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Register a new userLarge X-ray Flares on Stars Detected with MAXI/GSC: A Universal Correlation between the Duration of a Flare and its X-ray Luminosity
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23 giant flares from 13 active stars (eight RS CVn systems, one Algol system, three dMe stars and one YSO) were detected during the first two years of our all-sky X-ray monitoring with the gas propotional counters (GSC) of the Monitor of All-sky X-ray Image (MAXI). The observed parameters of all of these MAXI/GSC flares are found to be at the upper ends for stellar flares with the luminosity of 10^(31-34) ergs s-1 in the 2-20 keV band, the emission measure of 10^(54-57) cm-3, the e-folding time of 1 hour to 1.5 days, and the total radiative energy released during the flare of 10^(34-39) ergs. Notably, the peak X-ray luminosity of 5(3-9)*10^33 ergs s-1 in the 2-20 keV band was detected in one of the flares on II Peg, which is one of the, or potentially the, largest ever observed in stellar flares. X-ray flares were detected from GT Mus, V841 Cen, SZ Psc, and TWA-7 for the first time in this survey. Whereas most of our detected sources are multiple-star systems, two of them are single stars (YZ CMi and TWA-7). Among the stellar sources within 100 pc distance, the MAXI/GSC sources have larger rotation velocities than the other sources. This suggests that the rapid rotation velocity may play a key role in generating large flares. Combining the X-ray flare data of nearby stars and the sun, taken from literature and our own data, we discovered a universal correlation of tau~L_X^0.2 for the flare duration tau and the intrinsic X-ray luminosity L_X in the 0.1-100 keV band, which holds for 5 and 12 orders of magnitude in tau and L_X, respectively. The MAXI/GSC sample is located at the highest ends on the correlation.
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We present a large X-ray flare from a nearby weak-lined T Tauri star TWA-7 detected with the Gas Slit Camera (GSC) on the Monitor of All-sky X-ray Image (MAXI). The GSC captured X-ray flaring from TWA-7 with a flux of $3times10^{-9}$ ergs cm$^{-2}$ s$^{-1}$ in 2--20 keV band during the scan transit starting at UT 2010-09-07 18:24:30.The estimated X-ray luminosity at the scan in the energy band is 3$times10^{32}$ ergs s$^{-1}$,indicating that the event is among the largest X-ray flares fromT Tauri stars.Since MAXI GSC monitors a target only during a scan transit of about a minute per 92 min orbital cycle, the luminosity at the flare peak might have been higher than that detected. At the scan transit, we observed a high X-ray-to-bolometric luminosity ratio, log $L_{rm X}/L_{rm bol}$ = $-0.1^{+0.2}_{-0.3}$; i.e., the X-ray luminosity is comparable to the bolometric luminosity. Since TWA-7 has neither an accreting disk nor a binary companion, the observed event implies that none of those are essential to generate such big flares in T Tauri stars.
From a sample of GRBs detected by the $Fermi$ and $Swift$ missions, we have extracted the minimum variability time scales for temporal structures in the light curves associated with the prompt emission and X-ray flares. A comparison of this variability time scale with pulse parameters such as rise times,determined via pulse-fitting procedures, and spectral lags, extracted via the cross-correlation function (CCF), indicate a tight correlation between these temporal features for both the X-ray flares and the prompt emission. These correlations suggests a common origin for the production of X-ray flares and the prompt emission in GRBs.
Monitor of All sky X-ray Image (MAXI) discovered a new outburst of an X-ray transient source named MAXI J1421-613. Because of the detection of three X-ray bursts from the source, it was identified as a neutron star low-mass X-ray binary. The results of data analyses of the MAXI GSC and the Swift XRT follow-up observations suggest that the spectral hardness remained unchanged during the first two weeks of the outburst. All the XRT spectra in the 0.5-10 keV band can be well explained by thermal Comptonization of multi-color disk blackbody emission. The photon index of the Comptonized component is $approx$ 2, which is typical of low-mass X-ray binaries in the low/hard state. Since X-ray bursts have a maximum peak luminosity, it is possible to estimate the (maximum) distance from its observed peak flux. The peak flux of the second X-ray burst, which was observed by the GSC, is about 5 photons cm$^{-2}$ s$^{-1}$. By assuming a blackbody spectrum of 2.5 keV, the maximum distance to the source is estimated as 7 kpc. The position of this source is contained by the large error regions of two bright X-ray sources detected with Orbiting Solar Observatory-7 (OSO-7) in 1970s. Besides this, no past activities at the XRT position are reported in the literature. If MAXI J1421-613 is the same source as (one of) them, the outburst observed with MAXI may have occurred after the quiescence of 30-40 years.
We study properties of luminous X-ray pulsars using a simplified model of the accretion column. The maximal possible luminosity is calculated as a function of the neutron star (NS) magnetic field and spin period. It is shown that the luminosity can reach values of the order of $10^{40},{rm erg/s}$ for the magnetar-like magnetic field ($Bgtrsim 10^{14},{rm G}$) and long spin periods ($Pgtrsim 1.5,{rm s}$). The relative narrowness of an area of feasible NS parameters which are able to provide higher luminosities leads to the conclusion that $Lsimeq 10^{40},,{rm erg/s}$ is a good estimate for the limiting accretion luminosity of a NS. Because this luminosity coincides with the cut-off observed in the high mass X-ray binaries luminosity function which otherwise does not show any features at lower luminosities, we can conclude that a substantial part of ultra-luminous X-ray sources are accreting neutron stars in binary systems.
The massive binary system Eta Carinae is characterized by intense colliding winds that form shocks and emit X-rays. The system is highly eccentric ($esimeq0.9$), resulting in modulated X-ray emission during its 5.54 year orbit. The X-ray flux increases in the months prior to periastron passage, exhibiting strong flares, then rapidly declines to a flat minimum lasting a few weeks, followed by a gradual recovery. We present Neutron Star Interior Composition Explorer (NICER) telescope spectra obtained before, during, and after the 2020 X-ray minimum, and perform spectral analysis to establish the temporal behavior of X-ray flux and X-ray-absorbing column density ($N_{rm H}(t)$) for the 2-10 keV and 5-10 keV energy ranges. The latter range is dominated by the stellar wind collision region and, therefore, these spectral parameters - in particular, $N_{rm H}(t)$ - serves as a potentially stringent constraint on the binary orientation. We compare the observed $N_{rm H}(t)$ results to the behavior predicted by a simple geometrical model in an attempt to ascertain which star is closer to us at periastron: the more massive primary ($omega simeq 240$-$270^circ$), or the secondary ($omega simeq 90^circ$). We find that the variations in column density, both far from periastron and around periastron passage, support the latter configuration ($omega simeq 90^circ$). The 2020 X-ray minimum showed the fastest recovery among the last five minima, providing additional evidence for a recent weakening of the primary stars wind.
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