An investigation of a magnetic cataclysmic variable with a period of 14.1 ks

Magnetic cataclysmic variables(CVs) contain a white dwarf(WD) with a magnetic field strong enough to control the accretion flow from a late type secondary. In this paper, we identify a magnetic CV(CXOGSG J215544.4+380116) from the Chandra archive data. The X-ray light curves show a significant period of 14.1 ks, and the X-ray spectra can be described by a multi-temperature hot thermal plasma, suggesting the source is a magnetic CV. The broad dip in the X-ray light curve is due to the eclipse of the primary magnetic pole, and the additional dip in the bright phase of the soft and medium bands may be caused by the accretion stream crossing our line of sight to the primary pole.Follow-up optical spectra show features of an M2–M4 dwarf dominating the red band and a WD which is responsible for the weak upturn in the blue band. The mass(~ 0.4 M⊙) and radius(~0.4 R⊙) for the M dwarf are obtained using CV evolution models and empirical relations between the orbital period and the mass/radius. The estimated low X-ray luminosity and accretion rate may suggest the source is a low-accretion-rate polar. In addition, Very Large Array observations reveal a possible radio counterpart to the X-ray source, but with a low significance. Further radio observations with high quality are needed to confirm the radio counterpart and explore the properties of this binary system.

The age of the halo as determined from halo field stars

The age of the Galactic halo is a critical parameter that can constrain the origin of the stellar halo.In general, the Galactic stellar halo is believed to be very old. However, different independent measurements and techniques based on various types of stars are required so that the age estimates of the Galactic halo are accurate, robust, and reliable. In this work, we provide a novel approach to determine the age of the halo with turn-off stars. We first carefully select 63 field halo turn-off stars from the literature. Then, we compare them with the GARSTEC model, which takes the process of atomic diffusion into account in the B- V vs. metallicity plane. Finally, we run Monte Carlo simulations to consider the uncertainty of the color index and obtain the age of 10.5 ± 1.5 Gyr. This result is in agreement with previous studies. Future works are needed to collect more turn-off samples with more accurate photometry to reduce the uncertainty of the age.

New open cluster candidates discovered in the XSTPS-GAC survey

The Xuyi Schmidt Telescope Photometric Survey of the Galactic Anti-center(XSTPS-GAC)is a photometric sky survey that covers nearly 6000 deg^2 towards the Galactic Anti-center(GAC) in the g,r,i bands. Half of its survey field is located on the Galactic Anti-center disk,which makes XSTPSGAC highly suitable to search for new open clusters in the GAC region. In this paper,we report new open cluster candidates discovered in this survey,as well as properties of these open cluster candidates,such as age,distance and reddening,derived by isochrone fitting in the color-magnitude diagram(CMD).These open cluster candidates are stellar density peaks detected in the star density maps by applying the method from Koposov et al. Each candidate is inspected in terms of its true color image composed from three XSTPS-GAC band images. Then its CMD is checked,in order to identify whether the central region stars have a clear isochrone-like trend differing from background stars. The parameters derived from isochrone fitting for these candidates are mainly based on three band photometry of XSTPS-GAC.Moreover,when these new candidates are able to be seen clearly in 2 MASS data,their parameters are also derived based on the 2 MASS(J-H,J) CMD. There are a total of 320 known open clusters rediscovered and 24 new open cluster candidates discovered in this work. Furthermore,the parameters of these new candidates,as well as another 11 previously known open clusters,are properly determined for the first time.

Machine learning classification of Gaia Data Release 2

Machine learning has increasingly gained more popularity with its incredibly powerful ability to make predictions or calculate suggestions for large amounts of data. We apply machine learning classification to 85 613 922 objects in the Gaia Data Release 2, based on a combination of Pan-STARRS 1 and AllWISE data. The classification results are cross-matched with the Simbad database, and the total accuracy is 91.9%. Our sample is dominated by stars,～98%, and galaxies make up 2%. For the objects with negative parallaxes, about 2.5% are galaxies and QSOs, while about 99.9% are stars if the relative parallax uncertainties are smaller than 0.2. Our result implies that using the threshold of 0 < σ_π/π < 0.2 could yield a very clean stellar sample.

LAMOST Time-Domain survey:first results of four K2 plates

From Oct.2019 to Apr.2020,LAMOST performed a time-domain(TD)spectroscopic survey of four K2 plates with both low-and medium-resolution observations.The low-resolution spectroscopic survey acquired 282 exposures(≈46.6 h)over 25 nights,yielding a total of about 767000 spectra,and the medium-resolution survey took 177 exposures(≈49.1 h)over 27 nights,collecting about 478000 spectra.More than 70%/50%of low-resolution/medium-resolution spectra have signal-to-noise ratio higher than 10.We determine stellar parameters(e.g.,Teff,log g,[Fe/H])and radial velocity(RV)with different methods,including LASP,DD-Payne and SLAM.In general,these parameter estimations from different methods show good agreement,and the stellar parameter values are consistent with those of APOGEE.We use the Gaia DR2 RV values to calculate a median RV zero point(RVZP)for each spectrograph exposure by exposure,and the RVZP-corrected RVs agree well with the APOGEE data.The stellar evolutionary and spectroscopic masses are estimated based on the stellar parameters,multi-band magnitudes,distances and extinction values.Finally,we construct a binary catalog including about 2700 candidates by analyzing their light curves,fitting the RV data,calculating the binarity parameters from medium-resolution spectra and cross-matching the spatially resolved binary catalog from Gaia EDR3.The LAMOST TD survey is expected to represent a breakthrough in various scientific topics,such as binary systems,stellar activity,stellar pulsation,etc.

Detecting and Monitoring Tidal Dissipation of Hot Jupiters in the Era of SiTian

Transit Timing Variation(TTV)of hot Jupiters provides direct observational evidence of planet tidal dissipation.Detecting tidal dissipation through TTV needs high precision transit timings and long timing baselines.In this work,we predict and discuss the potential scientific contribution of the SiTian Survey in detecting and analyzing exoplanet TTV.We develop a tidal dissipation detection pipeline for the SiTian Survey that aims at time-domain astronomy with 721 m optical telescopes.The pipeline includes the modules of light curve deblending,transit timing acquisition and TTV modeling.SiTian is capable of detecting more than 25,000 exoplanets among which we expect~50 sources to show evidence of tidal dissipation.We present detection and analysis of tidal dissipating targets,based on simulated SiTian light curves of XO-3b and WASP-161 b.The transit light curve modeling gives consistent results within 1σto input values of simulated light curves.Also,the parameter uncertainties predicted by Markov Chain Monte Carlo are consistent with the distribution obtained from simulating and modeling the light curve 1000 times.The timing precision of SiTian observations is~0.5 minutes with one transit visit.We show that differences between TTV origins,e.g.,tidal dissipation,apsidal precession and multiple planets,would be significant,considering the timing precision and baseline.The detection rate of tidal dissipating hot Jupiters would answer a crucial question of whether the planet migrates at an early formation stage or random stages due to perturbations,e.g.,planet scattering or secular interaction.SiTian identified targets would be constructive given that the sample would extend tenfold.

Galactic halo age estimated from LAMOST DR4 and Gaia DR1

The stellar halo is one of the major components in the Milky Way. Research on its age can provide critical constraints on the origin of the stellar halo and further on the formation of our Galaxy.So far, different approaches and samples have been used to estimate the age of the Galactic halo. In our previous paper, we carefully selected 63 field halo turn-off stars within 1 kpc from the literature using a kinematic approach, then estimated the age of the halo. In this following work, we not only update the data from LAMOST DR4 and Gaia DR1, but also try a different method to select a clean halo sample by combining the metallicity and orbital parameters. Then we compare this halo turn-off sample with the GARSTEC model in the B-V vs. metallicity plane. After Monte Carlo simulations are performed, the age is estimated to be 10.5±1.4 Gyr, highly consistent with our previous result and other studies. However,due to the limited common sources between LAMOST DR4 and Gaia DR1, the final sample in this paper is still quite small. The estimated age will be more robust with the much larger Gaia DR2.

Epidemiological Investigation of Human Papillomavirus Infection in Men Attending a Sexually Transmitted Disease Clinic in Hangzhou Area

Objective To investigate the epidemiological characteristics of human papillomavirus (HPV) infection in men attending a sexually transmitted diseases (STD) clinic in Hangzhou area. Methods Male subjects (n=375) aged 18-70 years, attending the STD clinic were recruited. Urethral swabs were assessed for HPV DNA using polymerase chain reaction (PCR) with the consensus primers MY09/11. HPV genotypes of positive PCR products were determined by restriction fragment length polymorphisms and direct sequence analysis. Results Of the 375 swabs collected, 305 (81.3%) yielded sufficient DNA for the subsequent HPV analysis. Among the 305 subjects, the prevalence of HPV was 13.8%. Nononcogenic HPV types were found in 8.5% (26/305) of subjects, oncogenic types in 4.3% (13/305), and multiple types in 1.0% (3/305). The prevalence of HPV infection was higher in subjects from urban area than in those from rural area (P<0.05). The prevalence was also higher in those who received fewer years of education (P<0.05) and those who had more sex partners (P<0.05). Conclusions HPV infection among men at high risk is not uncommon. The detection rate of HPV DNA is significantly related to some sociodemographic factors, such as residence, educational level and the number of sex partners.

X-ray activity from different types of stars

X-ray emission is an important indicator of stellar activity.In this paper,we study stellar Xray activity using the XMM-Newton and LAMOST data for different types of stars.We provide a sample including 1259 X-ray-emitting stars,of which 1090 have accurate stellar parameter estimations.Our sample size is much larger than those used in previous works.We find a bimodal distribution of the X-ray to optical flux ratio(log(fX/fV)) for G and K stars.We interpret that this bimodality is due to two subpopulations with different coronal heating rates.Furthermore,using the full widths at half maxima calculated from Hα and Hβ lines,we show that these stars in the inactive peaks have smaller rotational velocities.This is consistent with the magnetic dynamo theory that presumes stars with low rotational velocities have low levels of stellar activity.We also examine the correlation between log(fX/fV) and luminosity of the excess emission in the Hα line,and find a tight relation between the coronal and chromospheric activity indicators.

An active M star with X-ray double flares disguised as an ultra-luminous X-ray source

Here we present research on an ultra-luminous X-ray source(ULX) candidate 2XMM J140229.91+542118.8. The X-ray light curves of this ULX candidate in M101 exhibit features of a flare star.More importantly, the Chandra light curve displays unusual X-ray double flares, which is comprised of two close peaks. The X-ray(0.3–11.0 ke V) flux of the first peak was derived from the two-temperature APEC model as ～ 1.1 ± 0.1 × 10-12 erg cm-2s-1. The observed flux at its first peak increased by about two orders of magnitude in X-ray as compared to quiescence. The slope of the second fast decay phase is steeper than the slope of the first fast decay phase, indicating that the appearance of a second flare accelerated the cooling of the first flare in a way we do not understand yet. We also observed its optical counterpart using a 2.16 m telescope administered by National Astronomical Observatories, Chinese Academy of Sciences.By optical spectral fitting, it is confirmed to be a late type d Me2.5 star. According to the spectral type and apparent magnitude of its optical counterpart, we estimate the photometric distance to be ～ 133.4 ± 14.2pc. According to the X-ray spectral fitting, a possible explanation is provided. However, more similar close double flares are needed to confirm whether this accelerated cooling event is a unique coincidence or a common physical process during double flaring.

Three X-ray flares near primary eclipse of the RS CVn binary XY UMa

We report on an archival X-ray observation of the eclipsing RS CVn binary XY UMa(P_(orb) ≈0.48 d). In two Chandra ACIS observations spanning 200 ks and almost five orbital periods, three flares occurred. We find no evidence for eclipses in the X-ray flux. The flares took place around times of primary eclipse, with one flare occurring shortly(< 0.125 P_(orb)) after a primary eclipse, and the other two happening shortly(< 0.05 P_(orb)) before a primary eclipse. Two flares occurred within roughly one orbital period(?φ≈1.024 P_(orb)) of each other. We analyze the light curve and spectra of the system, and investigate coronal length scales during both quiescence and flares, as well as the timing of the flares. We explore the possibility that the flares are orbit-induced by introducing a small orbital eccentricity, which is quite challenging for this close binary.

Gaia calibrated UV luminous stars in LAMOST

We take advantage of Gaia Data Release 2 to present 275 and 1774 ultraviolet luminous stars in the far ultraviolet(FUV) and near ultraviolet(NUV), respectively.These stars exceed their expected values by 5σ with respect to over one million ultraviolet stars in the log g vs.Teffdiagram.Galactic extinction is corrected with a 3 D dust map.In order to limit the Lutz-Kelker bias to an insignificant level, we select stars with relative uncertainties in luminosity less than 40% and trigonometric parallaxes less than 20%.We cross-identified our sample with the catalogs of RR Lyr stars and possible white dwarf main-sequence binaries, and find they compose ～62% and ～16% of our sample in the FUV and NUV, respectively.This catalog provides a unique sample to study stellar activity, spectrally unresolved compact main-sequence binaries and variable stars.

Stellar activity cycles as revealed by long-term beat-like patterns from light curves of Kepler

Stellar activity cycles have been found on many stars through different methods.Although in debate,an empirical relation has been found where a period of stellar activity cycle Pcyc is positively correlated with rotation period Prot along some segregated branches.In this work,we built a systematic process to search for the Kepler objects that are analogous to the long-term solar brightness variations and investigate their possible origins.After a rigorous selection,we obtained 43 objects that all exhibit clear long-term beat-like patterns and their variability(amplitude of rotational modulation)periods and rotation periods were derived.Due to the 4-year duration of the Kepler mission,our targets are located in the region where the cycle periods are longer than~1 yr and shorter than~2.5 yr.However,all of our targets exhibit clear and notable quasi-periodic variations in variability,yet we cannot confirm any linear trends that were predicted by previous research independently.This may suggest that in the Kepler realm of the Pcyc-Prot diagram,these relations would mix with each other or the beat-like patterns may not be related to the stellar cycle unless other evidence is provided.

Sharp Chandra View of ROSAT All-Sky Survey Bright Sources I. Improvement of Positional Accuracy

The ROSAT All-Sky Survey(RASS) represents one of the most complete and sensitive soft Xray all-sky surveys to date. However, the deficient positional accuracy of the RASS Bright Source Catalog(BSC) and subsequent lack of firm optical identifications affect multi-wavelength studies of X-ray sources.The widely used positional errors σpos based on the Tycho Reference Catalog(Tycho-1) have previously been applied for identifying objects in the optical band. The considerably sharper Chandra view covers a fraction of RASS sources, whose σpos could be improved by utilizing the sub-arcsec positional accuracy of Chandra observations. We cross-match X-ray objects between the BSC and Chandra sources extracted from the Advanced CCD Imaging Spectrometer(ACIS) archival observations. A combined list of counterparts(BSCx ACIS) with Chandra spatial positions weighted by the X-ray flux of multiple counterparts is employed to evaluate and improve the former identifications of BSC when used with other surveys. Based on these identification evaluations, we suggest that the point-source likeness of BSC sources and INS(isolated neutron star) candidates should be carefully reconsidered.

A label-free colorimetric assay for detection of c-Myc mRNA based on peptide nucleic acid and silver nanoparticles

A label-free colorimetric protocol based on peptide nucleic acid/silver nanoparticles(PNA/Ag NPs) has been initially proposed for specific recognition of m RNA.Making use of the controlled silver nanoparticles aggregation/dispersion by PNA/PNA–RNA complex, proto-oncogene c-Myc m RNA detection can be achieved. Moreover, the PNA/Ag NPs platform can undergo color change in response to target c-Myc m RNA with single-base-mismatch sensitivity, which could further help in visually identify single nucleotide differences in target genes.