G. Torricelli-Ciamponi

Interacting coronae of two T Tauri stars: first observational evidence for solar-like helmet streamers

Context. The young binary system V773 Tau A exhibits a persistent radio flaring activity that gradually increases from a level of a few mJy at apoastron to more than 100 mJy at periastron. Interbinary collisions between very large (>15 R∗) magnetic structures anchored on the two rotating stars of the system have been proposed to be the origin of these periodic radio flares. Magnetic structures extended over tens of stellar radii, that can also account for the observed fast decay of the radio flares, seem to correspond to the typical solar semi-open quite extended magnetic configurations called helmet streamers. Aims. We aim to find direct observational evidence for the postulated, solar-like, coronal topologies. Methods. We performed seven-consecutive-day VLBI observations at 8.4 GHz using an array consisting of the VLBA and the 100-m Effelsberg telescope. V773 Tau A was phase-referenced to QSO B0400+258. Results. Two distinctive structures appear in the radio images here presented. They happen to be associated with the primary and secondary stars of the V773 Tau A system. In one image (Fig. 2B) the two features are extended up to 18 R∗ each and are nearly parallel revealing the presence of two interacting helmet streamers. One image (Fig. 2E) taken a few hours after a flare monitored by the 100-m Effelsberg telescope shows one elongated fading structure substantially rotated with respect to those seen in the B run. The same decay scenario is seen in Fig. 2G for the helmet streamer associated with the other star. Conclusions. This is the very first direct evidence revealing that even if the flare origin is magnetic reconnection due to interbinary collision, both stars independently emit in the radio range with structures of their own. These structures are helmet streamers, observed for the first time in stars other than the Sun. The complete extent of each helmet streamer above the stellar surface is about 24 R∗ which implies that they can practically interact throughout the whole orbit, even rather close to apoastron where the stellar separation is 52 R∗. However, the radio flares become stronger when the stars approach. Around periastron the stellar separation is only 30 R∗, nearly covered by a single streamer: the two streamers overlap producing the observed giant flares.

Synchrotron emission from the T Tauri binary system V773 Tauri A

The pre-main sequence binary system V773 Tau A shows remarkable flaring activity around periastron passage. Here, we present the observation of such a flare at a wavelengt h of 3 mm (90 GHz) performed with the Plateau de Bure Interferometer. ⋆ We examine different possible causes for the energy losses responsible for the e-folding time of 2.31± 0.19 hours of that flare. We exclude synchrotron, collisional, an d inverse Compton losses because they are not consistent with observational constraints, and we propose that the fading of the emission is due to the leakage of electrons themselves at each reflection between the two mirror points of the magnetic stru cture partially trapping them. The magnetic structure compatible with both our leakage model and previous observations is that of a helmet streamer that, as in the solar case, can occur at t he top of the X-ray-emitting, stellar-sized coronal loops of one o f the stars. The streamer may extend up to∼ 20 R∗ and interact with the corona of the other star at periastron passage, causing r ecurring flares. The inferred magnetic field strength at the t wo mirror points of the helmet streamer is in the range 0.12 - 125 G, and the corresponding Lorentz factor,γ, of the partially trapped electrons is in the range 20<γ< 632. We therefore rule out that the emission could be of gyro-synchrotron nature: the derived high Lorentz factor proves that the nature of the emission at 90 GHz from this pre-main binary system is synchrotron radiation.

Intrinsic physical properties and Doppler boosting effects in LS I +61°303

Our aim is to show how variable Doppler boosting of an intrinsically variable jet can explain the long-term modulation of 1667 \pm 8 days observed in the radio emission of LSI+61303. The physical scenario is that of a conical, magnetized plasma jet having a periodical (P1) increase of relativistic particles, Nrel, at a specific orbital phase, as predicted by accretion in the eccentric orbit of LSI+61303. Jet precession (P2) changes the angle, eta, between jet axis and line of sight, thereby inducing variable Doppler boosting. The problem is defined in spherical geometry, and the optical depth through the precessing jet is calculated by taking into account that the plasma is stratified along the jet axis. The synchrotron emission of such a jet was calculated and we fitted the resulting flux density Smodel(t) to the observed flux density obtained during a 6.5-year monitoring of LSI+61303 by the Green Bank radio interferometer. Our physical model for the system LSI+61303 is not only able to reproduce the long-term modulation in the radio emission, but it also reproduces all the other observed characteristics of the radio source, the orbital modulation of the outbursts, their orbital phase shift, and their spectral index properties. Moreover, a correspondence seems to exist between variations in the ejection angle induced by precession and the rapid rotation in position angle observed in VLBA images. We conclude that the peak of the long-term modulation occurs when the jet electron density is around its maximum and the approaching jet is forming the smallest possible angle with the line of sight. This coincidence of maximum number of emitting particles and maximum Doppler boosting of their emission occurs every 1667 days and creates the long-term modulation observed in LSI+61303.

Origin of the long-term modulation of radio emission of LS I +61 303

One of the most unusual aspects of the X-ray binary LSI +61 303 is that at each orbit (P1=26.4960 \pm 0.0028 d) one radio outburst occurs whose amplitude is modulated with Plong, a long-term period of more than 4 yr. It is still not clear whether the compact object of the system or the companion Be star is responsible for the long-term modulation. We study here the stability of Plong. Such a stability is expected if Plong is due to periodic (P2) Doppler boosting of periodic (P1) ejections from the accreting compact object of the system. On the contrary it is not expected if Plong is related to variations in the mass loss of the companion Be star. We built a database of 36.8 yr of radio observations of LSI +61 303 covering more than 8 long-term cycles. We performed timing and correlation analysis. In addition to the two dominant features at P1 and P2, the timing analysis gives a feature at Plong=1628 \pm 48 days. The determined value of Plong agrees with the beat of the two dominant features, i.e. Pbeat=1/(\nu1 -\nu2)=1626 \pm 68 d. The correlation coefficient of the radio data oscillates at multiples of Pbeat. Cycles in varying Be stars change in length and disappear after 2-3 cycles following the well-studied case of the binary system zeta Tau. On the contrary, in LSI +61 303 the long-term period is quite stable and repeats itself over the available 8 cycles. The long-term modulation in LSI +61 303 accurately reflects the beat of periodical Doppler boosting (induced by precession) with the periodicity of the ejecta. The peak of the long-term modulation occurs at the coincidence of the maximum number of ejected particles with the maximum Doppler boosting of their emission; this coincidence occurs every 1/(\nu1 - \nu2) and creates the long-term modulation observed in LSI +61 303.

A possible interpretation of the Seyfert-like component in 3C 273 X-ray spectrum

Context. Recent studies of the X-ray spectrum of the quasar 3C 273, analyzing both BeppoSAX data and XMM-Newton data, have shown that, depending on the state of the source, Seyfert-like spectral features, including a significant soft excess, can be detected superimposed on the generally overwhelming beamed non-thermal emission (“jet” component). Aims. In order to explain the Seyfert-like component and the soft excess suggested by the X-ray spectrum analysis, we apply to 3C 273 a recently developed model for non-thermal emission of X-rays from AGN disk coronae, originally intended for the explanation of Seyfert type X-ray emission. Methods. We perform an analysis of BeppoSAX data at different epochs, fitting the spectrum with a composition of a beamed “jet” component and our coronal model with a reflected component and an iron emission line. Results. We obtained reasonable fits of BeppoSAX spectra at different epochs by changing the relative weight of our emission model and of the jet power-law component. We find that in all these cases the low energy (≤2 keV) part of the X-ray spectrum is accounted for by our Seyfert-like model; we also verify the existence of a positive correlation between the Seyfert-like component 2–10 keV flux and the 0.2–2 keV flux of the so-called soft excess. Conclusions. The results outlined above confirm the plausibility of our non-thermal flare-like coronal model as a possible interpretation of Seyfert-like emission in AGNs. We can conclude that the interpretation of the Seyfert-like component in terms of our non-thermal corona model is offering, as a natural result, a description of the soft excess origin, which is still the subject of lively debate.

Understanding the periodicities in radio and GeV emission from LS I +61°303

Context. One possible scenario to explain the emission from the stellar binary system LS I + 61°303 is that the observed flux is emitted by precessing jets powered by accretion. Accretion models predict two ejections along the eccentric orbit of LS I + 61°303: one major ejection at periastron and a second, lower ejection towards apastron. Our GeV gamma-ray observations show two peaks along the orbit (orbital period P 1 ) but reveal that at apastron the emission is also affected by a second periodicity, P 2 . Strong radio outbursts also occur at apastron, which are affected by both periodicities (i.e. P 1 and P 2 ), and radio observations show that P 2 is the precession of the radio jet. Consistently, a long-term modulation, equal to the beating of P 1 and P 2 , affects both radio and gamma-ray emission at apastron but it does not affect gamma-ray emission at periastron. Aims. If there are two ejections, why does the one at periastron not produce a radio outburst there? Is the lack of a periastron radio outburst somehow related to the lack of P 2 from the periastron gamma-ray emission? Methods. We develop a physical model in which relativistic electrons are ejected twice along the orbit. The ejecta form a conical jet that is precessing with P 2 . The jet radiates in the radio band by the synchrotron process and the jet radiates in the GeV energy band by the external inverse Compton and synchrotron self-Compton processes. We compare the output fluxes of our physical model with two available large archives: Owens Valley Radio Observatory (OVRO) radio and Fermi Large Area Telescope (LAT) GeV observations, the two databases overlapping for five years. Results. The larger ejection around periastron passage results in a slower jet, and severe inverse Compton losses result in the jet also being short. While large gamma-ray emission is produced, there is only negligible radio emission. Our results are that the periastron jet has a length of 3.0 × 10 6 r s and a velocity β ~ 0.006, whereas the jet at apastron has a length of 6.3 × 10 7 r s and β ~ 0.5. Conclusions. In the accretion scenario the observed periodicities can be explained if the observed flux is the intrinsic flux, which is a function of P 1 , times the Doppler factor, a function of β cos( f ( P 2 )). At periastron, the Doppler factor is scarcely influenced by P 2 because of the low β . At apastron the larger β gives rise to a significant Doppler factor with noticeable variations induced by jet precession.

Stellar envelopes as sources of broad line region emission: New possibilities allowed

In Active Galactic Nuclei (AGNs) the presence of a star cluster around the central black hole can have several effects on the dynamics and the emission of the global system. In this paper we analyze the interaction of stellar atmospheres with a wind outflowing from the central region of the AGN nucleus. Even a small mass loss from stars, as well as possible star collisions, can give a non-negligible contribution in feeding matter into the AGN nuclear wind. Moreover, stellar mass loss can produce envelopes surrounding stars that turn out to be suitable for reproducing the observed emission from the Broad Line Region (BLR). In this framework, the envelope can be confined by the bow shock arising from the interaction between the expanding stellar atmosphere and the AGN nuclear wind.