CONTEMPORANEOUS XMM-NEWTON INVESTIGATION OF A GIANT X-RAY FLARE AND QUIESCENT STATE FROM A COOL M-CLASS DWARF IN THE LOCAL CAVITY

Figure 2 shows the background-subtracted X-ray light curve with 1 ks binning for source 2XMM J043527.2−144301. The source light curve clearly shows two flares that occurred after an initial quiescent period of at least 65 ks (including a time interval in the middle of the observation affected by proton flare). The first flare was very strong; in the energy range (0.2–7.5) keV the peak count rate reached in the light curve was (0.11–0.12) counts s⁻¹, depending on the choice of binning. However, during the quiescent period the source was very faint, with an average count rate of 0.0021 counts s⁻¹, corresponding to an increase of more than 52 in the count rate from quiescent to peak. The second flare in the same energy band had peak count rate of 0.055 counts s⁻¹. Both flares show fast-rise, exponential-decay profile with a rise time of ∼3 ks, and similar e-folding decay times of (4.3 ± 0.7) ks and (4.5 ± 1.0) ks for the first and second flares, respectively.

Zoom In Zoom Out Reset image size

We compared the XMM-Newton position of source 2XMM J043527.2−144301 with error radius of 4'' with available optical/IR catalogs. We found a faint 2MASS counterpart (2MASS J04352724−1443017) only 048 from the XMM-Newton position, with K_s = 14.35 ± 0.08 and (J − K_s) = 1.16 ± 0.10 (Figure 3, Table 1). There is only one source in a 4' radius in the near-infrared band of the DSS image, which is not visible at other wavelengths (Figure 3).

Zoom In Zoom Out Reset image size

We also found a counterpart of our source in the USNO-B1.0 catalog and Guide Star Catalog GSC-2.3 with offset of 09. The USNO-B1.0 counterpart has R-band magnitude of 19.37 and near-IR band magnitude of 17.9. Whereas the GSC counterpart is identified only in near-IR (0.8 μm band), there is no detection in F (red), J (blue), and V (green) photographic bands. GSC-2.3 catalog is based on ground-based photographic plate material. The majority of stars visible in these camera plates are very red late-type stars, in agreement with the 2MASS characterization (see the next paragraph).

To assign a spectral class to source 2XMM J043527.2− 144301, we compared the 2MASS colors of this source with the colors of other M dwarfs⁴ and found that the (J − H), (H − K_s), and (J − K_s) colors are best matched by the spectral class M8.5V. To confirm the spectral class, in Figure 4 we compare source 2XMM J043527.2−144301 with data from Gizis et al. (2000, their Figure 4) on a color–color diagram. In particular, the figure shows typical (J − H) versus (H − K_s) value for M7–M7.5 (horizontal striped region) and M8 or later (vertical striped region) dwarfs. Most M7–M7.5 dwarfs are around (H − K_s, J − H) = (0.45, 0.62) whereas M8–M8.5 dwarfs are around (H − K_s, J − H) = (0.42, 0.74) with similar J − K_s colors (although, as the vertical striped region indicates, a few M8 or later stars may have significantly higher numbers). The black star in the figure shows the colors (H − K_s, J − H) = (0.42 ± 0.10, 0.74 ± 0.07) of the 2MASS counterpart of the source 2XMM J043527.2−144301, indicating a spectral class of M8–M8.5. We note that to properly confirm the spectral type of the source dedicated spectroscopic observations are necessary.

Zoom In Zoom Out Reset image size

To determine the distance of the source, we estimated the absolute magnitude from the 2MASS (J − K_s) color using the relation from Gizis et al. (2000), M_k = 7.593 + 2.25 × (J − K_s), with a scatter of σ = 0.36 mag. The relation is valid for only M7 and later dwarfs over the color range. The source 2XMM J043527.2−144301 value of (J − K_s) = (1.16 ± 0.10) gives M_k = 10.21 ± 0.22, and using the standard distance modulus equation $(m_{k}-M_{k}=5\log \frac{d_{k}}{10})$, we get d_k = (67 ± 13) pc. We note that this distance is significantly shorter than the measured distance of MBM20 (112 ± 15 pc <d < 161 ± 21 pc), indicating that the source is not part of MBM20, but in front of it. As discussed in the next sections, this is consistent with the spectral analysis of the flares and their inferred luminosity. In our subsequent analysis we use the distance derived from the 2MASS (J − K_s), although we will also briefly discuss the possibility that, instead, source 2XMM J043527.2−144301 is part of MBM20.

Using the relation for Bolometric Correction (B.C.)_k from Legget et al. (2001), we also obtained a bolometric magnitude of M_bol = (13.32 ±  0.43), which leads to a bolometric luminosity of L_bol = (1.4 ± 0.6) × 10³⁰ erg s⁻¹. We then used the near-IR colors to estimate the radius, effective temperature, and mass of the source 2XMM J043527.2−144301. Assuming a typical age of ultracool dwarfs of τ < 2 to 3 Gyr, we used the relation from Dahn (2002), R/R_☉ = 0.088 + 0.00070(16.2 − M_bol)^2.9, which predicts radii for objects with age between 1 Gyr and 5 Gyr over the range 12–16.5 in M_bol, to calculate the radius of the source:

$$\begin{equation} \frac{R}{R_{\odot }}=0.103\pm 0.006. \end{equation} \tag{ 1 }$$

With the calculated values of luminosity and radius, we also determined the effective temperature of the source as T_eff = (2479 ± 275) K, which is typical for an ultracool M dwarf. Using the relationships for mass versus M_k and mass versus spectral type from Chabrier & Baraffe (2000—their Figures 9 and 10, respectively), we found that the mass of 2XMM J043527.2−144301 is in the range (0.079 M_☉–0.083 M_☉), suggesting that the object is near the stellar/substellar transition.

To characterize the process responsible for the X-ray emission from source 2XMM J043527.2−144301, we analyzed its pn spectra during the quiescent and flare states. Source and background spectra in the (0.2–7.5) keV band were produced with SAS task xmmselect, using the same regions used to extract the light curve. The SAS tasks rmfgen and arfgen were used to produce redistribution matrices files (RMFs) and ancillary response files (ARFs).

Spectral fitting of the background-subtracted spectrum was performed with XSPEC v 12.4. We applied standard stellar emission models such as absorbed one-temperature (1T) and two-temperature (2T) thermal plasma models. For plasma thermal emission, the APEC (Smith et al. 2001) model was used. We also repeated the fits with other spectral models for thermal equilibrium plasma such as Raymond–Smith (RAYMOND) and Mewe–Kaastra–Liedahl (MeKaL), but we did not find any significant change in the fit parameters or reduced χ².

During the quiescent state the source was very faint, with net counts (after removing background) of (40 ± 14) in 18990 s (exposure corrected time) in the energy range (0.2–7.5) keV, for a quiescent count rate of (2.1 ± 0.7) × 10⁻³ counts s⁻¹. For model fitting, the source spectrum (during the quiescent state) was grouped to have 10 counts channel⁻¹ (before background subtraction). The spectrum is well fitted (Figure 5) by an absorbed cool thermal plasma model; however, the value of the temperature varies significantly depending on the binning and interval used for the fit. Combining all results, the best value for the temperature is kT = (1.7 ± 0.8) keV, while the absorption is poorly constrained. Due to the poor constraints on the fit with the 1T model, no significant improvement was obtained using a 2T plasma model. We note that the spectrum is also affected by an excess of counts at low energy, probably due to residual background.

Zoom In Zoom Out Reset image size

During the flare, 2XMM J043527.2−144301 was one of the brightest sources in the field. A weaker flare was also observed during the decay of the first giant flare. We analyzed the source spectrum during the first (strong) and second (weak) flares separately and did not find any significant difference between the two. We therefore only report here the spectral analysis of the sum of the two.

The first flare occurred during the observing time 70.0 to 80.0 ks (exposure corrected time of 6.14 ks). The source had net counts of (267 ± 16) and (276 ± 17) in the energy bands (0.2–2.0) kev and (0.2–7.5) keV, respectively, corresponding to average count rates of (4.3 ± 0.3) × 10⁻² counts s⁻¹ and (4.5 ± 0.3) × 10⁻² counts s⁻¹. The second flare had net counts of (90 ± 9) for an exposure corrected time of 3.35 ks (observing time 80.0 to 86.0 ks) in the energy range of 0.2–2.0 keV, corresponding to a count rate of (2.7 ± 0.3) × 10⁻² counts s⁻¹.

Since most of the emission from the source is in the energy range of 0.2–2.0 keV, as expected from a late type dwarf, we limited our spectral fitting to the 0.2–2.0 keV energy range. We grouped the spectrum to have 15 counts channel⁻¹. The 1T thermal plasma models gave unacceptably high values for reduced chi square (χ² = 63, n = 16); however, the spectral model with 2T thermal plasma resulted in better fits. The fit results are summarized in Table 2 and fits are shown in Figure 5. In the fits, the absorption is still poorly constrained, but is consistent with very low neutral hydrogen column density (NH).

To set a limit on NH, we repeated the fits with both free and fixed absorption. The fits with free absorption were statistically compatible with those with NH fixed at zero. We also repeated the fits by increasing the value of the fixed NH. By looking at the quality of the fit (value of χ²), we were able to set an upper limit on the neutral hydrogen column density of NH < 2.8 × 10²⁰ cm⁻². Note that the effect of the variation in NH on the other fit parameters is reflected in the results reported in Table 2. Using IRAS100 μm data (Galeazzi et al. 2007) we estimated the neutral hydrogen column density of MBM20 in the same direction to be 1.7 × 10²¹ cm⁻², supporting the notion that the source is, indeed, in front of MBM20.

We also extracted the spectrum of the peak emission of the first flare. We defined the peak period as the interval in which the count rate was above 50% of the maximum value (i.e., greater than 0.057 counts s⁻¹). For the peak emission the exposure corrected time is 2.79 ks (from 71.2 to 75.4 ks). The 2T thermal plasma model results in a good fit (Table 2) with the temperature of the cooler component similar to that obtained from the sum of the two flares; however, the temperature of the hotter component increases from ∼29 MK to ∼46 MK.

During the quiescent period the flux of the source in the energy band (0.2–2.0) keV was (9.1 ± 2.5) × 10⁻¹⁵ erg s⁻¹ cm⁻², where uncertainties are 90% confidence range. Assuming the distance d_k = (67 ± 13) pc, inferred from 2MASS colors, this leads to a luminosity of (4.9 ± 2.4) × 10²⁷ erg s⁻¹.

The average flux of the source during the first flare in the energy band (0.2–2.0) keV was (1.07 ± 0.14) × 10⁻¹³ erg s⁻¹ cm⁻², and the corresponding luminosity (5.8 ± 2.4) × 10²⁸ erg s⁻¹. Integrating the X-ray luminosity during the time of the first flare, including the fraction overlapping with the second flare, the total emitted X-ray energy was (6.7 ± 2.8) × 10³² erg.

For the peak emission of the first flare, the flux in the energy band (0.2–2.0) keV was (1.5 ± 0.4) × 10⁻¹³ erg s⁻¹ cm⁻² and the luminosity (8.2 ± 3.7) × 10²⁸ erg s⁻¹, while the total X-ray energy released during peak emission was (3.5 ± 1.5) × 10³² erg. Using the 2T thermal plasma model parameters, the count rate of the flare at the peak (∼0.12 counts s⁻¹) can also be converted to a flux of (2.8 ± 0.4) × 10⁻¹³ erg s⁻¹ cm⁻² and corresponding luminosity of (1.5 ± 0.6) × 10²⁹ erg s⁻¹.

The average flux of the second flare in the energy band (0.2–2.0) keV was (6.8 ± 2.1) × 10⁻¹⁴ erg s⁻¹ cm⁻² and the corresponding luminosity (3.7 ± 1.8) × 10²⁸ erg s⁻¹. Removing the contribution of the first flare, the net energy released from the second flare was (1.3 ± 1.1) × 10³² erg. Note that the total X-ray energy released during the first flare was about six times the total X-ray energy released during the second one. The total X-ray energy emitted by the source through both flares was (7.7 ± 3.3) × 10³² erg.

We also compared the X-ray and bolometric luminosity of source 2XMM J043527.2−144301 to find its activity level. We found the log of the ratio to be log L_x/L_bol = (−2.5 ± 0.7) in the quiescent state and log L_x/L_bol = (−1.0 ± 0.6) at the flare peak, indicating a high activity level of the flare. These values are comparable to what has been observed by Hambaryan et al. (2004) for M9 dwarf IRXS J115928.5−524717.

We compared our results with previous investigations of other ultracool M dwarfs. Table 3 summarizes the luminosities and coronal temperatures of all the observations available in X-rays. The luminosity of ultracool M dwarfs in a low activity period varies from 2.5 × 10²⁵ erg s⁻¹ to 2.0 × 10²⁷ erg s⁻¹. Our value for luminosity (4.9 ± 2.4) × 10²⁷ erg s⁻¹ is close to the upper range of luminosities observed so far in the quiescent state for ultracool M dwarfs. The same is true for the flare mean and peak emission luminosities.

The luminosities and coronal temperatures of our source in the quiescent and flaring periods are of the same order as observed for M8 dwarf LP 412-31. Stelzer et al. (2006) observed a giant flare from LP 412-31 lasting for ∼2 ks with peak luminosity of 4.7 × 10²⁹ erg s⁻¹. They reported an increase in the count rate by a factor 200–300 from quiescent to peak of flare. The emitted flare energy in soft X-rays was 3.0 × 10³² erg, similar to the value we observed for 2XMM J043527.2−144301.

We note that, if instead of the measured distance the source is associated with MBM20, based on the cloud distance all the values reported would be at least three times bigger. This would be inconsistent with data from previous investigations, strengthening the conclusion that, indeed, the source is not part of MBM20, but in front of it.