Bright X‐Ray Flares in Orion Young Stars from COUP: Evidence for Star‐Disk Magnetic Fields?

Abstract

We have analyzed a number of intense X-ray flares observed in the Chandra Orion Ultradeep Project (COUP), a 13 day observation of the Orion Nebula Cluster (ONC), concentrating on the events with the highest statistics (in terms of photon flux and event duration). Analysis of the flare decay allows to determine the physical parameters of the flaring structure, particularly its size and (using the peak temperature and emission measure of the event) the peak density, pressure, and minimum confining magnetic field. A total of 32 events, representing the most powerful '1% of COUP flares, have sufficient statistics and are sufficiently well resolved to grant a detailed analysis. A broad range of decay times are present in the sample of flares, with lc (the 1/e decay time) ranging from 10 to 400 ks. Peak flare temperatures are often very high, with half of the flares in the sample showing temperatures in excess of 100 MK. Significant sustained heating is present in the majority of the flares. The magnetic structures that are found, from the analysis of the flare's decay, to confine the plasma are in a number of cases very long, with semilengths up to '10 12 cm, implying the presence of magnetic fields of hundreds of G (necessary to confine the hot flaring plasma) extending to comparable distance from the stellar photosphere. These very large sizes for the flaring structures (length L 3 R Ã ) are not found in more evolved stars, where, almost invariably, the same type of analysis results in structures with L R Ã . As the majority of young stars in the ONC are surrounded by disks, we speculate that the large magnetic structures that confine the flaring plasma are actually the same type of structures that channel the plasma in the magnetospheric accretion paradigm, connecting the star's photosphere with the accretion disk.

Results from Droxo

“…To explain the observed long-lasting fluorescent emission, the electron acceleration mechanism should be sustained over several days. While we have no detailed physical mechanism to propose for this, we note that many of the long-lasting flares observed in YSOs by Favata et al (2005a) require sustained heating of the flaring material to explain the observed duration of the events, which in some cases lasted a few days. The thermodynamic cooling time of the flaring plasma would be much shorter; therefore, to explain the observed long flaring events in YSOs, magnetic reconnection must be ongoing, on time scales of days, similar to what is observed for the Fe 6.4 keV fluorescent emission from Elias 29.…”

“…Calvet & Hartmann 1992). The observation of very intense long-duration flares in YSOs, which implies flaring loop lengths of tens of stellar radii (Favata et al 2005a), provides evidence of flaring associated to these accretion streams.…”

“…accelerated particles will impact the stellar photosphere and heat and "evaporate" the chromospheric plasma, providing the hot material that emits the X-ray flare observed between 0.1 and 10 keV. Indeed, the flares for which long flaring loops were derived in Favata et al (2005a) appear to take place in CTTs with no significant ongoing accretion (hence their "accretion" tubes are not mass-loaded).…”

The role of magnetic reconnection on jet/accretion disk systems

“…However, some COUP (Chandra Orion Ultra-deep Project) sources have revealed strong flares that were related to peculiar gigantic magnetic loops linking the magnetosphere of the central star with the inner region of the accretion disk. It has been argued that this X-ray emission could be due to magnetic reconnection in these gigantic loops (Favata et al 2005). In this section we examine this in more detail, investigating the role of magnetic reconnection events in the inner disc/corona of YSOs to explain the X-ray flares and check if magnetic reconnection may be related to the thermal jets of these sources.…”

The classification of flaring states of blazars

“…In its form for time-tagged data, this algorithm was used for example to characterize the X-ray light curves of young pre-main sequence stars observed by the Chandra Orion Ultra-Deep Project (COUP, Getman et al 2005) in the Orion Nebula Cluster (ONC). A modified version of the S98 algorithm, based on a Maximum Likelihood rather than a Bayesian approach, was recently employed in other studies of stellar X-ray light curves (Wolk et al 2005;Favata et al 2005;Stelzer et al 2006). We will refer to this algorithm as the Maximum Likelihood Blocks (MLBs) and we introduce here a variant that is suitable for the analysis of binned light curves.…”