This illustration shows how an extremely rapidly rotating neutron star, which has formed from the collapse of a very massive star, can produce incredibly powerful magnetic fields. (Illustration: NASA/CXC/M.Weiss)
In 2013, astronomers announced they had discovered a magnetar exceptionally close to the supermassive black hole at the center of the Milky Way using a suite of space-borne telescopes including NASA's Chandra X-ray Observatory.
Magnetars are dense, collapsed stars (called "neutron stars") that possess enormously powerful magnetic fields. At a distance that could be as small as 0.3 light years
(or about 2 trillion miles) from the 4-million-solar mass black hole
in the center of our Milky Way galaxy, the magnetar is by far the
closest neutron star to a supermassive black hole ever discovered and is
likely in its gravitational grip.
Since its discovery two years ago when it gave off a burst of X-rays,
astronomers have been actively monitoring the magnetar, dubbed SGR
1745-2900, with Chandra and the European Space Agency's XMM-Newton. The
main image of the graphic shows the region around the Milky Way's black
hole in X-rays from Chandra (red, green, and blue are the low, medium, and high-energy X-rays respectively).
The inset contains Chandra's close-up look at the area right around the
black hole, showing a combined image obtained between 2005 and 2008
(left) when the magnetar was not detected, during a quiescent period,
and an observation in 2013 (right) when it was caught as a bright point
source during the X-ray outburst that led to its discovery.
A new study uses long-term monitoring observations to reveal that the
amount of X-rays from SGR 1745-2900 is dropping more slowly than other
previously observed magnetars, and its surface is hotter than expected.
The team first considered whether "starquakes" are able to explain
this unusual behavior. When neutron stars, including magnetars, form,
they can develop a tough crust on the outside of the condensed star.
Occasionally, this outer crust will crack, similar to how the Earth's
surface can fracture during an earthquake. Although starquakes can
explain the change in brightness and cooling seen in many magnetars, the
authors found that this mechanism by itself was unable to explain the
slow drop in X-ray brightness and the hot crustal temperature. Fading in
X-ray brightness and surface cooling occur too quickly in the starquake
model.
The researchers suggest that bombardment of the surface of the
magnetar by charged particles trapped in twisted bundles of magnetic
fields above the surface may provide the additional heating of the
magnetar's surface, and account for the slow decline in X-rays. These
twisted bundles of magnetic fields can be generated when the neutron
star forms.
The researchers do not think that the magnetar's unusual behavior is
caused by its proximity to a supermassive black hole, as the distance is
still too great for strong interactions via magnetic fields or gravity.
Astronomers will continue to study SGR 1745-2900 to glean more clues
about what is happening with this magnetar as it orbits our galaxy's
supermassive black hole.
These results appear in Monthly Notices of the Royal Astronomical
Society in a paper led by the PhD student Francesco Coti Zelati
(Universita' dell' Insubria, University of Amsterdam, INAF-OAB), within a
large international collaboration including N. Rea (University of the
Amsterdam, CSIC-IEEC), A. Papitto, D. Viganò (CSIC-IEEC), J. A. Pons
(Universitat d'Alacant), R. Turolla (Universita' di Padova, MSSL), P.
Esposito (INAF, CfA), D. Haggard (Amherst college), F. K. Baganoff
(MIT), G. Ponti (MPE), G. L. Israel, S. Campana (INAF), D. F. Torres
(CSIC-IEEC, ICREA), A. Tiengo (IUSS, INAF), S. Mereghetti (INAF), R.
Perna (Stony Brook University), S. Zane (MSSL), R. P. Mignani (INAF,
University of Zielona Gora), A. Possenti, L. Stella (INAF).
NASA's Marshall Space Flight Center in Huntsville, Alabama, manages
the Chandra program for the agency's Science Mission Directorate in
Washington. The Smithsonian Astrophysical Observatory in Cambridge,
Massachusetts, controls Chandra's science and flight operations.
Fast Facts for SGR 1745-2900:
Scale: Main Image is 8 arcmin across (about 61 light years); Inset image is about 14 arcsec across (1.8 light years)
Category: Black Holes, Milky Way GalaxyCoordinates (J2000): RA 17h 45m 40s | Dec -29° 00' 28.00"
Constellation: Sagittarius
Observation Date: Main Image: 43 pointings from September 21, 1999 to May 18, 2009; Inset: 25 pointings between 29 Apr 2013 and 30 Aug 2014
Observation Time: Main Image: 278 hours (11 days 14 hours);
Obs. ID: Main Image: 242, 1561, 2943, 2951-2954, 3392, 3393, 3549, 3663, 3665, 4683, 4684, 5360, 5950-5954, 6113, 6363, 6639, 6640-6646, 7554-7759, 9169-9174, 10556; Inset: 14702-14704, 14943-14946, 15040-15045, 15651, 15654, 16508, 16210-16217, 16597
Instrument: ACISReferences: Coti Zelati, F. et al, 2015, MNRAS 449, 2685; arXiv:1503.01307
Color Code: Energy: Red (2-3.3 keV), Green (3.3-4.7 keV), Blue (4.7-8 keV)
Distance Estimate: About 26,000 light years
Source: NASA’s Chandra X-ray Observatory
