X-ray emission occurs from manycelestial objects. These emissions can have apattern, occur intermittently, or as atransient astronomical event. InX-ray astronomy many sources have been discovered by placing anX-ray detector above theEarth's atmosphere. Often, the first X-ray source discovered in manyconstellations is anX-ray transient. These objects show changing levels of X-ray emission.NRL astronomer Dr. Joseph Lazio stated:^[1] " ... the sky is known to be full of transient objects emitting at X- and gamma-ray wavelengths, ...". There are a growing number of recurrent X-ray transients. In the sense of traveling as a transient, the only stellar X-ray source that does not belong to a constellation is theSun. As seen from Earth, the Sun moves from west to east along theecliptic, passing over the course of one year through the twelve constellations of theZodiac, andOphiuchus.

SCP 06F6 is (or was) anastronomical object of unknown type, discovered on February 21, 2006, in the constellationBoötes^[2] during a survey ofgalaxy cluster CL 1432.5+3332.8 with theHubble Space Telescope'sAdvanced Camera for Surveys Wide Field Channel.^[3]

The European X-ray satelliteXMM Newton made an observation in early August 2006 which appears to show an X-ray glow aroundSCP 06F6,^[4] two orders of magnitude more luminous than that of supernovae.^[5]

Most astronomical X-ray transient sources have simple and consistent time structures; typically a rapid brightening followed by gradual fading, as in anova orsupernova.

GRO J0422+32^[6] is an X-ray nova andblack hole candidate that was discovered by theBATSE instrument on theCompton Gamma Ray Observatory satellite on Aug 5 1992.^[7][8] During the outburst, it was observed to be stronger than theCrab Nebula gamma-ray source out to photon energies of about 500keV.^[9]

XTE J1650-500 is a transient binary X-ray source located in theconstellationAra. The binary period is 0.32 d.^[10]

"Soft X-ray transients" are composed of some type of compact object (probably a neutron star) and some type of "normal", low-mass star (i.e. a star with a mass of some fraction of the Sun's mass). These objects show changing levels of low-energy, or "soft", X-ray emission, probably produced somehow by variable transfer of mass from the normal star to the compact object. In effect the compact object "gobbles up" the normal star, and the X-ray emission can provide the best view of how this process occurs.^[11]

Soft X-ray transients Cen X-4 and Apl X-1 were discovered byHakucho,Japan's firstX-ray astronomysatellite.

X-ray bursters are one class ofX-ray binary stars exhibiting periodic and rapid increases inluminosity (typically a factor of 10 or greater) peaked in theX-ray regime of theelectromagnetic spectrum. These astrophysical systems are composed of anaccretingcompact object, typically aneutron star or occasionally ablack hole, and a companion 'donor' star; the mass of the donor star is used to categorize the system as either a high mass (above 10solar masses) or low mass (less than 1 solar mass) X-ray binary, abbreviated as LMXB and HMXB, respectively. X-ray bursters differ observationally from other X-ray transient sources (such asX-ray pulsars andsoft X-ray transients), showing a sharp rise time (1 – 10 seconds) followed by spectral softening (a property of coolingblack bodies). Individual bursts are characterized by an integrated flux of 10^39-40 ergs.^[12]

Agamma-ray burst (GRB) is a highlyluminous flash ofgamma rays — the most energetic form ofelectromagnetic radiation.GRB 970228 was a GRB detected on Feb 28 1997 at 02:58UTC. Prior to this event, GRBs had only been observed at gamma wavelengths. For several years physicists had expected these bursts to be followed by a longer-livedafterglow at longer wavelengths, such asradio waves,x-rays, and evenvisible light. This was the first burst for which such an afterglow was observed.^[13]

A transient x-ray source was detected which faded with apower law slope in the days following the burst. This x-rayafterglow was the first GRB afterglow ever detected.^[14]

For some types ofX-ray pulsars, the companion star is aBe star that rotates very rapidly and apparently sheds a disk of gas around its equator. The orbits of theneutron star with these companions are usually large and very elliptical in shape. When the neutron star passes nearby or through the Be circumstellar disk, it will capture material and temporarily become an X-ray pulsar. The circumstellar disk around the Be star expands and contracts for unknown reasons, so these are transient X-ray pulsars that are observed only intermittently, often with months to years between episodes of observable X-ray pulsation.

SAX J1808.4-3658 is a transient, accreting millisecondX-ray pulsar that is intermittent. In addition,X-ray burst oscillations and quasi-periodic oscillations in addition to coherent X-ray pulsations have been seen from SAX J1808.4-3658, making it a Rosetta stone for interpretation of the timing behavior oflow-mass X-ray binaries.

There are a growing number of recurrent X-ray transients, characterized by short outbursts with very fast rise times (~ tens of minutes) and typical durations of a few hours that are associated with OBsupergiants and hence define a new class of massive X-ray binaries: Supergiant Fast X-ray Transients (SFXTs).^[15] XTE J1739–302 is one of these. Discovered in 1997, remaining active only one day, with an X-ray spectrum well fitted with a thermalbremsstrahlung (temperature of ~20 keV), resembling the spectral properties of accreting pulsars, it was at first classified as a peculiar Be/X-ray transient with an unusually short outburst.^[16] A new burst was observed on Apr 8 2008 withSwift.^[16]

The quietSun, although less active than active regions, is awash withdynamic processes andtransient events (bright points,nanoflares and jets).^[17]

Acoronal mass ejection (CME) is an ejected plasma consisting primarily ofelectrons andprotons (in addition to small quantities of heavier elements such as helium, oxygen, and iron), plus the entraining coronal closedmagnetic field regions. Small-scale energetic signatures such as plasma heating (observed as compact soft X-ray brightening) may be indicative of impending CMEs. The soft X-ray sigmoid (an S-shaped intensity of soft X-rays) is an observational manifestation of the connection between coronal structure and CME production.^[18]

The first detection of aCoronal mass ejection (CME) as such was made on Dec 1 1971 by R. Tousey of theUS Naval Research Laboratory using the 7th Orbiting Solar Observatory (OSO 7).^[19] Earlier observations ofcoronal transients or even phenomena observed visually duringsolar eclipses are now understood as essentially the same thing.

The largest geomagnetic perturbation, resulting presumably from a "prehistoric" CME, coincided with the first-observedsolar flare, in 1859. The flare was observed visually byRichard Christopher Carrington and thegeomagnetic storm was observed with the recording magnetograph atKew Gardens. The same instrument recorded acrotchet, an instantaneous perturbation of the Earth's ionosphere by ionizing softX-rays. This could not easily be understood at the time because it predated the discovery of X-rays (byRoentgen) and the recognition of theionosphere (byKennelly andHeaviside).

Unlike Earth's aurorae, which are transient and only occur at times of heightened solar activity,Jupiter's aurorae are permanent, though their intensity varies from day to day. They consist of three main components: the main ovals, which are bright, narrow (< 1000 km in width) circular features located at approximately 16° from the magnetic poles;^[20] the satellite auroral spots, which correspond to the footprints of the magnetic field lines connecting their ionospheres with the ionosphere of Jupiter, and transient polar emissions situated within the main ovals.^[20][21] The auroral emissions were detected in almost all parts of the electromagnetic spectrum from radio waves to X-rays (up to 3 keV).

The X-ray monitor ofSolwind, designated NRL-608 or XMON, was a collaboration between theNaval Research Laboratory andLos Alamos National Laboratory. The monitor consisted of 2 collimated argon proportional counters. The instrument bandwidth of 3-10 keV was defined by the detector window absorption (the window was 0.254 mm beryllium) and the upper level discriminator. The active gas volume (P-10 mixture) was 2.54 cm deep, providing good efficiency up to 10 keV. Counts were recorded in 2 energy channels. Slat collimators defined a FOV of 3° x 30° (FWHM) for each detector; the long axes of the FOVs were perpendicular to each other. The long axes were inclined 45 degrees to the scan direction, allowing localization of transient events to about 1 degree.

ThePHEBUS experiment recorded high energy transient events in the range 100 keV to 100 MeV. It consisted of two independent detectors and their associatedelectronics. Each detector consisted of a bismuth germinate (BGO) crystal 78 mm indiameter by 120 mm thick, surrounded by a plastic anti-coincidence jacket. The two detectors were arranged on the spacecraft so as to observe 4πsteradians. The burst mode was triggered when the count rate in the 0.1 to 1.5 MeV energy range exceeded the background level by 8 σ (standard deviations) in either 0.25 or 1.0 seconds. There were 116 channels over the energy range.^[22]

Also on board theGranat International Astrophysical Observatory were fourWATCH instruments that could localize bright sources in the 6 to 180 keV range to within 0.5° using a Rotation Modulation Collimator. Taken together, the instruments' three fields of view covered approximately 75% of the sky. The energy resolution was 30%FWHM at 60 keV. During quiet periods, count rates in two energy bands (6 to 15 and 15 to 180 keV) were accumulated for 4, 8, or 16 seconds, depending on onboard computer memory availability. During a burst or transient event, count rates were accumulated with atime resolution of 1 s per 36 s.^[22]

TheCompton Gamma Ray Observatory (CGRO) carries the Burst and Transient Source Experiment (BATSE) which detects in the 20 keV to 8 MeV range.

WIND was launched on Nov 1 1994. At first, the satellite had a lunar swingby orbit around the Earth. With the assistance of the Moon's gravitational field Wind's apogee was kept over the day hemisphere of the Earth and magnetospheric observations were made. Later in the mission, the Wind spacecraft was inserted into a special "halo" orbit in the solar wind upstream from the Earth, about the sunward Sun-Earth equilibrium point (L1). The satellite has a spin period of ~ 20 seconds, with the spin axis normal to the ecliptic. WIND carries the Transient Gamma-Ray Spectrometer (TGRS) which covers the energy range 15 keV - 10 MeV, with an energy resolution of 2.0 keV @ 1.0 MeV (E/delta E = 500).

The third USSmall Astronomy Satellite (SAS-3) was launched on May 7, 1975, with 3 major scientific objectives: 1) determine bright X-ray source locations to an accuracy of 15 arcseconds; 2) study selected sources over the energy range 0.1-55 keV; and 3) continuously search the sky for X-ray novae, flares, and other transient phenomena. It was a spinning satellite with pointing capability. SAS 3 was the first to discover X-rays from a highly magnetic WD binary system, AM Her, discovered X-rays from Algol and HZ 43, and surveyed the soft X-ray background (0.1-0.28 kev).

Tenma was the second Japanese X-ray astronomy satellite launched on Feb 20 1983. Tenma carriedGSFC detectors which had an improved energy resolution (by a factor of 2) compared to proportional counters and performed the first sensitive measurements of the iron spectral region for many astronomical objects. Energy Range: 0.1 keV - 60 keV. Gas Scintillator Proportional Counter: 10 units of 80 cm² each, FOV ~ 3deg (FWHM), 2 - 60 keV. Transient Source Monitor: 2 - 10 keV.

India's first dedicatedastronomysatellite, scheduled for launch on board thePSLV in mid 2010,^[23]Astrosat will monitor the X-ray sky for new transients, among other scientific focuses.

• X-ray astronomy
• X-ray astrophysical sources

• HETE-2: High Energy Transient Explorer
• BATSE: Burst and Transient Source Explorer