Do black holes come in size medium?

Nov. 29, 2013 — Black holes can be petite, with masses only about 10 times that of our sun -- or monstrous, boasting the equivalent in mass up to 10 billion suns. Do black holes also come in size medium? NASA's Nuclear Spectroscopic Telescope Array, or NuSTAR, is busy scrutinizing a class of black holes that may fall into the proposed medium-sized category.

"Exactly how intermediate-sized black holes would form remains an open issue," said Dominic Walton of the California Institute of Technology, Pasadena. "Some theories suggest they could form in rich, dense clusters of stars through repeated mergers, but there are a lot of questions left to be answered."

The largest black holes, referred to as supermassive, dominate the hearts of galaxies. The immense gravity of these black holes drags material toward them, forcing the material to heat up and release powerful X-rays. Small black holes dot the rest of the galactic landscape. They form under the crush of collapsing, dying stars bigger than our sun.

Evidence for medium-sized black holes lying somewhere between these two extremes might come from objects called ultraluminous X-ray sources, or ULXs. These are pairs of objects in which a black hole ravenously feeds off a normal star. The feeding process is somewhat similar to what happens around supermassive black holes, but isn't as big and messy. In addition, ULXs are located throughout galaxies, not at the cores.

The bright glow of X-rays coming from ULXs is too great to be the product of typical small black holes. This and other evidence indicates the objects may be intermediate in mass, with 100 to 10,000 times the mass of our sun. Alternatively, an explanation may lie in some kind of exotic phenomenon involving extreme accretion, or "feeding," of a black hole.

NuSTAR is joining with other telescopes to take a closer look at ULXs. It's providing the first look at these objects in focused, high-energy X-rays, helping to get better estimates of their masses and other characteristics.

In a new paper from Walton and colleagues accepted for publication in the Astrophysical Journal, the astronomers report serendipitously finding a ULX that had gone largely unnoticed before. They studied the object, which lies in the Circinus spiral galaxy 13 million light-years away, not only with NuSTAR but also with the European Space Agency's XMM-Newton satellite. Archival data from NASA's Chandra, Swift and Spitzer space telescopes as well as Japan's Suzaku satellite, were also used for further studies. "We went to town on this object, looking at a range of epochs and wavelengths," said Walton.

The results indicate the black hole in question is about 100 times the mass of the sun, putting it right at the border between small and medium black holes.

In another accepted Astrophysical Journal paper, Matteo Bachetti of the Institut de Recherche en Astrophysique et Planetologie and colleagues looked at two ULXs in NGC 1313, a spiral galaxy known as the "Topsy Turvy galaxy," also about 13 million light-years way.

These are among the best-studied ULXs known. A single viewing with NuSTAR showed that the black holes didn't fit with models of medium-size black holes. As a result, the researchers now think both ULXs harbor small, stellar-mass black holes. One of the objects is estimated to be big for its size category, at 70 to 100 solar masses.

"It's possible that these objects are ultraluminous because they are accreting material at a high rate and not because of their size," said Bachetti. "If intermediate-mass black holes are out there, they are doing a good job of hiding from us."

NuSTAR is a Small Explorer mission led by Caltech and managed by NASA's Jet Propulsion Laboratory, Pasadena, Calif., for NASA's Science Mission Directorate in Washington. The spacecraft was built by Orbital Sciences Corporation, Dulles, Va. Its instrument was built by a consortium including Caltech; JPL; the University of California, Berkeley; Columbia University, New York; NASA's Goddard Space Flight Center, Greenbelt, Md.; the Danish Technical University in Denmark; Lawrence Livermore National Laboratory, Livermore, Calif.; ATK Aerospace Systems, Goleta, Calif., and with support from the Italian Space Agency (ASI) Science Data Center.

NuSTAR's mission operations center is at UC Berkeley, with the ASI providing its equatorial ground station located at Malindi, Kenya. The mission's outreach program is based at Sonoma State University, Rohnert Park, Calif. NASA's Explorer Program is managed by Goddard. JPL is managed by Caltech for NASA.

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Exercise Controls Weight in White Girls Better Than in Black Girls: Study

AppId is over the quota
AppId is over the quota

WEDNESDAY, June 6 (HealthDay News) -- Exercise appears less likely to prevent obesity among black teenage girls than their white peers, a new study shows.

British researchers who gauged the effect of exercise on more than 1,100 girls, aged 12 to 14, surmised that black teen girls may be less sensitive than white teen girls to the effects of physical activity to prevent obesity.

"Higher levels of physical activity were associated with lower risk of obesity among white girls but not among black girls," wrote study authors James White and Russell Jago.

This is of concern because obesity rates are increasing at a greater rate among black teen girls than other U.S. youths, putting them at greater risk for heart disease, according to background information in the study. Black American girls were 80 percent more likely than white girls to be overweight in 2007-2010, the U.S. Department of Health and Human Services' Office of Minority Health reported. And about four out of five black American women are overweight or obese.

"At present, we don't know whether these differences can be attributed to genetics," said White, a researcher at Cardiff University in Wales. While other studies have found black girls consume more calories than white girls, he said this study took those differences into account, suggesting there may be other reasons. "These may be genetic, but we don't really know," he said.

However, at least one expert believes the study results may reflect lifestyle differences, which can be addressed, not just genetics.

White and Jago, of the University of Bristol in England, evaluated data on girls who participated in the long-running National Heart, Lung, and Blood Institute Growth and Health Study.

The researchers were looking at factors associated with obesity and the development of heart disease risk factors.

For this study, published in the June issue of the Archives of Pediatric and Adolescent Medicine, the research team focused on 538 black girls and 610 white girls for whom they had data on physical activity, obesity and other measures.

The girls' overall level of physical activity was measured for three days with a device that could detect frequency of movement and speed. Their TV viewing hours, height, weight, body-mass index (BMI), percent of body fat and daily calorie intake were also recorded. BMI, a calculation based on height and weight, is a reference used to determine obesity.

At age 12, the activity levels were higher in white girls than in black girls. The black girls had a higher BMI and body fat percentage, and ate more calories daily. They also watched more TV: The median -- meaning half watched more, half watched less -- was 44.3 hours a week for black girls vs. 24.5 for white girls.

Median daily caloric intake was slightly higher for black girls -- 1,912 vs. 1,906 for white girls.

At 12 years of age, 14 percent of the black girls were obese, compared to 4.3 percent of the white girls. At 14 years, 15.6 percent of the black girls and 5.1 percent of the white girls were obese.

That no link was found between physical activity levels and obesity among black girls is consistent with other research showing that fat-burning rates in response to physical activity are lower in black girls during puberty and adulthood than in whites.

Lifestyle issues, rather than genetics, may partly explain the results, said Pete McCall, an exercise physiologist with the American Council on Exercise.

Noting that black girls watch much more TV than white girls, he said, "That would be one area to change." They could shut off the TV and get exercise, "but the neighborhood may not be conducive," he said.

Parent education and income might affect the results, too, McCall speculated. More than half the parents of white girls had a college degree, but fewer than one-quarter of the black girls' parents did. The incomes of white girls' parents were higher, too.

"College education and income levels [of the parents] can play a role in the amount of physical activity a person gets," he added.

The authors noted that the black girls began menstruating earlier than the whites, which could also influence weight gain.

Encouraging black teen girls to exercise is still important, but different approaches may be needed to prevent obesity in black girls, the authors said.

"Our findings require replication before any real recommendations can be made," White said. "They do, however, suggest that black girls should be particularly attentive in controlling their caloric intake."

More information

To learn more about obesity and black Americans, visit the U.S. Office of Minority Health.

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