December 2, 1998

X-ray image of the soft gamma-ray repeater (SGR) 1900+14 taken last September by Beppo-SAX, an astronomy satellite of the Italian Space Agency.

Cross-section of a neutron star. Beneath a rigid, solid crust of mostly heavy nuclei lies a liquid interior that consists predominantly of neutrons. Neutron stars are extremely compact: A mass roughly 1.5 times that of the Sun is squeezed into a volume just 12 miles across.

Intensity plot versus time of the radiation from SGR 1900+14 during an outburst of August 27, 1998 (see text). Note the extremely rapid initial rise of the burst, followed by gradual fading over the course of roughly five minutes and the presence of pulsation spikes. The pulsations are 5.16 seconds in duration and reveal the star's rotation period.

Image Credits: Chryssa Kouveliotou, Universities Space Research Association (USRA), and Peter Woods, University of Alabama, Huntsville (image 1); NASA Marshall Space Flight Center (image 2); the Ulysses Mission, NASA, and the European Space Agency (ESA) (image 3).

This past August 27, an unusually intense wave of gamma rays and X-rays, the most energetic forms of electromagnetic radiation, arrived at the solar system from a mysterious star some 20,000 light-years away in the constellation Aquila (the Eagle).

The high-energy radiation ionized atoms on the night side of the Earth's upper atmosphere (i.e., the gamma rays and X-rays knocked electrons off atmospheric atoms), which typically happens only on the day side and is caused by ultraviolet radiation from the Sun. Furthermore, the gamma rays and X-rays saturated detectors onboard a number of orbiting science and weather satellites.

The cause of the burst of high-energy radiation was a super-strong flare on the surface of a rotating neutron star known as SGR 1900+14. The star, which is extremely compact (image 2), is believed to have the strongest magnetic field ever measured in the universe -- roughly 10¹⁵ times that of the Earth's (10¹⁵ is shorthand for a one followed by 15 zeros).

Due to the fact that the magnetic field lines of SGR 1900+14 are not rigidly anchored to the neutron star (like the magnetic field lines in a bar magnet), but are capable of shifting and moving, they get twisted and entangled until eventually they break and reconnect. Because the field is so strong, this breaking and reconnecting releases enormous amounts of pent-up energy in a brief moment, and gives rise to a powerful stellar flare. The flare is accompanied by a burst of gamma rays and X-rays such as those observed on August 27, followed by gradual fading (image 3). The flare also heats the star's surface, shatters the neutron star's strong, rigid outer crust (causing "star quakes"), and probably ejects into space electrically charged particles at near the speed of light. (Flares also happen on the Sun, but on a considerably weaker scale than on SGR 1900+14 since the solar magnetic fields are roughly a trillion times weaker.)

Four other stars similar to SGR 1900+14, with extreme magnetic fields, are known. During these stars' dormant, non-flaring phases, they pulsate faintly in X-rays with periods of several seconds. The pulsations correspond to the stars' rotation periods. In the case of SGR 1900+14, the period is 5.16 seconds.

Every few months to years, and quite unpredictably, the faint X-ray glow is interrupted by a major outburst, followed again by dormancy. Because of the repeating nature of the outbursts, astronomers have named such stars "soft gamma ray repeaters" or "SGRs" ("soft" means that the gamma rays are close to the low-energy end of the gamma ray spectral range). Another name for these objects is "magnetar," in reference to these stars' extraordinarily strong magnetic fields.

More Cool Stuff

We obtained the information for this article from NASA press release 98-193 of September 29, 1998; a NASA Space Science News article, "Crusty Young Star Makes Its Presence Felt;" and a Web article on magnetars, SGRs, and very strong magnetic fields by Robert C. Duncan of the University of Texas at Austin:

http://www1.msfc.nasa.gov/NEWSROOM/news/releases/1998/98-193.html
http://science.msfc.nasa.gov/newhome/headlines/ast29sep98_1.htm
http://solomon.as.utexas.edu/~duncan/magnetar.html

Soft gamma ray repeaters (SGRs) are not to be confused with gamma ray bursts (GRBs), which occur far beyond the Milky Way galaxy, do not repeat, are even more energetic than SGRs, and have different causes. We have featured gamma ray bursts in earlier Observations of the Week:

http://observe.ivv.nasa.gov/nasa/ootw/1997/ootw_970115/ob970115.html
http://observe.ivv.nasa.gov/nasa/ootw/1997/ootw_971119/ob971119.html
http://observe.ivv.nasa.gov/nasa/ootw/1998/ootw_981007/ob981007.html

As part of its Learning Technologies Project (LTP), NASA supports a number of educational Web sites that have excellent material on the space sciences:

http://observe.ivv.nasa.gov/nasa/education/edu/edudocs/topic_space.html