1 Introduction

Late-type stars exhibit strong signs of magnetic activity such as H${\alpha}$, Ca II or X-ray emission from hot thermal plasma confined in magnetic fields on the star. For fully convective stars (with spectral type $\sim$M3 and later) the change in interior structure is expected to result in a change of the field sustaining dynamo and, therefore, of the emission properties. However, no clear change in common activity diagnostics is found at these spectral types. Gizis et al. (2000), Basri (2001) and Mohanty & Basri (2002) have observed a decline in H${\alpha}$ emission for old (>1 Gyr) L-type objects in the field, suggesting that a decline in dynamo activity sets in beyond the boundary where the objects become fully convective and close to the borderline to substellar masses. An X-ray study of K- and early M-stars within the solar neighborhood by Fleming et al. (1995) did not unveil any change in X-ray activity for objects on the transition towards fully convective energy transport. It is unclear to date how activity in fully convective and substellar objects depends on parameters such as rotation or age. Studying these relations is essential to an understanding of the underlying dynamo mechanism.

An important measure for magnetic activity on late-type stars of all ages is X-ray emission which is usually explained by emission from a hot plasma. Within the last few years, X-ray emission has also been detected from Brown Dwarfs (BD) and BD Candidates (see e.g. Neuhäuser & Comerón 1998; Neuhäuser et al 1999). In these early studies with ROSAT only BDs in the Chamaeleon, Taurus and $\rho$ Ophiuchus star forming regions were detected, while all older substellar objects in stellar associations such as the Pleiades and in the field are X-ray quiet down to the ROSAT detection limit. More recent observations with Chandra revealed X-ray emission from further young BDs in Orion (Garmire et al. 2000; Feigelson et al. 2002), $\rho$ Oph (Imanishi et al. 2001), and IC 348 (Preibisch & Zinnecker 2001; Preibisch & Zinnecker 2002). To date only one old field BD is known to emit X-rays, namely LP 944-20 which was detected only during a flare (Rutledge et al. 2000). Here, we extend the investigations with a study of X-ray emission from the very low-mass (VLM) members in the $\sigma$ Orionis association, and an update of the X-ray emitting BD population in Taurus.

The $\sigma$ Ori star forming region was discovered by Walter (1997), and was shown to be rich in X-ray sources. Photometric and spectroscopic observations by Béjar et al. (1999), Zapatero et al. (2000), and Béjar et al. (2001) have revealed $\sim$80 objects in the low mass regime close to and below the hydrogen burning mass limit (HBML) in the OB1b association near the multiple star $\sigma$ Orionis at a mean distance modulus of DM = 7.73 measured by HIPPARCOS, corresponding to 352 pc (Béjar et al. 1999). The age of this cluster is estimated to be 1-7 Myrs where the upper age limit is given by constraints of the central star $\sigma$ Orionis: being of spectral type O9.5 and still in the hydrogen burning phase, it cannot be older than 5-7 Myrs (see Béjar et al. 2001; Barrado y Navascues et al. 2001, and references therein). According to Béjar et al. (2001), the position of all members in the H-R diagram is best compatible with an isochrone for 5 Myrs. Based on measurements of lithium abundances in $\sigma$ Ori members Zapatero et al. (2002) give as most likely age 2-4 Myrs with an upper limit of 8 Myrs. At this young age, even VLM objects are still very luminous. In addition, the $\sigma$ Ori association is hosted in a region of very low extinction ( E_B-V=0.05; Lee 1968). This provides excellent conditions to study activity in faint VLM objects despite the considerable distance to the cluster.

Taurus-Auriga is one of the nearest (d = 140 pc; Elias 1978) and most-studied regions of star formation. The X-ray emission from late-type pre-main sequence stars in Taurus-Auriga was recently discussed by Stelzer & Neuhäuser (2001) (hereafter SN01). SN01, whilst concentrating on the G- to early M-type T Tauri Stars, put forth the ROSAT PSPC detection of several objects with spectral types beyond M5, i.e. near the substellar limit. Four additional BDs have been discovered in that region (Martín et al. 2001b) since. In this paper we present a detailed analysis of the X-ray activity of all VLM objects in Taurus-Auriga including the new BDs. In the following we used the term "very-low mass'' for objects with spectral type later than M4.

In Sect. 2, we outline the criteria we used when selecting our data samples. In Sect. 3, we describe the ROSAT data analysis. We present the results for both $\sigma$ Orionis and Taurus in Sect. 4. In Sect. 5, we perform a comparative study of the X-ray properties of all young BDs and BD candidates detected so far, including tests for variability and an investigation of correlations with other activity parameters. A summary of our results is given in Sect. 6.