X-ray Luminosity Lx Research Articles

Investigation of Coronal Properties of X-Ray Bright G-dwarf Stars Based on the Solar Surface Magnetic Field–Corona Relationship

Abstract We investigated the coronal properties of G-dwarf stars including the Sun over a wide range of X-ray luminosity L X (3 × 1026 to 2 × 1030 erg s−1). We analyzed the archival data of 10 X-ray bright (L X > 1028 erg s−1) G-dwarf stars to derive their emission measure (EM) and the coronal temperature (T) during the periods when no prominent stellar flares were observed. We attempted to explain the relation on the basis of our understanding of the present Sun: a steady corona model based on the so-called Rosner-Tucker-Vaiana (RTV) scaling laws and the observed power-law distribution function of surface magnetic features. We derived a theoretical scaling law of the EM–T relation for a star with multiple active regions, and applied it to the observations combined with data in the literature. We found that with the solar parameters, our scaling law seems to be consistent with the data of slowly rotating stars. However, more X-ray-bright stars are located well above the scaling law based on the solar parameter. The scaling law may explain the observations if those stars show a power-law distribution function of active regions with the same power-law index but a 10–100 times larger coefficient. This suggests that X-ray bright stars show more active regions for a given size than the Sun. Since our samples include rapidly rotating stars, we infer that the offset of the X-ray bright stars from the present Sun-based scaling law is due to the enhancement of the surface magnetic field generation by their rapid rotation.

Multi-epoch X-ray imaging of globular cluster M62 with Chandra

ABSTRACT Using archival spectral-imaging data with a total exposure of ∼144 ks obtained by Chandra, 43 X-ray sources are detected within the half-light radius of globular cluster M62 (NGC 6266). Based on the X-ray colour–luminosity diagram or the positional coincidences with known sources, we have classified these sources into different groups of compact binaries including cataclysmic variable (CV), quiescent low-mass X-ray binary (qLMXB), millisecond pulsar, and black hole (BH). Candidates of the X-ray counterparts of 12 CVs, 4 qLMXBs, 2 MSPs, and 1 BH are identified in our analysis. The data used in our analysis consist of two frames separated by 12 yr, which enable us to search for the long-term variability as well as the short-term X-ray flux variability within each observation window. Evidence for the short-term variability and long-term variability have been found in 7 and 12 sources, respectively. For a number of bright sources with X-ray luminosities Lx ≳ 1032 erg s−1, we have characterized their spectral properties in further details. By comparing the X-ray population in M62 with those in several other prototypical globular clusters, we found the proportion of bright sources is larger in M62 that can possibly be a result of their active dynamical formation processes.

The corona of GJ 1151 in the context of star–planet interaction

ABSTRACT The low-mass star GJ 1151 has been reported to display variable low-frequency radio emission, which has been interpreted as a signpost of coronal star–planet interactions with an unseen exoplanet. Here we report the first X-ray detection of GJ 1151’s corona based on the XMM–Newton data. We find that the star displays a small flare during the X-ray observation. Averaged over the observation, we detect the star with a low coronal temperature of 1.6 MK and an X-ray luminosity of LX = 5.5 × 1026 erg s−1. During the quiescent time periods excluding the flare, the star remains undetected with an upper limit of $L_{\mathrm{ X},\, \mathrm{ qui}} \le 3.7\times 10^{26}$ erg s−1. This is compatible with the coronal assumptions used in a recently published model for a star–planet interaction origin of the observed radio signals from this star.

Analysis of the X-ray emission from OB stars III: low-resolution spectra of OB stars

This paper is the third in our series of papers devoted to the investigation of X-ray emission from OB stars. In our two previous papers, we study the high-resolutionX-ray spectra of 32 O stars and 25 B stars to investigate the correlations between the properties of X-ray emission and stellar parameters.We checked if the X-ray hardness and post-shock plasma temperature grow with increasing stellar magnetic field, mass loss rate and terminal wind velocity. Our previous analysis of high-resolution spectra showed that the correlations are weak or even absent. In the present paper, we analyzed low-resolution X-ray spectra, using model-independent X-ray hardness values for checking the above mentioned dependencies. We establish that X-ray luminosities LX weakly depend on the stellar magnetic field. At the same time, LX ∝ Ṁ0.5 and , where ṁ is the mass loss rate and Ekin is the kinetic energy of the wind. The X-ray luminosities decrease with growing magnetic confinement parameter η. We also argue that there is an additional (probably non-thermal) component contributed to the stellar X-ray emission.

The X-ray emission in young radio active galactic nuclei

ABSTRACT In this work, we investigated the X-ray emission for a sample of young radio active galactic nuclei (AGNs) by combining data from Chandra/XMM–Newton with data for other wavebands. We find strong correlations between the X-ray luminosity LX at 2–10 keV and the radio luminosities LR at 5 GHz for the VLBI radio-core, VLA radio-core and Faint Images of the Radio Sky at Twenty-Centimeters (FIRST) component, indicating that both parsec- and kiloparsec-scale radio emissions are strongly correlated with X-ray emission in these sources. We find an approximately linear dependence of the radio on the X-ray luminosity in the sources with radiatively efficient accretion flows (i.e. the Eddington ratio Redd ≳ 10−3), with b ∼ 1 ($L_{\rm R}\, \propto \, L_{\rm X} ^{b}$) and $\xi _{\rm RX}\, \sim$ 1 in the fundamental plane using the VLBI data. The dependence is consistent with the re-analysed results of a previous study of Fan and Bai at Redd ≳ 10−3, but is significantly different from the theoretical prediction of accretion flow as the origin of X-ray emission. In contrast to the case for radio-quiet quasars, there is no significant correlation between Γ and the Eddington ratio. Our results seem to indicate that the X-ray emission of high-accretion young radio AGNs may be from the jet. We constructed the spectral energy distributions (SEDs) for 18 sources (most of which are in radiatively efficient accretion), namely nine galaxies and nine quasars with high-quality X-ray data, and find that the X-ray emission of most quasars is more luminous than that of normal radio-quiet quasars. This is clearly seen from the quasar composite SED, in which the X-ray emission is apparently higher than that of radio-quiet quasars, probably supporting jet-related X-ray emission in young radio AGNs. The possibility that the X-ray emission is from self-synchrotron Compton is discussed.

On the peculiar torque reversals and the X-ray luminosity history of the accretion-powered X-ray pulsar 4U 1626–67

ABSTRACT The X-ray luminosity (Lx) and the rotational properties of 4U 1626–67 have been measured at regular intervals during the last four decades. It has been recorded that the source underwent torque reversals twice. We have tried to understand whether these eccentrical sign-switches of the spin period derivative ($\dot{P}$) of 4U 1626–67 could be accounted for with the existing torque models. We have found that the observed source properties are better estimated with the distances close to the lower limit of the previously predicted distance range (5−13 kpc). Furthermore, assuming an inclined rotator, we have considered the partial accretion/ejection from the inner disc radius that leads to different Lx–$\dot{P}$ profiles than the aligned rotator cases. We have concluded that the oblique rotator assumption with the inclination angle χ ∼ (10°−30°) brings at least equally best fitting to the observed Lx and $\dot{P}$ of 4U 1626–67. More importantly, the estimated change of the mass accretion rate, which causes the change in observed Lx of 4U 1626–67 is much less than that is found in an aligned rotator case. In other words, without the need for a substantial modification of mass accretion rate from the companion star, the range of the observed Lx could be explained naturally with an inclined magnetic axis and rotation axis of the neutron star.

On the Jet Production Efficiency in a Sample of the Youngest Radio Galaxies

Abstract Here we discuss the jet production efficiency in a sample of 17 young radio galaxies with measured redshifts, kinematic ages, and nuclear X-ray fluxes, for which the observed luminosities of compact jets/lobes and accretion disks correspond to the same episode of AGN activity. For the targets, we analyze the available optical data, estimating the bolometric luminosities of the accretion disks L bol, and the black hole masses; we also derive the minimum jet kinetic luminosities, P j. With this information we investigate the distribution of our sample in the three-dimensional space of the accretion rate λ Edd ≡ L bol/L Edd, the nuclear X-ray luminosity L X considered here as a limit for the emission of the disk coronae, and P j, expressing the latter two parameters either in the Eddington units, or in the units of the disk luminosity. We find that (i) the accretion rate λ Edd in our sample is distributed within a narrow range λ Edd ∼ 0.01–0.2; (ii) the normalized jet power P j/L Edd formally correlates with the accretion rate λ Edd, with some saturation at the largest values λ Edd > 0.05; (iii) the jet production efficiency spans a range from η jet ≲ 10−3 up to ∼0.2 at maximum, which is below the level expected for magnetically arrested disks around maximally spinning black holes; and (iv) there is a diversification in η jet on the hardness–intensity diagram L X/L bol − λ Edd, with the jets being produced most efficiently during the high/hard states, and suppressed during the soft states.

A Carbon-rich Hot Bubble in the Planetary Nebula NGC 5189

Abstract We present the discovery of extended X-ray emission from the planetary nebula (PN) NGC 5189 around the [WO1]-type WD 1330−657 with XMM-Newton. The X-ray-emitting gas fills the cavities detected in the Hubble Space Telescope [O iii] narrowband image and presents a limb-brightened morphology toward the outer edges of the east and west lobes. The bulk of the X-ray emission is detected in the soft (0.3–0.7 keV) band with the XMM-Newton EPIC spectra dominated by the C vi Lyα line at 0.37 keV (=33.7 Å). Spectral analysis resulted in carbon and neon abundances 38 and 6 times their solar values, with a plasma temperature of kT = 0.14 ± 0.01 keV (T = 1.6 × 106 K) and X-ray luminosity of L X = (2.8 ± 0.8) × 1032 erg s−1. NGC 5189 is an evolved and extended PN (≲0.70 pc in radius), thus, we suggest that the origin of its X-ray emission is consistent with the born-again scenario in which the central star becomes carbon-rich through an eruptive very late thermal pulse, subsequently developing a fast, carbon-rich wind powering the X-ray emission as suggested for A 30 and A 78.

Probing the building blocks of galaxies: Sub-galactic scaling relations between X-ray luminosity, SFR and stellar mass

AbstractIt is well known that X-ray luminosity (Lx) originating from high mass X-ray binaries (HMXBs) is tightly correlated with the host galaxy’s star formation rate (SFR). We explore this connection using a sample representative of the star-formation activity in the local Universe (Star-Formation Reference Survey; SFRS) along with a comprehensive set of star-formation (radio, FIR, 24μm, 8 μm, Hα, UV, SED fitting) and stellar mass (K-band, 3.6 μm, SED fitting) indicators, and Chandra observations. We investigate the Lx–SFR and Lx– stellar mass (M*) scaling relations down to sub-galactic scales of ∼lkpc2. This way we extend these relations to extremely low SFR (∼10−6M⊙.yr−1) and M* (∼104M⊙). We also quantify their scatter and their dependence on the age of the local stellar populations as inferred from the different age sensitive SFR indicators. These results are particularly important for setting the benchmark for the formation of X-ray binaries in vigorous, but low SFR objects such as galaxies in the early Universe.

Hot, dense He iioutflows during the 2017 outburst of the X-ray transientSwift J1357.2−0933

AbstractTime-resolved SALT spectra of the short-period, dipping X-ray transient, Swift J1357.2−0933, during its 2017 outburst has revealed broad Balmer and He ii λ4686 absorption features, blueshifted by ∼600 km s−1. Remarkably these features are also variable on the ∼500 s dipping period, indicating their likely association with structure in the inner accretion disc. We interpret this as arising in a dense, hot (≳30 000 K) outflowing wind seen at very high inclination, and draw comparisons with other accretion disc corona sources. We argue against previous distance estimates of 1.5 kpc and favour a value ≳6 kpc, implying an X-ray luminosity LX ≳ 4 × 1036 erg s−1. Hence it is not a very faint X-ray transient. Our preliminary 1D Monte Carlo radiative transfer and photoionization calculations support this interpretation, as they imply a high intrinsic LX, a column density NH ≳ 1024 cm−2, and a low covering factor for the wind. Our study shows that Swift J1357.2−0933 is truly remarkable amongst the cohort of luminous, Galactic X-ray binaries, showing the first example of He ii λ4686 absorption, the first (and only) variable dip period and is possibly the first black hole ‘accretion disc corona’ candidate.

Discovery of a radio transient in M81

AbstractWe report the discovery of a radio transient in the spiral galaxy M81. The transient was detected in early 2015 as part of a two-year survey of M81 made up of 12 epochs using the Karl G. Jansky Very Large Array. While undetected on 2014 September 12, the source was first detected on 2015 January 2, from which point it remained visible at an approximately constant luminosity of LR, ν = 1.5 ± 0.1 × 1024 erg s−1 Hz−1 at the observing frequency of 6 GHz for at least 2 months. Assuming this is a synchrotron event with a rise-time between 2.6 and 112 d, the peak luminosity (at equipartition) corresponds to a minimum energy of 1044 ≲ Emin ≲ 1046 erg and jet power of Pmin ∼ 1039 erg s−1, which are higher than most known X-ray binaries. Given its longevity, lack of short-term radio variability, and the absence of any multiwavelength counterpart (X-ray luminosity Lx ≲ 1036 erg s−1), it does not behave like known Galactic or extragalactic X-ray binaries. The M81 transient radio properties more closely resemble the unidentified radio transient 43.78+59.3 discovered in M82, which has been suggested to be a radio nebula associated with an accreting source similar to SS 433. One possibility is that both the new M81 transient and the M82 transient may be the birth of a short-lived radio bubble associated with a discrete accretion event similar to those observed from the ULX Holmberg II X-1. However, it is not possible to rule out other identifications including long-term supernova shockwave interactions with the surrounding medium from a faint supernova or a background active galaxy.

SDSS-IV eBOSS Spectroscopy of X-Ray and WISE AGNs in Stripe 82X: Overview of the Demographics of X-Ray- and Mid-infrared-selected Active Galactic Nuclei

Abstract We report the results of a Sloan Digital Sky Survey IV eBOSS program to target X-ray sources and mid-infrared-selected Wide-field Infrared Survey Explorer (WISE) active galactic nucleus (AGN) candidates in a 36.8 deg2 region of Stripe 82. About half this survey (15.6 deg2) covers the largest contiguous portion of the Stripe 82 X-ray survey. This program represents the largest spectroscopic survey of AGN candidates selected solely by their WISE colors. We combine this sample with X-ray and WISE AGNs in the field identified via other sources of spectroscopy, producing a catalog of 4847 sources that is 82% complete to r ∼ 22. Based on X-ray luminosities or WISE colors, 4730 of these sources are AGNs, with a median sample redshift of z ∼ 1. About 30% of the AGNs are optically obscured (i.e., lack broad lines in their optical spectra). BPT analysis, however, indicates that 50% of the WISE AGNs at z < 0.5 have emission line ratios consistent with star-forming galaxies, so whether they are buried AGNs or star-forming galaxy contaminants is currently unclear. We find that 61% of X-ray AGNs are not selected as mid-infrared AGNs, with 22% of X-ray AGNs undetected by WISE. Most of these latter AGNs have high X-ray luminosities (L x > 1044 erg s−1), indicating that mid-infrared selection misses a sizable fraction of the highest luminosity AGNs, as well as lower luminosity sources where AGN-heated dust is not dominating the mid-infrared emission. Conversely, ∼58% of WISE AGNs are undetected by X-rays, though we do not find that they are preferentially redder than the X-ray-detected WISE AGNs.

HADES RV Programme with HARPS-N at TNG

Aims. We extend the relationship between X-ray luminosity (Lx) and rotation period (Prot) found for main-sequence FGK stars, and test whether it also holds for early M dwarfs, especially in the non-saturated regime (Lx ∝ Prot−2) which corresponds to slow rotators. Methods. We use the luminosity coronal activity indicator (Lx) of a sample of 78 early M dwarfs with masses in the range from 0.3 to 0.75 M⊙ from the HArps-N red Dwarf Exoplanet Survey (HADES) radial velocity (RV) programme collected from ROSAT and XMM-Newton. The determination of the rotation periods (Prot) was done by analysing time series of high-resolution spectroscopy of the Ca II H & K and Hα activity indicators. Our sample principally covers the slow rotation regime with rotation periods from 15 to 60 days. Results. Our work extends to the low mass regime the observed trend for more massive stars showing a continuous shift of the Lx∕Lbol versus Prot power law towards longer rotation period values, and includes a more accurate way to determine the value of the rotation period at which the saturation occurs (Psat) for M dwarf stars. Conclusions. We conclude that the relations between coronal activity and stellar rotation for FGK stars also hold for early M dwarfs in the non-saturated regime, indicating that the rotation period is sufficient to determine the ratio Lx∕Lbol.

X-ray emission of z > 2.5 active galactic nuclei can be obscured by their host galaxies

We present a multiwavelength study of seven active galactic nuclei (AGN) at spectroscopic redshift >2.5 in the 7 Ms Chandra Deep Field South that were selected for their good far-infrared (FIR) and submillimeter (submm) detections. Our aim is to investigate the possibility that the obscuration observed in the X-rays can be produced by the interstellar medium (ISM) of the host galaxy. Based on the 7 Ms Chandra spectra, we measured obscuring column densities NH, X in excess of 7 × 1022 cm−2 and intrinsic X-ray luminosities LX > 1044 erg s−1 for our targets, as well as equivalent widths for the Fe Kα emission line EWrest ≳ 0.5−1 keV. We built the UV-to-FIR spectral energy distributions (SEDs) by using broadband photometry from the CANDELS and Herschel catalogs. By means of an SED decomposition technique, we derived stellar masses (M* ∼ 1011 M⊙), IR luminosities (LIR > 1012 L⊙), star formation rates (SFR ∼ 190−1680 M⊙ yr−1) and AGN bolometric luminosities (Lbol ∼ 1046 erg s−1) for our sample. We used an empirically calibrated relation between gas masses and FIR/submm luminosities and derived Mgas ∼ 0.8−5.4 × 1010 M⊙. High-resolution (0.3−0.7″) ALMA data (when available, CANDELS data otherwise) were used to estimate the galaxy size and hence the volume enclosing most of the ISM under simple geometrical assumptions. These measurements were then combined to derive the column density associated with the ISM of the host, which is on the order of NH, ISM ∼ 1023−24 cm−2. The comparison between the ISM column densities and those measured from the X-ray spectral analysis shows that they are similar. This suggests that at least at high redshift, significant absorption on kiloparsec scales by the dense ISM in the host likely adds to or substitutes that produced by circumnuclear gas on parsec scales (i.e., the torus of unified models). The lack of unobscured AGN among our ISM-rich targets supports this scenario.

Stellar and AGN Feedback in Isolated Early-type Galaxies: The Role in Regulating Star Formation and ISM Properties

Abstract How galaxies maintain the inefficiency of star formation with physically self-consistent models is a central problem in understanding galaxy evolution. Although numerous theoretical models have been proposed in recent decades, the debate continues. By means of high-resolution two-dimensional hydrodynamical simulations, we study the three feedback effects (the stellar wind heating, supernova (SN) feedback, and active galactic nucleus (AGN) feedback) in suppressing star formation activities on the evolution of early-type galaxies with different stellar masses. The AGN feedback models are updated from work by Yuan et al. The gas sources arise exclusively from the mass losses of dying low-mass stars for most of our models. We find that SN feedback can keep star formation at a significantly low level for low-mass elliptical galaxies for a cosmological evolution time. For high-mass galaxies, AGN feedback can efficiently offset radiative cooling and thus regulate star formation activity. Such a suppression of star formation is extremely efficient in the inner regions of galaxies. Asymptotic giant branch heating cannot account for this suppression for low- or high-mass galaxies. The X-ray temperature T X and luminosity L X of hot plasma can be in agreement with the observed data with the inclusion of effective feedback processes. These results thus suggest that we can use T X and L X to probe the role of different feedback processes. The inclusion of additional gas sources can cause the mass scale between SN and AGN feedback to dominate in suppressing star formation decrease to an observationally inferred value of a few 1010 M ⊙.

Time Evolution of the X-Ray and γ-Ray Fluxes of the Crab Pulsar

Abstract We studied the evolution of the X-ray and γ-ray spectra of the Crab pulsar utilizing the 11-year observations from the Rossi X-ray Timing Explorer (RXTE) and 9-year observations from the Fermi Gamma-ray Space Telescope (FGST). By fitting the spectrum of each observation, we obtained the corresponding flux and then analyzed the long-term evolution of the X-ray (or γ-ray) luminosities as well as their correlations with the spin-down power of the pulsar. The X-ray flux in 5–60 keV obtained by the Proportional Counter Array of RXTE decreases with a rate of (−2.4 ± 0.4) × 10−14 erg cm−2 s−1 per day. The X-ray flux in 15–250 keV obtained by the High Energy X-ray Timing Experiment of RXTE and the γ-ray flux in 0.1–300 GeV by the Large Area Telescope onboard FGST show similar decreasing trends but are insignificant statistically. The 5–60 keV X-ray luminosity L X is correlated with the spin-down power L sd by , which is similar to the statistical results for young pulsars.

The Clusters Hiding in Plain Sight (CHiPS) Survey: A First Discovery of a Massive Nearby Cluster around PKS 1353−341

Abstract We introduce the first result of the Clusters Hiding in Plain Sight (CHiPS) survey, which aims to discover new nearby massive galaxy clusters that were incorrectly identified as isolated point sources in the ROSAT All-sky Survey. We present a Chandra X-ray observation of our first newly discovered low-redshift (z = 0.223) galaxy cluster with a central X-ray bright point source, PKS 1353−341. After removing the point source contribution to the cluster core (L nuc ∼ 1.8 × 1044 erg s−1), we determine various properties of the cluster. The presence of a relaxed X-ray morphology, a central temperature drop, and a central cooling time around 400 Myr indicates that it is a strong cool-core cluster. The central galaxy appears to be forming stars at the rate of 6.2 ± 3.6 M ⊙ yr−1, corresponding to ∼1% of the classical cooling prediction. The supermassive black hole in the central galaxy appears to be accreting at ∼0.1% of the Eddington rate with a total power output of ∼5 × 1045 erg s−1, split nearly equally between radiative and mechanical power. We see weak evidence of localized excess entropy at a radius of 200 kpc, which, if true, could imply a recent (∼180 Myr) energetic outburst in the core that has risen buoyantly to a larger radius. Comparing the cluster’s bulk properties with those of other known clusters (e.g., the total mass M 500 is , and the bolometric X-ray luminosity L X is 7 × 1044 erg s−1), we show that this cluster, which is massive enough that it was detected (but not confirmed) by the Planck survey, is also sufficiently luminous that it would have been identified as a cluster in the ROSAT All-sky Survey if it did not have such a bright central point source. This discovery demonstrates the potential of the CHiPS survey to find massive nearby clusters with extreme central properties that may have been missed or misidentified by previous surveys.

Dissecting the X-Ray Emission in the Young Planetary Nebula NGC 7027

Abstract We present an analysis of a second observation of the young planetary nebula (PN) NGC 7027 by the Chandra X-ray Observatory. This latest 59.2 ks exposure with ACIS-S was acquired approximately 14 years after the initial 18.2 ks ACIS-S observation, and the improved photon statistics allow us to perform a detailed spatial and spectral analysis of the X-ray emission. Comparison with multiwavelength imaging of NGC 7027 reveals a strong anti-correlation between extinction across the nebula and the soft-band X-ray emission. Dissecting the X-ray emission into low- and high-extinction regions results in more robust characterization of the plasma spectral properties. We determine that the X-ray emitting plasma has a temperature of ∼3.6 MK, is deficient in Fe, and has an X-ray luminosity of L X ∼ 7 × 1031 erg s−1, all of which are generally consistent with the plasma properties found in PN hot bubbles. We find no evidence of evolution in the X-ray surface brightness over the 14 year baseline between CXO observations. Our analysis underscores the importance of accounting for nebular absorption of the X-ray emission in accurately determining plasma properties for hot bubbles within PNe.

Large X-ray flares on stars detected with MAXI/GSC: A universal correlation between the duration of a flare and its X-ray luminosity

Abstract Twenty-three giant flares from thirteen active stars (eight RS CVn systems, one Algol system, three dMe stars, and one young stellar object) 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 these MAXI/GSC flares are found to be at the upper ends for stellar flares with the luminosity of 1031–34 erg s−1 in the 2–20 keV band, the emission measure of 1054–57 cm−3, the e-folding time of 1 hr to 1.5 d, and the total radiative energy released during the flare of 1034–39 erg. Notably, the peak X-ray luminosity of $5^{+4}_{-2} \times 10^{33}\:$erg 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 $\tau \propto L_{\rm X}^{0.2}$ for the flare duration τ and the intrinsic X-ray luminosity LX in the 0.1–100 keV band, which holds for 5 and 12 orders of magnitude in τ and LX, respectively. The MAXI/GSC sample is located at the highest ends of the correlation.

TRACING THE LOWEST PROPELLER LINE IN MAGELLANIC HIGH-MASS X-RAY BINARIES

ABSTRACT We have combined the published observations of high-mass X-ray binary (HMXB) pulsars in the Magellanic Clouds with a new processing of the complete archival data sets from the XMM-Newton and Chandra observatories in an attempt to trace the lowest propeller line below which accretion to polar caps is inhibited by the centrifugal force and the pulsations from the most weakly magnetized pulsars cease. Previously published data reveal that some of the faster-spinning pulsars with spin periods of P S < 12 s, detected at relatively low X-ray luminosities L X , appear to define such a line in the P S –L X diagram, characterized by a magnetic moment of μ = 3 × 1029 G cm3. This value implies the presence of surface magnetic fields of B ≥ 3 × 1011 G in the compact objects of this class. Only a few quiescent HMXBs are found below the propeller line: LXP4.40 and SXP4.78, for which XMM-Newton and Chandra null detections respectively placed firm upper limits on their X-ray fluxes in deep quiescence; and A0538-66, for which many sub-Eddington detections have never measured any pulsations. On the other hand, the data from the XMM-Newton and Chandra archives show clearly that, during routine observation cycles, several sources have been detected below the propeller line in extremely faint, nonpulsating states that can be understood as the result of weak magnetospheric emission when accretion to the poles is centrifugally stalled or severely diminished. We also pay attention to the anomalous X-ray pulsar CXOU J010043.1-721134 that was reported in HMXB surveys. Its pulsations and locations near and above the propeller line indicate that this pulsar could be accreting from a fossil disk.