 erg s−1. Spectra of CXB3 would be consistent with thermal emission as seen in white dwarf symbiotic systems, but the high X-ray luminosity in the light curve is more typically seen in symbiotic X-ray binary systems. Optical spectra of both objects taken with Gemini GMOS indicate CXB3 as containing an M-type star and CX332 having a carbon star counterpart. Both targets show at most marginal evidence of H α emission favouring a symbiotic X-ray binary interpretation for both sources, though we cannot rule out a white dwarf for either case.

Superflares on the late-type giant KIC 2852961

Context. The most powerful superflares reaching 1039 erg bolometric energy are from giant stars. The mechanism behind flaring is thought to be the magnetic reconnection, which is closely related to magnetic activity (including starspots). However, it is poorly understood how the underlying magnetic dynamo works and how the flare activity is related to the stellar properties that eventually control the dynamo action. Aims. We analyze the flaring activity of KIC 2852961, a late-type giant star, in order to understand how its flare statistics are related to those of other stars with flares and superflares, and to understand the role of the observed stellar properties in generating flares. Methods. We searched for flares in the full Kepler dataset of KIC 2852961 using an automated technique together with visual inspection. We cross-matched the flare-like events detected by the two different approaches and set a final list of 59 verified flares during the observing term. We calculated flare energies for the sample and performed a statistical analysis. Results. The stellar properties of KIC 2852961 are revised and a more consistent set of parameters are proposed. The cumulative flare energy distribution can be characterized by a broken power law; that is to say, on the log-log representation the distribution function is fitted by two linear functions with different slopes, depending on the energy range fitted. We find that the total flare energy integrated over a few rotation periods correlates with the average amplitude of the rotational modulation due to starspots. Conclusions. Flares and superflares seem to be the result of the same physical mechanism at different energy levels, also implying that late-type stars in the main sequence and flaring giant stars have the same underlying physical process for emitting flares. There might be a scaling effect behind the generation of flares and superflares in the sense that the higher the magnetic activity, the higher the overall magnetic energy released by flares and/or superflares.

Flare-like Variability of the Mg ii λ2798 Å Emission Line and UV Fe ii Band in the Blazar CTA 102

Abstract We report on the detection of a statistically significant flare-like event in the Mg ii λ 2798 Å emission line and the UV Fe ii band of CTA 102 during the outburst of fall 2017. The ratio between the maximum and minimum of λ3000 Å continuum flux for the observation period (2010−2017) is 179 ± 15. Respectively, the max/min ratios 8.1 ± 10.5 and 34.0 ± 45.5 confirmed the variability of the Mg ii emission line and of the Fe ii band. The highest levels of emission line fluxes recorded coincide with a superluminal jet component traversing through a stationary component located ∼0.1 mas from the 43 GHz core. Additionally, comparing the Mg ii line profile in the minimum of activity against the one in the maximum, we found that the latter is broader and blueshifted. As a result of these findings, we can conclude that the non-thermal continuum emission produced by material in the jet moving at relativistic speeds is related to the broad emission line fluctuations. Consequently, these fluctuations are also linked to the presence of broad-line region (BLR) clouds located ∼25 pc from the central engine, outside the inner parsec, where the canonical BLR is located. Our results suggest that during strong activity in CTA 102, the source of non-thermal emission and broad-line clouds outside the inner parsec introduces uncertainties in the estimates of black hole (BH) mass. Therefore, it is important to estimate the BH mass, using single-epoch or reverberation mapping techniques, only with spectra where the continuum luminosity is dominated by the accretion disk.

Flare Characteristics from X-ray Light Curves

A new methodology is given to determine basic parameters of flares from their X-ray light curves. Algorithms are developed from the analysis of small X-ray flares occurring during the deep solar minimum of 2009, between Solar Cycles 23 and 24, observed by the Polish Solar Photometer in X-rays (SphinX) on the Complex Orbital Observations Near-Earth of Activity of the Sun-Photon (CORONAS-Photon) spacecraft. One is a semi-automatic flare detection procedure that gives start, peak, and end times for single (“elementary”) flare events under the assumption that the light curve is a simple convolution of a Gaussian and exponential decay functions. More complex flares with multiple peaks can generally be described by a sum of such elementary flares. Flare time profiles in the two energy ranges of SphinX (1.16 – 1.51 keV, 1.51 – 15 keV) are used to derive temperature and emission measure as a function of time during each flare. The result is a comprehensive catalogue – the SphinX Flare Catalogue – which contains 1600 flares or flare-like events and is made available for general use. The methods described here can be applied to observations made by Geosynchronous Operational Environmental Satellites (GOES), the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) and other broad-band spectrometers.

Long-term optical spectral monitoring of NGC 7469

We present the results of the long-term (20-year period, from 1996 to 2015) optical spectral monitoring of the Seyfert 1 galaxy NGC 7469. The variation in the light-curves of the broad He II {\lambda}4686A H{\beta} and H{\alpha} lines, and the continuum at 5100A and 6300A have been explored. The maximum of activity was in 1998, and the variability in the continuum and lines seems to have two periods of around 1200 and 2600 days, however these periodicities should be taken with caution because of the red-noise. Beside these periods, there are several short-term (1-5 days) flare-like events in the light-curves. There are good correlations between the continuum fluxes and H{\alpha} and H{\beta} line fluxes, but significantly smaller correlation between the He II and continuum. We found that the time-lags between the continuum and broad lines are different for H{\beta} (~20 l.d.) and H{\alpha} (~3 l.d.), and that He II also has a smaller lag (~2-3 l.d.). The H{\alpha} and H{\beta} line profiles show a slight red asymmetry, and the broad line profiles did not changed in the 20-year period. Using the lags and widths of H{\alpha} and H{\beta} we estimated the central black hole mass and found that it is ~(1-6)x$10^7 M_{\odot}$, which is in agreement with previous reverberation estimates.

CATACLYSMIC VARIABLES OBSERVED DURING K2 CAMPAIGNS 0 AND 1

ABSTRACT There are 15 cataclysmic variables (CVs) observed in the first two campaigns of the K2 mission. In this paper, the eight CVs showing distinct features are analyzed in detail. Among these eight, modulations during quiescence are evident at the known orbital periods in the SU UMa stars QZ Vir and RZ Leo, and at our newly determined orbital periods for 1RXS J0632+2536 and WD 1144+011. The periodogram analysis for the quiescent light curve of QZ Vir reveals multi-period modulations and the coexistence of orbital and superhump periods. The phased orbital light curves for the other three CVs in quiescence display wide (about half cycle) and shallow (<0.5 mag) eclipse features. Besides these modulations, their quiescent light curves reveal several transient events: a sudden decrease of system light in 1RXS J0632+2536, a low-level flare-like event in QZ Vir, a short brightening event in RZ Leo, and a temporary disappearance of the orbital modulation in WD 1144+011. The two known dwarf novae UV Gem and TW Vir and the CVs USNO-B1.01144-00115322 and CSS 130516:111236:111236.7+002807 show outbursts, including one complete and three incomplete normal outbursts and two complete superoutbursts. An incomplete but typical normal outburst confirms the dwarf nova identification of the USNO-B1.01144-00115322. The one complete normal outburst in UV Gem may provide the orbital period, since its modulations are shorter than the previously observed superhump period. The superoutburst of CSS 130516:111236.7+002807, along with the corresponding superhump period, indicates that this object is an SU UMa star. The derived superhump period of CSS 130516:111236:111236.7+002807 is 1.44 hr, implying that this new SU UMa star is close to the period minimum.

The first study of the light-travel time effect in massive LMC eclipsing binaries

Aims: New CCD observations for semidetached and detached eclipsing binaries from the Large Magellanic Cloud were carried out using the Danish 1.54-m telescope located at the La Silla Observatory in Chile. The selected systems were monitored for their times of minima, which were required to be able to study the period changes taking place in them. In addition, many new times of minima were derived from the photometric surveys OGLE-II, OGLE-III, and MACHO. Methods: The O-C diagrams of minima timings were analysed using the hypothesis of the light-travel time effect, i.e. assuming the orbital motion around a common barycenter with the distant component. Moreover, the light curves of these systems were also analysed using the program PHOEBE, which provided the physical parameters of the stars. Results: For the first time, in this study we derived the relatively short periods of modulation in these systems, which relates to third bodies. The orbital periods resulted from 3.6 to 11.3 yr and the eccentricities were found to be up to 0.64. This is the first time that this kind of analysis for the set of extragalactic sources has been performed. The Wolf-Rayet system OGLE-LMC-ECL-08823 is the most mysterious one, owing to the resultant high mass function. Another system, OGLE-LMC-ECL-19996, was found to contain a third body with a very high mass (M3,min = 26M0). One system (OGLE-LMC-ECL-09971) is suspicious because of its eccentricity, and another one (OGLE-LMC-ECL-20162) shows some light curve variability, with a possible flare-like or microlensing-like event. Conclusions: All of these results came only from the photometric observations of the systems and can be considered as a good starting point for future dedicated observations.

Interplay between heartbeat oscillations and wind outflow in microquasar IGR J17091-3624

During the bright outburst in 2011, the black hole candidate IGR J17091-3624 exhibited strong quasi-periodic flare-like events (on timescales of tens of seconds) in some characteristic states, the so-called heartbeat state. From the theoretical point of view, these oscillations may be modeled by the process of accretion disk instability, driven by the dominant radiation pressure and enhanced heating of the plasma. Although the mean accretion rate in this source is probably below the Eddington limit, the oscillations will still have large amplitudes. As the observations show, the source can exhibit strong wind outflow during the soft state. This wind may help to partially or even completely stabilize the heartbeat. Using our hydrodynamical code GLADIS, we modeled the evolution of an accretion disk responsible for X-ray emission of the source. We accounted for a variable wind outflow from the disk surface. We examined the data archive from the Chandra and XMM-Newton satellites to find the observed limitations on the wind physical properties, such as its velocity and ionization state. We also investigated the long-term evolution of this source, which lasted over about 600 days of observations, using the data collected by the Swift and RXTE satellites. During this long period, the oscillations pattern and the observable wind properties changed systematically. We found that this source probably exhibits observable outbursts of appropriate timescales and amplitudes as a result of the disk instability. Our model requires a substantial wind component to explain the proper variability pattern, and even complete suppression of flares in some states. The wind mass-loss rate extracted from the data agrees quantitatively well with our scenario.

Understanding the spin-down rate changes of PSR B0919+06

We study the spin-down properties of PSR B0919+06 based on almost 30 years of radio observations. We confirm that the time derivative of the rotational frequency |$\dot{\nu }$| is modulated quasi-periodically and show that it exhibits a repeating double-peaked structure throughout the entire observation span. We model the |$\dot{\nu }$| variation of the pulsar assuming two spin-down rates with sudden switches between them in time. Our results show that the double-peak structure in |$\dot{\nu }$| has a repetition time of about 630 d until MJD 52000 (2001 April) and 550 d since then. During this cycle, the pulsar spin varies from the lower spin-down rate to the upper spin-down rate twice with different amounts of time spent in each state, resulting in a further quasi-stable secondary modulation of the two-state switching. This particular spin-down state switching is broadly consistent with free precession of the pulsar; however, strong evidence linked with this mechanism is not clearly established. We also confirm that the pulsar occasionally emits groups of pulses which appear early in pulse phase, so-called flares, and these events significantly contribute to the pulse-profile shape. We find the |$\dot{\nu }$| modulation and the pulse-shape variations are correlated throughout the observations. However, the flare state is not entirely responsible for this correlation. In addition to the flare state, we detect flare-like events from the pulsar in single-pulse observations. During these events, the shift in pulse phase is small compared to that of the main flare state and clearly visible only in single-pulse observations.

Suzaku observations of Mrk 335: confronting partial covering and relativistic reflection

We report on the deepest X-ray observation of the narrow-line Seyfert 1 galaxy Mrk 335 in the low-flux state obtained with Suzaku. The data are compared to a 2006 high-flux Suzaku observation when the source was ~10-times brighter. Describing the two flux levels self-consistently with partial covering models would require extreme circumstances, as the source would be subject to negligible absorption during the bright state and ninety-five per cent covering with near Compton-thick material when dim. Blurred reflection from an accretion disc around a nearly maximum spinning black hole (a>0.91, with preference for a spin parameter as high as ~ 0.995) appears more likely and is consistent with the long-term and rapid variability. Measurements of the emissivity profile and spectral modelling indicate the high-flux Suzaku observation of Mrk 335 is consistent with continuum-dominated, jet-like emission (i.e. beamed away from the disc). It can be argued that the ejecta must be confined to within ~25 rg if it does not escape the system. During the low-flux state the corona becomes compact and only extends to about 5 rg from the black hole, and the spectrum becomes reflection-dominated. The low-frequency lags measured at both epochs are comparable indicating that the accretion mechanism is not changing between the two flux levels. Various techniques to study the spectral variability (e.g. principal component analysis, fractional variability, difference spectra, and hardness ratio analysis) indicate that the low-state variability is dominated by changes in the power law flux and photon index, but that changes in the ionisation state of the reflector are also required. Most notably, the ionisation parameter becomes inversely correlated with the reflected flux after a long-duration flare-like event during the observation.

Magnetic activity and orbital periods of five low-mass eclipsing binaries

We report 15 new VRI light curves of five low-mass eclipsing binaries (NSVS 02502726, NSVS 07453183, NSVS 11868841, NSVS 06550671 and NSVS 10653195) that were observed between 2010 and 2012. We analysed our new data together with three published spectroscopic observations and seven published light curves using a modified version of the Wilson-Devinney program. Orbital solutions of the five low-mass eclipsing binaries were revised and new star-spot parameters were obtained. We found that spot locations on the five low-mass eclipsing binaries changed over several years. However, the star-spots for NSVS 07453183 and NSVS 06550671 were stable for several months. More interestingly, for NSVS 02502726, the spots within a star-spot longitude region of 180A degrees-360A degrees indicated a magnetic activity cycle of 5.9(+/- 0.2) yr. Moreover, we detected the first flare-like event on NSVS 07453183 at phase 0.39. The observations of the chromospheric activity indicators (H beta and H gamma lines) revealed that NSVS 10653195 and NSVS 06550671 were active. For NSVS 02502726, we found a weak continuous secular decrease at a smaller rate of dp/dt = -2.1(0.8) x 10(-7) d yr(-1) than the previous result. For NSVS 07453183, the O-C times appeared to increase at cycle 6000, and this was followed by a decrease at cycle 6500.

Spectral optical monitoring of a double-peaked emission line AGN Arp 102B

Here we present results of the long-term (1987-2010) optical spectral monitoring of the broad line radio galaxy Arp 102B, a prototype of active galactic nuclei with the double-peaked broad emission lines, usually assumed to be emitted from an accretion disk. To explore the structure of the broad line region (BLR), we analyze the light curves of the broad H\alpha and H\beta lines and the continuum flux. We aim to estimate the dimensions of the broad-line emitting regions and the mass of the central black hole. We use the CCF to find lags between the lines and continuum variations. We investigate in more details the correlation between line and continuum fluxes, moreover we explore periodical variations of the red-to-blue line flux ratio using Lomb-Scargle periodograms. The line and continuum light curves show several flare-like events. The fluxes in lines and in the continuum are not showing a big change (around 20%) during the monitoring period. We found a small correlation between the line and continuum flux variation, that may indicate that variation in lines has weak connection with the variation of the central photoionization source. In spite of a low line-continuum correlation, using several methods, we estimated a time lag for H\beta around 20 days. The correlation between the H\beta and H\alpha flux variation is significantly higher than between lines and continuum. During the monitoring period, the H\beta and H\alpha lines show double-peaked profiles and we found an indication for a periodical oscillation in the red-to-blue flux ratio of the H\alpha line. The estimated mass of the central black hole is \sim 1.1 \times 10^8 M\odot that is in an agreement with the mass estimated from the M-\sigma* relation.

The photometric variability of the WC9-type Wolf-Rayet star WR 103

We discuss a collection of archival multi-colour photometric data of the variable WC9-type Wolf-Rayet star WR 103 = HD 164270 observed over a time interval of eleven years. The photometric systems used are Walraven VBLUW, Bessel UBV and Stromgren uvby. The purpose is to search for periodicity and to disentangle continuum and line emission variations. The star turns out to be stochastically variable in all time intervals under consideration. The time scale of the variations hovers between a few hours to a few days. The continuum light amplitude varies from ∼ 0.m1 in the visual to ∼ 0.m2 in the UV. Emission-line variations at the level of 1–5 % percent are detectable in all pass bands, but are largest in the Stromgren b and Walraven V filters, due to the prominent presence of the Ciii emission lines (blended with a much weaker Oii line) at 4650 and 5696 A emission lines, respectively. The relative large light amplitude of WR 103 resembles that of WN8-type stars; a possible link between the two is discussed. Stellar (multi-mode) pulsations are likely the cause of the photometric variability. We also discuss the exceptional status of WR 103 within the class of WC9-type stars which are almost photometrically stable. A striking phenomenon observed for the first time in WR 103: a three days lasting flux enhancement of the Ciii line by at least 10 % was observed in August 1998. Such strong spectroscopic flare-like events are very seldom observed in WR stars. So far, the one of WR 103 had the longest duration ever observed.

FLARE-LIKE VARIABILITY OF THE Mg II λ2800 EMISSION LINE IN THE γ-RAY BLAZAR 3C 454.3

We report the detection of a statistically significant flare-like event in the Mg II\lambda 2800\AA\ emission line of 3C 454.3 during the outburst of autumn 2010. The highest levels of emission line flux recorded over the monitoring period (2008 - 2011) coincide with a superluminal jet component traversing through the radio core. This finding crucially links the broad-emission line fluctuations to the non-thermal continuum emission produced by relativistically moving material in the jet and hence to the presence of broad-line region clouds surrounding the radio core. If the radio core were located at several parsecs from the central black hole then our results would suggest the presence of broad-line region material outside the inner parsec where the canonical broad-line region is envisaged to be located. We briefly discuss the implications of broad-emission line material ionized by non-thermal continuum on the context of virial black hole mass estimates and gamma-ray production mechanisms.

CHARACTERIZATION OF THE X-RAY LIGHT CURVE OF THE γ Cas-LIKE B1e STAR HD 110432

HD 110432 (BZ Cru; B1Ve) is the brightest member of a small group of "gamma Cas analogs" that emit copious hard X-ray flux, punctuated by ubiquitous "flares." To characterize the X-ray time history of this star, we made a series of six RXTE multi-visit observations in 2010 and an extended observation with the XMM-Newton in 2007. We analyzed these new light curves along with three older XMM-Newton observations from 2002--2003. Distributed over five months, the RXTE observations were designed to search for long X-ray modulations over a few months. These observations indeed suggest the presence of a long cycle with P = 226 days and an amplitude of a factor of two. We also used X-ray light curves constructed from XMM-Newton observations to characterize the lifetimes, strengths, and interflare intervals of 1615 flare-like events in the light curves. After accounting for false positive events, we infer the presence of 955 (2002-2003) and 386 (2007) events we identified as flares. Similarly, as a control we measured the same attributes for an additional group of 541 events in XMM-Newton light curves of gamma Cas, which after a similar correction yielded 517 flares. We found that the flare properties of HD 110432 are mostly similar to our control group. In both cases the distribution of flare strengths are best fit with log-linear relations. Both the slopes of these distributions and the flaring frequencies themselves exhibit modest fluctuations. We discovered that some flares in the hard X-ray band of HD 110432 were weak or unobserved in the soft band and vice versa. The light curves also occasionally show rapid curve drop offs that are sustained for hours. We discuss the existence of the long cycle and these flare properties in the backdrop of two rival scenarios to produce hard X-rays, a magnetic star-disk interaction and the accretion of blobs onto a secondary white dwarf.

The peculiar 2011 outburst of the black hole candidate IGR J17091−3624, a GRS 1915+105-like source?

We report on the long-term monitoring campaign of the black hole candidate IGR J17091-3624 performed with INTEGRAL and Swift during the peculiar outburst started on January 2011. We have studied the two month spectral evolution of the source in detail. Unlike the previous outbursts, the initial transition from the hard to the soft state in 2011 was not followed by the standard spectral evolution expected for a transient black hole binary. IGR J17091-3624 showed pseudo periodic flare-like events in the light curve, closely resembling those observed from GRS 1915+105. We find evidence that these phenomena are due to the same physical instability process ascribed to GRS 1915+105. Finally we speculate that the faintness of IGR J17091-3624 could be not only due to the high distance of the source but to the high inclination angle of the system as well.

Clouds and red giants interacting with the base of AGN jets

Extragalactic jets are formed close to supermassive black-holes in the center of galaxies. Large amounts of gas, dust, and stars cluster in the galaxy nucleus, and interactions between this ambient material and the jet base should be frequent, having dynamical as well as radiative consequences. This work studies the dynamical interaction of an obstacle, a clump of matter or the atmosphere of an evolved star, with the innermost region of an extragalactic jet. Jet mass-loading and the high-energy outcome of this interaction are briefly discussed. Relativistic hydrodynamical simulations with axial symmetry have been carried out for homogeneous and inhomogeneous obstacles inside a relativistic jet. These obstacles may represent a medium inhomogeneity or the disrupted atmosphere of a red giant star. Once inside the jet, an homogeneous obstacle expands and gets disrupted after few dynamical timescales, whereas in the inhomogeneous case, a solid core can smoothen the process, with the obstacle mass-loss dominated by a dense and narrow tail pointing in the direction of the jet. In either case, matter is expected to accelerate and eventually get incorporated to the jet. Particles can be accelerated in the interaction region, and produce variable gamma-rays in the ambient matter, magnetic and photon fields. The presence of matter clumps or red giants into the base of an extragalactic jet likely implies significant jet mass-loading and slowing down. Fast flare-like gamma-ray events, and some level of persistent emission, are expected due to these interactions.

Mass loss, destruction and detection of Sun-grazing and -impacting cometary nuclei

[Abridged] Sun-grazing comets almost never re-emerge, but their sublimative destruction near the sun has only recently been observed directly, while chromospheric impacts have not yet been seen, nor impact theory developed. Employing simple analytic models to describe comet destruction near the Sun and to enable the estimation of observable signatures, we find analytic solutions for the mass as a function of distance from the Sun, for insolation sublimation, impact ablation and explosion. Sun-grazers are found to fall into three regimes based on initial mass and perihelion: sublimation-, ablation-, and explosion-dominated. Most sun-grazers are destroyed sublimatively, and our analytic results are similar to numerical models. Larger masses (>10^11g) with small perihelion (q<1.01Rsun) ablation dominates but results are sensitive to nucleus strength, Pc, and entry angle to the vertical, phi. Nuclei with initial mass >~10^10g (Pc/10^6 (dyne/cm^2) sec (phi))^3 are fully ablated before exploding, though the hot wake itself explodes. For most sun-impactors sec(phi)~1. For small perihelion the ablation regime applies to moderate masses ~10^13-16 g impactors unless Pc is very low. For higher masses, or smaller perihelia, nuclei reach higher densities where ram pressure causes catastrophic explosion. For perihelion < 1.01Rsun, initial mass > 10^11 g nuclei are destroyed by ablation or explosion (depending on phi and Pc) in the chromosphere, producing flare-like events with cometary abundance spectra. For all plausible masses and physical parameters, nuclei are destroyed above the photosphere.