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Chandra Illuminates Difference between Distant Double Quasars

quasars
Image: COURTESY OF NASA/SAO/CXC/P. GREEN ET AL.

Since its discovery more than two decades ago, the double quasar system Q2345+007A,B has confounded astronomers. Often such apparent quasar pairs are in fact optical illusions caused by so-called gravitational bending. Massive objects located between a single quasar and earth can bend and focus the quasar's light such that the image splits into two. But in the case of Q2345+007A,B, scientists have so far been unable to identify enough intervening material capable of acting as a gravitational lens. Now new findings from the Chandra X-ray Observatory suggest that the twins are in fact distinct objects formed by merging galaxies.

Because previous studies did not find any stars or galaxies capable of gravitational bending between Q2345+007A,B and earth, scientists suggested that a new type of cluster--one containing only hot gas and dark matter--could bend the quasar's light. Optical and ultraviolet telescopes would not be able to locate such a dark cluster, so Paul Green of the Harvard-Smithsonian Center for Astrophysics and colleagues trained the Chandra X-ray Observatory on Q2345+007A,B. The team detected no evidence of a light-bending dark cluster. Furthermore, they discovered that the x-ray spectra of the two quasars were significantly different (unlike spectra taken at optical and ultraviolet wavelengths). "This may mean that the pair Q2345+007A,B actually consists of two separate quasars," Green says.

The new findings do not completely resolve the coupled quasar mystery, however. Though it's likely that the quasars seem comparable because they formed very close together, their incredibly similar optical spectra--"every bump and wiggle" as Green puts it--still seem like an unlikely coincidence. The differing x-ray spectra may be due to the fact that such observations can probe closer to the quasars' central black holes and illuminate more subtle individual differences. Green notes that "the quasars that make up these nearly identical twins appear to have been hatched in the same nest."

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