Researchers investigate two ultra-luminous X-ray sources in the galaxy NGC 925
DSS image of the galaxy NGC 925, where the red stars indicate the positions of ULX-1 and ULX-2. Insets: XMM-Newton/EPIC-pn (left-top), Chandra/ACIS-S (left-bottom) and DSS images (right). The images have different scales. Credit: Pintore et al., 2018.
European researchers have investigated two ultraluminous X-ray sources (ULXs) located in the barred spiral galaxy NGC 925. The study provides hints on the real nature of these two sources and could be helpful in improving our general understanding of ULXs. The research is detailed in a paper published June 29 on the arXiv pre-print server.
ULXs are point sources in the sky that are so bright in X-rays that each emits more radiation than a million suns emit at all wavelengths. They are less luminous than active galactic nuclei, but more consistently luminous than any known stellar process. Although numerous studies of ULXs have been conducted, the basic nature of these sources still remains unsolved.
Usually there is one ULX per galaxy in galaxies which host them, however some galaxies were found to contain many such sources. Located some 28 million light years away from the Earth, NGC 925 contains two ultraluminous X-ray sources designated NGC 925 ULX-1 and NGC 925 ULX-2. These two sources has been recently studied by a team of scientists led by Fabio Pintore of the Institute of Space Astrophysics and Cosmic Physics (ISAF) in Milan, Italy.
As part of the study the researchers analyzed observational data provided by ESA's XMM-Newton and NASA's Nuclear Spectroscopic Telescope Array (NuSTAR) spacecraft. The research also includes archival data obtained by NASA's Chandra X-ray space observatory.
"NGC 925 ULX-1 and ULX-2 are two ultraluminous X-ray sources in the galaxy NGC 925, at a distance of 8.5 Mpc. For the first time, we analyzed high quality, simultaneous XMM-Newton and NuSTAR data of both sources," the paper reads.
The researchers found that NGC 925 ULX-1 reached a peak luminosity up to 40 duodecillion erg/s, which makes it one of the brightest known ULXs.
According to the paper, spectral properties of NGC 925 ULX-1 can be associated to a single optically thick Comptonization component, with electron temperature of about 3.5 keV and seed photon temperature of 0.15 keV. The authors noted that these parameters indicate that NGC 925 ULX-1 is as a broadened disc ultraluminous X-ray source.
The finding therefore does not support the previous proposed hypothesis that NGC 925 ULX-1 could be an intermediate mass black hole.
When it comes to NGC 925 ULX-2, Pintore's team was not able to ultimately classify this source, taking into account the available data. The researchers noted that spectral properties of NGC 925 ULX-2 are not consistent with the thermal soft state of an X-ray binary as well as with the intermediate mass black hole scenario.
In concluding remarks the scientists underlined that in order to disclose the nature of NGC 925 ULX-2, future more detailed observations of this source are necessary.
"Hence, further and deeper X-ray and optical observations of this source are strongly needed to better constrain its nature," the authors of the paper concluded.
Abstract
NGC 925 ULX-1 and ULX-2 are two ultraluminous X-ray sources in the galaxy NGC 925, at a distance of 8.5 Mpc. For the first time, we analyzed high quality, simultaneous XMM-Newton and NuSTAR data of both sources. Although at a first glance ULX-1 resembles an intermediate mass black hole candidate (IMBH) because of its high X-ray luminosity ((2−4)×1040 erg s−1) and its spectral/temporal features, a closer inspection shows that its properties are more similar to those of a typical super-Eddington accreting stellar black hole and we classify it as a `broadened disc' ultraluminous X-ray source. Based on the physical interpretation of this spectral state, we suggest that ULX-1 is seen at small inclination angles, possibly through the evacuated cone of a powerful wind originating in the accretion disc. The spectral classification of ULX-2 is less certain, but we disfavour an IMBH accreting at sub-Eddington rates as none of its spectral/temporal properties can be associated to either the soft or hard state of Galactic accreting black hole binaries.
Ultra-luminous X-ray sources in starburst galaxies
December 22, 2014, Harvard-Smithsonian Center for Astrophysics
The galaxy NGC 1068, seen here in X-ray (red), optical (green) and radio (blue), is actively forming stars and contains three ultra-luminous X-ray sources ULXs. Astronomers investigating the connections between young stars and ULXs have …more
Ultra-luminous X-ray sources (ULXs) are point sources in the sky that are so bright in X-rays that each emits more radiation than a million suns emit at all wavelengths. ULXs are rare. Most galaxies (including our own Milky Way) have none, and those galaxies that do host a ULX usually contain only one. ULXs are also mysterious objects. They can't be normal stars because their huge luminosities should then tear them apart. Most astronomers think that ULXs are black holes more than about ten solar masses in size (so-called intermediate mass black holes) that are accreting matter onto a surrounding disk and emitting X-rays. Bright X-ray emission is not unusual - the nuclei of galaxies also are bright X-ray emitters - but they are super-massive black holes, while ULXs are neither super-massive nor located in galactic nuclei.
The origin of ULXs is a puzzle as well. Supernovae, the explosive deaths of massive stars, can make stellar-mass-sized black holes, but how they then grow to ULX-size is not understood. Astronomers nevertheless think that, if they are on the right track, star formation activity could be a signpost for ULXs because supernovae are short-lived young stars. Indeed, observations of ULXs indicate that they are ten times more likely to be found in star forming galaxies than in old, red galaxies.
CfA astronomers Stefano Mineo and Andy Goulding and their colleagues used the Chandra X-ray Observatory to search for ULXs in a sample of seventeen luminous infrared galaxies that are exceptionally bright because of their extreme star formation activity. If star formation does signal the presence of ULXs, or even produce them, then these objects should have many. The team discovered fifty-three ULXs (with an uncertainty of about 30% ) among the 139 X-ray sources present in this sample. They report, however, that this ULX figure is actually ten times smaller than would be expected if ULXs correlated with simple star formation activity. They offer several possible explanations for this deficiency, including a surfeit of elements heavier than helium in these galaxies (these elements can suppress the birth of black holes). But the most likely scenario, they argue, is that large amounts of gas in these galaxies are present and absorbing X-rays, with the result that many of the ULXs present are not detected.
Their conclusion implies that deep X-ray surveys of galaxies must take absorbing gas into account when estimating their internal X-ray properties and how this radiation affects the galaxies' properties and evolution.
Ultra-luminous x-ray sources
March 19, 2012, Harvard-Smithsonian Center for Astrophysics
The ring galaxy NGC 922 as seen in the optical. X-ray studies have discovered twelve Ultra-Luminous X-ray sources in this galaxy - an unusually large number. Credit: David Malin
(PhysOrg.com) -- An ultra-luminous X-ray source (ULX) emits more radiation in the X-rays than do a million suns at all wavelengths. ULXs are rare: Most galaxies, including our own Milky Way, have none, and galaxies that do host a ULX usually have only one. ULXs are also mysterious: If they were stars with about the mass of the sun, their huge luminosities should tear them apart. Most astronomers today think that ULXs are black holes of more than about ten solar-masses that are accreting material onto a disk, and emitting their X-rays in a beam. When there is beaming towards us, the total luminosity is much less than we would infer from our measurements than if the emission were in all directions. This explanation helps understand the physics of ULXs, but their origins are still obscure.
Stellar mass black holes are made in the explosive deaths of massive stars. Because astronomers believe it is easier to make massive black holes from stars that are deficient in elements heavier than helium, they suspect that ULXs are more likely to be present in galaxies deficient in these elements. Several years ago, twelve ULXs - a record number - were indeed found in an element-deficient galaxy, lending support to this model. But that galaxy had other peculiarities as well. It had recently suffered a collision that knocked its inner nuclear region away, leaving just the outer galaxy behind in the form of a ring. Maybe the collision that produced the ring also helped to spawn the ULXs?
To find out, CfA astronomers Andrea Prestwich, Jose Galache, and Andreas Zezas, with six colleagues, observed the ring galaxy NGC 922 with the Chandra X-ray Observatory. They scientists found that this ring galaxy also had an abundance of ULXs: seven of them. But NGC 922 is not deficient in heavy elements. The team concludes that, at least in these two objects, the heavy element abundance is not a factor. Moreover, they find that the relative number of ULXs in the two galaxies scales with their relative star formation rates. The team has undertaken a larger survey of element-deficient galaxies in an effort to probe the phenomenon more widely, but the new results are an important first step.
Andromeda galaxy pops up ultraluminous X-ray sources
February 27, 2012 By Francis Reddy, JPL/NASA
Andromeda galaxy pops up ultraluminous x-ray sources
This image from Swift's X-Ray Telescope captures both of the known ULXs in M31. The first, dubbed CXOM31 J004253.1+411422, was discovered with NASA's Chandra X-ray Observatory on Dec. 17, 2009, and appears to be a stellar-mass black hole. …more
(PhysOrg.com) -- Researchers using NASA's Chandra X-ray Observatory, Hubble Space Telescope, Swift Gamma-ray Burst Explorer and the European Space Agency's XMM-Newton Observatory have been studying an object known as an ultraluminous X-ray source -- ULX, for short -- in the neighboring Andromeda galaxy (M31). Scientists have long debated the nature of these super-bright X-ray sources, and Chandra's 2009 discovery of a nearby ULX in Andromeda sparked intense interest.
Why are these sources so bright in X-rays? Are they normal stellar-mass black holes gorging on unusually large amounts of gas? Or are they long-sought "intermediate mass" black holes, dozens of times more massive than their stellar counterparts but smaller than the monster black holes found in the centers of most galaxies?
Andromeda galaxy pops up ultraluminous X-ray sources
The locations of two M31 ULXs are shown on this optical image of our galactic neighbor. M31 lies 2.5 million light-years away in the constellation Andromeda and is the nearest large spiral galaxy to our own. Under a clear, dark sky, it can …more
Although astronomers have been studying M31 extensively by X-ray satellites since the 1980s, no ULX had been seen there until 2009.
Adding to the intrigue is a new ULX in M31, discovered just last month by XMM-Newton. Previously, M31's lack of ULXs had suggested to some scientists that these intense beacons didn't form in tranquil spiral galaxies like Andromeda and our own home galaxy, the Milky Way. The appearance of two ULXs within such a relatively short period is truly remarkable.
The Chandra-discovered ULX, dubbed CXOM31 J004253.1+411422, is now the subject of two studies published in the journals Astronomy and Astrophysics and the Monthly Notices of the Royal Astronomical Society. These latest reports suggest that this ULX is a stellar-mass black hole, but one that's consuming great amounts of matter.
The locations of two M31 ULXs are shown on this optical image of our galactic neighbor. M31 lies 2.5 million light-years away in the constellation Andromeda and is the nearest large spiral galaxy to our own. Under a clear, dark sky, it can be seen as a misty patch with the naked eye.
