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X-ray observations play a crucial role in time-domain astronomy. The Einstein Probe (EP), a recently launched X-ray astronomical satellite, emerges as a forefront player in the field of time-domain astronomy and high-energy astrophysics. With a focus on systematic surveys in the soft X-ray band, EP aims to discover high-energy transients and monitor variable sources in the universe. To achieve these objectives, a quick and reliable classification of observed sources is essential. In this study, we developed a machine learning classifier for autonomous source classification using data from the EP-WXT Pathfinder Lobster Eye Imager for Astronomy (LEIA) and EP-WXT simulations. The proposed Random Forest classifier, built on selected features derived from light curves, energy spectra, and location information, achieves an accuracy of approximately 95% on EP simulation data and 98% on LEIA observational data. The classifier is integrated into the LEIA data processing pipeline, serving as a tool for manual validation and rapid classification during observations. This paper presents an efficient method for the classification of X-ray sources based on single observations, along with implications of most effective features for the task. This work facilitates rapid source classification for the EP mission and also provides valuable insights into feature selection and classification techniques for enhancing the efficiency and accuracy of X-ray source classification that can be adapted to other X-ray telescope data.

The Einstein Probe (EP) achieved its first detection and localization of a bright X-ray flare, EP240219a, on February 19, 2024, during its commissioning phase. Subsequent targeted searches triggered by the EP240219a alert identified a faint, untriggered gamma-ray burst (GRB) in the archived data of Fermi/GBM, Swift/BAT, Insight-HXMT/HE and INTEGRAL/SPI-ACS. The EP/WXT light curve reveals a long duration of approximately 160 seconds with a slow decay, whereas the Fermi/GBM light curve shows a total duration of approximately 70 seconds. The peak in the Fermi/GBM light curve occurs slightly later with respect to the peak seen in the EP/WXT light curve. Our spectral analysis shows that a single cutoff power-law model effectively describes the joint EP/WXT-Fermi/GBM spectra in general, indicating coherent broad emission typical of GRBs. The model yielded a photon index of $\sim -1.70 \pm 0.05$ and a peak energy of $\sim 257 \pm 134$ keV. After detection of GRB 240219A, long-term observations identified several candidates in optical and radio wavelengths, none of which was confirmed as the afterglow counterpart during subsequent optical and near-infrared follow-ups. The analysis of GRB 240219A classifies it as an X-ray rich GRB with a high peak energy, presenting both challenges and opportunities for studying the physical origins of X-ray flashes (XRFs), X-ray rich GRBs (XRRs), and classical GRBs (C-GRBs). Furthermore, linking the cutoff power-law component to non-thermal synchrotron radiation suggests that the burst is driven by a Poynting flux-dominated outflow., Comment: 14 pages, 8 figures, 3 tables

The nature of the minute-to-hour long Fast X-ray Transients (FXTs) localised by telescopes such as Chandra, Swift, and XMM-Newton remains mysterious, with numerous models suggested for the events. Here, we report multi-wavelength observations of EP240315a, a 1600 s long transient detected by the Einstein Probe, showing it to have a redshift of z=4.859. We measure a low column density of neutral hydrogen, indicating that the event is embedded in a low-density environment, further supported by direct detection of leaking ionising Lyman-continuum. The observed properties are consistent with EP240315a being a long-duration gamma-ray burst, and these observations support an interpretation in which a significant fraction of the FXT population are lower-luminosity examples of similar events. Such transients are detectable at high redshifts by the Einstein Probe and, in the (near) future, out to even larger distances by SVOM, THESEUS, and Athena, providing samples of events into the epoch of reionisation., Comment: 41 pages, 7 figures, submitted

Since their first discovery in the late 1960s, Gamma-ray bursts have attracted an exponentially growing interest from the international community due to their central role in the most highly debated open questions of the modern research of astronomy, astrophysics, cosmology, and fundamental physics. These range from the intimate nuclear composition of high density material within the core of ultra-dense neuron stars, to stellar evolution via the collapse of massive stars, the production and propagation of gravitational waves, as well as the exploration of the early Universe by unveiling first stars and galaxies (assessing also their evolution and cosmic re-ionization). GRBs have stimulated in the past $\sim$50 years the development of cutting-edge technological instruments for observations of high energy celestial sources from space, leading to the launch and successful operations of many different scientific missions (several of them still in data taking mode nowadays). In this review, we provide a brief description of the GRB-dedicated missions from space being designed and developed for the future. The list of these projects, not meant to be exhaustive, shall serve as a reference to interested readers to understand what is likely to come next to lead the further development of GRB research and associated phenomenology., Comment: Accepted for publication on Universe. Invited review, contribution to the Universe Special Issue "Recent Advances in Gamma Ray Astrophysics and Future Perspectives", P. Romano eds. (https://www.mdpi.com/journal/universe/special_issues/7299902Z97)

Complementary metal-oxide-semiconductor (CMOS) detectors are a competitive choice for current and upcoming astronomical missions. To understand the performance variations of CMOS detectors in space environment, we investigate the total ionizing dose effects on custom-made large-format X-ray CMOS detectors. Three CMOS detector samples were irradiated with a Co-60 source with a total dose of 70 krad and 105 krad. We test and compare the performance of these detectors before and after irradiation. After irradiation, the dark current increases by roughly 20 to 100 times, and the readout noise increases from 3 e- to 6 e-. The bias level at 50 ms integration time decreases by 13 to 18 Digital Number (DN) at -30 degree. The energy resolution increases from about 150 eV to about 170 eV at 4.5 keV at -30 degree. The conversion gain of the detectors varies for less than 2% after the irradiation. Furthermore, there are about 50 pixels whose bias at 50 ms has changed by more than 20 DN after the exposure to the radiation and about 30 to 140 pixels whose readout noise has increased by over 20 e- at -30 degree at 50 ms integration time. These results demonstrate that the performances of large-format CMOS detectors do not suffer significant degeneration in space environment., Comment: accepted by JATIS

We report on results of the on-ground X-ray calibration of the Lobster Eye Imager for Astronomy (LEIA), an experimental space wide-field (18.6*18.6 square degrees) X-ray telescope built from novel lobster eye mirco-pore optics. LEIA was successfully launched on July 27, 2022 onboard the SATech-01 satellite. To achieve full characterisation of its performance before launch, a series of tests and calibrations have been carried out at different levels of devices, assemblies and the complete module. In this paper, we present the results of the end-to-end calibration campaign of the complete module carried out at the 100-m X-ray Test Facility at IHEP. The PSF, effective area and energy response of the detectors were measured in a wide range of incident directions at several X-ray line energies. The distributions of the PSF and effective areas are roughly uniform across the FoV, in large agreement with the prediction of lobster-eye optics. The mild variations and deviations from the prediction of idealized lobster-eye optics can be understood to be caused by the imperfect shapes and alignment of the micro-pores as well as the obscuration by the supporting frames, which can be well reproduced by MC simulations. The spatial resolution of LEIA defined by the FWHM of the focal spot ranges from 4-8 arcmin with a median of 5.7. The measured effective areas are in range of 2-3 $cm^2$ at ~1.25 keV across the entire FoV, and its dependence on photon energy is in large agreement with simulations. The gains of the CMOS sensors are in range of 6.5-6.9 eV/DN, and the energy resolutions in the range of ~120-140 eV at 1.25 keV and ~170-190 eV at 4.5 keV. These results have been ingested into the calibration database and applied to the analysis of the scientific data acquired by LEIA. This work paves the way for the calibration of the Wide-field X-Ray Telescope modules of the Einstein Probe mission., Comment: 24 pages, 13 figures. Submitted to Experimental Astronomy

3C 273 is one of the nearest high-luminosity quasars. Although classified as a blazar, 3C 273 also has some features in Seyferts, whose X-ray may originate from the corona. Since both jet and corona produce power-law spectra in X-ray, the spectrum cannot completely distinguish their contributions to 3C 273 in the low state. X-ray polarimetric observations provide the chance to constrain the X-ray radiation origin of 3C 273 in the low state. We perform general relativistic radiative transfer simulations with the code MONK to compute the X-ray polarization in 2-10 keV from the jets, sphere coronae, and slab coronae for 3C 273. We find that the radiation from the jet in 2-10 keV has a larger polarization degree than that of the corona: the polarization degree in 2-10 keV from the corona is unpolarized, while these are 4.1%-15.8% for the jet with a vertical or radial magnetic field and $\leq$5.0% for the jet with toroidal magnetic field. The X-ray polarization of the corona and jet is sensitive to optical depth and geometry, and the main driver for this dependence is the number of scatterings. These results show that X-ray polarization can effectively constrain the X-ray radiation origin of 3C 273 in the low state., Comment: Accepted by MNRAS

Complementary metal-oxide-semiconductor (CMOS) sensors are a competitive choice for future X-ray astronomy missions. Typically, CMOS sensors on space astronomical telescopes are exposed to a high dose of irradiation. We investigate the impact of irradiation on the performance of two scientific CMOS (sCMOS) sensors between -30 to 20 degree at high gain mode (7.5 times), including the bias map, readout noise, dark current, conversion gain, and energy resolution. The two sensors are irradiated with 50 MeV protons with a total dose of 5.3*10^10 p/cm^2. After the exposure, the bias map, readout noise and conversion gain at various temperatures are not significantly degraded, nor is the energy resolution at -30 degree. However, after the exposure the dark current has increased by hundreds of times, and for every 20 degree increase in temperature, the dark current also increases by an order of magnitude. Therefore, at room temperature, the fluctuations of the dark currents dominate the noise and lead to a serious degradation of the energy resolution. Moreover, among the 4k * 4k pixels, there are about 100 pixels whose bias at 50 ms has changed by more than 10 DN (~18 e-), and about 10 pixels whose readout noise has increased by over 15 e- at -30 degree. Fortunately, the influence of the dark current can be reduced by decreasing the integration time, and the degraded pixels can be masked by regular analysis of the dark images. Some future X-ray missions will likely operate at -30 degree, under which the dark current is too small to significantly affect the X-ray performance. Our investigations show the high tolerance of the sCMOS sensors for proton radiation and prove their suitability for X-ray astronomy applications., Comment: accepted by JATIS

Segment Anything Model (SAM), a vision foundation model trained on large-scale annotations, has recently continued raising awareness within medical image segmentation. Despite the impressive capabilities of SAM on natural scenes, it struggles with performance decline when confronted with medical images, especially those involving blurry boundaries and highly irregular regions of low contrast. In this paper, a SAM-based parameter-efficient fine-tuning method, called SAMIHS, is proposed for intracranial hemorrhage segmentation, which is a crucial and challenging step in stroke diagnosis and surgical planning. Distinguished from previous SAM and SAM-based methods, SAMIHS incorporates parameter-refactoring adapters into SAM's image encoder and considers the efficient and flexible utilization of adapters' parameters. Additionally, we employ a combo loss that combines binary cross-entropy loss and boundary-sensitive loss to enhance SAMIHS's ability to recognize the boundary regions. Our experimental results on two public datasets demonstrate the effectiveness of our proposed method. Code is available at https://github.com/mileswyn/SAMIHS ., Comment: 5 pages, 3 figures, 2 tables

In recent years, tremendous progress has been made on scientific Complementary Metal Oxide Semiconductor (sCMOS) sensors, making them a promising device for future space X-ray missions. We have customized a large-format sCMOS sensor, G1516BI, dedicated for X-ray applications. In this work, a 200 nm thick aluminum layer is successfully sputtered on the surface of this sensor. This Al-coated sensor, named EP4K, shows consistent performance with the uncoated version. The readout noise of the EP4K sensor is around 2.5 e- and the dark current is less than 0.01 e-/pixel/s at -30 degree. The maximum frame rate is 20 Hz in the current design. The ratio of single pixel events of the sensor is 45.0%. The energy resolution can reach 153.2 eV at 4.51 keV and 174.2 eV at 5.90 keV at -30 degree. The optical transmittance of the aluminum layer is approximately 1e-8 to 1e-10 for optical lights from 365 to 880 nm, corresponding to an effective aluminum thickness of around 140 to 160 nm. The good X-ray performance and low optical transmittance of this Al-coated sCMOS sensor make it a good choice for space X-ray missions. The Lobster Eye Imager for Astronomy (LEIA), which has been working in orbit for about one year, is equipped with four pieces of EP4K sensors. Furthermore, 48 pieces of EP4K sensors are used on the Wide-field X-ray Telescope (WXT) on the Einstein Probe (EP) satellite, which will be launched at the end of 2023., Comment: published on PASP

In recent years, scientific CMOS (sCMOS) sensors have been vigorously developed and have outperformed CCDs in several aspects: higher readout frame rate, higher radiation tolerance, and higher working temperature. For silicon image sensors, image lag will occur when the charges of an event are not fully transferred inside pixels. It can degrade the image quality for optical imaging, and deteriorate the energy resolution for X-ray spectroscopy. In this work, the image lag of a sCMOS sensor is studied. To measure the image lag under low-light illumination, we constructed a new method to extract the image lag from X-ray photons. The image lag of a customized X-ray sCMOS sensor GSENSE1516BSI is measured, and its influence on X-ray performance is evaluated. The result shows that the image lag of this sensor exists only in the immediately subsequent frame and is always less than 0.05% for different incident photon energies and under different experimental conditions. The residual charge is smaller than 0.5 e- with the highest incident photon charge around 8 ke-. Compared to the readout noise level around 3 e-, the image lag of this sensor is too small to have a significant impact on the imaging quality and the energy resolution. The image lag shows a positive correlation with the incident photon energy and a negative correlation with the temperature. However, it has no dependence on the gain setting and the integration time. These relations can be explained qualitatively by the non-ideal potential structure inside the pixels. This method can also be applied to the study of image lag for other kinds of imaging sensors., Comment: accepted by NIM A

Scientific Complementary Metal Oxide Semiconductor (sCMOS) sensors are finding increasingly more applications in astronomical observations, thanks to their advantages over charge-coupled devices (CCDs) such as a higher readout frame rate, higher radiation tolerance, and higher working temperature. In this work, we investigate the performance at the individual pixel level of a large-format sCMOS sensor, GSENSE1516BSI, which has 4096 * 4096 pixels, each of 15 {\mu}m in size. To achieve this, three areas on the sCMOS sensor, each consisting of 99 * 99 pixels, are chosen for the experiment. The readout noise, conversion gain and energy resolutions of the individual pixels in these areas are measured from a large number (more than 25,000) of X-ray events accumulated for each of the pixels through long time exposures. The energy resolution of these pixels can reach 140 eV at 6.4 keV at room temperature and shows a significant positive correlation with the readout noise. The accurate gain can also be derived individually for each of the pixels from its X-ray spectrum obtained. Variations of the gain values are found at a level of 0.56% statistically among the 30 thousand pixels in the areas studied. With the gain of each pixel determined accurately, a precise gain correction is performed pixel by pixel in these areas, in contrast to the standardized ensemble gain used in the conventional method. In this way, we could almost completely eliminate the degradation of energy resolutions caused by gain variations among pixels. As a result, the energy resolution at room temperature can be significantly improved to 124.6 eV at 4.5 keV and 140.7 eV at 6.4 keV. This pixel-by-pixel gain correction method can be applied to all kinds of CMOS sensors, and is expected to find interesting applications in X-ray spectroscopic observations in the future., Comment: accepted by PASP

In recent years, scientific Complementary Metal Oxide Semiconductor (sCMOS) devices have been increasingly applied in X-ray detection, thanks to their attributes such as high frame rate, low dark current, high radiation tolerance and low readout noise. We tested the basic performance of a backside-illuminated (BSI) sCMOS sensor, which has a small pixel size of 6.5 um * 6.5 um. At a temperature of -20C, The readout noise is 1.6 e, the dark current is 0.5 e/pixel/s, and the energy resolution reaches 204.6 eV for single-pixel events. The effect of depletion depth on the sensor's performance was also examined, using three versions of the sensors with different deletion depths. We found that the sensor with a deeper depletion region can achieve a better energy resolution for events of all types of pixel splitting patterns, and has a higher efficiency in collecting photoelectrons produced by X-ray photons. We further study the effect of depletion depth on charge diffusion with a center-of-gravity (CG) model. Based on this work, a highly depleted sCMOS is recommended for applications of soft X-ray spectroscop., Comment: 11 pages, 13 figures. Accepted for publication in JInst

The $\gamma$-ray narrow-line Seyfert 1 (NLS1) galaxies can be considered to be the third class of $\gamma$-ray active galactic nuclei possessing relativistic jets. In this paper, we present multi-band high resolution Very Long Baseline Array (VLBA) images of the $\gamma$-ray NLS1, SDSS J211852.96$-$073227.5 (J2118$-$0732, $z=0.26$). We find a core-jet radio morphology and significant flux density variations in the radio core. The high brightness temperature estimated from VLBA images and core variability demonstrate that it exhibits substantial relativistic beaming effects. From considering radio emission in several bands, we find that the source has an inverted spectrum above 1 GHz but a steep spectrum at low frequencies from 74 MHz to 1 GHz; these may arise from the present activity and the old diffuse/extended emission, respectively. The core-jet morphology, significant flux density variations, and beaming effect make J2118$-$0732 resemble a blazar. Considering the low mass of its central black hole and ongoing merger environment, J2118$-$0732 may represent a low-mass, low-power counterpart of blazars, and may finally evolve to a blazar., Comment: 12 pages, 6 figures. Published in ApJ

In recent years, the performance of Scientifc Complementary Metal Oxide Semiconductor (sCMOS) sensors has been improved signifcantly. Compared with CCD sensors, sCMOS sensors have various advantages, making them potentially better devices for optical and X-ray detection, especially in time-domain astronomy. After a series of tests of sCMOS sensors, we proposed a new dedicated high-speed, large-format X-ray detector in 2016 cooperating with Gpixel Inc. This new sCMOS sensor has a physical size of 6 cm by 6 cm, with an array of 4096 by 4096 pixels and a pixel size of 15 um. The frame rate is 20.1 fps under current condition and can be boosted to a maximum value around 100 fps. The epitaxial thickness is increased to 10 um compared to the previous sCMOS product. We show the results of its frst taped-out product in this work. The dark current of this sCMOS is lower than 10 e/pixel/s at 20C, and lower than 0.02 e/pixel/s at -30C. The Fixed Pattern Noise (FPN) and the readout noise are lower than 5 e in high-gain situation and show a small increase at low temperature. The energy resolution reaches 180.1 eV (3.1%) at 5.90 keV for single-pixel events and 212.3 eV (3.6%) for all split events. The continuous X-ray spectrum measurement shows that this sensor is able to response to X-ray photons from 500 eV to 37 keV. The excellent performance, as demonstrated from these test results, makes sCMOS sensor an ideal detector for X-ray imaging and spectroscopic application., Comment: 20 pages. published in PASP

In recent years, scientific CMOS (sCMOS) sensors have found increasing applications to X-ray detection, including X-ray astronomical observations. In order to examine the performance of sCMOS sensors, we have developed X-ray cameras based on sCMOS sensors. Two cameras, CNX22 and CNX 66, have been developed using sCMOS sensors with a photosensitive area of 2 cm * 2 cm and 6 cm * 6 cm, respectively. The designs of the cameras are presented in this paper. The CNX22 camera has a frame rate of 48 fps, whereas CNX66 has a frame rate of currently 20 fps, that can be boosted to 100 fps in the future. The operating temperature of the sCMOS sensor can reach to -20C for CNX22 and -30C for CNX66 with a peltier cooler device. In addition to the commonly used mode of saving original images, the cameras provide a mode of real-time extraction of X-ray events and storage their information, which significantly reduces the requirement for data storage and offline analysis work. For both cameras, the energy resolutions can reach less than 200 eV at 5.9 keV using single-pixel events. These cameras are suitable for X-ray spectroscopy applications in laboratories and calibration for the space X-ray telescopes., Comment: 7 pages, 5 figures, published by Proc SPIE 2022

The Einstein Probe (EP) is a mission designed to monitor the sky in the soft X-ray band. It will perform systematic surveys and characterisation of high-energy transients and monitoring of variable objects at unprecedented sensitivity and monitoring cadences. It has a large instantaneous field-of-view (3,600 sq. deg.), that is realised via the lobster-eye micro-pore X-ray focusing optics. EP also carries a conventional X-ray focusing telescope with a larger effective area to perform followup observations and precise positioning of newly-discovered transients. Alerts for transient objects will be issued publicly and timely. The scientific goals of EP are concerned with discovering faint, distant or rare types of high-energy transients and variable sources. Within the confines of a modest-sized mission, EP will cover a wide range of scientific topics, from the nearby to high-redshift Universe. The Einstein Probe is a mission of the Chinese Academy of Sciences, and also an international collaborative project. This paper presents the background, scientific objectives, and the mission design including the micro-pore optics and CMOS technologies adopted, the instruments and their expected performance, and the mission profile. The development status of the project is also presented., Comment: 29 pages, 23 figures; Invited chapter for Handbook of X-ray and Gamma-ray Astrophysics (Eds. C. Bambi and A. Santangelo, Springer Singapore, expected in 2022)

The X-ray variability of active galactic nuclei (AGN) carries crucial information about the X-ray radiation mechanism. We performed a systematic study of the X-ray short-term (1-100 ks timescale) variability for a large sample of 78 Seyferts with 426 deep XMM-Newton observations. In this paper, we present the time-averaged spectra and rms spectra for the entire sample, which show a variety of properties. Based on the spectral shape, we divide the rms spectra into five subtypes and the time-averaged spectra into four subtypes. The most common shape of the rms spectra is concave-down where the rms peaks at $\sim$ 1 keV. We find that different sources can show similar time-averaged spectra and rms spectra. However, there is no one-to-one mapping between the subtypes of the time-averaged spectra and rms spectra, as similar time-averaged spectra can be accompanied by different rms spectra, and vice versa. This is likely because different physical mechanisms can produce similar rms spectra. For every subtype of the time-averaged spectra, we report its preferred subtypes of the rms spectra in both low- and high-frequency bands. We also compare the statistical properties for different subtypes, such as the black hole mass and Eddington ratio. Finally, we investigate the rms in the Fe K$\alpha$ line regime and find that those with a broad and extended red-wing profile tend to show stronger variability than those showing a narrow or relatively symmetric profile. Our results demonstrate the necessity of performing joint spectral and variability modeling in order to understand the mechanism of the X-ray emission in AGN. All of the rms spectra have been made publicly available., Comment: 25 pages, 10 figures, 4 tables, accepted for publication in The Astrophysical Journal, data available under https://gohujingwei.github.io/rms.html

Active Galactic Nuclei (AGN) are generally considered as the scaled-up counterparts of X-ray binaries (XRBs). It is known that the power spectral density (PSD) of the X-ray emission of XRBs shows significant evolution with spectral states. It is not clear whether AGN follow a similar evolutionary trend, however, though their X-ray emission and the PSD are both variable. In this work, we study a sample of nine AGN with multiple long observations with XMM-Newton, which exhibit significant X-ray spectral variation. We perform Bayesian PSD analysis to measure the PSD shape and variation. We find that a large change in the X-ray energy spectrum (mainly the change of flux state) is often accompanied by a large change in the PSD shape. The emergence of a high-frequency break in the PSD also depends on the spectral state. Among the four sources with significant high-frequency PSD breaks detected, three show the break only in the high-flux state, while the remaining one shows it only in the low-flux state. Moreover, the X-ray rms variability in different spectral states of an AGN is found to vary by as much as 1.0 dex. These results suggest that the different variability properties observed are likely caused by different physical processes dominating different spectral states. Our results also indicate that the intrinsic PSD variation can introduce a significant fraction of the dispersion as reported for the correlations between various X-ray variability properties and the black hole mass., Comment: 25 pages, 11 figures, accepted for publication in ApJ

We present the identifications of a flux-limited sample of highly variable X-ray sources on long time-scales from the second catalogue of the XMM$-$Newton SLew survey (XMMSL2). The carefully constructed sample, comprising 265 sources (2.5 per cent) selected from the XMMSL2 clean catalogue, displayed X-ray variability of a factor of more than 10 in 0.2$-$2 keV compared to the ROSAT All Sky Survey. Of the sample sources, 94.3 per cent are identified. The identification procedure follows a series of cross-matches with astronomical data bases and multiwavelength catalogues to refine the source position and identify counterparts to the X-ray sources. Assignment of source type utilizes a combination of indicators including counterparts offset, parallax measurement, spectral colours, X-ray luminosity, and light-curve behaviour. We identified 40 per cent of the variables with stars, 10 per cent with accreting binaries, and at least 30.4 per cent with active galactic nuclei. The rest of the variables are identified as galaxies. It is found that the mean effective temperatures of the highly variable stars are lower than those of less variable stars. Our sample of highly variable AGN tend to have lower black hole masses, redshifts, and marginally lower soft X-ray luminosities compared to the less variable ones, while no difference was found in the Eddington ratio distributions. Five flaring events are tidal disruption events published previously. This study has significantly increased the number of variable sources in XMMSL2 with identifications and provides greater insight on the nature of many of the sources, enabling further studies of highly variable X-ray sources., Comment: 20 pages, 18 figures, published in MNRAS

A dozen X-ray supernova shock breakout (SN SBO) candidates were reported recently based on XMM-Newton archival data, which increased the X-ray selected SN SBO sample by an order of magnitude. Assuming they are genuine SN SBOs, we study the luminosity function (LF) by improving upon the method used in our previous work. The light curves and the spectra of the candidates were used to derive the maximum volume within which these objects could be detected with XMM-Newton by simulation. The results show that the SN SBO LF can be described by either a broken power law (BPL) with indices (at the 68$\%$ confidence level) of $0.48 \pm 0.28$ and $2.11 \pm 1.27$ before and after the break luminosity at $\log (L_b/\rm erg\,s^{-1})=$ $45.32 \pm 0.55$ or a single power law (SPL) with index of $0.80 \pm 0.16$. The local event rate densities of SN SBOs above $5\times 10^{42}$ $\rm erg\,s^{-1}$ are consistent for two models, i.e., $4.6 ^{+1.7}_{-1.3} \times 10^4$ and $4.9 ^{+1.9}_{-1.4} \times 10^4$ $\rm Gpc^{-3}\,yr^{-1}$ for BPL and SPL models, respectively. The number of fast X-ray transients of SN SBO origin can be significantly increased by the wide-field X-ray telescopes such as the Einstein Probe., Comment: 13 pages, 5 figures, 3 tables. Accepted for publication in ApJ

The jet in active galactic nuclei (AGN) is a key ingredient in understanding the co-evolution of galaxies and their central supermassive black holes (SMBHs). Unfortunately, the mechanism of jet launching and collimation is still elusive. The observational evidence of decreasing radio loudness with increasing Eddington ratio implies that the jet should be coupled with the accretion process. To further explore the relationship between the jet and accretion, it is necessary to extend our knowledge of the jet to an extreme end of the Eddington ratio distribution of AGN. Using Very Long Baseline Array (VLBA), we report the detection of the parsec-scale radio structure in Mrk 335, a radio-quiet narrow-line Seyfert 1 galaxy with an Eddington ratio close to/above unity. The VLBA image at 1.5 GHz reveals an elongated structure extending $\sim20$ parsec in north-south direction with a peak flux density of $1.98\pm0.05$ mJy/beam and radio brightness temperatures as high as $6\times10^{7}$ K. This feature provides a strong evidence of a parsec-scale (bipolar) jet launched from a highly accreting SMBH. We discuss the result by comparing Mrk 335 with other highly accreting systems, e.g. Galactic black holes and tidal disruption events, and recall the discovery of collimated corona in the vicinity of SMBH in Mrk 335 by previous X-ray observations, whose relation to the parsec-scale radio jet should be explored by future simultaneous X-ray spectroscopy and high resolution radio observations., Comment: Published in MNRAS, 9 pages, 2 figures

As a space X-ray imaging mission dedicated to time-domain astrophysics, the Einstein Probe (EP) carries two kinds of scientific payloads, the wide-field X-ray telescope (WXT) and the follow-up X-ray telescope (FXT). FXT utilizes Wolter-I type mirrors and the pn-CCD detectors. In this work, we investigate the in-orbit background of FXT based on Geant4 simulation. The impact of various space components present in the EP orbital environment are considered, such as the cosmic photon background, cosmic ray primary and secondary particles (e.g. protons, electrons and positrons), albedo gamma rays, and the low-energy protons near the geomagnetic equator. The obtained instrumental background at 0.5-10 keV, which is mainly induced by cosmic ray protons and cosmic photon background, corresponds to a level of $\sim$3.1$\times$10$^{-2}$ counts s$^{-1}$ keV$^{-1}$ in the imaging area of the focal plane detector (FPD), i.e. 3.7$\times$10$^{-3}$ counts s$^{-1}$ keV$^{-1}$ cm$^{-2}$ after normalization. Compared with the instrumental background, the field of view (FOV) background, which is induced by cosmic photons reflected by the optical mirror, dominates below 2 keV. Based on the simulated background level within the focal spot (a 30$^{\prime\prime}$-radius circle), the sensitivity of FXT is calculated, which could theoretically achieve several $\mu$crab (in the order of 10$^{-14}$ erg cm$^{-2}$ s$^{-1}$) in 0.5-2 keV and several tens of $\mu$crab (in the order of 10$^{-13}$ erg cm$^{-2}$ s$^{-1}$) in 2-10 keV for a pointed observation with an exposure of 25 minutes. This sensitivity becomes worse by a factor of $\sim2$ if additional 10% systematic uncertainty of the background subtraction is included., Comment: 38 pages, 19 figures, accepted for publication in Astroparticle Physics

A black hole X-ray binary produces hard X-ray radiation from its corona and disk when the accreting matter heats up. During an outburst, the disk and corona co-evolves with each other. However, such an evolution is still unclear in both its geometry and dynamics. Here we report the unusual decrease of the reflection fraction in MAXI J1820+070, which is the ratio of the coronal intensity illuminating the disk to the coronal intensity reaching the observer, as the corona is observed to contrast during the decay phase. We postulate a jet-like corona model, in which the corona can be understood as a standing shock where the material flowing through. In this dynamical scenario, the decrease of the reflection fraction is a signature of the corona's bulk velocity. Our findings suggest that as the corona is observed to get closer to the black hole, the coronal material might be outflowing faster., Comment: 11 pages, 5 figures; published in Nature Communications; An artist's impression of Figure 5 is updated here, and the animation is available at http://youbei.work/research/

KUG 1141+371 is a Seyfert 1 galaxy that shows a simultaneous flux increase in the optical and UV bands in the past decade. For instance, the latest Swift observation in 2019 shows that the UVW2 flux of the AGN in KUG 1141+371 has increased by over one order of magnitude since 2009. Meanwhile, the soft X-ray flux of KUG 1141+371 also shows a steady increase by one order of magnitude since 2007. The significant multi-wavelength luminosity change is likely due to a boost of mass accretion rate from approximately 0.6% of the Eddington limit to 3.2%, assuming a black hole mass of $10^{8}M_{\odot}$. In this work, we conduct detailed multi-epoch X-ray spectral analysis focusing on the variability of the X-ray continuum emission and the puzzling soft excess emission. In addition, our SED models also suggest a simultaneous increase of disc temperature and a decreasing inner disc radius along with the increasing accretion rate. Finally, we discuss possible connection between KUG 1141+371 and black hole transients in outburst., Comment: 18 pages, 14 figures, accepted by MNRAS

A long-standing question in active galactic nucleus (AGN) research is how the corona is heated up to produce X-ray radiation much stronger than that arising from the viscous heating within the corona. In this paper, we carry out detailed investigations of magnetic-reconnection heating to the corona, specifically, studying how the disc and corona are self-consistently coupled with the magnetic field, and how the emergent spectra depend on the fundamental parameters of AGN. It is shown that diverse spectral shapes and luminosities over a broad bandpass from optical to X-ray can be produced from the coupled disc and corona within a limited range of the black hole mass, accretion rate and magnetic field strength. The relative strength of X-ray emission with respect to optical/ultraviolet (UV) depends on the strength of the magnetic field in the disc, which, together with accretion rate, determines the fraction of accretion energy transported and released in the corona. This refined disc-corona model is then applied to reproduce the broad-band spectral energy distributions (SEDs) of a sample of 20 bright local AGNs observed simultaneously in X-ray and optical/UV. We find that, in general, the overall observed broad-band SEDs can be reasonably reproduced, except for rather hard X-ray spectral shapes in some objects. The radiation pressure-dominant region, as previously predicted for the standard accretion disc in AGN, disappears for strong X-ray sources, revealing that AGN accretion discs are indeed commonly stable as observed. Our study suggests the disc-corona coupling model involving magnetic fields to be a promising approach for understanding the broad-band spectra of bright AGNs., Comment: 14 pages, 10 figures, 2 tables, published in MNRAS

We report the discovery of a Tidal Disruption Event (TDE) candidate occurring in the Active Galactic Nucleus SDSS J022700.77-042020.6. A sudden increase in flux of J0227-0420 during the second half of 2009 is clearly shown in the long-term optical, UV, and NIR light curves. A plateau phase, following an initial decline, is seen in the NUV and optical u, g, r, i light curves. Moreover, we find possible evidence that the plateau phase in the NUV band may lag behind the optical ones by approximately 70-80 days with also a much shorter duration, i.e. $\sim$7-15 days against $\sim$40-50 days. The long-term NUV/optical (after the plateau phase), NIR and MIR light curves can be well characterized with a form of $L(t)\propto t^{-\beta}$, consistent with the expectation of a TDE. The plateaus can be explained if the stellar streams collide with the pre-existing AGN disk at different radii. Though the overall fallback rate decreases, the material in the outer disk gradually drifts inward and increases the local accretion rate at the inner region, producing the optical and UV plateaus. The possible lag between the optical and NUV plateaus can then be attributed to viscosity delay. The index $\beta$ of the NIR $J, H, K_s$ bands ($\sim1.4-3.3$) is steeper than that of the UV/optical ($\sim0.7-1.3$) and MIR bands ($\sim0.9-1.8$), which may suggest that a certain fraction of the dust in the inner region of the dusty torus may be sublimated during the TDE phase. Our results indicate that, due to collisions between stellar debris and pre-existing disk, the light curves of TDEs occurring in AGN may show distinctive features, which may shed new light on the accretion process., Comment: 15 pages, 7 figures, submitted to ApJ after minor revision

We present multiwavelength studies of a transient X-ray source, XMMSL1 J131952.3+225958, associated with the galaxy NGC 5092 at $z=0.023$ detected in the XMM-Newton SLew survey (XMMSL). The source brightened in the 0.2--2 keV band by a factor of $>20$ in 2005 as compared with previous flux limits and then faded by a factor of $>200$ as observed with it XMM-Newton in 2013 and with it Swift in 2018. At the flaring state, the X-ray spectrum can be modeled with a blackbody at a temperature of $\sim$ 60 eV and an overall luminosity of $\sim$ $1.5 \times 10^{43}$ erg s$^{-1}$. A UV flare and optical flare were also detected with the Galaxy Evolution Explorer and the Sloan Digital Sky Survey, respectively, within several months of the X-ray flare, whose nonstellar UV--optical spectrum can be described with a blackbody at a temperature of $\sim$ $(1-2) \times 10^4$ K and a luminosity of $\sim$ $(2-6) \times 10^{43}$ erg s$^{-1}$. Interestingly, mid-infrared monitoring observations of NGC 5092 with the Wide-field Infrared Survey Explorer 5--13 yr later show a continuous flux decline. These dramatic variability properties, from the X-ray through UV and optical to infrared, appear to be orderly, suggestive of a stellar tidal disruption event (TDE) by a massive black hole, confirming the postulation by Kanner et al.(2013). This TDE candidate belongs to a rare sample with contemporaneous bright emission detected in the X-ray, UV, and optical, which are later echoed by dust-reprocessed light in the mid-infrared. The black hole has a mass of $\sim$ $5 \times 10^{7} \rm M_{\odot}$, residing in a galaxy that is dominated by a middle-aged stellar population of 2.5 Gyr., Comment: 24 pages, 13 figures, accepted for publication in ApJ

We report the discovery of large amplitude X-ray variability in the low luminosity AGN (LLAGN) MGC 7589, and present possible observational evidence for accretion mode transition in this source. Long-term X-ray flux variations by a factor of more than 50 are found using X-ray data obtained by Swift/XRT and XMM-Newton over 17 years. Results of long-term monitoring data in the UV, optical and infrared bands over ~20 years are also presented. The Eddington ratio increased from $10^{-3}$ to $\sim0.13$, suggesting a transition of the accretion flow from an ADAF to a standard thin accretion disc. Further evidence supporting the thin disc in the high luminosity state is found by the detection of a significant soft X-ray component in the X-ray spectrum. The temperature of this component ($\sim19^{+15}_{-7}$eV, fitted with a blackbody model) is in agreement with the predicted temperature of the inner region for a thin disc around a black hole (BH) with mass of $\sim10^{7}\,M_{\mathrm{Sun}}$. These results may indicate that NGC 7589 had experienced accretion mode transition over a timescale of a few years, suggesting the idea that similar accretion processes are at work for massive black hole and black hole X-ray binaries., Comment: 10 pages, 2 figures, resubmitted to MNRAS after revision

Quasars are high-luminosity active galactic nuclei believed to be powered by accretion of interstellar matter onto a super-massive black hole (SMBH) therein. Most of the observed energy is released in an accretion disk of inspiralling gas surrounding the SMBH. An enormous amount of fueling material is expected to be transported inwards. However, basic questions remain unanswered as to whether and how the accretion disks are supplied with external gas, since no disk-feeding inflow has hitherto been observed clearly. Here we report the discovery of highly redshifted broad absorption lines arising from neutral hydrogen and helium atoms in a small sample of quasars. Their absorption troughs show a broad range of Doppler velocities from zero extending continuously inward up to as high as $\sim 5,000$ km s$^{-1}$. We thus see through streams of cold gas moving with a radially inward velocity component that spans an immense gradient---a result of gravitational acceleration by the central SMBH. Extensive photo-ionization modeling for the archetypical object SDSS J103516.20+142200.6 indicates the inflowing gas to be dense, thick and moderately ionized, with a characteristic distance to the SMBH of $\sim 1,000$ gravitational radii, possibly overlapping or close to the outer accretion disk. Our results present the first compelling evidence for the long-sought inflow directly feeding quasars' accretion disks with external materials, likely originating from the dusty torus at a parsec scale. Our approach provides a new tool to probe the bulk of the so far elusive fueling inflows in quasars. Their studies may help address some of the fundamental questions concerning accretion physics, the onset and sustainment of quasar activity, and the SMBH growth at centers of most galaxies., Comment: 38 pages, 15 figures, pre-print of an article published in Nature

Two bright X-ray transients were reported from the Chandra Deep Field South archival data, namely CDF-S XT1 and XT2. Whereas the nature of the former is not identified, the latter was suggested as an excellent candidate for a rapidly spinning magnetar born from a binary neutron star (BNS) merger. Here we propose a unified model to interpret both transients within the framework of the BNS merger magnetar model. According to our picture, CDF-S XT2 is observed from the "free zone" where the magnetar spindown powered X-ray emission escapes freely, whereas CDF-S XT1 originates from the "trapped zone" where the X-ray emission is initially blocked by the dynamical ejecta and becomes transparent after the ejecta is pushed to a distance where Thomson optical depth drops below unity. We fit the magnetar model to the light curves of both transients and derived consistent parameters for the two events, with magnetic field, initial spin period and X-ray emission efficiency being ($B_p=10^{16}\,G$, $P=1.2\,\rm ms$, $\eta = 0.001$) and ($B_p=10^{15.8}\,G$, $P=4.4\, \rm ms$, $\eta = 0.001$) for XT1 and XT2, respectively. The "isotropic equivalent" ejecta mass of XT1 is $M_{\rm ej} \sim 10^{-3}$ $M_{\odot}$, while it is not constrained for XT2. Our results suggest that more extreme magnetar parameters are required to have XT1 detected from the trapped zone. The model parameters for both events are generally consistent with those derived from SGRB X-ray plateau observations. The host galaxy properties of both transients are also consistent with those of SGRBs. The event rate densities of both XT1 and XT2 are consistent with that of BNS mergers., Comment: 8 pages, 5 figures, accepted for publication in ApJ

A new, complete sample of 14,584 broad-line AGNs at $z<0.35$ is presented, which are uncovered homogeneously from the complete database of galaxies and quasars observed spectroscopically in the Sloan Digital Sky Survey Seventh Data Release. The stellar continuum is properly removed for each spectrum with significant host absorption line features, and careful analyses of the emission-line spectra, particularly in the H$\rm \alpha$ and H$\rm \beta$ wavebands, are carried out. The broad Balmer emission line, particularly, H$\rm \alpha$, is used to indicate the presence of an AGN. The broad H$\rm \alpha$ lines have luminosities in a range of $10^{38.5}$-$10^{44.3}$ erg s$^{-1}$, and line widths (FWHMs) of 500-34,000 km s$^{-1}$. The virial black hole masses, estimated from the broad line measurements, span a range of $10^{5.1}$-$10^{10.3}$ $M_\odot$, and the Eddington ratios vary from $-3.3$ to $1.3$ in logarithmic scale. Other quantities such as multi-wavelength photometric properties and flags denoting peculiar line profiles are also included in this catalog. We describe the construction of this catalog and briefly discuss its properties. The catalog is publicly available online. This homogeneously selected AGN catalog, along with the accurately measured spectral parameters, provide the most updated, largest AGN sample data, which will enable further comprehensive investigations of the properties of the AGN population in the low-redshift universe., Comment: 22 pages, 11 figures, 3 tables; accepted for publication in ApJS

As hybrids of narrow-line Seyfert 1 (NLS1) galaxies and blazars, {\gamma}-ray emitting NLS1s are important probes of jet physics in the high Eddington-ratio regime. Only very few of them are known to date; the majority of them below redshift z = 0.5. Here we present the identification of the {\gamma}-ray emitting AGN TXS 0943+105 (SDSS J094635.06+101706.1) as a high-redshift NLS1 galaxy. It turns out to be one of the radio-loudest NLS1s known, highly variable at all wavelengths, and shows widely extended radio emission at a (projected) > 100 kpc scale. It is a known strong {\gamma}-ray emitter with a luminous flare reported previously. At redshift z=1.004, this is the most distant {\gamma}-NLS1 known to date., Comment: 6 pages, 5 figures, MNRAS Letters in press

The broadband spectral energy distribution (SED) of Active Galactic Nuclei (AGN) is investigated for a well-selected sample composed of $23$ Seyfert 1 galaxies observed simultaneously in the optical/UV and X-ray bands with the Neil Gehrels {\it Swift} Observatory. The optical to UV continuum spectra are modeled, for the first time, with emission from an accretion disk with a generalized radial temperature profile, in order to account for the intrinsic spectra which are found to be generally redder than the model prediction of the standard Shakura-Sunyaev disk (SSD) ($F_\nu\propto\nu^{+1/3}$). The power-law indices of the radial temperature profile ($T_{\rm eff}(R)\propto R^{-p}$, $R$ is the radius of the accretion disk) are inferred to be $p=0.5$ -- $0.75$ (a median of $0.63$), deviating from the canonical $p=0.75$ for the SSD model as widely adopted in previous studies. A marginal correlation of a flatter radial temperature profile (a smaller $p$ value) with increasing the Eddington ratio is suggested. Such a model produces generally a lower peak of accretion disk emission and thus a smaller bolometric luminosity in some of the AGN, particularly those with high Eddington ratios, than that based on the SSD model by a factor of several. The broadband SED, the bolometric correction factors and their dependence on some of the AGN parameters are re-visited. We suggest that such non-standard SSD disks may operate in AGN and are at least partly responsible for the reddened optical/UV spectra as observed. One possible explanation for these flattened temperature profiles is the mass loss process in form of disk winds/outflows., Comment: 22 pages, 18 figures, 6 tables, accepted for publication in MNRAS

In this paper we describe the potential of the enhanced X-ray Timing and Polarimetry (eXTP) mission for studies related to accretion flows in the strong field gravity regime around both stellar-mass and supermassive black-holes. eXTP has the unique capability of using advanced 'spectral-timing-polarimetry' techniques to analyze the rapid variations with three orthogonal diagnostics of the flow and its geometry, yielding unprecedented insight into the inner accreting regions, the effects of strong field gravity on the material within them and the powerful outflows which are driven by the accretion process., Comment: Accepted for publication on Sci. China Phys. Mech. Astron. (2019)

We present spectropolarimetric and spectrophotometric observations of the peculiar active galactic nucleus (AGN) SDSS J120300.19+162443.7 (hereafter J1203+1624) at $z=0.1656$. Its optical total flux spectra clearly show broad emission lines (BELs) in H$\alpha$ and H$\beta$. After removal of narrow emission lines (NELs), the full width at half maximum of the lorentzian BEL is $FWHM_{\rm BEL} \approx 1,270$ km s$^{-1}$, fulfilling the conventional definition of a narrow line Seyfert 1 (NLS1) galaxy. However, its NELs are unprecedentedly strong when compared to type 1AGNs. This, together with its large MIR excess ($g-W_4 = 13.172$ mag), implies that the observer and the narrow emission line region (NELR) might see a different ionization continuum. Our optical spectropolarimetry confirms its type 2 nature by detecting a polarized blue continuum and Balmer BELs ($FWHM_{\rm Polarized~BEL} \approx 1,183$ km s$^{-1}$), with a high polarization degree of $> 20$% in the blue wing. The mass and Eddington rate of the central black hole is estimated based on both transmitted and scattered AGN radiation, which is $M_{\bullet} < 2.9 \times 10^7 M_\odot$ and $L_{\rm bol}/L_{\rm Edd} > 1.5$. Severe extinction of the AGN emission also enables a clear view of the compact host galaxy. Discovery of J1203+1624 suggests that NLS1 follows AGN unfication scheme, studying of its analogs could blaze a new trail for exploring the connection between black hole growth and star formation in the host galaxy. Interesting features of J1203+1624 like the peculiar NELs, inflowing scattering clouds within sublimation radius are worth detailed follow-ups in the future., Comment: 9 pages, 6 figures

The {\gamma}-ray detection from several radio-loud (RL) narrow-line Seyfert 1 (NLS1) galaxies has enabled us to study powerful relativistic jets in active galactic nuclei (AGNs) with smaller black hole masses and higher accretion rates than classical blazars. However, the sample of those {\gamma}-ray detected NLS1s available is still not large enough for a comprehensive and statistical study. We provide a summary of our detections and follow-up studies of three {\gamma}-ray-emitting NLS1s: SDSS J211852.96-073227.5 with flaring {\gamma}-ray radiation (Yang et al. 2018, Paliya et al. 2018) and SDSS J122222.55+041315.7 with the highest redshift by far (z~1) (Yao et al. 2015a), along with the prototype 1H 0323+342 (Zhou et al. 2007, Yao et al. 2015b). And we will discuss their multiwavelength properties and variability properties, including implications from high-energy observations in {\gamma}-rays and X-rays, infrared and radio properties, and correlated variability between several wavebands., Comment: 6 pages, 2 figures, accepted for publication in Proceedings of Science. Contributed article in the conference on "Revisiting narrow-line Seyfert 1 galaxies and their place in the Universe" held in Padova university, Italy, 9-13 April 2018

As a newly discovered class of radio-loud active galactic nuclei (AGNs), the \gamma-ray detected narrow-line Seyfert 1 galaxies (NLS1s) launch powerful jets which are generally found only in blazars and radio galaxies. However, their black hole (BH) masses as estimated from the broad emission lines are one order of magnitude or more lower than those in blazars. This brings new challenges for explaining the radio loudness triggering in AGNs. It is still under debate whether their BH masses from the commonly used virial method are underestimated. Here we present an estimate of the BH mass for the \gamma-ray detected NLS1 1H 0323+342, an archetype of this class, from the X-ray variability which is inclination independent. Our results independently confirm that this \gamma-ray detected NLS1 harbors a $(2.8-7.9)\times10^6\,M_{\odot}$ BH similar to those in normal NLS1s rather than those in blazars., Comment: Published in ApJ

We identified a peculiar proximate sub-damped Lya absorption system (sub-DLA) at z=3.234 with a neutral hydrogen column density of $N_{H} =10^{19.70\pm0.15}~\rm cm^{-2}$ toward the quasar SDSS J125903.26+621211.5 in two epoch optical spectra of the Sloan Digital Sky Survey. We detected Lya residue in the proximate sub-DLA trough at $> 8\sigma$ level. To our surprise, extreme ultraviolet (EUV) continuum emission is significantly ($> 4\sigma$) detected in the corresponding Lymann limit absorption edge at both of the FUV and NUV bands by the Galaxy Evolution Explorer. The high neutral hydrogen column density should allow a negligible transmission of both the Lya line photons and EUV continuum photons due to the high optical depth of the gas. The possible scenarios of foreground galaxy contamination, partial coverage, emission from the quasar host galaxy, and extended Lya emission are excluded in turn, and we speculate that the residual Lya and EUV emission is due to photons scattering (broad Lya and the continuum emission) of electrons residing at a spatial scale larger than that of the proximate sub-DLA. Electron scattering is wavelength-independent, the scattered light is therefore a copy of the incident spectrum that might originate in the accretion disk. With the assistances from the neutral hydrogen absorbers as the "natural coronagraph" and the scatterers as the "natural mirror", we yielded a very hard EUV spectral index of $\alpha = 0.65\pm 0.25$ ($F_{\nu}\propto \nu^{\alpha}$), consistent with the standard picture of the locally heated accretion disk in the inner EUV-emitting radii, as well as in the outer near-infrared-emitting radii suggested by Kishimoto et al. (2008)., Comment: 14 pages, 4 figures

A new sample of 204 low-mass black holes (LMBHs) in active galactic nuclei (AGNs) is presented with black hole masses in the range of (1-20) * 10^5 M_sun. The AGNs are selected from a systematic search among galaxies in the Seventh Data Release (DR 7) of the Sloan Digital Sky Survey (SDSS), and careful analyses of their optical spectra and precise measurement of spectral parameters. Combining them with our previous sample selected from the SDSS DR 4 makes it the largest LMBH sample so far, totaling over 500 objects. Some of the statistical properties of the combined LMBH AGN sample are briefly discussed, in the context of exploring the low-mass end of the AGN population. Their X-ray luminosities follow the extension of the previously known correlation with the [O III] luminosity. The effective optical-to-X-ray spectral indices \alpha_OX, albeit with a large scatter, are broadly consistent with the extension of the relation with the near-UV luminosity L_2500\AA. Interestingly, a correlation of \alpha_OX with black hole mass is also found in the sense that \alpha_OX is statistically flatter (stronger X-ray relative to optical) for lower black hole mass. Only 26 objects, mostly radio loud, were detected in radio at 20 cm in the FIRST survey, giving a radio loud fraction of 4%. The host galaxies of LMBHs have stellar masses in the range of 10^8.8-10^12.4 M_sun and optical colors typical of Sbc spirals. They are dominated by young stellar populations that seem to have undergone a continuous star formation history., Comment: 5 tables, 16 figures, ApJS accepted

We report on the identification of a new {\gamma}-ray-emitting narrow-line Seyfert 1 (NLS1) galaxy, SDSS J211852.96-073227.5 (hereinafter J2118-0732). The galaxy, at a redshift of 0.26, is associated with a radio source of flat/inverted spectrum at high radio frequencies. The analysis of its optical spectrum obtained in the Sloan Digital Sky Survey (SDSS) revealed a small linewidth of the broad component of the H{\beta} line (full width at half-maximum = 1585 km s^{-1}), making it a radio-loud NLS1 galaxy -- an intriguing class of active galactic nuclei with exceptional multiwavelength properties. A new {\gamma}-ray source centred at J2118-0732 was sporadically detected during 2009--2013 in form of flares by the Fermi-LAT. Our XMM-Newton observations revealed a flat X-ray spectrum described by a simple power law, and a flux variation by a factor of ~2.5 in five months. The source also shows intraday variability in the infrared band. Its broad-band spectral energy distribution can be modelled by emission from a simple one-zone leptonic jet model, and the flux drop from infrared to X-rays in five months can be explained by changes of the jet parameters, though the exact values may be subject to relatively large uncertainties. With the NLS1-blazar composite nucleus, the clear detection of the host galaxy, and the synchronous variations in the multiwavelength fluxes, J2118-0732 provides a new perspective on the formation and evolution of relativistic jets under the regime of relatively small black hole masses and high accretion rates., Comment: 13 pages, 7 figures, Final version, Published in MNRAS