Class Of X-ray Sources Research Articles

The Peculiar X-Ray Transient Swift J0840.7−3516: An Unusual Low-mass X-Ray Binary or a Tidal Disruption Event?

Abstract We report on the X-ray properties of the new transient Swift J0840.7−3516, discovered with Swift/BAT in 2020 February, using extensive data from Swift, MAXI, NICER, and NuSTAR. The source flux increased for ∼103 s after the discovery, decayed rapidly over ∼5 orders of magnitude in five days, and then remained almost constant over nine months. Large-amplitude short-term variations on timescales of 1–104 s were observed throughout the decay. In the initial flux rise, the source showed a hard power-law-shaped spectrum with a photon index of ∼1.0 extending up to ∼30 keV, above which an exponential cutoff was present. The photon index increased in the following rapid decay and became ∼2 at the end of the decay. A spectral absorption feature at 3–4 keV was detected in the decay. It is not straightforward to explain all the observed properties by any known class of X-ray sources. We discuss the possible nature of the source, including a Galactic low-mass X-ray binary with multiple extreme properties and a tidal disruption event by a supermassive black hole or a Galactic neutron star.

The Swift bulge survey: motivation, strategy, and first X-ray results

ABSTRACT Very faint X-ray transients (VFXTs) are X-ray transients with peak X-ray luminosities (LX) of L$_X \lesssim 10^{36}$ erg s−1, which are not well understood. We carried out a survey of 16 deg2 of the Galactic Bulge with the Swift Observatory, using short (60 s) exposures, and returning every 2 weeks for 19 epochs in 2017–18 (with a gap from 2017 November to 2018 February, when the Bulge was in sun-constraint). Our main goal was to detect and study VFXT behaviour in the Galactic Bulge across various classes of X-ray sources. In this work, we explain the observing strategy of the survey, compare our results with the expected number of source detections per class, and discuss the constraints from our survey on the Galactic VFXT population. We detected 91 X-ray sources, 25 of which have clearly varied by a factor of at least 10. In total, 45 of these X-ray sources have known counterparts: 17 chromospherically active stars, 12 X-ray binaries, 5 cataclysmic variables (and 4 candidates), 3 symbiotic systems, 2 radio pulsars, 1 active galactic nuclei, and a young star cluster. The other 46 are of previously undetermined nature. We utilize X-ray hardness ratios, searches for optical/infrared counterparts in published catalogues, and flux ratios from quiescence to outburst to constrain the nature of the unknown sources. Of these 46, 7 are newly discovered hard transients, which are likely VFXT X-ray binaries. Furthermore, we find strong new evidence for a symbiotic nature of four sources in our full sample, and new evidence for accretion power in six X-ray sources with optical counterparts. Our findings indicate that a large subset of VXFTs is likely made up of symbiotic systems.

The fates of merging supermassive black holes and a proposal for a new class of X-ray sources

ABSTRACT We perform N-body simulations on some of the most massive galaxies extracted from a cosmological simulation of hierarchical structure formation with total masses in the range 1012 M⊙ < Mtot < 3 × 1013 M⊙ from 4 ≥ z ≥ 0. After galactic mergers, we track the dynamical evolution of the infalling black holes (BHs) around their host’s central BHs (CBHs). From 11 different simulations, we find that, of the 86 infalling BHs with masses >104 M⊙, 36 merge with their host’s CBH, 13 are ejected from their host galaxy, and 37 are still orbiting at z = 0. Across all galaxies, 33 BHs are kicked to a higher orbit after close interactions with the CBH binary or multiple, after which only one of them merged with their hosts. These orbiting BHs should be detectable by their anomalous (not low-mass X-ray binary) spectra. The X-ray luminosities of the orbiting massive BHs at z = 0 are in the range $10^{28}-10^{43}\, \mathrm{erg}~\mathrm{s}^{-1}$, with a currently undetectable median value of $10^{33}\, \mathrm{erg}~\mathrm{s}^{-1}$. However, the most luminous ∼5 per cent should be detectable by existing X-ray facilities.

12 YEARS OF X-RAY VARIABILITY IN M31 GLOBULAR CLUSTERS, INCLUDING 8 BLACK HOLE CANDIDATES, AS SEEN BYCHANDRA

We examined 134 Chandra observations of the population of X-ray sources associated with globular clusters (GCs) in the central region of M31. These are expected to be X-ray binary systems (XBs), consisting of a neutron star or black hole accreting material from a close companion. We created long-term lightcurves for these sources, correcting for background, interstellar absorption and instrumental effects. We tested for variability by examining the goodness of fit for the best fit constant intensity. We also created structure functions (SFs) for every object in our sample, the first time this technique has been applied to XBs. We found significant variability in 28 out of 34 GCs and GC candidates; the other 6 sources had 0.3--10 keV luminosities fainter than ~2E+36 erg/s, limiting our ability to detect similar variability. The SFs of XBs with 0.3--10 keV luminosities ~2--50 E+36 generally showed considerably more variability than the published ensemble SF of AGN. Our brightest XBs were mostly consistent with the AGN SF; however, their 2--10 keV fluxes could be matched by <1 AGN per square degree. These encouraging results suggest that examining the long term lightcurves of other X-ray sources in the field may provide an important distinction between X-ray binaries and background galaxies, as the X-ray emission spectra from these two classes of X-ray sources are similar. Additionally, we identify 3 new black hole candidates (BHCs) using additional XMM-Newton data, bringing the total number of M31 GC BHCs to 9, with 8 covered in this survey.

Swift monitoring of the central X-ray source in RCW 103

The X-ray source 1E 161348-5055 lies at the centre of the 2-kyr-old supernova remnant RCW 103. Owing to its 24-ks modulation, orders-of-magnitude flux variability over a few months/years, and lack of an obvious optical counterpart, 1E 161348-5055 defies assignment to any known class of X-ray sources. Starting from April 2006, Swift observed 1E 161348-5055 with its X-ray telescope for 2 ks approximately once per month. During the five years covered, the source has remained in a quiescent state, with an average observed flux of 1.7e-12 erg/cm^2/s (1-10 keV), about 20 times lower than the historical maximum attained in its 1999-2000 outburst. The long time-span of the Swift data allows us to obtain an accurate measure of the period of 1E 161348-5055 [P = 24030.42(2) s] and to derive the first upper limit on its period derivative (|dP/dt| < 1.6e-9 s/s at 3 sigma).

Charge Transfer Reactions

Charge transfer, or charge exchange, describes a process in which an ion takes one or more electrons from another atom. Investigations of this fundamental process have accompanied atomic physics from its very beginning, and have been extended to astrophysical scenarios already many decades ago. Yet one important aspect of this process, i.e. its high efficiency in generating X-rays, was only revealed in 1996, when comets were discovered as a new class of X-ray sources. This finding has opened up an entirely new field of X-ray studies, with great impact due to the richness of the underlying atomic physics, as the X-rays are not generated by hot electrons, but by ions picking up electrons from cold gas. While comets still represent the best astrophysical laboratory for investigating the physics of charge transfer, various studies have already spotted a variety of other astrophysical locations, within and beyond our solar system, where X-rays may be generated by this process. They range from planetary atmospheres, the heliosphere, the interstellar medium and stars to galaxies and clusters of galaxies, where charge transfer may even be observationally linked to dark matter. This review attempts to put the various aspects of the study of charge transfer reactions into a broader historical context, with special emphasis on X-ray astrophysics, where the discovery of cometary X-ray emission may have stimulated a novel look at our universe.

The peculiar X-ray source in NGC 5281

Recent observations with Chandra and XMM-Newton have shown that the X-ray source 1WGA J1346.5–6255 is associated with the Be star HD 119682, a member of the open cluster NGC 5281, and displays all the characteristics of the new class of X-ray sources known as γ -Cas analogues. We present a detailed spectroscopic study of this open cluster, finding an age of 40 Myr and a sequence of evolved stars. These results imply that HD 119682 is a blue straggler in NGC 5281, as it is much more massive than any other cluster member, and its membership is strongly suggested by proper motion analysis. This is the second γ -Cas analogue found to be blue straggler in an open cluster, suggesting that evolution plays a role in the formation of these systems and that a supernova explosion has not occurred in them.

What can we learn about quasars from αOXmeasurements in Galactic black hole binaries?

We draw a comparison between active galactic nuclei (AGN) and Galactic black hole binaries (GBHs) using a uniform description of spectral energy distribution of these two classes of X-ray sources. We parametrize spectra of GBHs with an αGBH parameter which we define as a slope of a nominal power-law function between 3 and 20 keV. We show that this parameter can be treated as an equivalent of the X-ray loudness, αOX, used to describe AGN spectra. We do not find linear correlation between the αGBH and disc flux (similar to that between αOX and optical/UV luminosity found in AGN). Instead, we show that αGBH follows a welldefined pattern during a GBH outburst. We find that αGBH tend to cluster around 1, 1.5 and 2, which corresponds to the hard, very high/intermediate and soft spectral states, respectively. We conclude that majority of the observed Type 1 radio quiet AGN are in the spectral state corresponding to the very high/intermediate state of GBHs. The same conclusion is valid for radio-loud AGN. We also study variations of the spectral slopes (αGBH and the X-ray photon index, � ) as a function of disc and Comptonization fluxes. We discuss these dependencies in ⎫

The discovery of an expanding X-ray source in the HH 154 protostellar jet

Context. Protostellar jets are a new class of X-ray sources which has been discovered with both XMM-Newton and Chandra. The mechanism responsible for the X-ray emission is still not clear. Self-shocking in jets, shocks where the jet hits the surrounding medium, reflected or scattered stellar X-ray emission have all been invoked as possible explanations. Aims. One key diagnostic discriminating among physical emission mechanisms is the motion of the X-ray source: hydrodynamical numerical models of continuous protostellar jets plowing through a uniform medium show an X-ray emitting shock front moving at several hundreds km s -1 . In the nearest X-ray emitting protostellar jet, HH 154, this is detectable, with the spatial resolution of the Chandra X-ray observatory, over a few years baseline, allowing a robust discrimination among different mechanisms. Methods. We have performed, in October 2005, a deep Chandra X-ray observation of HH 154. Comparison with the previous (2001) Chandra observation allows to detect proper motion down to the level predicted by models of X-ray emitting shocks in the jet. Results. The 2005 Chandra observation of HH 154 shows unexpected morphological changes of the X-ray emission in comparison with the 2001 data. Two components are present: a stronger, point-like component with no detectable motion and a weaker component which has expanded in size by approximately 300 AU over the 4 years time base of the two observations. This expansion corresponds to approximately 500 km s -1 , very close to the velocity of the X-ray emitting shock in the simple theoretical models. Conclusions. The 2005 data show a more complex system than initially thought (and modeled), with multiple components with different properties. The observed morphology is possibly indicating a pulsed jet propagating through a non-homogeneous medium, likely with medium density decreasing with distance from the driving source. Detailed theoretical modeling and deeper X-ray observations will be needed to understand the physics of this fascinating class of sources.

An Unusual Spectral State of an Ultraluminous Very Soft X-Ray Source during Outburst

We report the results of Chandra and XMM-Newton observations of a new outburst of an ultraluminous X-ray source in M101. During a Chandra monitoring observation of M101, M101 ULX-1 was found to be in outburst in 2004 December, the second outburst in 2004. The peak bolometric luminosity is about 3 × 1040 ergs s-1 (7 × 1039 ergs s-1 in 0.3-7 keV). The outburst spectra are very soft and can generally be fitted with a blackbody model, with temperatures of 40-80 eV, similar to supersoft X-ray sources in the Milky Way and in the Magellanic Clouds. In one Chandra observation, the source spectrum appears to be harder with a temperature of 150 eV. Such a spectral state is rarely seen in M101 ULX-1, and no X-ray source in the Milky Way shows this kind of spectrum. However, such an unusual spectral state very likely belongs to a new class of X-ray sources, quasi-soft X-ray sources, recently discovered in nearby galaxies. M101 ULX-1 returned to supersoft state in a subsequent XMM-Newton observation. Based on the two outbursts in 2004, the extremely high luminosity (Lbol = 1040-1041 ergs s-1), very soft X-ray spectra (kT = 40-150 eV), transient behavior, and state transition provide strong evidence that M101 ULX-1 harbors an intermediate-mass black hole.

A Possible New Population of Sources with Extreme X-Ray/Optical Ratios

We describe a possible new class of X-ray sources that have robust detections in ultradeep Chandra data yet have no detections at all in our deep multiband Great Observatories Origins Deep Survey Hubble Space Telescope Advanced Camera for Surveys (ACS) images, which represent the highest quality optical imaging obtained to date on these fields. These extreme X-ray/optical ratio sources (EXOs) have values of FX/Fopt at least an order of magnitude above those generally found for other active galactic nuclei (AGNs), even those that are harbored by reddened hosts. We thus infer two possible scenarios: (1) if these sources lie at redshifts z 6, then their hosts need to be exceedingly underluminous or more reddened, compared with other known sources, or (2) if these sources lie above z ~ 6-7, such that even their Ly? emission is redshifted out of the bandpass of our ACS z850 filter, then their optical and X-ray fluxes can be accounted for in terms of relatively normal ~L* hosts and moderate-luminosity AGNs.

X-Ray Binaries: A Laboratory for Frontier Physics

The goal of this paper is to discuss the behaviour of the X-ray binary systems, in order to present to the readers an updated panorama of this important class of X-ray sources. They have in common the binary nature, but rather different characteristics: millisecond pulsations in low mass binary systems, seconds-hundreds seconds pulsations in transient high mass X-ray systems, pulsations limited in a narrow ∼6-12s band in the enigmatic class of systems named anomalous X-ray pulsars. However, all these systems are characterized by a neutron star as collapsed object. A few words will be devoted also to those systems having a black hole as collapsed object. Some comments on radio pulsars-SNRs and X-ray pulsars-SNRs associations will be given too.

The Constellation X‐ray mission

Constellation-X combines both the large collecting area and sensitive spectrometers required to obtain high spectral resolution, broad bandpass (0.25—40 keV) spectroscopy for all classes of X-ray sources, over a wide range of luminosity and redshift. It represents a major advance in sensitivity, providing up to a factor of 100 increase in throughput over currently planned high resolution (R > 300) X-ray spectroscopy missions. Constellation-X is the X-ray equivalent of the Keck Telescope. When observations begin towards the end of the next decade, they will mark the start of a new era with high resolution X-ray spectra obtained for all classes of X-ray sources over a wide range of luminosity and distance. With its increased capabilities, Constellation-X will address many fundamental astrophysics questions such as observing the formation and evolution of clusters of galaxies; constraining the Baryon content of the Universe; determining the spin and mass of super massive black holes in AGN; and probing strong gravity in the vicinity of black holes.

X-ray Emissions from Comets Detected in the Röntgen X-ray Satellite All-Sky Survey

After the unexpected discovery of x-rays emitted from comet C/1996 B2 (Hyakutake) with the Röntgen X-ray Satellite (ROSAT) in March 1996, x-ray emissions from comets C/1990 K1 (Levy), C/1990 N1 (Tsuchiya-Kiuchi), 45P (Honda–Mrkos–Pajdušáková), and C/1991 A2 (Arai), optically 300 to 30,000 times fainter than Hyakutake, were discovered in archival ROSAT data. These findings establish comets as a class of x-ray sources and allow their properties to be studied over a wide optical brightness range. The results indicate that charge exchange between highly charged heavy ions in the solar wind and cometary neutrals is the dominant process for the x-ray emission. Comets may thus be used as probes for monitoring the heavy-ion content of the solar wind.

Magnetic cataclysmic variables: optical counterparts of X-ray and EUV sources

The magnetic cataclysmic variables (mCVs) are a small but astrophysically important class of X-ray source. This paper discusses some of the work carried out by the author and his collaborators on these objects, which were discovered as part of various optical identification programs of both X-ray and extreme ultraviolet sources observed by the HEAO-1 and ROSAT satellites. Definitions are given of the two classes of mCVs; the Polars and the Intermediate Polars. The reasons why mCVs are significant sources of X-ray and EUV radiation are also discussed. Individual systems which have been the subject of intensive follow-up programmes at SAAO are then described, and the implications arising from these observations are discussed.

A new transient pulsar in the Small Magellanic Cloud with an unusual x-ray spectrum

This article reports the discovery of a luminous (3.5 x 10(exp 37) ergs/sec over the 0.2 to 2 keV band) transient X-ray pulsar in the Small Magellanic Cloud (SMC) with an extremely soft component to its X-ray spectrum. This is the first time that a spectrum of this type has been seen in this class of X-ray source. The pulse period is 2.7632 s, and the pulse modulation appears to vary with energy from nearly unpulsed in the low-energy band of the ROSAT Position Sensitive Proportional Counter (PSPC) (0.07 to 0.4 keV) to about 50% in the high-energy band (1.0 to 2.4 keV). The object, RX J0059.2-7138, also shows flickering variability in its X-ray emission on timescales of 50 to 100s. The pulse-phase-averaged PSPC X-ray spectrum can be well described by a two-component source model seen through an absorbing column density of approximately 10(exp 21) atoms cm(exp -2). One spectral component is a power law with photon index 2.4. The other component is significantly softer and can be described by either a steeply falling power law or a blackbody with a temperature KT(sub BB) approximately 35 eV. Ths component is transient, but evidently upulsed, and, for the blackbody model fits, requires a large bolometric luminosity: near, or even several times greater than, the Eddington luminosity for a 1.4 solar mass object. When these characteristics of its soft emission are considered, RX J0059.2-7138 appears quite similar to other X-ray sources in the magellanic Clouds, such as CAL 83, CAL 87, and RX J0527.8-6954, which show only extreme ultrasoft (EUS) X-ray spectra. The discovery of RX J0059.2-7138, a probably high-mass X-ray binary, clearly indicates that EUS spectra may arise from accretion-powered neutron-star X-ray sources. This result lends support to the idea that some of the 'pure' EUS sources may be shrouded low-mass X-ray binaries rather than accreting white dwarfs.

X-ray emission from the ring nebula NGC6888

Soft X-ray emission from the ring nebula NGC6888, formed by the stellar wind of the central Wolf–Rayet star HD192163, has been found after thorough processing of Einstein observation data. A new class of X-ray sources, predicted some 20 years ago, has thus been discovered. The spectral shape agrees completely with theoretical predictions, but the emission flux is more than an order of magnitude less than the predictions.

Observations of SN 1987A from Ginga

AbstractResults of the observations of SN 1987A from Ginga are presented. The first detection of X-rays from the SN 1987A was July, 1987. The energy spectrum is quite unnusual for any of the known classes of X-ray sources, and apparently consists of two seperate components; a soft and a hard component. The soft component is significantly time-variable, and also showed a flarelike increase in January, 1988. Whereas, the intensity of the hard component has remained fairly stationary for more than 300 days. The origins of the two components are also discussed.

X-Ray Observation of SN1987A from Ginga

AbstractAn unusual hard X-ray source was discovered in an error box of 0.2° × 0.3° including SN1987A from the X-ray astronomy satellite Ginga. The energy spectrum is quite unusual for any known classes of X-ray source, and apparently consists of two separate components, a soft component and a very hard component. This source is considered to be identified with SN1987A. The X-ray emergence occurred in July, 1987, or possibly even earlier. The soft component is significantly time-variable and also showed a flare-like increase in January, 1988, while the intensity of the hard component has remained relatively unchanged for more than 200 days.

Discovery of an unusual hard X-ray source in the region of supernova 1987A

We have discovered a new hard X-ray source near supernova 1987 A, in the Large Magellanic Cloud, from the X-ray astronomy satellite Ginga. The present error box of 0.2° × 0.3° includes the supernova. The energy spectrum is very hard above 10 keV and unusual for any of the known classes of X-ray source. The flux in the range 10–30 keV as of 2–3 September is ˜5×10–11ergcm–2 s–1. The source intensity increased steadily throughout July, August and early September, but an observation in late September revealed no further increase. The positional agreement, a steady brightening and an unusual hard spectrum support the identification of the source as SN1987A.