---
abstract: 'We report the first detection of X-ray emission from a brown dwarf in the Pleiades, the M7-type Roque 14, obtained using the EPIC detectors on [[*XMM-Newton*]{}]{}. This is the first X-ray detection of a brown dwarf intermediate in age between $\approx 12$ and $\approx 320$ Myr. The emission appears persistent, although we cannot rule out flare-like behaviour with a decay time-scale $> 4$ ks. The time-averaged X-ray luminosity of $\approx 3.3 \pm 0.8 \times 10^{27}$ , and its ratios with the bolometric ([$L_{\rm X}/L_{\rm bol}$]{} $\approx 10^{-3.05}$) and H$\alpha$ (/ $\approx 4.0$) luminosities suggest magnetic activity similar to that of active main-sequence M dwarfs, such as the M7 old-disc star VB 8, though the suspected binary nature of Roque 14 merits further attention. No emission is detected from four proposed later-type Pleiades brown dwarfs, with upper limits to in the range 2.1–3.8 $\times 10^{27}$ and to [$\log (L_{\rm X}/L_{\rm bol})$]{} in the range $-3.10$ to $-2.91$.'
author:
- |
    K.R. Briggs$^{1}$[^1] and J.P. Pye$^2$[^2]\
    $^1$Paul Scherrer Institut, CH-5232 Villigen PSI, Switzerland\
    $^2$Department of Physics and Astronomy, University of Leicester, Leicester LE1 7RH, UK\
date:  Accepted 2004 June 8th 
title: 'X-ray emission from a brown dwarf in the Pleiades'
---

X-rays: stars – stars: low-mass and brown dwarfs, activity, coronae – stars: individual: Roque 14 – open clusters and associations: individual: the Pleiades

Introduction {#intro}
============

Magnetic activity, generating chromospheric H$\alpha$ and coronal X-ray and radio emissions, is a ubiquitous feature of main-sequence (MS) late-type stars (spectral types $\approx$F5–M7). Studies of these diagnostic emissions have found consistency in the character of magnetic activity throughout this range, despite the expected change in dynamo mechanism demanded by the absence of a radiative interior in stars of spectral types $\approx$ M3 and later. However, recent studies suggest the magnetic activity of ‘ultracool’ objects, with spectral types $\approx$ M8 and later, is quite different.

The observed persistent (‘non-flaring’) levels of X-ray ([$L_{\rm X}/L_{\rm bol}$]{}) and H$\alpha$ () emission from MS late-type stars increase with decreasing Rossby number, $Ro =  P / \tau_{\rm C}$, where $P$ is the rotation period and $\tau_{\rm C}$ is the convective turnover time, until reaching respective ‘saturation’ plateaus of [$L_{\rm X}/L_{\rm bol}$]{} $\sim 10^{-3}$ and $\sim 10^{-3.5}$ (e.g. Delfosse [et al. ]{}1998 for M dwarfs). The fraction of field stars showing persistent chromospheric emission levels close to saturation increases toward later spectral types, peaking around M6–7 (Gizis [et al. ]{}2000).

However, around spectral type M9 persistent H$\alpha$ emission levels begin to plummet dramatically (Gizis [et al. ]{}2000). Among L-type dwarfs no rotation–activity connection is found: continues to fall steeply toward later spectral types despite most L-dwarfs being fast rotators (Mohanty & Basri 2003). A proposed explanation is that magnetic fields diffuse with increasing efficiency in the increasingly neutral atmospheres of cooler dwarfs (Meyer & Meyer-Hofmeister 1999; Mohanty [et al. ]{}2002), overwhelming the importance of a rotation-driven dynamo efficiency in chromospheric heating. Magnetic activity is still observed, however, in the forms of H$\alpha$ flaring on some L dwarfs (e.g. Liebert [et al. ]{}2003), and flaring and apparently-persistent radio emission from several ultracool dwarfs (Berger [et al. ]{}2001; Berger 2002).

Detections of X-ray emission from ultracool field dwarfs are scarce. The M7 old-disc star VB 8 shows persistent and flaring X-ray emission levels of [$L_{\rm X}/L_{\rm bol}$]{}$\approx 10^{-4.1}$–$10^{-2.8}$, similar to those of active M dwarfs (Fleming [et al. ]{}1993; Schmitt, Fleming & Giampapa 1995; Fleming, Giampapa & Garza 2003). However, the persistent levels of X-ray emission from the M8 old-disc star VB 10 and the M9 $\sim$320 Myr-old brown dwarf LP 944-20 are at least an order of magnitude lower – [$L_{\rm X}/L_{\rm bol}$]{}$\approx 10^{-5.0}$ (Fleming [et al. ]{}2003) and [$L_{\rm X}/L_{\rm bol}$]{}$< 10^{-5.7}$ (Rutledge [et al. ]{}2000), respectively – despite the latter being a fast rotator ([$v \sin i$]{}$= 30$ ). Yet transient strong magnetic activity is evidenced by the flaring X-ray emission, with peak [$L_{\rm X}/L_{\rm bol}$]{}$\approx
10^{-3.7}$–$10^{-1.0}$, that has been observed on both VB 10 (Fleming, Giampapa & Schmitt 2000) and LP 944-20, and on the M9 field dwarfs LHS 2065 (Schmitt & Liefke 2002) and 1RXS J115928.5-524717 (Hambaryan [et al. ]{}2004). Interestingly the temperature of the dominant X-ray-emitting plasma appears to be low, $T \approx 10^{6.5}$ K, whether it is measured in the persistent (VB 10) or flaring (LP 944-20 and 1RXS J115928.5-524717) emission state. While such low temperatures are typical for the persistent coronae of inactive stars – M dwarfs (Giampapa [et al. ]{}1996) and the Sun (Orlando, Peres & Reale 2000) alike – the temperatures of flaring plasma are significantly higher, with $T >
10^{7.0}$ K (Güdel [et al. ]{}2004; Reale, Peres & Orlando 2001).

As very young substellar objects ($t \la 5$ Myr) may have photospheres as warm as MS M5–6 dwarfs, an individual brown dwarf may experience a transition from ‘stellar-like’ to ‘ultracool’ magnetic activity as it cools. Brown dwarfs in star-forming regions are routinely observed to emit X-rays at high levels, [$L_{\rm X}/L_{\rm bol}$]{}$\ga 10^{-3.5}$, arising from plasma at $T \ga 10^{7.0}$ K, similar to those of dMe stars and higher-mass young stars (e.g. Neuhäuser & Comerón 1998; Imanishi, Tsujimoto & Koyama 2001; Mokler & Stelzer 2002; Preibisch & Zinnecker 2002; Feigelson [et al. ]{}2002). The $\approx
12$ Myr-old, low-mass brown dwarf TWA 5B, of ultracool spectral type M8.5–9, exhibits apparently persistent X-ray emission at [$L_{\rm X}/L_{\rm bol}$]{} $\approx 10^{-3.4}$, like younger brown dwarfs, but with $T \approx 10^{6.5}$ K (Tsuboi [et al. ]{}2003), like field ultracool dwarfs and LP 944-20.

At around spectral type M8–9, between the ages of $\sim 10$ and $\sim 300$ Myr for brown dwarfs, persistent X-ray emission levels appear to fall by a factor $\sim 100$ and coronal temperatures appear constrained to $T \la 10^{6.5}$ K, even during flares and at high emission levels. The population of brown dwarfs in the Pleiades cluster, $135$ pc away (Pan, Shao & Kulkarni 2004), of age $\approx
125$ Myr (Stauffer, Schultz & Fitzpatrick 1998) and spanning spectral types M6.5–early-L (Mart[í]{}n [et al. ]{}1998), is therefore crucial to understanding the evolution of substellar magnetic activity and the conflict of a rotationally-driven magnetic dynamo against atmospheric neutrality. [[*ROSAT*]{}]{} observations of brown dwarfs in the Pleiades detected no X-ray emission at the level of [$L_{\rm X}/L_{\rm bol}$]{}$\ga 10^{-2.5}$ (Neuhäuser [et al. ]{}1999, and see Section 4.4).

We present a deeper X-ray (0.3–4.5 keV) observation of five candidate brown dwarfs in the Pleiades (described in Section 2), using the the more sensitive [[*XMM-Newton*]{}]{} observatory (Section 3). We detect X-ray emission from the M7 Roque 14, investigate its temporal and spectral nature, and place upper limits on the X-ray emission levels of the undetected brown dwarfs: Teide 1, Roque 9, Roque 11 and Roque 12 (Section 4). We discuss the relative X-ray and H$\alpha$ emissions of these objects in the context of magnetic activity on ultracool dwarfs and its evolution (Section 5) and close by summarising our findings (Section 6).

Sample of brown dwarfs in the Pleiades
======================================

Five objects in our [[*XMM-Newton*]{}]{} field have been proposed as candidate brown dwarf members of the Pleiades on the basis of optical and near-infrared (NIR) photometry in the [*IZJHK*]{} bands (Zapatero Osorio [et al. ]{}1997a; Pinfield [et al. ]{}2000; 2003). Observed and derived physical parameters are listed in Table \[tbl\_sample\]. All except Roque 9 have published spectral types consistent with those expected of $\approx$ 125 Myr-old brown dwarfs. Further evidence for or against membership of the Pleiades and hence substellar status is summarised below:

[**Teide 1**]{} is the on-axis target of the [[*XMM-Newton*]{}]{} observation. Its status as a brown dwarf member of the Pleiades is well-established on the basis of its proper motion (Rebolo, Zapatero Osorio & Mart[í]{}n 1995), the detection of Li in its spectrum and its radial velocity (Rebolo [et al. ]{}1996). It shows H$\alpha$ emission with variable equivalent width, $EW_{\rm H\alpha} = 3.5$–8.6 Å (Rebolo [et al. ]{}1995; 1996). It has been suspected to have a lower-mass companion from its position on a $JK$ colour-magnitude diagram (Pinfield [et al. ]{}2003).

[**Roque 11**]{} has an anomolous position on a $JHK$ colour-colour diagram (Pinfield [et al. ]{}2003) but its radial velocity of $-3.5
\pm 7$ is consistent with those of other Pleiades members, and its Na [i]{} absorption is lower than that of field stars of the same spectral type, indicating lower gravity, and hence youth (Zapatero Osorio [et al. ]{}1997b).

[**Roque 12**]{} has a radial velocity consistent with Pleiades membership (Festin 1998), low Na [i]{} absorption and is a strong H$\alpha$ emitter with $EW_{\rm H\alpha} = 19.7$ Å (Martín [et al. ]{}1998).

[**Roque 14**]{} has low Na [i]{} absorption and strong H$\alpha$ emission with $EW_{\rm H\alpha} = 17.0$ Å (Zapatero Osorio [et al. ]{}1997b). It has been suspected to be a near-equal mass binary on the basis of its position in an $IK$ colour-magnitude diagram (Pinfield [et al. ]{}2003) but no comparably bright companion with separation $> 0.1$ arcsec has been found (Mart[í]{}n [et al. ]{}2000).

[**Roque 9**]{} has no published spectral type but we estimate a spectral type of M8 as its NIR photometry is similar to those of Teide 1 and Roque 11.

We shall use the term “brown dwarf” to refer to all five objects, but note that the evidence in support of substellar status varies across the sample, and is weak for Roque 9.

Observation and data analysis {#sec_obs}
=============================

The [[*XMM-Newton*]{}]{} observation, 0094780101, was centred on Teide 1 (J2000: $\alpha=03^{\rm h} 47^{\rm m} 18\fs0$, $\delta=+24\degr 22\arcmin
31\arcsec$) and conducted on 2000 September 1 in orbit 134. The Thick optical blocking filter was placed in front of all three EPIC detectors: the [pn]{} (Str[" u]{}der [et al. ]{}2001) was exposed for 40.6 ks and each MOS (M1 and M2; Turner [et al. ]{}2001) for 33.0 ks, beginning 7.5 ks later. The data were processed using the [science analysis system]{} ([sas]{}) v5.4.1[^3] and each EPIC eventlist was further filtered to exclude flagged ‘bad’ events, and uncalibrated event patterns ($> 12$ for MOS; $> 4$ for [pn]{}). Several short intervals affected by high background were also excluded. We considered only events with PI in the range 300–4500 (nominally energies of 0.3–4.5 keV, and PI is reported in units of eV or keV from this point) to reduce background contamination.

We extracted an image with $4 \times 4$ arcsec square pixels from each detector and performed source detection in each image. The procedure, using tasks available in the [sas]{}, is described in detail in Briggs & Pye, in preparation. In brief, potential sources were located using a wavelet detection package ([ewavelet]{}), and masked out of the photon image while it was adaptively smoothed (using [asmooth]{}) to generate a model of the background. The spatial variation of vignetting and quantum efficiency was modelled in an exposure map (produced using [eexpmap]{}). The images, background and exposure maps[^4] of the three EPIC instruments were also mosaicked to optimize the sensitivity of the analysis and [ewavelet]{} was used to locate potential sources in the EPIC image. In each image, at the position of each [ewavelet]{} source, a maximum likelihood fitting of the position-dependent instrument point spread function (PSF) was applied ([emldetect]{}) to parametrize each source and those with [*ML*]{} $> 6$ were retained. [emldetect]{} additionally reconstructed a smooth model of the input image by adding PSFs to the background map with the location, normalization and extent of the parametrized sources. We repeated the source detection procedure on 20 images generated from this model image using Poisson statistics, and 20 images generated from the source-free background map, and found in both cases a mean number of false detections per image of 5 with [ *ML*]{} $> 6$ and 2 with [*ML*]{} $> 7$.

Approximately 130 X-ray sources were found, of which 34 were associated with proposed members of the Pleiades. Matching of the [emldetect]{} source positions with NIR positions of these members (Pinfield [et al. ]{}2000) revealed a boresight shift in the EPIC image of $\alpha_{\rm X} \cos\delta_{\rm X} - \alpha_{\rm NIR} \cos\delta_{\rm
  NIR} = 3.7$ arcsec and $\delta_{\rm X} - \delta_{\rm NIR} = -0.8$ arcsec, which is accounted for in the remainder of this work. The corrected positions of sources associated with the Pleiades were all within 6 arcsec of the NIR positions. The probabilities of a positional coincidence within 6 arcsec of a brown dwarf with a spurious detection or a source unrelated to the Pleiades are $\approx 10^{-3}$ and $\approx 0.02$, respectively.

ML-fitting was performed in each image at the NIR position of each brown dwarf. The successful detection of Roque 14 is described further in Section 4.1, and investigations of the temporal and spectral nature of its emission are pursued in Sections 4.2 and 4.3, respectively. In no other case was an [*ML*]{} value $> 1$ found. Upper limits to the X-ray luminosity of these objects are calculated in Section 4.4.

Results {#sec_res}
=======

An X-ray detection of Roque 14
------------------------------

Roque 14 was returned as an X-ray source by [ewavelet]{} in the M2, [pn]{}, and mosaicked-EPIC images. [emldetect]{} determined [*ML*]{} values of 2.1, 5.9 and 7.0 (1.6, 3.0 and $3.3 \sigma$) in these respective images, confirming the detection at [*ML*]{} $> 6$ ($3.0 \sigma$) only in the mosaicked-EPIC image[^5] (Fig. \[fig\_img\_x\]a). The best-fitting X-ray source position in the EPIC image was 0.8 arcsec offset from the NIR position. We estimated the [1-$\sigma$]{} uncertainty in the relative positions as the sum in quadrature of the statistical fitting uncertainty, 0.73 arcsec, an uncertainty in the EPIC absolute pointing of 1.0 arcsec, and an uncertainty in the optical position of 0.5 arcsec; thus the positional offset of the source is $\approx 0.6 \sigma$. The total number of EPIC source counts determined by [emldetect]{} was $24.7 \pm 5.9$. We calculate the X-ray luminosity after consideration of the source spectrum in Section 4.3.

Further analysis of the X-ray emission from Roque 14 has been conducted both including and excluding periods affected by high background, and no significant improvement has been found by excluding those periods. Therefore, to maximize the number of events available, we report the results of including the short periods of high background.

We extracted source events from a circle centred on the best-fitting source position with radius 10.75 arcsec, which encloses only 54 per cent of the source counts (Ghizzardi 2002) but optimises the signal:noise ratio, and background events from the surrounding annulus, 8 times larger than the source extraction region. M1 and M2 eventlists were combined as these instruments have near-identical sensitivity, spectral response and exposure length. A total of 32 counts in all EPIC instruments (23 from [pn]{} and 9 from the two [MOS]{} cameras) was extracted from the source region and 140 (99 [pn]{}; 41 [MOS]{}) counts from the background region. Thus we expect 17.5 (12.4 [pn]{}; 5.1 [MOS]{}) background counts in the source region. As the background region will contain $\approx 46$ per cent of the $\approx 25$ source counts, we expect a contribution of source counts ($\approx 1.4$) similar to the [1-$\sigma$]{} uncertainty ($\approx 1.2$) in the estimated mean background counts. The probability of 32 counts or more appearing in the source region as a result of a Poissonian fluctuation in a mean background of 17.5 counts is 0.0011 ($3.2 \sigma$), in support of the [emldetect]{} detection.

The events extracted from the source region were used to examine the temporal and spectral behaviour of Roque 14. Events were extracted from a larger source-free region on the same CCD as Roque 14 to make a better model of the time and energy distribution of the background.

Transient or persistent emission?
---------------------------------

Examination of the arrival times of the photons in the source eventlist suggests a concentration, particularly of lower-energy (0.3–1.4 keV) events, in the $\approx 10$ ks interval 10–20 ks after the start of the [pn]{} exposure (Fig. \[fig\_r14\_ev\]).

We investigated the statistical significance of this possible variability using the Kolmogorov–Smirnov (K–S) statistic. The K–S statistic is the maximum vertical difference between the cumulative distribution functions (CDFs) of an observed dataset and a test distribution (or two observed datasets). As our test model must account for the significant number of background events in the source extraction region, which is subject to Poissonian fluctuations about its expected number, we have performed Monte Carlo simulations to assess the confidence level of deviation from the test model as a function of K–S value.

Our test-model CDF was constructed from the arrival times of events in the background extraction region and arrival times of a number of source events (such that the ratio of source:background counts was as estimated from the observed data) chosen at uniform intervals in the CDF of a constant count-rate source. Sets of arrival times for [pn]{} and [MOS]{} events were constructed separately and then merged before calculating the test-model CDF for all EPIC data (Fig. \[fig\_cdf\_mod\]a).

Each trial dataset in our simulations was composed of the observed numbers of [pn]{} (23) and [MOS]{} (9) events. The number of background events was drawn from a Poisson distribution with mean value as estimated from the data. Arrival times for these background events were drawn at random from the observed distribution for background events. Arrival times for the remaining events were drawn at random from the modelled distribution for source events. [MOS]{} and [pn]{} datasets were again constructed separately before being combined. For each trial dataset the K–S statistic was calculated. The CDF of these K–S values gives the confidence level of deviation from the test model as a function of measured K–S value (Fig. \[fig\_nhp\_ks\]a).

The simulated distribution of K–S values for a test model of a constant source count-rate, based on 10000 trials, is practically indistinguishable from the standard calculated distribution. The K–S value of the observed dataset for this test model is 0.110 (Fig.\[fig\_cdf\_mod\]c), which corresponds to just 20 per cent confidence that the source count rate is not constant. Following the same procedure we have additionally tested models in which the source emission is purely transient (Fig. \[fig\_cdf\_mod\]a), with the profile of fast (here, instantaneous) rise and exponential decay typical of flares on late-type stars, beginning 10 ks after the start of the [pn]{} observation. For short decay time-scales, when the source and background models are starkly different (Fig. \[fig\_cdf\_mod\]a), Fig. \[fig\_nhp\_ks\]a shows that the standard calculation significantly overestimates the simulated confidence level of deviation from the test-model. The K–S test rules out flare models with decay time-scale of 3 ks or less at the 90 per cent confidence level, but permits those with time-scale 4 ks or longer (Fig. \[fig\_cdf\_mod\]c).

We therefore cannot exclude the possibility that the observed emission from Roque 14 was due to a flare-like outburst similar to those with decay time-scales of $\sim 5$ ks seen from the M9 ultracool dwarfs LP 944-20 (Rutledge [et al. ]{}2000), LHS 2065 (Schmitt & Liefke 2002), and RXS J115928.5-524717 (Hambaryan [et al. ]{}2004).

Temperature of X-ray emitting plasma
------------------------------------

Fig. \[fig\_r14\_ev\] suggests the excess of counts was chiefly in the low-energy (0.3–1.4 keV) band, indicative of plasma with temperature $T \la 10^{7.25}$ K typical in coronae on late-type stars.

We attempted to constrain the source temperature by implementing a K–S test of the PI values of the observed events, as described above for arrival times. Model distributions of PI values were tested for an isothermal optically-thin plasma source, with an array of temperatures, $T$, at intervals of 0.25 dex in $\log T$ in the range $6.0 \le \log T$ (K) $\le 8.0$ (Fig. \[fig\_cdf\_mod\]b). Each source model was generated in XSPEC (Arnaud 1996) using an [[apec]{}]{} model (Smith [et al. ]{}2001) with solar abundances and absorbing column density typical for the Pleiades, [$N_{\rm H}$]{}$= 2.0 \times 10^{20}$ cm$^{-2}$ (Stauffer 1984; Paresce 1984), convolved with the [pn]{} response matrix and ancilliary response at the source position.

While it was possible to rule out models with $T < 10^{6.5}$ K at the 90 per cent confidence level, due to the small number of counts and the hard spectrum of the significant number of background events we were unable to exclude high source temperatures, even up to $10^{8.0}$ K (Fig. \[fig\_cdf\_mod\]d).

For a plasma temperature of $10^{7.0}$ K the sensitivity ratio of [pn]{} to [MOS]{} is 3.6 in the 0.3–4.5 keV energy band and the [MOS]{} count-to-flux conversion factor is $6.4 \times 10^{-12}$ per (0.3–4.5 keV) cs$^{-1}$. Thus, the total [MOS]{}-equivalent effective exposure time for Roque 14 was 117.6 ks, its [MOS]{}-equivalent count-rate was $2.1 \pm 0.5 \times 10^{-4}$ cs$^{-1}$, and its time-averaged X-ray luminosity was $3.3 \pm 0.8 \times 10^{27}$ , with [$L_{\rm X}/L_{\rm bol}$]{}$\approx 10^{-3.05}$. The assumption of a plasma temperature of $10^{6.5}$ K would give $\approx 3.5 \pm 0.8 \times 10^{27}$ .

Upper limits to X-ray emission from undetected brown dwarfs
-----------------------------------------------------------

No significant X-ray emission was detected from any of the remaining four brown dwarfs[^6] . At the NIR position of each brown dwarf, we counted events detected by each EPIC instrument in the 0.3–1.4 keV energy range[^7]. A radius of 8 arcsec was used to minimize counts from nearby sources to Roque 11 and Teide 1. The expected number of background counts was estimated from the value at the source position in the reconstructed image generated by [emldetect]{} in the 0.3–1.4 keV band. This enabled us to account for stray counts from nearby sources, and we note that the respective backgrounds for Teide 1 and Roque 11 were thus 25 and 36 per cent higher than the values at those positions in the [asmooth]{} background map. Upper limits to the source counts at 95 per cent confidence were calculated using the Bayesian method described by Kraft, Burrows & Nousek (1991).

These upper limits were corrected for the enclosed energy fraction at the source off-axis angle (0.40–0.46 for a radius of 8 arcsec; Ghizzardi 2002) and converted to [MOS]{}-equivalent count-rates by dividing by the value at the source position in the [MOS]{}-equivalent EPIC exposure map. The assumption of a $10^{7.0}$ K plasma with solar abundances and [$N_{\rm H}$]{}$=2 \times 10^{20}$ cm$^{-2}$ gives a [MOS]{} count-to-flux conversion factor of $7.2 \times 10^{-12}$ per (0.3–1.4 keV) cs$^{-1}$ and upper limits to the 0.3–4.5 keV X-ray luminosity in the range 1.9–3.4 $\times 10^{27}$ , with upper limits to [$L_{\rm X}/L_{\rm bol}$]{} in the range $10^{-3.10}$–$10^{-2.91}$ (listed in Table \[tbl\_activity\]).

To optimise our sensitivity to the detection of X-ray emission from these brown dwarfs and put the tightest possible constraint on their mean X-ray luminosity, we constructed a composite image of all the available EPIC data in $1 \times 1$ arcmin squares centred on the NIR positions of the four brown dwarfs (Fig. \[fig\_img\_x\]b). The total [MOS]{}-equivalent exposure time for the composite brown dwarf was 439.3 ks. [emldetect]{} detected the source $\approx 25$ arcsec west of Roque 11, but no source at the position of the composite brown dwarf. We calculated an upper limit to the mean X-ray luminosity of the four brown dwarfs of $1.1 \times 10^{27}$ using the method described above. This corresponds to a mean [$L_{\rm X}/L_{\rm bol}$]{} of $10^{-3.4}$. Deeper observations or a composite analysis of a larger sample are required to determine if the mean activity level of Pleiades brown dwarfs lies well below the saturated level.

Considerably lower 2-$\sigma$ ($\approx 95$ per cent confidence) upper limits, to in the range 5.2–9.5 $\times 10^{26}$ , and to [$L_{\rm X}/L_{\rm bol}$]{} in the range $10^{-3.83}$–$10^{-3.43}$, have been reported for these objects (excepting Roque 9) by Neuhäuser [et al. ]{}(1999; henceforth N99) using a number of observations of the Pleiades by the [[*ROSAT*]{}]{} PSPC. All five brown dwarfs were included in 7 separate exposures longer than 1.5 ks of 4 different PSPC pointings in the [[*ROSAT*]{}]{} public archive; the total exposure time of each field ranged from 22.5–39.9 ks. They were best-observed in the “Pleiades Center” field, at off-axis angles in the range 14–24 arcmin. The longest of the three exposures of this field, 22.4 ks from a total of 35.4 ks, was not used in N99, probably due to a faulty aspect solution (Hodgkin, Jameson & Steele 1995). We have recalculated 95 per cent confidence upper limits by applying the Bayesian method as described above, using the broad-band (0.1–2.4 keV) images, background maps and exposure maps of the 6 remaining archival exposures, and merging data from exposures of the same field. We used a variety of extraction radii in each field and further combined data from different fields obtained with similar enclosed energy fraction (calculated using Boese 2000, equation 9), to find the strictest upper limit for each brown dwarf. We were also mindful of avoiding nearby X-ray sources detected by [[*XMM-Newton*]{}]{}. Optimum enclosed energy fractions ranged from 0.35–0.75. We converted PSPC count-rates to unabsorbed fluxes in the 0.1–2.4 keV band using a conversion factor of $1.0 \times 10^{-11}$ per cs$^{-1}$, appropriate for a $10^{7.0}$ K plasma with low absorption, as used in N99. Fluxes in the 0.3–4.5 keV energy band would be 5 per cent higher. A distance of 135 pc was used to calculate luminosities (N99 used 125 pc).

The tightest upper limits to the X-ray luminosity we could apply were 1.2–2.5 $\times 10^{28}$ for Roque 9, 14, 11, and 12, and 4.3 $\times 10^{28}$ for Teide 1, which may be contaminated by stray counts from the X-ray-bright K5 Pleiad HII 1348, $\approx 50$ arcsec away (Briggs & Pye 2003a). Upper limits to [$L_{\rm X}/L_{\rm bol}$]{} were in the range $10^{-2.4}$–$10^{-1.8}$. Hence, we conclude that the [[*ROSAT*]{}]{} observations were not sufficient to detect Roque 14 at the level detected here by [[*XMM-Newton*]{}]{}, and the current [[*XMM-Newton*]{}]{} observation places the strictest upper limits thus far to the X-ray emission levels of Pleiades brown dwarfs of spectral type M7.5–8.

Discussion
==========

X-ray emission from ultracool dwarfs
------------------------------------

We have detected X-ray emission only from the M7-type Roque 14 at [$L_{\rm X}/L_{\rm bol}$]{} $\approx 10^{-3.05}$, and placed an upper limit to the mean emission level from the four later-type Pleiades brown dwarfs at [$L_{\rm X}/L_{\rm bol}$]{} $<
10^{-3.4}$. This is consistent both with the pattern of emission from active main-sequence M stars, which scales with bolometric luminosity, and with the idea that persistent X-ray emission levels fall around spectral type M8 as magnetic field dissipates more easily in cooler, more neutral lower atmospheres.

We can make some comparison of the character of the magnetic activity of Roque 14 with that of other low-mass stars based on the relative levels of chromospheric and coronal emission. Fleming (1988) has reported a mean ratio of / $\approx 6.7$ for a sample of the active, frequently-flaring, dMe stars and Reid, Hawley & Mateo (1995) have reported a value of $\sim 3$ for M0–6 field stars outside of flares. Unfortunately, and are both typically variable and simultaneous measurements are scarce. Observed values and calculated ratios of and for the H$\alpha$- and X-ray-detected dwarfs VB 8, VB 10, LHS 2065, and LP 944-20 are listed in Table \[tbl\_field\]. We stress that these / ratios are not calculated from simultaneous measurements of and , but from a simple ranking of the observed values of each. Nevertheless, a striking change in non-flaring emission appears to take place between the M7 star VB 8, in which $>$ and the M8 and M9 dwarfs VB 10 and LP 944-20, in which $<$ (noted by Fleming [et al. ]{}2003). This suggests that the efficiency of persistent coronal heating decays more quickly in response to the increasingly neutral atmosphere of ultracool dwarfs than the efficiency of chromospheric heating. Within the same sample, excluding LP 944-20, transition region heating, inferred from C [iv]{} emission, appears to remain as efficient as chromospheric heating (Hawley & Johns-Krull 2003).

The ratio of the single measurements of and from Roque 14 is 4.0. If Roque 14’s measured chromospheric and coronal emissions are interpreted as persistent this suggests that its magnetic activity is of similar character to that of VB 8 and active main-sequence M dwarfs. Although we cannot exclude that Roque 14’s observed X-ray emission is solely the result of a flare with decay time-scale $\sim 5$ ks, like the observed high-level X-ray emission from M8 old-disc star VB 10 and M9 dwarfs LP 944-20, LHS 2065 and RXS J115928.5-524717, the high H$\alpha$ emission level of Roque 14 supports a persistently higher level of magnetic activity than on these cooler dwarfs.

Roque 14 has been suspected to be a near-equal-mass binary, but /$>2$ even if the X-ray emission is interpreted as coming from two stars[^8]. An observation by the [*Hubble Space Telescope*]{} NICMOS camera does not support Roque 14’s binarity, finding no companion of comparable brightness at separations $> 0.1$ arcsec, or 13.5 AU (Mart[í]{}n [et al. ]{}2000). While interaction between two close binary components could be influential in the X-ray production mechanism, the emission is at a similar level to that produced by magnetic activity on single active MS M dwarfs.

The observed strong H$\alpha$ emission from the M7.5 Roque 12 hints at activity similar to that of Roque 14. In contrast, the measured H$\alpha$ emission levels of the M8 Pleiades brown dwarfs Teide 1 and Roque 11 are very like that of the M8 VB 10, and we speculate that the magnetic activity of these objects is already in the regime where $<$ and their persistent X-ray emission levels are more than an order of magnitude lower than that of Roque 14. Deeper X-ray observations are required to test this prediction.

The evolution of X-ray emission from brown dwarfs
-------------------------------------------------

Very young brown dwarfs in star-forming regions are now routinely observed to emit X-rays at levels of [$L_{\rm X}/L_{\rm bol}$]{}$\sim 10^{-4}$–$10^{-3}$ that arise from hot ($T >
10^{7}$ K) plasma (e.g. Imanishi [et al. ]{}2001; Preibisch & Zinnecker 2002; Feigelson [et al. ]{}2002). A growing body of evidence indicates that young brown dwarfs experience accretion and outflows just like young low-mass stars (Jayawardhana, Mohanty & Basri 2003). Their X-ray emission is very likely to be produced by similar means to T Tauri stars. This is probably largely coronal emission as a result of magnetic activity, as in older low-mass stars, and as young substellar objects may have photospheres as warm as 2900 K (e.g. Baraffe [et al. ]{}1998), their atmospheric conditions are ripe for the efficient coronal heating seen on active MS M5–6 stars. However, H$\alpha$ emission may arise predominantly from material accreting on to the young brown dwarf, rather than from a hot chromosphere. X-ray emission levels appear to rise with H$\alpha$ emission levels up to $EW_{{\rm H}\alpha} \approx 20$ Å, as would be expected for H$\alpha$ emission predominantly from a magnetically-powered chromosphere, but there is no detection of X-rays from a young brown dwarf with $EW_{{\rm H}\alpha} 
> 30$ Å (Tsuboi [et al. ]{}2003). This is consistent with the scenario emerging from X-ray surveys of T Tauri stars wherein samples of stars showing signs of strong accretion, such as high $EW_{{\rm H}\alpha}$, appear to show lower levels of X-ray emission than weakly-accreting objects of similar mass (e.g. Flaccomio, Micela & Sciortino 2003).

X-ray emission has been previously detected from just two brown dwarfs older than 5 Myr. While any accretion is expected to have ceased by the age of the 12 Myr-old TWA 5B, this object has a much lower mass than Roque 14, 0.014–0.043 M$_{\sun}$ (Neuhäuser [et al. ]{}2000), and is unlikely to be a good model for its youthful X-ray activity. Conversely, LP 944-20 appears to have a similar mass to Roque 14, 0.056–0.064 M$_{\sun}$ (Tinney 1998). It has cooled to a spectral type of M9, so its lower atmosphere is highly neutral, which is probably the key reason why its persistent X-ray emission level is $\sim 100$ times lower than those of many very young brown dwarfs and the detected level from Roque 14. However, we should be wary of assuming that the activity levels of all coeval brown dwarfs are the same, and that Roque 14 and LP 944-20 are representative of their respective ages. There is a large spread in the observed / values of young brown dwarfs, a significant number being undetected at upper limits $< 10^{-4}$ (e.g. Imanishi [et al. ]{}2001). Roque 14 has one of the highest H$\alpha$ emission levels among Pleiades brown dwarf candidates (Zapatero Osorio [et al. ]{}1997b), while LP 944-20 has one of the lowest among M9 field dwarfs (Mohanty & Basri 2003). So, while we may expect the X-ray emission level of brown dwarfs to decrease as they age from $\approx 125$ to $\approx 320$ Myr, as apparently observed by comparing Roque 14 and LP 944-20, the effect is probably exaggerated in choosing these two objects as representative. Larger, and necessarily deeper, surveys of the X-ray emission of brown dwarfs in the field and in the Pleiades (and other clusters) are required to make further progress in understanding the evolution of the magnetic activity of substellar and ultracool objects.

Summary
=======

We have observed five candidate brown dwarfs in the Pleiades with [[*XMM-Newton*]{}]{}, detecting X-ray emission from the M7 Roque 14. The low number of counts and significant contribution of background counts prevent meaningful constraint of the temperature and exclusion of transient emission. Assuming a plasma temperature of $10^{7.0}$ K and nominal absorbing column to the Pleiades of $2 \times 10^{20}$ cm$^{-2}$, the time-averaged X-ray luminosity of $=3.3 \pm 0.8
\times 10^{27}$ , and its ratios with the bolometric ([$L_{\rm X}/L_{\rm bol}$]{} $\approx 10^{-3.05}$) and H$\alpha$ (/ $\approx 4.0$) luminosities resemble those of active main-sequence M dwarfs, as have been observed on the M7 old-disc star VB 8.

We have placed the tightest upper limits thus far on the X-ray emission levels of the four later-type Pleiades brown dwarfs: $<2.1$–$3.8 \times 10^{27}$ , and [$L_{\rm X}/L_{\rm bol}$]{} $<
10^{-3.10}$–$10^{-2.91}$.

The [[*XMM-Newton*]{}]{} data do not exclude that Roque 14’s observed high level of X-ray emission is solely the result of a flare with decay time-scale $\sim 5$ ks like the observed high-level emission from M8–9 field dwarfs VB 10, LP 944-20, LHS 2065 and RXS J115928.5-524717, but the high H$\alpha$ emission level of Roque 14 is supportive of a persistently higher level of magnetic activity than on these cooler dwarfs. However, the similarity of the low H$\alpha$ emission levels of Teide 1 and Roque 11 to that of the M8 old-disc star VB 10 prompts us to speculate that the persistent X-ray emission levels of M8 brown dwarfs in the Pleiades may be low, with [$L_{\rm X}/L_{\rm bol}$]{}$\la 10^{-5}$, like those of VB 10 and the M9, $\approx 320$ Myr-old brown dwarf LP 944-20.

Deeper X-ray observations, less contaminated by background, are required to confirm a persistent high level of X-ray emission from Roque 14, to assess its coronal temperature, and to probe the typical X-ray emission levels of Pleiades brown dwarfs. Coordinated programmes to study X-ray, H$\alpha$ and radio emission from ultracool and brown dwarfs of a range of ages are required to enable significant progress toward understanding the mechanisms and evolution of magnetic activity on these low-mass, cool objects.

Acknowledgments {#acknowledgments .unnumbered}
===============

JPP acknowledges the financial support of the UK Particle Physics and Astronomy Research Council (PPARC). The authors thank David Burrows for providing source code for calculating upper limits, and Manuel Güdel for useful discussions. This work uses data obtained by [[*XMM-Newton*]{}]{}, an ESA science mission with instruments and contributions directly funded by ESA Member States and the USA (NASA), as part of the [[*XMM-Newton*]{}]{} Survey Science Centre Guaranteed Time programme. The work also made use of archival material from the SIMBAD and VIZIER systems at CDS, Strasbourg, NASA’s Astrophysics Data System, the [[*ROSAT*]{}]{} Data Archive of the Max-Planck-Institut für extraterrestrische Physik (MPE) at Garching, Germany, and the Leicester Database and Archive Service (LEDAS).

Arnaud K. A., 1996, in Jacoby G.H., Barnes J., eds, ASP Conf. Ser. Vol. 101, Astronomical Data Analysis Software and Systems V. Astron. Soc. Pac., San Francisco, p. 67

Baraffe I., Chabrier G., Allard F., Hauschildt P. H., 1998, A&A, 337, 403

Berger E. et al., 2001, Nat, 410, 338

Berger E., 2002, ApJ, 572, 503

Boese F. G., 2000, A&AS, 141, 507

Briggs K. R., Pye J. P., 2003a, MNRAS, 345, 714

Briggs K. R., Pye J. P., 2003b, Adv. Sp. Res., 32, 1081

Delfosse X., Forveille T., Perrier C., Mayor M., 1998, A&A, 331, 581

Feigelson, E. D., Broos, P., Gaffney, J. A., Garmire, G., Hillenbrand, L. A., Pravdo, S. H., Townsley, L., Tsuboi, Y. 2002, [ApJ]{}, 574, 258

Festin L., 1998, MNRAS, 298, L34

Flaccomio E., Micela G., Sciortino S., 2003, A&A, 397, 611

Fleming T. A., 1988, Ph.D. thesis, Univ. Arizona

Fleming T. A., Giampapa M. S., Schmitt J. H. M. M., Bookbinder J. A., 1993, ApJ, 410, 387

Fleming T. A., Giampapa M. S., Schmitt J. H. M. M., 2000, ApJ, 533, 372

Fleming T. A., Giampapa M. S., Garza D., 2003, ApJ, 594, 982

Ghizzardi S., 2002, technical document, EPIC-MCT-TN-012

Giampapa M. S., Rosner R., Kashyap V., Fleming T. A., Schmitt J. H. M. M., Bookbinder J. A., 1996, ApJ, 463, 707

Gizis J. E., Monet D. G., Reid I. N., Kirkpatrick J. D., Liebert J., Williams R. J., 2000, AJ, 120, 1085

G[" u]{}del M., Audard M., Reale F., Skinner S. L., Linsky J. L., 2004, A&A, 416, 713

Hambaryan V., Staude A., Schwope A. D., Scholz R.-D., Kimeswenger S., Neuh[" a]{}user R., 2004, A&A, 415, 265

Hawley S. L., Johns-Krull C. M., 2003, ApJ, 588, L109

Hodgkin S. T., Jameson R. F., Steele I. A., 1995, MNRAS, 274, 869

Imanishi K., Tsujimoto M., Koyama K., 2001, ApJ, 563, 361

Jayawardhana R., Mohanty S., Basri G., 2003, ApJ, 592, 282

Kraft R. P., Burrows D. N., Nousek J. A. 1991, [ApJ]{}, 374, 344

Liebert J., Kirkpatrick J. D., Cruz K. L., Reid I. N., Burgasser A., Tinney C. G., Gizis J. E., 2003, AJ, 125, 343

Mart[í]{}n E. L., 1999, MNRAS, 302, 59

Mart[í]{}n E. L., Ardila D. R., 2001, AJ, 121, 2758

Mart[í]{}n E. L., Basri G., Zapatero Osorio M. R., Rebolo R., L[' o]{}pez R. J. G., 1998, ApJ, 507, L41

Mart[í]{}n E. L., Brandner W., Bouvier J., Luhman K. L., Stauffer J., Basri G., Zapatero Osorio M. R., Barrado y Navascu[' e]{}s D., 2000, ApJ, 543, 299

Meyer F., Meyer-Hofmeister E., 1999, A&A, 341, L23

Mohanty S., Basri G., Shu F., Allard F., Chabrier G., 2002, ApJ, 571, 469

Mohanty S., Basri G., 2003, ApJ, 583, 451

Mokler F., Stelzer B., 2002, A&A, 391, 1025

Neuhäuser R., Comerón F. 1998, Sci, 282, 83

Neuh[" a]{}user R. et al. 1999, [A&A]{}, 343, 883

Neuh[" a]{}user R., Guenther E. W., Petr M. G., Brandner W., Hu[' e]{}lamo N., Alves J. 2000, [A&A]{}, 360, L39

Orlando S., Peres G., Reale F., 2000, ApJ, 528, 524

Pan X., Shao M., Kulkarni S. R., 2004, Nat, 427, 326

Paresce F., 1984, AJ, 89, 1022

Pinfield D. J., Hodgkin S. T., Jameson R. F., Cossburn M. R., Hambly N. C., Devereux N., 2000, MNRAS, 313, 347

Pinfield D. J., Dobbie P. D., Jameson R. F., Steele I. A., Jones H. R. A., Katsiyannis A. C., 2003, MNRAS, 342, 1241

Preibisch T., Zinnecker H., 2002, [AJ]{}, 123, 1613

Reale F., Peres G., Orlando S., 2001, ApJ, 557, 906

Rebolo R., Zapatero Osorio M. R., Mart[í]{}n E. L., 1995, Nat, 377, 129

Rebolo R., Mart[í]{}n E. L., Basri G., Marcy G. W., Zapatero Osorio M. R., 1996, ApJ, 469, L53

Reid N., Hawley S. L., Mateo M. 1995, [MNRAS]{}, 272, 828

Rutledge R. E., Basri G., Mart[í]{}n E. L., Bildsten L., 2000, ApJ, 538, L141

Schmitt J. H. M. M., Liefke C., 2002, A&A, 382, L9

Schmitt J. H. M. M., Fleming T. A., Giampapa M. S. 1995, [ApJ]{}, 450, 392

Smith R. K., Brickhouse N. S., Liedahl D. A., Raymond J. C., 2001, ApJ, 556, L91

Stauffer J. R., 1984, ApJ, 280, 189

Stauffer J. R., Schultz, G., Kirkpatrick J. D. 1998, [ApJ]{}, 499, L199

Str[" u]{}der L. et al., 2001, A&A, 365, L18

Tinney C. G. 1998, [MNRAS]{}, 296, L42

Tinney C. G., Reid I. N., 1998, MNRAS, 301, 1031

Tsuboi Y., Maeda Y., Feigelson E. D., Garmire G. P., Chartas G., Mori K., Pravdo S. H., 2003, ApJ, 587, L51

Turner M. J. L. et al. 2001, [A&A]{}, 365, L27

Zapatero Osorio M. R., Rebolo R., Mart[í]{}n E. L., 1997a, A&A, 317, 164

Zapatero Osorio M. R., Rebolo R., Mart[í]{}n E. L., Basri G., Magazzu A., Hodgkin S. T., Jameson R. F., Cossburn M. R., 1997b, ApJ, 491, L81

Zapatero Osorio M. R., Rebolo R., Mart[í]{}n E. L., Hodgkin S. T., Cossburn M. R., Magazz[\` u]{} A., Steele I. A., Jameson R. F., 1999, A&AS, 134, 537

\[lastpage\]

[^1]: briggs@astro.phys.ethz.ch

[^2]: pye@star.le.ac.uk

[^3]: http://xmm.vilspa.esa.es/.

[^4]: The [pn]{} exposure map must be scaled by a factor to account for its higher sensitivity compared to M1 and M2. This factor is dependent on the source spectrum and discussed in Section 4.3.

[^5]: [*ML*]{} values output by [emldetect]{} have been corrected as advised in [*XMM News 29*]{}.

[^6]: A source detected by a previous analysis 15 arcsec from Roque 9 only in the 0.8–1.5 keV band is not recovered in this procedure, and was in any case considered more likely to be the chance alignment of a background source (Briggs & Pye 2003b).

[^7]: For a $T = 10^{7.0}$ K plasma with solar abundances and [$N_{\rm H}$]{}$=2 \times 10^{20}$ cm$^{-2}$ the 0.3–1.4 keV energy band enables detection at a given signal:noise ratio against the observed background spectrum for the lowest total of 0.3–4.5 keV source counts. The required number of source counts is only slowly increasing with the upper energy bound up to 4.5 keV and we do not find better constraint of the Roque 14 source variability or plasma temperature using this stricter energy cut.

[^8]: As is calculated here from $EW_{{\rm H}\alpha}$, as a ratio with the continuum, it should be little changed whether there is one source or are two similar sources.
