We report on continued monitoring observations of the Galactic center carried out by the X-ray telescope aboard the Swift satellite in 2008 and 2009. This campaign revealed activity of the five known X-ray transients AX J1745.6-2901, CXOGC J174535.5-290124, GRS 1741-2853, XMM J174457-2850.3 and CXOGC J174538.0-290022. All these sources are known to undergo very faint X-ray outbursts with 2-10 keV peak luminosities of Lx,peak~1E34-1E36 erg/s, although the two confirmed neutron star low-mass X-ray binaries AX J1745.6-2901 and GRS 1741-2853 can also become brighter (Lx,peak~1E36-1E37 erg/s). We discuss the observed long-term lightcurves and X-ray spectra of these five enigmatic transients. In 2008, AX J1745.6-2901 returned to quiescence following an unusually long accretion outburst of more than 1.5 years. GRS 1741-2853 was active in 2009 and displayed the brightest outburst ever recorded for this source, reaching up to a 2-10 keV luminosity of Lx~1E37 (D/7.2 kpc)^2 erg/s. This system appears to undergo recurrent accretion outbursts approximately every 2 years. Furthermore, we find that the unclassified transient XMM J174457-2850.3 becomes bright only during short episodes (days) and is often found active in between quiescence (Lx~1E32 erg/s) and its maximum outburst luminosity of Lx~1E36 erg/s. CXOGC J174535.5-290124 and CXOGC J174538.0-290022, as well as three other very-faint X-ray transients that were detected by Swift monitoring observations in 2006, have very low time-averaged mass-accretion rates of ~< 2E-12 Msun/yr. Despite having obtained two years of new data in 2008 and 2009, no new X-ray transients were detected.
We present here the main characteristics of the BHC IGR J17091-3624 outbursts occurred several times since 1994. Since 2003, the source has been extensively observed by INTEGRAL and Swift. In particular, we report results on the last 2011 outburst that showed a rare variability behaviour observed before only in the galactic BH GRS 1915+105 but at a different level of flux. Several hypotheses have been proposed in order to explain this particular behaviour. They are all discussed here, in the light of their apparent contradiction. Finally, based on all available informations, we attempt to give…
Supergiant fast X-ray transients (SFXTs) are a class of high-mass X-ray binaries with possible counterparts in the high energy gamma rays. The Swift SFXT Project has conducted a systematic investigation of the properties of SFTXs on timescales ranging from minutes to years and in several intensity states (from bright flares, to intermediate intensity states, and down to almost quiescence). We also performed broad-band spectroscopy of outbursts, and intensity-selected spectroscopy outside of outbursts. We demonstrated that while the brightest phase of the outburst only lasts a few hours, furthe…
Monitoring of pulse period variations in accreting binary pulsars is an important tool to study the interaction between the magnetosphere of the neutron star and the accretion disk. While the X-ray flux of the brightest X-ray pulsars have been successfully monitored over many years (e.g. with RXTE/ASM, CGRO/BATSE, Swift/BAT), the possibility to monitor their pulse timing properties continuously has so far been very limited. In our work we use Swift/BAT observations to study one of the most enigmatic X-ray pulsars, Hercules X-1. For the first time, a quasi-continuous monitoring of the pulse per…
A multifrequency campaign on the BL Lac object PG 1553+113 was organized by the Whole Earth Blazar Telescope (WEBT) in 2013 April-August, involving 19 optical, two near-IR, and three radio telescopes. The aim was to study the source behaviour at low energies during and around the high-energy observations by the Major Atmospheric Gamma-ray Imaging Cherenkov (MAGIC) telescopes in April-July. We also analyse the UV and X-ray data acquired by the Swift and XMM-Newton satellites in the same period. The WEBT and satellite observations allow us to detail the synchrotron emission bump in the source sp…
The Enigmatic Type Icn Supernova 2024abvb Located ~22 kpc from Its Host Galaxy
The University of Melbourne（墨尔本大学）；University of California, Santa Cruz（加州大学圣克鲁兹分校）；University of Hawai’i at Manoa（夏威夷大学马诺阿分校）；The Australian National University（澳大利亚国立大学）；Kavli Institute of Cosmology Cambridge（卡弗里宇宙学剑桥研究所）；Institut d’Estudis Espacials de Catalunya (IEEC)（加泰罗尼亚空间研究学院）；OzGrav, Centre for Gravitational Astrophysics, College of Science, The Australian National University（澳大利亚国立大学理学院引力天体物理中心）；ARC Centre of Excellence for Gravitational Wave Discovery (OzGrav)（ARC引力波发现卓越中心）；Australian National Centre for the Public Awareness of Science, The Australian National University（澳大利亚国立大学公众科学意识国家中心）；Department of Astronomy, The Ohio State University（俄亥俄州立大学天文学系）
Context: Monitoring of pulse period variations in accreting binary pulsars is an important tool to study the interaction between the magnetosphere of the neutron star and the accretion disk. While the X-ray flux of the brightest X-ray pulsars have been successfully monitored over many years (e.g. with RXTE/ASM, CGRO/BATSE, Swift/BAT), the possibility to monitor their pulse timing properties continuously has so far been very limited. Aims: In our work we show that the Swift/BAT observations can be used to monitor coherent pulsations of bright X-ray sources and use the Swift archival data to stu…
We report on the identification of the new Galactic Center (GC) transient Swift J174540.7-290015 as a likely low mass X-ray binary (LMXB) located at only 16 arcsec from Sgr A*. This transient was detected on 2016 February 6th during the Swift GC monitoring, and it showed long-term spectral variations compatible with a hard to soft state transition. We observed the field with XMM-Newton on February 26th for 35 ks, detecting the source in the soft state, characterised by a low level of variability and a soft X-ray thermal spectrum with a high energy tail (detected by INTEGRAL up to ~50 keV), typ…
Since their discovery by the Beppo-SAX satellite in 1997, gamma-ray burst afterglows have attracted an ever-growing interest. They have allowed redshift measurements that have confirmed that gamma-ray bursts are located at cosmological distances. Their study covers a huge range both in time (from one minute to several months after the trigger) and energy (from the GeV to radio domains). The purpose of this review is first to give a short historical account of afterglow research and describe the main observational results with a special attention to the early afterglow revealed by Swift. We the…
A significant fraction ($\sim 30$ %) of the high-energy gamma-ray sources listed in the second $\textit{Fermi}$ LAT (2FGL) catalog are still of unknown origin, being not yet associated with counterparts at lower energies. In order to investigate the nature of these enigmatic sources, we present here an extensive search of X-ray sources lying in the positional uncertainty region of a selected sample of these Unidentified Gamma-ray Sources (UGSs) that makes use of all available observations performed by the $\textit{Swift}$ X-ray Telescope before March 31, 2013, available for 205 UGSs. To detect…
New generation TeV gamma-ray telescopes have discovered many new sources, including several enigmatic unidentified TeV objects. HESS J0632+057 is a particularly interesting unidentified TeV source since: it is a point source, it has a possible hard-spectrum X-ray counterpart and a positionally consistent Be star, it has evidence of long-term VHE flux variability, and it is postulated to be a newly detected TeV/X-ray binary. We have obtained Swift X-ray telescope observations of this source from MJD 54857 to 54965, in an attempt to ascertain its nature and to investigate the hypothesis that it'…
In this paper we present the results obtained with a monitoring programme (23 days long) performed with Swift-XRT on the local Seyfert galaxy Mrk 915. The light-curve analysis shows a significant count rate variation (about a factor of 2-3) on a time-scale of a few days, while the X-ray colours show a change in the spectral curvature below 2 keV and the presence of two main spectral states. From the spectral analysis we find that the observed variations can be explained by the change of the intrinsic nuclear power (about a factor of 1.5) coupled with a change of the properties of an ionized ab…
Tiānguān ($ζ$ Tau) as a binary system consisting of a Be-star and an accreting White Dwarf: opening a gate to understanding enigmatic $γ$ Cas analogues
Swift J0243.6+6124 was discovered during a giant X-ray outburst in October 2017. While there are numerous studies in the X-ray band, very little is known about the optical counterpart. We have performed an spectral and photometric analysis of the optical counterpart of this intriguing source. We find that the optical counterpart to Swift J0243.6+6124 is a V = 12.9, O9.5Ve star, located at a distance of $\sim5$ kpc. The optical extinction in the direction of the source is $A_V=3.6$ mag. The rotational velocity of the O-type star is 210 km s$^{-1}$. The long-term optical variability agrees with …
The burst-only Swift J1749.4-2807 source was discovered in a high X-ray-active state, while during an {INTEGRAL observations of the Galactic bulge on 2010 April 10. Pulsations at 518 Hz were discovered in the RXTE data, confirming previous suggestions of possible associations between burst-only sources and accreting millisecond X-ray pulsars. The subsequent discovery of X-ray eclipses made Swift J1749.4$-$2807 the first eclipsing accreting millisecond X-ray pulsar. We obtain additional information on Swift J1749.4-2807 and other burst-only sources. We report on the results of a monitoring camp…
BL Lacs are an enigmatic class of active galactic nuclei (AGNs), characterized by the non-thermal continuum typically attributed to synchrotron and inverse Compton emission. Depending on the frequency location of the maxima of these components, they are subdivided into three subclasses LBLs, IBLs, and HBLs. We present the results of a set of observations of eight BL Lac objects of LBL and IBL type performed by the XRT and UVOT detectors onboard the Swift satellite between January 2005 and November 2006. We are mainly interested in measuring the spectral parameters, and particularly the steepne…
GRB 090709A is a long gamma-ray burst (GRB) discovered by Swift, featuring a bright X-ray afterglow as well as a faint infrared transient with very red and peculiar colors. The burst attracted a large interest because of a possible quasi-periodicity at P=8.1 s in the prompt emission, suggesting that it could have a different origin with respect to standard, long GRBs. In order to understand the nature of this burst, we obtained a target of opportunity observation with XMM-Newton. X-ray spectroscopy, based on XMM-Newton and Swift data, allowed us to model the significant excess in photoelectric…
We present results of investigation of the poorly studied X-ray pulsar Swift J1816.7--1613 during its transition from the type I outburst to the quiescent state. Our studies are based on the data obtained from X-ray observatories \textit{Swift}, \textit{NuSTAR} and \textit{Chandra} alongside with the latest IR data from UKIDSS/GPS and \textit{Spitzer}/GLIMPSE surveys. The aim of the work is to determine parameters of the system: the strength of the neutron star magnetic field and the distance to the source, which are required for the interpretation of the source behaviour in the framework of p…
We report on a new NuSTAR observation and on the ongoing Swift XRT monitoring campaign of the peculiar source 1E 161348-5055, located at the centre of the supernova remnant RCW 103, which is recovering from its last outburst in June 2016. The X-ray spectrum at the epoch of the NuSTAR observation can be described by either two absorbed blackbodies ($kT_{BB_1}$ ~ 0.5 keV, $kT_{BB_2}$ ~ 1.2 keV) or an absorbed blackbody plus a power law ($kT_{BB_1}$ ~ 0.6 keV, $Γ$ ~ 3.9). The observed flux was ~ 9 $\times$ 10$^{-12}$ erg s$^{-1}$ cm$^{-2}$, ~ 3 times lower than what observed at the outburst onset…
We present a long-term study of the 2011 outburst of the magnetar Swift J1834.9-0846 carried out using new Chandra observations, as well as all the available Swift, RXTE, and XMM-Newton data. The last observation was performed on 2011 November 12, about 100 days after the onset of the bursting activity that had led to the discovery of the source on 2011 August 07. This long time span enabled us to refine the rotational ephemeris and observe a downturn in the decay of the X-ray flux. Assuming a broken power law for the long-term light curve, the break was at ~46 d after the outburst onset, when…
We report the results from our analysis of a large set of archival data acquired with the X-ray telescope (XRT) onboard Swift, covering the sky region surrounding objects from the first Fermi Large Area Telescope (LAT) catalogue of high-energy sources (1FHL), which still lack an association. Of the 23 regions analysed, ten did not show any evidence of X-ray emission, but 13 were characterised by the presence of one or more objects emitting in the 0.3-10 keV band. Only in a couple of cases is the X-ray counterpart located outside the Fermi positional uncertainty, while in all other cases the as…
We present a 30-day monitoring campaign of the optical counterpart of the bright X-ray transient Swift J1745-26, starting only 19 minutes after the discovery of the source. We observe the system peaking at i' ~17.6 on day 6 (MJD 56192) to then decay at a rate of ~0.04 mag/day. We show that the optical peak occurs at least 3 days later than the hard X-ray (15-50 keV) flux peak. Our measurements result in an outburst amplitude greater than 4.3 magnitudes, which favours an orbital period < 21 h and a companion star with a spectral type later than ~ A0. Spectroscopic observations taken with the GT…
MeV blazars are a sub--population of the blazar family, exhibiting larger--than--average jet powers, accretion luminosities and black hole masses. Because of their extremely hard X--ray continua, these objects are best studied in the X-ray domain. Here, we report on the discovery by the $Fermi$ Large Area Telescope and subsequent follow-up observations with $NuSTAR$, $Swift$ and GROND of a new member of the MeV blazar family: PMN J0641$-$0320. Our optical spectroscopy provides confirmation that this is a flat--spectrum radio quasar located at a redshift of $z=1.196$. Its very hard $NuSTAR$ spe…
MAXI J1421-613 is an X-ray burster discovered by Monitor of All-sky X-ray Image (MAXI) on 9 January 2014 and is considered to be a low-mass X-ray binary. A previous study analyzing follow-up observation data obtained by Suzaku on 31 January to 3 February 2014 reported that an annular emission of ~3'-9' radius was found around the transient source. The most plausible origin of the annular emission is a dust scattering echo by the outburst of MAXI J1421-613. In this paper, we confirm the annular emission by analyzing the data of the Swift follow-up observation which was conducted by the photon c…
In this work, the temporal and spectral properties of the poorly studied X-ray pulsar Swift J1808.4$-$1754 were investigated in the 0.8-79 keV energy range based on the data from the NuSTAR and Swift observatories collected during the 2014 outburst. Strong pulsations with a period of $909.73\pm0.03$ s were detected in the source light curve, with the pulsed fraction demonstrating a nonmonotonic dependence on the energy with a local minimum around 17-22 keV. Phase lags in one of the pulse profile components, reaching the maximal value approximately at the same energy, were discovered. The pulse…
Swift J1357.2-0933 is the first confirmed very faint black hole X-ray transient and has a short estimated orbital period of 2.8 hr. We observed Swift J1357.2-0933 for ~50 ks with XMM-Newton in 2013 July during its quiescent state. The source is clearly detected at a 0.5-10 keV unabsorbed flux of ~3x10^-15 erg cm-2 s-1. If the source is located at a distance of 1.5 kpc (as suggested in the literature), this would imply a luminosity of ~8x10^29 erg s-1, making it the faintest detected quiescent black hole LMXB. This would also imply that there is no indication of a reversal in the quiescence X-r…
We report on the spectral behavior of the first Galactic ultraluminous X-ray pulsar Swift J0243.6+6124 with NuSTAR observations during its 2017-2018 outburst. At sub-Eddington levels, the source spectrum is characterized by three emission components, respectively from the accretion column, the hot spot, and a broad iron line emission region. When the source is above the Eddington limit, the hot spot temperature increases and the spectrum features two more blackbody components. One blackbody component has a radius of 10-20 km and is likely originated from the top of the accretion column. The ot…
In 2006 February, shortly after its launch, Swift began monitoring the center of the Milky Way with the onboard X-Ray Telescope using short 1-ks exposures performed every 1-4 days. Between 2006 and 2014, over 1200 observations have been obtained, amounting to ~1.2 Ms of exposure time. This has yielded a wealth of information about the long-term X-ray behavior of the supermassive black hole Sgr A*, and numerous transient X-ray binaries that are located within the 25'x25' region covered by the campaign. In this review we highlight the discoveries made during these first nine years, which include…
We present the first results for the dust-scattering rings of GRB 221009A, coined as the GRB of the century, as observed by the Neil Gehrels Swift satellite. We perform analysis of both time resolved observations and stacked data. The former approach enable us to study the expansion of the most prominent rings, associate their origin with the prompt X-ray emission of the GRB and determine the location of the dust layers. The stacked radial profiles increase the signal-to-noise ratio of the data and allows detection of fainter and overlapping peaks in the angular profile. We find a total of 16 …
We have discovered a \sim420d modulation, with associated X-ray dips, in RXTE-ASM/MAXI/Swift-BAT archival light-curves of the short-period (3.2h) black-hole X-ray transient, Swift J1753.5-0127. This modulation only appeared at the end of a gradual rebrightening, approximately 3 years after the initial X-ray outburst in mid-2005. The same periodicity is present in both the 2-20 keV and 15-50 keV bands, but with a \sim0.1 phase offset (\sim40d). Contemporaneous photometry in the optical and near-IR reveals a weaker modulation, but consistent with the X-ray period. There are two substantial X-ray…
High-dispersion spectroscopic monitoring of HESS J0632+057 has been carried out over four orbital cycles in order to search for orbital modulation, covering the entire orbital phase. We have measured radial velocity of H$α$ emission line with the method introduced by Shafter et al. (1986), which has been successfully applied to some Be stars. The velocity is seen to increase much earlier than expected for the orbital period of 315 days, and much more steeply than expected at around "apastron". The period of the H$α$ modulation is found to be as $308^{+26}_{-23}$ days. We have also analyzed Swi…
The nearby type-II AGN 1ES1927+654 went through a violent changing-look (CL) event beginning December 2017 during which the optical and UV fluxes increased by four magnitudes over a few months, and broad emission lines newly appeared in the optical/UV. By July 2018 the X-ray coronal emission had completely vanished, only to reappear a few months later. In this work we report the evolution of the radio, optical, UV and X-rays from the pre-flare state through mid-2021 with new and archival data from the Very Long Baseline Array (VLBA), the European VLBI Network, the Very Large Array (VLA), the T…
Swift J1818.0-1607 is a radio-loud magnetar with a spin period of 1.36 s and a dipolar magnetic field strength of B~3E14 G, which is very young compared to the Galactic pulsar population. We report here on the long-term X-ray monitoring campaign of this young magnetar using XMM-Newton, NuSTAR, and Swift from the activation of its first outburst in March 2020 until October 2021, as well as INTEGRAL upper limits on its hard X-ray emission. The 1-10 keV magnetar spectrum is well modeled by an absorbed blackbody with a temperature of kT_BB~1.1 keV, and apparent reduction in the radius of the emitt…
We present results from eight months of Green Bank Telescope 8.7-GHz observations and nearly 18 months Swift X-ray telescope observations of the radio magnetar SGR J1745-2900, which is located 2.4" from Sgr A*. We tracked the magnetar's radio flux density, polarization properties, pulse profile evolution, rotation, and single-pulse behavior. We identified two main periods of activity in SGR J1745-2900. The first is characterized by approximately 5.5 months of relatively stable evolution in radio flux density, rotation, and profile shape, while in the second these properties varied substantiall…
The HAWC gamma ray observatory is located at the Sierra Negra Volcano in Puebla, Mexico, at an altitude of 4,100 meters. HAWC is a wide field of view array of 300 water Cherenkov detectors that are continuously surveying ~ 2sr of the sky, operating since March 2015. The large collected data sample allows HAWC to perform an unbiased monitoring of the BL Lac Mrk 421. This is the closest and brightest known extragalactic high-synchrotron-peaked BL Lac in the gamma-ray/X- ray bands and is extensively monitored by the Large Area Telescope (LAT) on-board the Fermi satellite, and the BAT and XRT inst…
Through an optical campaign performed at 4 telescopes located in the northern and the southern hemispheres, we have obtained optical spectroscopy for 75 counterparts of unclassified or poorly studied hard X-ray emitting objects detected with Swift/BAT and listed in the 54 month Palermo BAT catalogue. All these objects have also observations taken with Swift/XRT, ROSAT or Chandra satellites which allowed us to reduce the high energy error box and pinpoint the most likely optical counterpart/s. We find that 69 sources in our sample are Active Galactic Nuclei (AGNs); of them, 35 are classified as…
The system RX J0806.3+1527 (HM Cnc) is a pulsating X-ray source with 100 per cent modulation on a period of 321.5 s (5.4 min). This period reflects the orbital motion of a close binary consisting of two interacting white dwarfs. Here we present a series of simultaneous X-ray (0.2-10 keV) and near-ultraviolet (2600 angstrom and 1928 angstrom) observations that were carried out with the Swift satellite. In the near-ultraviolet, the counterpart of RX J0806.3+1527 was detected at flux densities consistent with a blackbody with temperature 27E+3 K. We found that the emission at 2600 angstrom is mod…
We present X-ray observations of the new transient magnetar Swift J1834.9-0846, discovered with Swift BAT on 2011 August 7. The data were obtained with Swift, RXTE, CXO, and XMM-Newton both before and after the outburst. Timing analysis reveals singe peak pulsations with a period of 2.4823 s and an unusually high pulsed fraction, 85+/-10%. Using the RXTE and CXO data, we estimated the period derivative, dot{P}=8\times 10^{-12} s/s, and confirmed the high magnetic field of the source, B=1.4\times 10^{14} G. The decay of the persistent X-ray flux, spanning 48 days, is consistent with a power law…
SWIFT J1626.6-5156 is an X-ray pulsar that was discovered in December 2005 during an X-ray outburst. Although the X-ray data suggest that the system is a high-mass X-ray binary, very little information exists on the nature of the optical counterpart. We have analysed all RXTE observations since its discovery, archived optical spectroscopic and photometric data and obtained for the first time near-IR spectra. The K-band spectrum shows HeI 20581 A and HI 21660 A (Brackett-gamma) in emission, which confine the spectral type of the companion to be earlier than B2.5. The H-band spectrum exhibits th…
We report on the VERITAS detection of very-high-energy (VHE, $E > 100~\mathrm{GeV}$) $γ$-ray emission from the optically bright quasar OJ 287 which is located at a redshift of z = 0.306. OJ 287 has been observed to display regular optical outbursts with a period of approximately 12 years, with the last major optical outburst having occurred in 2015. To explain this periodicity, models involving a binary supermassive black hole system at the core of OJ 287, or a helical jet, have been developed. Motivated by elevated Swift-XRT count rates, VERITAS observed OJ 287 in February 2017, and detected …
The first wind nebula around a magnetar was recently discovered in X-rays around Swift~J1834.9$-$0846. We study this magnetar's global energetics and the properties of its particle wind or outflows. At a distance of $\sim4\;$kpc, Swift~J1834.9$-$0846 is located at the center of the supernova remnant (SNR) W41 whose radius is $\sim 19\;$pc, an order of magnitude larger than that of the X-ray nebula ($\sim2\;$pc). The association with SNR W41 suggests a common age of $\sim5-100\;$kyr, while its spin-down age is $4.9$~kyr. A small natal kick velocity may partly explain why a wind nebula was detec…
We aimed to detect a supernova (SN) shock breakout (SBO) with observations in time domain. The SBO marks the first escape of radiation from the blast wave that breaks through the photosphere of the star and launches the SN ejecta, and peaks in the ultraviolet and soft X-ray bands. The detection of a SBO allows determining the onset of the explosion with an accuracy from a few hours to a few seconds. Using the XRT and UVOT instruments onboard the Swift satellite we carried out a weekly cadenced, six months lasting monitoring of seven nearby (distance <50 Mpc) galaxies, namely NGC1084, NGC2207/I…
When neutron stars reside in transient X-ray binaries, their crustal layers become heated during accretion outbursts and subsequently cool in quiescence. Observing and modeling this thermal response has yielded valuable insight into the physics of neutron star crusts. However, one unresolved problem is the evidence in several neutron stars for an extra energy source, located at shallow depth in the crust, that is not accounted for by standard heating models. Its origin remains puzzling, and it is currently unclear whether this additional heating occurs in all neutron stars, and if the magnitud…
AX J1745.6-2901 and GRS 1741-2853 are two transient neutron star low-mass X-ray binaries that are located within ~10' from the Galactic center. Multi-year monitoring observations with the Swift/XRT has exposed several accretion outbursts from these objects. We report on their updated X-ray light curves and renewed activity that occurred in 2010-2013.
The Chandra X-ray Observatory (CXO) imaged the high-mass gamma-ray binary (HMGB) HESS J0632+057 with the Advanced CCD Imaging Spectrometer (ACIS). We analyzed the CXO data together with 967 ks of archival Swift-XRT observations. On arcsecond scales we find a hint of asymmetric extended emission. On arcminute scales, a region of extended emission ("the blob"), located $\sim$5.5' east of the binary, is seen in both the CXO-ACIS and the Swift-XRT images. The blob does not have a counterpart in the radio, NIR, IR, or optical images. The ACIS spectrum of the blob fits either an absorbed power-law m…
KS 1741-293 is a transient neutron star low-mass X-ray binary that is located at an angular distance of ~20' from the Galactic center. We map out the historic activity of the source since its discovery in 1989, characterize its most recent X-ray outbursts observed with Swift (2007, 2008, 2010, and 2011), and discuss its quiescent X-ray properties using archival Chandra data. KS 1741-293 is frequently active, exhibiting outbursts that typically reach a 2-10 keV luminosity of Lx~1E36 (D/6.2 kpc)^2 erg/s and last for several weeks-months. However, Swift also captured a very short and weak accreti…
The very faint X-ray transient Swift J174610-290018 at the Galactic center
INAF – Osservatorio Astronomico di Brera（意大利国家天体物理研究所布雷拉天文台）；Como Lake Center for Astrophysics (CLAP), DiSAT, Università degli Studi dell’Insubria（因苏布里亚大学迪萨特学院科莫湖天体物理学中心）；Max-Planck-Institut für extraterrestrische Physik（马克斯·普朗克地外物理研究所）；Anton Pannekoek Institute for Astronomy, University of Amsterdam（阿姆斯特丹大学安东·潘内库克天文学研究所）；INAF - Istituto di Astrofisica Spaziale e Fisica Cosmica（意大利国家天体物理研究所空间与宇宙物理研究所）；Columbia Astrophysics Laboratory, Columbia University（哥伦比亚大学哥伦比亚天体物理实验室）；Department of Astronomy, University of Illinois（伊利诺伊大学天文学系）
SWIFT J1658.2-4242 is an X-ray transient discovered recently in the Galactic plane, with severe X-ray absorption corresponding to an equivalent hydrogen column density of $N_{\rm H,abs}\sim2\times10^{23}$ cm$^{-2}$. Using new Chandra and XMM-Newton data, we discover a strong X-ray dust scattering halo around it. The halo profile can be well fitted by the scattering from at least three separated dust layers. During the persistent emission phase of SWIFT J1658.2-4242, the best-fit dust scattering $N_{\rm H,sca}$ based on the COMP-AC-S dust grain model is consistent with $N_{\rm H,abs}$. The best…
Here we propose a new gamma-ray burst (GRB) mission, the Next Generation Rapid-Response GRB Observatory (NGRG). As with Swift, GRBs are initially located with a coded-mask X-ray camera. However, the NGRG has two new features: First, a beam-steering system to begin optical observations within ~ 1 s after location; second, a near-IR (NIR) camera viewing the same sky, for sensitivity to extinguished bursts. These features allow measurement of the rise phase of GRB optical-NIR emission. Thus far, the rise time and transition between prompt and afterglow in the optical and NIR are rarely measured. …
We report on analysis of properties of the X-ray binary pulsar GROJ1750-27 based on X-ray (Chandra, Swift, and Fermi/GBM), and near-infrared (VVV and UKIDSS surveys) observations. An accurate position of the source is determined for the first time and used to identify its infrared counterpart. Based on the VVV data we investigate the spectral energy distribution (SED) of the companion, taking into account a non-standard absorption law in the source direction. A comparison of this SED with those of known Be/X-ray binaries and early type stars has allowed us to estimate a lower distance limit to…
XSS J12270-4859 (J12270) is an enigmatic source of unknown nature. Previous studies revealed that the source has unusual X-ray temporal characteristics, including repetitive short-term flares followed by spectral hardening, non-periodic dips, and dichotomy in activity; i.e. intervals filled with flares and those without. Together with a power-law X-ray spectrum, it is suggested to be a low-mass X-ray binary (LMXB). In order to better understand the object, we present the results of our near-infrared (NIR) photometry and linear polarimetry observations as well as X-ray spectroscopy observations…
We present simultaneous NuSTAR and Swift observations of the black hole transient IGR J17091-3642 during its 2016 outburst. By jointly fitting six NuSTAR and four Swift spectra, we found that during this outburst the source evolves from the hard to the hard/soft intermediate and back to the hard state, similar to the 2011 outburst. Unlike in the previous outburst, in this case we observed both a broad emission and an moderately broad absorption line in our observations. Our fits favour an accretion disc with an inclination angle of $\sim$$45°$ with respect to the line of sight and a high iron …
We re-assess the XMM-Newton and Swift observations of HLX1, to examine the evidence for its identification as an intermediate-mass black hole. We show that the X-ray spectral and timing properties are equally consistent with an intermediate-mass black hole in a high state, or with a foreground neutron star with a luminosity of about a few times 10^{32} erg/s ~ 10^{-6} L_{Edd}, located at a distance of about 1.5 to 3 kpc. Contrary to previously published results, we find that the X-ray spectral change between the two XMM-Newton observations of 2004 and 2008 (going from power-law dominated to th…
Through an optical campaign performed at 4 telescopes located in the northern and the southern hemispheres, plus archival data from two on-line sky surveys, we have obtained optical spectroscopy for 29 counterparts of unclassified or poorly studied hard X-ray emitting objects detected with Swift/BAT and listed in the 39 months Palermo catalogue. All these objects have also observations taken with Swift/XRT or XMM-EPIC which not only allow us to pinpoint their optical counterpart, but also to study their X-ray spectral properties (column density, power law photon index and F2-10 keV flux). We f…
The morphology of the circumnuclear gas accreting onto supermassive black holes in Seyfert galaxies remains a topic of much debate. As the innermost regions of Active Galactic Nuclei (AGN) are spatially unresolved, X-ray spectroscopy, and in particular line-of-sight absorption variability, is a key diagnostic to map out the distribution of gas. Observations of variable X-ray absorption in multiple Seyferts and over a wide range of timescales indicate the presence of clumps/clouds of gas within the circumnuclear material. Eclipse events by clumps transiting the line of sight allow us to explore…
