:Astrophysical X-ray source

{{Main|Galaxy groups and clusters}}

File:Bullet cluster.jpg of the Bullet Cluster. Exposure time was 140 hours. The scale is shown in megaparsecs. Redshift (z) = 0.3, meaning its light has wavelengths stretched by a factor of 1.3.]]

Clusters of galaxies are formed by the merger of smaller units of matter, such as galaxy groups or individual galaxies. The infalling material (which contains galaxies, gas and dark matter) gains kinetic energy as it falls into the cluster's gravitational potential well. The infalling gas collides with gas already in the cluster and is shock heated to between 10⁷ and 10⁸ K depending on the size of the cluster. This very hot gas emits X-rays by thermal bremsstrahlung emission, and line emission from metals (in astronomy, 'metals' often means all elements except hydrogen and helium). The galaxies and dark matter are collisionless and quickly become virialised, orbiting in the cluster potential well.

At a statistical significance of 8σ, it was found that the spatial offset of the center of the total mass from the center of the baryonic mass peaks cannot be explained with an alteration of the gravitational force law.{{cite journal |author= Clowe D |title = A Direct Empirical Proof of the Existence of Dark Matter |journal=Astrophys J |year=2006 |volume=648 |issue= 2 |pages = L109–L113 |doi = 10.1086/508162 |arxiv=astro-ph/0608407 |bibcode=2006ApJ...648L.109C|s2cid = 2897407 |display-authors=etal}}{{clear}}

{{Main|Quasar}}

A quasi-stellar radio source (quasar) is a very energetic and distant galaxy with an active galactic nucleus (AGN). QSO 0836+7107 is a Quasi-Stellar Object (QSO) that emits baffling amounts of radio energy. This radio emission is caused by electrons spiraling (thus accelerating) along magnetic fields producing cyclotron or synchrotron radiation. These electrons can also interact with visible light emitted by the disk around the AGN or the black hole at its center. These photons accelerate the electrons, which then emit X- and gamma-radiation via Compton and inverse Compton scattering.

On board the Compton Gamma Ray Observatory (CGRO) is the Burst and Transient Source Experiment (BATSE) which detects in the 20 keV to 8 MeV range. QSO 0836+7107 or 4C 71.07 was detected by BATSE as a source of soft gamma rays and hard X-rays. "What BATSE has discovered is that it can be a soft gamma-ray source", McCollough said. QSO 0836+7107 is the faintest and most distant object to be observed in soft gamma rays. It has already been observed in gamma rays by the Energetic Gamma Ray Experiment Telescope (EGRET) also aboard the Compton Gamma Ray Observatory.{{cite web |author=Dooling D |title=BATSE finds most distant quasar yet seen in soft gamma rays Discovery will provide insight on formation of galaxies |url=https://science.nasa.gov/science-news/science-at-nasa/1999/ast24nov99_1/}}{{clear}}

{{Main|Seyfert galaxy}}

Seyfert galaxies are a class of galaxies with nuclei that produce spectral line emission from highly ionized gas.{{cite book|first1=L. S.|last1=Sparke |author1-link=Linda Sparke|first2=J. S. III|last2= Gallagher |year=2007 |title=Galaxies in the Universe: An Introduction |publisher=Cambridge University Press|isbn=978-0-521-67186-6}} They are a subclass of active galactic nuclei (AGN), and are thought to contain supermassive black holes.

The following early-type galaxies (NGCs) have been observed to be X-ray bright due to hot gaseous coronae: NGC 315, 1316, 1332, 1395, 2563, 4374, 4382, 4406, 4472, 4594, 4636, 4649, and 5128.{{cite journal |doi=10.1086/163218 |vauthors=Forman W, Jones C, Tucker W |title=Hot coronae around early-type galaxies |journal=Astrophys. J. |date=June 1985 |volume=293 |issue=6 |pages=102–19 |bibcode=1985ApJ...293..102F |s2cid=122426629 |doi-access=free }} The X-ray emission can be explained as thermal bremsstrahlung from hot gas (0.5–1.5 keV).

{{Main|Ultraluminous X-ray source}}

Ultraluminous X-ray sources (ULXs) are pointlike, nonnuclear X-ray sources with luminosities above the Eddington limit of 3 × 10³² W for a {{Solar mass|20|link=y}} black hole.{{cite journal |vauthors=Feng H, Kaaret P |title=Spectral state transitions of the ultraluminous X-RAY sources X-1 and X-2 in NGC 1313 |journal=Astrophys J |year=2006 |volume=650 |issue=1 |pages=L75–L78 |doi=10.1086/508613 |bibcode=2006ApJ...650L..75F|arxiv = astro-ph/0608066 |s2cid=17728755 }} Many ULXs show strong variability and may be black hole binaries. To fall into the class of intermediate-mass black holes (IMBHs), their luminosities, thermal disk emissions, variation timescales, and surrounding emission-line nebulae must suggest this. However, when the emission is beamed or exceeds the Eddington limit, the ULX may be a stellar-mass black hole. The nearby spiral galaxy NGC 1313 has two compact ULXs, X-1 and X-2. For X-1 the X-ray luminosity increases to a maximum of 3 × 10³³ W, exceeding the Eddington limit, and enters a steep power-law state at high luminosities more indicative of a stellar-mass black hole, whereas X-2 has the opposite behavior and appears to be in the hard X-ray state of an IMBH.{{clear}}

{{Main|Black hole}}

File:Chandra image of Cygnus X-1.jpg, which was the first strong black hole candidate to be discovered.]]

Black holes give off radiation because matter falling into them loses gravitational energy which may result in the emission of radiation before the matter falls into the event horizon. The infalling matter has angular momentum, which means that the material cannot fall in directly, but spins around the black hole. This material often forms an accretion disk. Similar luminous accretion disks can also form around white dwarfs and neutron stars, but in these the infalling gas releases additional energy as it slams against the high-density surface with high speed. In case of a neutron star, the infall speed can be a sizeable fraction of the speed of light.{{clear}}

In some neutron star or white dwarf systems, the magnetic field of the star is strong enough to prevent the formation of an accretion disc. The material in the disc gets very hot because of friction, and emits X-rays. The material in the disc slowly loses its angular momentum and falls into the compact star. In neutron stars and white dwarfs, additional X-rays are generated when the material hits their surfaces. X-ray emission from black holes is variable, varying in luminosity in very short timescales. The variation in luminosity can provide information about the size of the black hole.

{{Main|Supernova remnant}}

A Type Ia supernova is an explosion of a white dwarf in orbit around either another white dwarf or a red giant star. The dense white dwarf can accumulate gas donated from the companion. When the dwarf reaches the critical mass of {{Solar mass|1.4|link=y}}, a thermonuclear explosion ensues. As each Type Ia shines with a known luminosity, Type Ia are used as "standard candles" to measure distances in the universe.

SN 2005ke is the first Type Ia supernova detected in X-ray wavelengths, and it is much brighter in the ultraviolet than expected.{{clear}}

{{See also|Stellar X-ray astronomy}}

{{Main|Vela X-1}}

Vela X-1 is a pulsing, eclipsing high-mass X-ray binary (HMXB) system, associated with the Uhuru source 4U 0900-40 and the supergiant star HD 77581. The X-ray emission of the neutron star is caused by the capture and accretion of matter from the stellar wind of the supergiant companion. Vela X-1 is the prototypical detached HMXB.{{cite journal |vauthors=Mauche CW, Liedahl DA, Akiyama S, Plewa T |title=Hydrodynamic and Spectral Simulations of HMXB Winds |journal=Prog Theor Phys Suppl|year=2007 |volume=169 |pages=196–199 |doi=10.1143/PTPS.169.196|bibcode = 2007PThPS.169..196M |arxiv = 0704.0237 |s2cid=17149878 }}

{{Main|Hercules X-1}}

File:Herx1 lc.gif

An intermediate-mass X-ray binary (IMXB) is a binary star system where one of the components is a neutron star or a black hole. The other component is an intermediate mass star.{{Cite journal|vauthors=Podsiadlowski P, Rappaport S, Pfahl E |title=Evolutionary Binary Sequences for Low- and Intermediate-Mass X-ray Binaries |journal=The Astrophysical Journal |volume=565 |issue=2 |pages=1107–1133 |year=2001 |arxiv=astro-ph/0107261|doi=10.1086/324686 |bibcode=2002ApJ...565.1107P|s2cid=16381236 }}

Hercules X-1 is composed of a neutron star accreting matter from a normal star (HZ Her) probably due to Roche lobe overflow. X-1 is the prototype for the massive X-ray binaries although it falls on the borderline, {{Solar mass|~2|link=y}}, between high- and low-mass X-ray binaries.{{cite journal |vauthors=Priedhorsky WC, Holt SS |title=Long-term cycles in cosmic X-ray sources |journal=Space Sci Rev|volume=45|year=1987 |issue=3–4 |page=291|doi=10.1007/BF00171997 |bibcode=1987SSRv...45..291P|s2cid=120443194 }}{{clear}}

{{Main|Scorpius X-1}}

The first extrasolar X-ray source was discovered on 12 June 1962.{{cite journal |author=Giacconi R |title=Nobel Lecture: The dawn of x-ray astronomy |journal=Rev Mod Phys|year=2003 |volume=75 |issue=3 |page=995 |doi=10.1103/RevModPhys.75.995 |bibcode=2003RvMP...75..995G|doi-access=free }} This source is called Scorpius X-1, the first X-ray source found in the constellation of Scorpius, located in the direction of the center of the Milky Way. Scorpius X-1 is some 9,000 ly from Earth and after the Sun is the strongest X-ray source in the sky at energies below 20 keV. Its X-ray output is 2.3 × 10³¹ W, about 60,000 times the total luminosity of the Sun.{{cite journal|title=The Dynamic Formation of Prominence Condensations|author=S. K. Antiochos|year= 1999|journal=Astrophys J|volume=512|issue=2|page=985|doi=10.1086/306804|bibcode=1999ApJ...512..985A|arxiv = astro-ph/9808199 |s2cid=1207793|display-authors=etal}} Scorpius X-1 itself is a neutron star. This system is classified as a low-mass X-ray binary (LMXB); the neutron star is roughly 1.4 solar masses, while the donor star is only 0.42 solar masses.{{cite journal|doi=10.1086/339224|title=The Mass Donor of Scorpius X-1 Revealed|author1=Steeghs, D. |author2=Casares, J |journal=Astrophys J|volume=568|issue=1|page=273|year=2002|bibcode=2002ApJ...568..273S|arxiv = astro-ph/0107343 |s2cid=14136652}}

{{Main|Sun|Solar X-ray astronomy}}

File:Sun in X-Ray.png on 8 May 1992 by the soft X-ray telescope on board the Yohkoh solar observatory spacecraft.]]

In the late 1930s, the presence of a very hot, tenuous gas surrounding the Sun was inferred indirectly from optical coronal lines of highly ionized species. In the mid-1940s radio observations revealed a radio corona around the Sun. After detecting X-ray photons from the Sun in the course of a rocket flight, T. Burnight wrote, "The sun is assumed to be the source of this radiation although radiation of wavelength shorter than 4 Å would not be expected from theoretical estimates of black body radiation from the solar corona." And, of course, people have seen the solar corona in scattered visible light during solar eclipses.

While neutron stars and black holes are the quintessential point sources of X-rays, all main sequence stars are likely to have hot enough coronae to emit X-rays.{{cite journal |vauthors=Gould RJ, Burbidge GR |title=High energy cosmic photons and neutrinos |journal=Annales d'Astrophysique|volume=28 |page=171|year=1965 |bibcode=1965AnAp...28..171G}} A- or F-type stars have at most thin convection zones and thus produce little coronal activity.{{cite journal |vauthors=Knigge C, Gilliland RL, Dieball A, Zurek DR, Shara MM, Long KS |title=A blue straggler binary with three progenitors in the core of a globular cluster? |journal=Astrophys J|year=2006 |volume=641 |issue=1 |pages=281–287 |doi=10.1086/500311 |bibcode=2006ApJ...641..281K|arxiv = astro-ph/0511645 |s2cid=11072226 }}

Similar solar cycle-related variations are observed in the flux of solar X-ray and UV or EUV radiation. Rotation is one of the primary determinants of the magnetic dynamo, but this point could not be demonstrated by observing the Sun: the Sun's magnetic activity is in fact strongly modulated (due to the 11-year magnetic spot cycle), but this effect is not directly dependent on the rotation period.

Solar flares usually follow the solar cycle. CORONAS-F was launched on 31 July 2001 to coincide with the 23rd solar cycle maximum.

The solar flare of 29 October 2003 apparently showed a significant degree of linear polarization (> 70% in channels E2 = 40–60 keV and E3 = 60–100 keV, but only about 50% in E1 = 20–40 keV) in hard X-rays,{{cite journal |author1=Zhitnik IA |author2=Logachev YI |author3=Bogomolov AV |author4=Denisov YI |author5=Kavanosyan SS |author6=Kuznetsov SN |author7=Morozov OV |author8=Myagkova IN |author9=Svertilov SI |author10=Ignat'ev AP |author11=Oparin SN |author12=Pertsov AA |author13=Tindo IP |title=Polarization, temporal, and spectral parameters of solar flare hard X-rays as measured by the SPR-N instrument onboard the CORONAS-F satellite |journal=Solar System Research|year=2006 |volume=40 |issue=2|page=93|doi=10.1134/S003809460602002X|bibcode = 2006SoSyR..40...93Z |s2cid=120983201 }} but other observations have generally only set upper limits.

File:Tracemosaic.jpg observatory: the blue, green, and red channels show the 17.1 nm, 19.5 nm, and 28.4 nm, respectively. These TRACE filters are most sensitive to emission from 1, 1.5, and 2 million degree plasma, thus showing the entire corona and detail of coronal loops in the lower solar atmosphere.]]

Coronal loops form the basic structure of the lower corona and transition region of the Sun. These highly structured and elegant loops are a direct consequence of the twisted solar magnetic flux within the solar body. The population of coronal loops can be directly linked with the solar cycle, it is for this reason coronal loops are often found with sunspots at their footpoints. Coronal loops populate both active and quiet regions of the solar surface. The Yohkoh Soft X-ray Telescope (SXT) observed X-rays in the 0.25–4.0 keV range, resolving solar features to 2.5 arc seconds with a temporal resolution of 0.5–2 seconds. SXT was sensitive to plasma in the 2–4 MK temperature range, making it an ideal observational platform to compare with data collected from TRACE coronal loops radiating in the EUV wavelengths.{{cite journal |author=Aschwanden MJ |title=Observations and models of coronal loops: From Yohkoh to TRACE, In: Magnetic coupling of the solar atmosphere |volume=188 |page=1|year=2002}}

Variations of solar-flare emission in soft X-rays (10–130 nm) and EUV (26–34 nm) recorded on board CORONAS-F demonstrate for most flares observed by CORONAS-F in 2001–2003 UV radiation preceded X-ray emission by 1–10 min.{{cite journal |vauthors=Nusinov AA, Kazachevskaya TV |title=Extreme ultraviolet and X-ray emission of solar flares as observed from the CORONAS-F spacecraft in 2001–2003 |journal=Solar System Research|year=2006 |volume=40 |issue=2 |page=111|doi=10.1134/S0038094606020043|bibcode = 2006SoSyR..40..111N |s2cid=122895766 }}{{clear}}

{{Main|White dwarf}}

When the core of a medium mass star contracts, it causes a release of energy that makes the envelope of the star expand. This continues until the star finally blows its outer layers off. The core of the star remains intact and becomes a white dwarf. The white dwarf is surrounded by an expanding shell of gas in an object known as a planetary nebula. Planetary nebula seem to mark the transition of a medium mass star from red giant to white dwarf. X-ray images reveal clouds of multimillion degree gas that have been compressed and heated by the fast stellar wind. Eventually the central star collapses to form a white dwarf. For a billion or so years after a star collapses to form a white dwarf, it is "white" hot with surface temperatures of ~20,000 K.

X-ray emission has been detected from PG 1658+441, a hot, isolated, magnetic white dwarf, first detected in an Einstein IPC observation and later identified in an Exosat channel multiplier array observation.{{cite journal |vauthors=Pravdo SH, Marshall FE, White NE, Giommi P |title=X-rays from the magnetic white dwarf PG 1658 + 441 |journal=Astrophys J|year=1986 |volume=300|page=819|doi=10.1086/163859 |bibcode=1986ApJ...300..819P|doi-access=free}} "The broad-band spectrum of this DA white dwarf can be explained as emission from a homogeneous, high-gravity, pure hydrogen atmosphere with a temperature near 28,000 K." These observations of PG 1658+441 support a correlation between temperature and helium abundance in white dwarf atmospheres.

A super soft X-ray source (SSXS) radiates soft X-rays in the range of 0.09 to 2.5 keV. Super soft X-rays are believed to be produced by steady nuclear fusion on a white dwarf's surface of material pulled from a binary companion.{{cite web |title=Max Planck Institute for Extraterrestrial Physics: Super Soft X-ray Sources – Discovered with ROSAT |url=http://www.mpe.mpg.de/~jcg/sss/sss_high.html}} This requires a flow of material sufficiently high to sustain the fusion.

Real mass transfer variations may be occurring in V Sge similar to SSXS RX J0513.9-6951 as revealed by analysis of the activity of the SSXS V Sge where episodes of long low states occur in a cycle of ~400 days.{{cite conference |vauthors=Simon V, Mattei JA |title=Activity of the super-soft X-ray source V Sge |conference=AIP Conference Proceedings|year=2002 |volume=637|page=333|doi=10.1063/1.1518226 |bibcode=2002AIPC..637..333S}}

HD 49798 is a subdwarf star that forms a binary system with RX J0648.0-4418. The subdwarf star is a bright object in the optical and UV bands. The orbital period of the system is accurately known. Recent XMM-Newton observations timed to coincide with the expected eclipse of the X-ray source allowed an accurate determination of the mass of the X-ray source (at least 1.2 solar masses), establishing the X-ray source as a rare, ultra-massive white dwarf.{{cite web |title=XMM-Newton weighs up a rare white dwarf and finds it to be a heavyweight |year=2009|url=http://sci.esa.int/science-e/www/object/index.cfm?fobjectid=45453}}

{{Main|Brown dwarf}}

According to theory, an object that has a mass of less than about 8% of the mass of the Sun cannot sustain significant nuclear fusion in its core.{{cite web |title=Brown Dwarfs |url=http://chandra.harvard.edu/xray_sources/browndwarf_fg.html}} This marks the dividing line between red dwarf stars and brown dwarfs. The dividing line between planets and brown dwarfs occurs with objects that have masses below about 1% of the mass of the Sun, or 10 times the mass of Jupiter. These objects cannot fuse deuterium.

File:Lp94420 duo m.jpg

With no strong central nuclear energy source, the interior of a brown dwarf is in a rapid boiling, or convective state. When combined with the rapid rotation that most brown dwarfs exhibit, convection sets up conditions for the development of a strong, tangled magnetic field near the surface. The flare observed by Chandra from LP 944-20 could have its origin in the turbulent magnetized hot material beneath the brown dwarf's surface. A sub-surface flare could conduct heat to the atmosphere, allowing electric currents to flow and produce an X-ray flare, like a stroke of lightning. The absence of X-rays from LP 944-20 during the non-flaring period is also a significant result. It sets the lowest observational limit on steady X-ray power produced by a brown dwarf star, and shows that coronas cease to exist as the surface temperature of a brown dwarf cools below about 2500 °C and becomes electrically neutral.{{clear}}

File:Chandra X-ray Observatory image of the brown dwarf TWA 5B.jpg

Using NASA's Chandra X-ray Observatory, scientists have detected X-rays from a low mass brown dwarf in a multiple star system.{{cite web|date=14 April 2003|title=X-rays from a Brown Dwarf's Corona|url=http://www.williams.edu/Astronomy/jay/chapter18_etu6.html|access-date=16 November 2009|archive-url=https://web.archive.org/web/20101230000830/http://www.williams.edu/Astronomy/jay/chapter18_etu6.html|archive-date=30 December 2010|url-status=dead}} This is the first time that a brown dwarf this close to its parent star(s) (Sun-like stars TWA 5A) has been resolved in X-rays. "Our Chandra data show that the X-rays originate from the brown dwarf's coronal plasma which is some 3 million degrees Celsius", said Yohko Tsuboi of Chuo University in Tokyo. "This brown dwarf is as bright as the Sun today in X-ray light, while it is fifty times less massive than the Sun", said Tsuboi. "This observation, thus, raises the possibility that even massive planets might emit X-rays by themselves during their youth!"

Electric potentials of about 10 million volts, and currents of 10 million amps – a hundred times greater than the most powerful lightning bolts – are required to explain the auroras at Jupiter's poles, which are a thousand times more powerful than those on Earth.

On Earth, auroras are triggered by solar storms of energetic particles, which disturb Earth's magnetic field. As shown by the swept-back appearance in the illustration, gusts of particles from the Sun also distort Jupiter's magnetic field, and on occasion produce auroras.

Saturn's X-ray spectrum is similar to that of X-rays from the Sun indicating that Saturn's X-radiation is due to the reflection of solar X-rays by Saturn's atmosphere. The optical image is much brighter, and shows the beautiful ring structures, which were not detected in X-rays.{{clear}}

Some of the detected X-rays, originating from solar system bodies other than the Sun, are produced by fluorescence. Scattered solar X-rays provide an additional component.

In the Röntgensatellit (ROSAT) image of the Moon, pixel brightness corresponds to X-ray intensity. The bright lunar hemisphere shines in X-rays because it re-emits X-rays originating from the sun. The background sky has an X-ray glow in part due to the myriad of distant, powerful active galaxies, unresolved in the ROSAT picture. The dark side of the Moon's disk shadows this X-ray background radiation coming from the deep space. A few X-rays only seem to come from the shadowed lunar hemisphere. Instead, they originate in Earth's geocorona or extended atmosphere which surrounds the orbiting X-ray observatory. The measured lunar X-ray luminosity of ~1.2 × 10⁵ W makes the Moon one of the weakest known non-terrestrial X-ray sources.

File:Comet Lulin Jan. 28-2009 Swift gamma.jpg was passing through the constellation Libra when Swift imaged it on 28 January 2009. This image merges data acquired by Swift's Ultraviolet/Optical Telescope (blue and green) and X-Ray Telescope (red). At the time of the observation, the comet was 99.5 million miles from Earth and 115.3 million miles from the Sun.]]

NASA's Swift Gamma-Ray Burst Mission satellite was monitoring Comet Lulin as it closed to 63 Gm of Earth. For the first time, astronomers can see simultaneous UV and X-ray images of a comet. "The solar wind – a fast-moving stream of particles from the sun – interacts with the comet's broader cloud of atoms.Cravens, T. E., Comet Hyakutake X-ray source: Charge transfer of solarwind heavy ions, Geophys. Res. Lett., 24, 105, 1997. This causes the solar wind to light up with X-rays, and that's what Swift's XRT sees", said Stefan Immler, of the Goddard Space Flight Center. This interaction, called charge exchange, results in X-rays from most comets when they pass within about three times Earth's distance from the sun. Because Lulin is so active, its atomic cloud is especially dense. As a result, the X-ray-emitting region extends far sunward of the comet.{{cite web |author=Reddy F |title=NASA's Swift Spies Comet Lulin |url=http://www.nasa.gov/mission_pages/swift/bursts/lulin.html}}{{clear}}

The celestial sphere has been divided into 88 constellations. The IAU constellations are areas of the sky. Each of these contains remarkable X-ray sources. Some of them are galaxies or black holes at the centers of galaxies. Some are pulsars. As with the astronomical X-ray sources, striving to understand the generation of X-rays by the apparent source helps to understand the Sun, the universe as a whole, and how these affect us on Earth.

File:PIA20061 - Andromeda in High-Energy X-rays, Figure 1.jpg – in high-energy X-ray and ultraviolet light (released 5 January 2016).]]

File:M31 Core in X-rays.jpg's galactic center appears to harbor an X-ray source characteristic of a black hole of a million or more solar masses. Seen above, the false-color X-ray picture shows a number of X-ray sources, likely X-ray binary stars, within Andromeda's central region as yellowish dots. The blue source located right at the galaxy's center is coincident with the position of the suspected massive black hole. While the X-rays are produced as material falls into the black hole and heats up, estimates from the X-ray data show Andromeda's central source to be very cold – only about million degrees, compared to the tens of millions of degrees indicated for Andromeda's X-ray binaries.]]

Multiple X-ray sources have been detected in the Andromeda Galaxy, using observations from the ESA's XMM-Newton orbiting observatory.{{clear}}

File:3C 295 Chandra.jpg, a strongly X-ray emitting galaxy cluster in the constellation Boötes. The cluster is filled with gas. Image is 42 arcsec across. RA 14^h 11^m 20^s Dec −52° 12' 21". Observation date: 30 August 1999. Instrument: ACIS. Aka: Cl 1409+524]]

3C 295 (Cl 1409+524) in Boötes is one of the most distant galaxy clusters observed by X-ray telescopes. The cluster is filled with a vast cloud of 50 MK gas that radiates strongly in X rays. Chandra observed that the central galaxy is a strong, complex source of X rays.{{clear}}

File:Ms0735 xray 420.jpg 07^h 41^m 50.20^s Dec +74° 14' 51.00" in Camelopardus. Observation date: 30 November 2003.]]

Hot X-ray emitting gas pervades the galaxy cluster MS 0735.6+7421 in Camelopardus. Two vast cavities – each 600,000 lyrs in diameter appear on opposite sides of a large galaxy at the center of the cluster. These cavities are filled with a two-sided, elongated, magnetized bubble of extremely high-energy electrons that emit radio waves.{{clear}}

{{Main|NGC 4151}}

File:2MASS NGC 4151 JHK.jpg image of NGC 4151.]]

The X-ray landmark NGC 4151, an intermediate spiral Seyfert galaxy has a massive black hole in its core.{{cite web |title=Hubble site news center: Fireworks Near a Black Hole in the Core of Seyfert Galaxy NGC 4151 |url=http://hubblesite.org/newscenter/archive/releases/1997/18/image/a/ }}{{clear}}

{{Main|Sirius}}

A Chandra X-ray image of Sirius A and B shows Sirius B to be more luminous than Sirius A.{{cite news |title=The Dog Star, Sirius, and its Tiny Companion |publisher=Hubble News Desk |date=13 December 2005 |url=http://hubblesite.org/newscenter/newsdesk/archive/releases/2005/36/image/a | access-date=4 August 2006 | archive-url= https://web.archive.org/web/20060712234359/http://hubblesite.org/newscenter/newsdesk/archive/releases/2005/36/image/a| archive-date= 12 July 2006 | url-status= live}} Whereas in the visual range, Sirius A is the more luminous.

File:Cassiopeia A Spitzer Crop.jpg, orange is visible data from the Hubble Space Telescope, and blue and green are data from the Chandra X-ray Observatory.]]

Regarding Cassiopea A SNR, it is believed that first light from the stellar explosion reached Earth approximately 300 years ago but there are no historical records of any sightings of the progenitor supernova, probably due to interstellar dust absorbing optical wavelength radiation before it reached Earth (although it is possible that it was recorded as a sixth magnitude star 3 Cassiopeiae by John Flamsteed on 16 August 1680{{cite journal |author=Hughes DW |title=Did Flamsteed see the Cassiopeia A supernova? |journal=Nature |volume=285|year=1980 |issue=5761 |page=132|doi=10.1038/285132a0|bibcode = 1980Natur.285..132H |s2cid=4257241 |doi-access=free }}). Possible explanations lean toward the idea that the source star was unusually massive and had previously ejected much of its outer layers. These outer layers would have cloaked the star and reabsorbed much of the light released as the inner star collapsed.

CTA 1 is another SNR X-ray source in Cassiopeia. A pulsar in the CTA 1 supernova remnant (4U 0000+72) initially emitted radiation in the X-ray bands (1970–1977). Strangely, when it was observed at a later time (2008) X-ray radiation was not detected. Instead, the Fermi Gamma-ray Space Telescope detected the pulsar was emitting gamma ray radiation, the first of its kind.{{cite web |author=Atkinson N |url=http://www.universetoday.com/2008/10/17/fermi-telescope-makes-first-big-discovery-gamma-ray-pulsar/ |title=Universe Today – Fermi Telescope Makes First Big Discovery: Gamma Ray Pulsar |date=17 October 2008}}{{clear}}

{{Main|Eta Carinae}}

File:ECARmulticolor4.tnl.jpg, Eta Carinae exhibits a superstar at its center as seen in this image from Chandra. The new X-ray observation shows three distinct structures: an outer, horseshoe-shaped ring about 2 light years in diameter, a hot inner core about 3 light-months in diameter, and a hot central source less than 1 light-month in diameter which may contain the superstar that drives the whole show. The outer ring provides evidence of another large explosion that occurred over 1,000 years ago.]]

Three structures around Eta Carinae are thought to represent shock waves produced by matter rushing away from the superstar at supersonic speeds. The temperature of the shock-heated gas ranges from 60 MK in the central regions to 3 MK on the horseshoe-shaped outer structure. "The Chandra image contains some puzzles for existing ideas of how a star can produce such hot and intense X-rays," says Prof. Kris Davidson of the University of Minnesota.{{cite web |title=Chandra Takes X-ray Image of Repeat Offender |url=https://science.nasa.gov/newhome/headlines/ast08oct99_1.htm |access-date=12 July 2017 |archive-url=https://web.archive.org/web/20090724092013/http://science.nasa.gov/newhome/headlines/ast08oct99_1.htm |archive-date=24 July 2009 |url-status=dead }}{{clear}}

File:3C75 in Radio+Xray.jpg, some 25,000 light years away from each other.]]

Abell 400 is a galaxy cluster, containing a galaxy (NGC 1128) with two supermassive black holes 3C 75 spiraling towards merger.{{clear}}

The Chamaeleon complex is a large star forming region (SFR) that includes the Chamaeleon I, Chamaeleon II, and Chamaeleon III dark clouds. It occupies nearly all of the constellation and overlaps into Apus, Musca, and Carina. The mean density of X-ray sources is about one source per square degree.{{cite journal |author1=Alcala JM |author2=Krautter J |author3=Schmitt JHMM |author4=Covino E |author5=Wichmann R |author6=Mundt R |title=A study of the Chamaeleon star forming region from the ROSAT all-sky survey. I. X-ray observations and optical identifications |journal=Astron. Astrophys. |date=November 1995 |volume=114 |issue=11 |pages=109–34 |bibcode=1995A&AS..114..109A }}

File:Chamaeleon I cloud.png false-color image in X-rays between 500 eV and 1.1 keV of the Chamaeleon I dark cloud. The contours are 100 μm emission from dust measured by the IRAS satellite.]]

The Chamaeleon I (Cha I) cloud is a coronal cloud and one of the nearest active star formation regions at ~160 pc. It is relatively isolated from other star-forming clouds, so it is unlikely that older pre-main sequence (PMS) stars have drifted into the field.{{cite journal |doi=10.1086/423613 |vauthors=Feigelson ED, Lawson WA |title=An X-ray census of young stars in the Chamaeleon I North Cloud |journal=Astrophys. J. |date=October 2004 |volume=614 |issue=10 |pages=267–83 |bibcode=2004ApJ...614..267F|arxiv = astro-ph/0406529 |s2cid=14535693 }} The total stellar population is 200–300. The Cha I cloud is further divided into the North cloud or region and South cloud or main cloud.{{clear}}

The Chamaeleon II dark cloud contains some 40 X-ray sources.{{cite journal |author1=Alcalá JM |author2=Covino E |author3=Sterzik MF |author4=Schmitt JHMM |author5=Krautter J |author6=Neuhäuser R |title=A ROSAT pointed observation of the Chamaeleon II dark cloud |journal=Astron. Astrophys. |date=March 2000 |volume=355 |issue=3 |pages=629–38 |bibcode=2000A&A...355..629A }} Observation in Chamaeleon II was carried out from 10 to 17 September 1993. Source RXJ 1301.9-7706, a new WTTS candidate of spectral type K1, is closest to 4U 1302–77.

"Chamaeleon III appears to be devoid of current star-formation activity."{{cite journal |vauthors=Yamauchi S, Hamaguchi K, Koyama K, Murakami H |title=ASCA Observations of the Chamaeleon II Dark Cloud |journal=Publ. Astron. Soc. Jpn. |date=October 1998 |volume=50 |issue=10 |pages=465–74 |bibcode=1998PASJ...50..465Y |doi = 10.1093/pasj/50.5.465 |doi-access=free }} HD 104237 (spectral type A4e) observed by ASCA, located in the Chamaeleon III dark cloud, is the brightest Herbig Ae/Be star in the sky.{{cite journal |vauthors=Hamaguchi K, Yamauchi S, Koyama K |title=X-ray Study of the Intermediate-Mass Young Stars Herbig Ae/Be Stars |journal=Astrophys J |year=2005 | arxiv=astro-ph/0406489v1|bibcode = 2005ApJ...618..360H |doi=10.1086/423192 | volume=618 |issue=1 | page=260|s2cid=119356104 }}

{{Main|Abell 2142}}

File:Abell2142 chandra xray.jpg.]]

The galaxy cluster Abell 2142 emits X-rays and is in Corona Borealis. It is one of the most massive objects in the universe.{{clear}}

{{Main|Antennae Galaxies}}

From the Chandra X-ray analysis of the Antennae Galaxies rich deposits of neon, magnesium, and silicon were discovered. These elements are among those that form the building blocks for habitable planets. The clouds imaged contain magnesium and silicon at 16 and 24 times respectively, the abundance in the Sun.{{clear}}

File:PKS 1127-145 X-rays.jpg

The jet exhibited in X-rays coming from PKS 1127-145 is likely due to the collision of a beam of high-energy electrons with microwave photons.{{clear}}

{{See also|3C 390.3}}

The Draco nebula (a soft X-ray shadow) is outlined by contours and is blue-black in the image by ROSAT of a portion of the constellation Draco.

Abell 2256 is a galaxy cluster of more than 500 galaxies. The double structure of this ROSAT image shows the merging of two clusters.{{clear}}

{{Main|X-rays from Eridanus}}

File:Orion-Eridanus Bubble.gif and Orion. Soft X-rays are emitted by hot gas (T ~ 2–3 MK) in the interior of the superbubble. This bright object forms the background for the "shadow" of a filament of gas and dust. The filament is shown by the overlaid contours, which represent 100 micrometre emission from dust at a temperature of about 30 K as measured by IRAS. Here the filament absorbs soft X-rays between 100 and 300 eV, indicating that the hot gas is located behind the filament. This filament may be part of a shell of neutral gas that surrounds the hot bubble. Its interior is energized by UV light and stellar winds from hot stars in the Orion OB1 association. These stars energize a superbubble about 1200 lys across which is observed in the optical (Hα) and X-ray portions of the spectrum.]]

Within the constellations Orion and Eridanus and stretching across them is a soft X-ray "hot spot" known as the Orion-Eridanus Superbubble, the Eridanus Soft X-ray Enhancement, or simply the Eridanus Bubble, a 25° area of interlocking arcs of Hα emitting filaments.{{clear}}

File:0087-HydraA-GalaxyCluster-ChandraXRay-19991030.jpg 09^h 18^m 06^s Dec −12° 05' 45" in Hydra. Observation date: 30 October 1999. Instrument: ACIS.]]

A large cloud of hot gas extends throughout the Hydra A galaxy cluster.{{clear}}

File:Arp270 xray 420.jpg. In the image, red represents low, green intermediate, and blue high-energy (temperature) X-rays. Image is 4 arcmin on a side. RA 10h 49 m 52.5s Dec +32° 59' 6". Observation date: 28 April 2001. Instrument: ACIS.]]

Arp260 is an X-ray source in Leo Minor at RA {{RA|10|49|52.5}} Dec {{DEC|+32|59|6}}.{{clear}}

File:Rass orion layout.jpg. On the left is Orion as seen in X-rays only. Betelgeuse is easily seen above the three stars of Orion's belt on the right. The X-ray colors represent the temperature of the X-ray emission from each star: hot stars are blue-white and cooler stars are yellow-red. The brightest object in the optical image is the full moon, which is also in the X-ray image. The X-ray image was actually obtained by the ROSAT satellite during the All-Sky Survey phase in 1990–1991.]]

In the adjacent images are the constellation Orion. On the right side of the images is the visual image of the constellation. On the left is Orion as seen in X-rays only. Betelgeuse is easily seen above the three stars of Orion's belt on the right. The brightest object in the visual image is the full moon, which is also in the X-ray image. The X-ray colors represent the temperature of the X-ray emission from each star: hot stars are blue-white and cooler stars are yellow-red.{{clear}}

File:Stephan's Quintet X-ray + Optical.jpg, a compact group of galaxies discovered about 130 years ago and located about 280 million light years from Earth, provides a rare opportunity to observe a galaxy group in the process of evolving from an X-ray faint system dominated by spiral galaxies to a more developed system dominated by elliptical galaxies and bright X-ray emission. Being able to witness the dramatic effect of collisions in causing this evolution is important for increasing our understanding of the origins of the hot, X-ray bright halos of gas in groups of galaxies.]]

Stephan's Quintet are of interest because of their violent collisions. Four of the five galaxies in Stephan's Quintet form a physical association, and are involved in a cosmic dance that most likely will end with the galaxies merging. As NGC 7318B collides with gas in the group, a huge shock wave bigger than the Milky Way spreads throughout the medium between the galaxies, heating some of the gas to temperatures of millions of degrees where they emit X-rays detectable with the NASA Chandra X-ray Observatory. NGC 7319 has a type 2 Seyfert nucleus.{{clear}}

{{Main|Perseus Cluster}}

File:Central regions Perseus galaxy cluster.jpg 03^h 19^m 47.60^s Dec +41° 30' 37.00" in Perseus. Observation dates: 13 pointings between 8 August 2002 and 20 October 2004. Color code: Energy (Red 0.3–1.2 keV, Green 1.2-2 keV, Blue 2–7 keV). Instrument: ACIS.]]

The Perseus galaxy cluster is one of the most massive objects in the universe, containing thousands of galaxies immersed in a vast cloud of multimillion degree gas.{{clear}}

File:Pictor A- Spectacular X-ray Jet Points Toward Cosmic Energy Booster (2000-pictor - pictor xray).jpg

Pictor A is a galaxy that may have a black hole at its center which has emitted magnetized gas at extremely high speed. The bright spot at the right in the image is the head of the jet. As it plows into the tenuous gas of intergalactic space, it emits X-rays. Pictor A is X-ray source designated H 0517-456 and 3U 0510-44.{{cite journal |doi=10.1086/190992 |vauthors=Wood KS, Meekins JF, Yentis DJ, Smathers HW, McNutt DP, Bleach RD |title=The HEAO A-1 X-ray source catalog |journal=Astrophys. J. Suppl. Ser. |date=December 1984 |volume=56 |issue=12 |pages=507–649 |bibcode=1984ApJS...56..507W |doi-access= }}{{clear}}

{{Main|Puppis A}}

File:Puppis A Chandra + ROSAT.jpg 08^h 23^m 08.16^s Dec −42° 41' 41.40" in Puppis. Observation date: 4 September 2005. Color code: Energy (Red 0.4–0.7 keV; Green 0.7–1.2 keV; Blue 1.2–10 keV). Instrument: ACIS.]]

Puppis A is a supernova remnant (SNR) about 10 light-years in diameter. The supernova occurred approximately 3700 years ago.{{clear}}

File:SgrAWest BEAR.jpg (or Sgr A) is a complex at the center of the Milky Way. It consists of three overlapping components, the SNR Sagittarius A East, the spiral structure Sagittarius A West, and a very bright compact radio source at the center of the spiral, Sagittarius A*.]]

The Galactic Center is at 1745–2900 which corresponds to Sagittarius A*, very near to radio source Sagittarius A (W24). In probably the first catalogue of galactic X-ray sources,{{cite journal |author=Ouellette GA |title=Development of a catalogue of galactic x-ray sources |journal=Astron J|year=1967 |volume=72|page=597|doi=10.1086/110278 |bibcode=1967AJ.....72..597O}} two Sgr X-1s are suggested: (1) at 1744–2312 and (2) at 1755–2912, noting that (2) is an uncertain identification. Source (1) seems to correspond to S11.{{cite journal |vauthors=Gursky H, Gorenstein P, Giacconi R |title=The Distribution of Galactic X-Ray Sources from Scorpio to Cygnus |journal=Astrophys J|year=1967 |volume=150 |page=L75|doi=10.1086/180097 |bibcode=1967ApJ...150L..75G|doi-access=free }}{{clear}}

{{Main|Cartwheel Galaxy}}

File:Cartwheel Galaxy- Astronomers Do Flips Over Cartwheel Galaxy (2940633427).jpg (purple); the Galaxy Evolution Explorer satellite (ultraviolet/blue); the Hubble Space Telescope (visible/green); the Spitzer Space Telescope (infrared/red). Image is 160 arcsec across. RA 0^h 37^m 41.10^s Dec −33° 42' 58.80" in Sculptor. Color code: Ultraviolet (blue), Optical (green), X-ray (purple), Infrared (red).]]

The unusual shape of the Cartwheel Galaxy may be due to a collision with a smaller galaxy such as those in the lower left of the image. The most recent star burst (star formation due to compression waves) has lit up the Cartwheel rim, which has a diameter larger than the Milky Way. There is an exceptionally large number of black holes in the rim of the galaxy as can be seen in the inset.{{clear}}

{{See also|Serpens}}

File:187900main colorpress1 lg.jpg spectrum from superheated iron atoms at the inner edge of the accretion disk orbiting the neutron star in Serpens X-1. The line is usually a symmetrical peak, but it exhibits the classic features of distortion due to relativistic effects. The extremely fast motion of the iron-rich gas causes the line to spread out. The entire line has been shifted to longer wavelengths (left, red) because of the neutron star's powerful gravity. The line is brighter toward shorter wavelengths (right, blue) because Einstein's special theory of relativity predicts that a high-speed source beamed toward Earth will appear brighter than the same source moving away from Earth.]]

As of 27 August 2007, discoveries concerning asymmetric iron line broadening and their implications for relativity have been a topic of much excitement. With respect to the asymmetric iron line broadening, Edward Cackett of the University of Michigan commented, "We're seeing the gas whipping around just outside the neutron star's surface,". "And since the inner part of the disk obviously can't orbit any closer than the neutron star's surface, these measurements give us a maximum size of the neutron star's diameter. The neutron stars can be no larger than 18 to 20.5 miles across, results that agree with other types of measurements."{{cite web |vauthors=Gibb M, Bhattacharyya S, Strohmayer T, Cackett E, Miller J |title=Astronomers Pioneer New Method for Probing Exotic Matter |url=http://imagine.gsfc.nasa.gov/docs/features/news/27aug07.html}}

"We've seen these asymmetric lines from many black holes, but this is the first confirmation that neutron stars can produce them as well. It shows that the way neutron stars accrete matter is not very different from that of black holes, and it gives us a new tool to probe Einstein's theory", says Tod Strohmayer of NASA's Goddard Space Flight Center.

"This is fundamental physics", says Sudip Bhattacharyya also of NASA's Goddard Space Flight Center in Greenbelt, Maryland, and the University of Maryland. "There could be exotic kinds of particles or states of matter, such as quark matter, in the centers of neutron stars, but it's impossible to create them in the lab. The only way to find out is to understand neutron stars."

Using XMM-Newton, Bhattacharyya and Strohmayer observed Serpens X-1, which contains a neutron star and a stellar companion. Cackett and Jon Miller of the University of Michigan, along with Bhattacharyya and Strohmayer, used Suzaku's superb spectral capabilities to survey Serpens X-1. The Suzaku data confirmed the XMM-Newton result regarding the iron line in Serpens X-1.{{clear}}

M82 X-1 is in the constellation Ursa Major at {{RA|09|55|50.01}} +{{DEC|69|40|46.0}}. It was detected in January 2006 by the Rossi X-ray Timing Explorer.

In Ursa Major at RA 10^h 34^m 00.00 Dec +57° 40' 00.00" is a field of view that is almost free of absorption by neutral hydrogen gas within the Milky Way. It is known as the Lockman Hole. Hundreds of X-ray sources from other galaxies, some of them supermassive black holes, can be seen through this window.{{clear}}

File:PIA24574-SS433-ULXray-20210709.jpg - possible ULX ray source