PHASE CONNECTING MULTI-EPOCH RADIO DATA FOR THE ULTRACOOL DWARF TVLM

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1 PHASE CONNECTING MULTI-EPOCH RADIO DATA FOR THE ULTRACOOL DWARF TVLM J.G. Doyle 1, A. Antonova 1,2, G. Hallinan 3, A. Golden 3 ABSTRACT Radio observations for a number of ultracool dwarfs show short duration pulses once or twice per rotation period. Here we evaluate the stability of these pulses for one such object, TVLM In particular, data taken in April and June 27 show that the pulses are confined to a narrow range in the rotation period. This suggests the presence of stable magnetic fields on timescales of at least 6 weeks. These pulses are produced by the electron cyclotron maser (ECM) instability, with one such pulse in the June 27 data showing a 5-fold increase in activity over one rotation period (p = hrs.). The similarly in the burst structure in datasets taken weeks/months apart point towards the stability of a stable electric fields structure which is somehow generated and sustained within the magnetosphere of the ultracool dwarf. Subject headings: stars: low-mass, brown dwarfs radio continuum: stars processes: magnetic fields, masers, polarization 1. INTRODUCTION The detection of radio emission from the coronae of a wide range of main sequence stars was a major breakthrough with the advent of radio astronomy. The quiescent emission suggested the presence of a non-thermal corona with large populations of electrons at high energies. This quiescent emission has generally been attributed to non-thermal gyrosynchrotron radiation. One possible explanation for its persistent nature and low degree of variability in M dwarfs is that it is due continuous unresolved low-level flaring together with 1 Armagh Observatory, College Hill, Armagh BT61 9DG, N. Ireland; tan@arm.ac.uk, jgd@arm.ac.uk. 2 Department of Astronomy, Faculty of Physics, St Kliment Ohridski, University of Sofia, 5 James Bourchier Str. BG-1164 Sofia, Bulgaria 3 Computational Astrophysics Laboratory, I.T. Building, National University of Ireland, Galway, Ireland; gregg@it.nuigalway.ie, agolden@it.nuigalway.ie.

2 2 efficient trapping of accelerated electrons in coronal loops. Given the above considerations, it was expected that radio emission from ultracool dwarfs (UCDs spectral type M8 and later) should be weak or absent, in accordance with the empirical correlation of Güdel & Benz (1993). However, recent results from a number of UCDs (Hallinan et al 26, 28) have raised questions regarding this interpretation. Less than a decade ago it was considered that the cool, dense atmosphere in UCDs implies low ionization fractions and thus high electrical resistivities, leading to a decoupling of magnetic lines from the upper atmosphere, and hence reduced or zero activity in the outer atmosphere. There are now a number of confirmed radio detections at and below the substellar boundary in recent years (Berger et al. 21, Berger 22, Burgasser & Putman 25, Berger et al. 25, Phan-Bao et al. 27, Antonova et al. 27) violating the Güdel & Benz relation by up to four orders of magnitude. Recently, we have detected periodic pulses of 1% circularly polarized radio emission from four ultracool dwarfs (Hallinan et al. 26, 27, 28; Antonova et al. 28), with the periodicity for two of these dwarfs confirmed to correspond to the rotation period of the dwarf (Lane et al. 27). Due to the high degree of polarization, the emission process must be coherent. The data collected to date, suggest that the pulses are produced at the poles by the electron cyclotron maser (hereafter ECM) instability, the same mechanism known to be responsible for the radio emission from the magnetized planets in our solar system (Zarka 1998). This mechanism is also the source of certain classes of solar and stellar bursts (Melrose & Dulk 1982, Bingham et al. 21, Osten & Bastian 26). It was also suggested to be operational in the magnetic chemically peculiar star CU Vir where 1% circularly polarized, bright and highly directive radio bursts were detected (Kellett et al. 27, Trigilio et al. 27), and for the compact binary RX J (Ramsay et al. 27). However, others, e.g. Berger et al. (28) argue towards the gyrosynchrotron interpretation as the radio emission mechanism for UCDs. The radio monitoring for a number of ultracool dwarfs has confirmed that the pulses can vary in brightness, disappear and reappear. If the magnetic field is stable, such pulses should always be confined to a particular range of phase of rotation of the dwarf governed by the topology of the magnetic field. This can be investigated through correlating the phase of the radio pulses from datsets obtained at different epochs. Here, we report on observations of TVLM (thereafter TVLM 513) taken in June 27, these being followup observations to the earlier detection of pulses in data acquired in 26. In addition, we re-analysis all of the archival data, plus in particular, data taken in late April 28 (Berger et al 28), i.e. 42 days earlier than our data.

3 3 2. TVLM : A potted History Berger (22) reported variable emission from TVLM 513 at an average flux of 38 µjy during 2hr observation in September 21. A brightening towards the end of the observation was interpreted as a flare (see later). Osten et al. (26) re-observed TVLM 513 in January 24 during a 12 hr observation period, sampling three frequencies, 8.4 GHz, 4.8 Hz and 1.4 GHz. The 8.4 GHz observation had an average flux of 228 µjy; however due to sequential sampling of three different frequencies, the time resolution was not sufficient to allow a variability study. In January 25, Hallinan et al. (26) observed the same target simultaneously at 8.4 GHz and 4.9 GHz for a total of 5 hr. These authors reported a periodicity at both frequencies of 2 hr (later confirmed as the rotation period of the dwarf, Lane et al. 27), with a 4 µmjy flux at 8.4 GHz. The first report of pulsed emission from TVLM 513 was by Hallinan et al. (27) who observed the target in May 26 for 1 hrs on consecutive days at 8.4 GHz and 4.9 GHz. Approximately 9 hr of data obtained in April 27 by Berger et al (28) claim a non-detection of pluses but instead reported non-periodicity flaring activity (see later). A summary of the radio observations of TVLM 513 is given in Table Observations & Results On 1 June 27, follow-up observations of TVLM 513 at 8.6 GHz were conducted for 8 hours using the VLA in full array mode with the A configuration. The flux density was determined using the extragalactic source (3C286) which was observed both at the start and the end of the observation. Phase was monitored using the source observed for 11 sec every 1 min. The time resolution of the observation was set to be 1.7 sec. Data reduction was carried out with the Astronomical Image Processing System (AIPS) software package. The visibility data were inspected for quality both before and after the standard calibration procedures, and noisy points removed. For imaging the data, we used the task IMAGR. We also CLEANed the region around each source and used the UVSUB routine to subtract the resulting source model of the background sources from the visibility data. TVLM 513 was detected with a mean flux level of 318 ± 9 µjy. This flux is 1µJy lower than the one found in January 25 and May 26, yet it is similar to that reported by Berger (22). The Stokes V map of the source did not produce a source at the position of the dwarf, thus the net polarization for the total time of the observation could only be constrained by an upper limit on the polarized flux (S V < 3 µjy) to be f c < 1%. However,

4 June April Stokes I 1.4 Stokes I Time (Hours) Time (Hours) June 27 Stokes V April 27 Stokes V Time (Hours) Time (Hours) Fig. 1. Light curves of TVLM 513 in Stokes I (upper panel) and Stokes V (lower panel) on June 1, 27 and April 2, 27. The data is plotted with a time resolution of 6 seconds. The vertical lines correspond to the rotational period. inspection of the light curve of TVLM 513 revealed a number of bright, highly polarized bursts of short duration (Figure 1), as in the May 26 observations of the same dwarf. The Stokes V map of a single 1 minute scan containing one such burst give a detection with a mean flux level S V = 348 ± 43 µjy. This is a second epoch observation of pulsed radio emission from TVLM 513, thus suggesting that the emission mechanism is stable on long times scales. In addition to the highly polarized and short lived bursts, several more gradual brightenings lasting 4 minutes can also be seen in the light curve. A closer inspection of the pairs of brightenings at around 2.8 and 6.8 hours and around 4.65 and 8.6 hours respectively in the total intensity light curve, revealed a separation of the emission peaks of 3.95 hours, which is very close to two rotational periods of the dwarf. The same correlation can be seen in the Stokes V map. In a visual inspection of the data, if we take the second part of the light curve (from 6 hours onwards) and shift in time by 3.92 hours (i.e. two rotational periods), the agreement between the two sections of the lightcurve is excellent. The peak around 8.6 hrs is additionally enhanced in brightness by the presence of a very strong burst, hence the larger difference in magnitude when compared to the peak at 4.65 hours.

5 5 The periodicity is more clearly seen in the Lomb-Scargle periodogram (Figure 2), which has two peaks at frequencies.5 hr 1 and 1 hr 1, corresponding to a period of 1.96 hours, as is the case for the January 25 and May 26 data. For comparison, we also give the Lomb-Scargle periodogram for the May 26 data. All three datasets have a high probability of significance. As mentioned in Section 2, data was obtained in April 27 for TVLM 513 as reported by Berger et al. (28). However, these authors failed to detect a periodicity, thus this data was obtained from the archive and reduced in a similar manner as the June 27 data. The resulting Stokes I and V lightcurves are also shown in Figure 1. As in the June 27 data, similar brightenings are present, separated by less than 2 hrs. The Lomb-Scargle periodogram again reveals two peaks at frequencies.5 hr 1 and 1 hr 1. This therefore contradicts the findings of Berger et al (28), who suggested that the reported brightenings/pulses were random flare events. In-fact, using these two datasets we can extract a very accurate period. Figure 3(a) shows the Stokes I and Figure 3(b) shows the Stokes V light-curve from the June 27 (light line) and from the April 27 data (dark line) phase folded to a period of ±.7 hours. In deriving the period, both Stokes I and V were used. There is excellent agreement between the two datasets which shows that the pulse locations are stable (although they may vary in intensity and duration). This has major implications for the stability of the field structure, at least over a time interval of 42 days. Using the above period, we identify peaks in each of the datasets from 21 to 27, plotting these over a two hour interval in Figure 4. Taking the center of the pulse at hr on 23 September 21, 14.6 hr on 13 January 25, 6.97 hrs on 2 May 26, 6.78 hr. on 2 April 27 and 8.58 hr. on 1 June 27, agrees very well with the above derived period. The interesting thing about Figure 4 is the overall correspondence between the pulses over a 6 yr. time span, although in detail changes do occur. For example, the pulses around phase 1 (see Figure 3) seen in both datasets taken in 27, show right polarization followed by left polarization while the reverse is true for the May 26 data. Figure 4 also highlights the importance of high cadence data. For example, in the September 21 dataset, Berger (22) smoothed the data over several minutes and reported the presence of a flare. However, as seen in Figure 4, this event consists of three pulses when binned over a shorter time interval. Thus heavy binning/smoothing of the data hides the individual peaks during the pulse phase and would result in a more flare-like lightcurve, leading to a mis-interpretation of the phenomena.

6 6 4. Discussion and Future Work The present study confirms that the electron cyclotron maser instability is indeed the dominant source of radio emission from TVLM 513. We have established to a high degree of accuracy the lack of evolution in the morphology of the periodic light curves over an interval of several weeks. Over longer time intervals, changes in the structure of the pulses are observable, although the pulses do still occur (within a few minutes) at the same orbital phase. The pulsed emission give vital information on the characteristic size and topology of magnetic fields in ultracool dwarfs. As we show, the pulses observed in April and June 27 do remain confined to a narrow range of rotational phase. This therefore confirms the presence of stable magnetic fields structures on timescales of at least 6 weeks. Such information is crucial to understanding the means by which persistent levels of ECM emission are sustained in the magnetospheres of UCDs. This persistent generation of ECM emission in turn, implies the presence of stable electric fields, which are somehow generated and sustained within the magnetosphere of the ultracool dwarf. Such electric fields may be a fundamental source of electron acceleration, and hence radio emission, for plasma trapped in a large-scale magnetic field. Berger (22) and Berger et al (28) interpret the TVLM 513 pulses as random flare events. This work clearly shows otherwise, furthermore we suggest that all of these pulses are produced by the ECM instability and one needs to be careful with regard to binning/smoothing of the data. Further monitoring, particularly at a higher cadence is required in order to investigate the location, extent, and morphology of the sub-pluses which are clearly observable in in each burst interval. Also, further refinement of the period could be obtained with additional I-band monitoring. Other questions that need to be addressed include, what changes are required in the ECM instability to produce the observed a 5-fold increase in activity over one rotation period? Armagh Observatory is grant-aided by the N. Ireland Dept. of Culture, Arts & Leisure. G.H. and A.G. gratefully acknowledge the support of Science Foundation Ireland (grant No. 7/RFP/PHYF553).

7 May 26 1 Power α = 1ε hrs April 27 4 Power 3 α = 1ε hrs June α = 1ε 12 Power hrs 1 Fig. 2. Lomb-Scargle periodogram for the April and June 27 datasets, plus for comparison the May 26 data. For each dataset, the false alarm probability is given.

8 1.2 1 Stokes I 8 April 27 June Stokes V April 27 June Rotational Phase Fig. 3. Stokes I and V from June 27 (light line) and April 27 (dark line) phase folded.

9 Sept. 21 Jan. 25 May 26 Apr. 27 June Time (Hours) Fig. 4. Stokes I data for a burst in each of the datasets from September 21 to June 27, with each dataset shifted in flux, see text for the times of the selected bursts.

10 1 REFERENCES Antonova, A., Doyle, J.G., Hallinan, G., Golden, A. & Koen, C., 27, A&A 472, 257 Antonova, A., Doyle, J.G., Hallinan, G., Bourke, S. & Golden, A., 28, A&A 487, 317 Berger, E., and 13 co-authors, 21, Nat, 41, 338 Berger, E., 22, ApJ 572, 53 Berger, E., and 14 co-authors, 25, ApJ, 627, 96 Berger, E., Gizis, J.E., Giampapa, M.S., Rutledge, R.E., Liebert, J., Martn, E., Basri, G., Fleming, T.A., Johns-Krull, C.M., Phan-Bao, N. & Sherry, W.H., 28, ApJ 673, 18 Bingham, R., Cairns, R.A. & Kellett, B.J., 21, A&A 37, 1 Burgasser, A.J. & Putman M.E., 25, ApJ, 626, 486 Güdel, M. & Benz, A.O., 1993, ApJ 45, L63 Hallinan, G., Antonova, A., Doyle, J.G., Bourke, S., Brisken, W.F. & Golden, A., 26, ApJ 653, 69 Hallinan, G., Lane, C., Bourke, S., Antonova, A., Doyle, J.G., Zavala, R.T., Brisken, W.F., Boyle, R.P., Vrba, F.J. & Golden, A., 27, ApJ 663, L25 Hallinan, G., Antonova, A., Doyle, J.G., Bourke, S., Lane, C. & Golden, A., 28, ApJ 684, 644 Kellett, B.J., Graffagnino, V., Bingham, R., Muxlow, T.W.B. & Gunn, A.G., 27, astroph/71214 Lane, C., Hallinan, G., Zavala, R.T., Butler, R.F., Boyle, R.P., Bourke, S., Antonova, A., Doyle, J.G., Vrba, F.J. & Golden, A., 27, ApJ 668, L163 Melrose, D.B. & Dulk, G.A., 1982, ApJ 259, 844 Osten, R.A. & Bastian, T.S., 26, ApJ 637, 116 Osten, R.A., Hawley, S.L., Bastian, T.S. & Reid, I.N., 26, ApJ 637, 518 Phan-Bao, N., Osten, R.A., Lim, J., Martín, E.L., Ho, Paul, T.P., 27, ApJ. 658, 553

11 11 Ramsay, G., Brocksopp, C., Wu, K., Slee, B. & Saxton, C.J., 27, MNRAS 382, 461 Reiners, A. & Basri, G., 27, ApJ 656, 1121 Trigilio, C., Leto, P., Umana, G., Buemi, C.S. & Leone, F., 28, MNRAS 384, 1437 Zarka, P., 1998, JGR 13, 2159 This preprint was prepared with the AAS L A TEX macros v5..

12 12 Table 1: A summary of the radio observations for TVLM 513 Date of. Instr/Array/ Integr. Time Average flux Period Data first reported: observ. Freq. (GHz) time (h) res. (s) (µmjy) VLA CD ±16 no Berger VLA CD ±11 no Osten et al VLA CD ±13 no VLA A ±18 2 h Hallinan et al VLA A ±16 2 h VLA A ± h Hallinan et al VLA A ± h VLA D ± h Berger et al VLA A ± h present