Researchers investigate two ultraluminous X-ray sources in the galaxy NGC 925

July 11, 2018 by Tomasz Nowakowski, Phys.org report
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 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 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.

Explore further: Ultra-luminous X-ray sources in starburst galaxies

More information: The two Ultraluminous X-ray sources in the galaxy NGC 925, arXiv:1806.11444 [astro-ph.HE] arxiv.org/abs/1806.11444

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.

Related Stories

Ultra-luminous X-ray sources in starburst galaxies

December 22, 2014

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 ...

Andromeda galaxy pops up ultraluminous X-ray sources

February 27, 2012

(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 ...

The search for medium-sized black holes

November 27, 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 ...

Pulsar discovered in an ultraluminous X-ray source

September 26, 2016

(Phys.org)—A team of European astronomers has discovered a new pulsar in a variable ultraluminous X-ray source (ULX) known as NGC 7793 P13. The newly found object is the third ultraluminous X-ray pulsar detected so far, ...

Arp 299: Galactic Goulash

June 26, 2017

What would happen if you took two galaxies and mixed them together over millions of years? A new image including data from NASA's Chandra X-ray Observatory reveals the cosmic culinary outcome.

Recommended for you

The surprising environment of an enigmatic neutron star

September 17, 2018

An unusual infrared emission detected by the Hubble Space Telescope from a nearby neutron star could indicate that the pulsar has features never before seen. The observation, by a team of researchers at Penn State, Sabanci ...

Ceres takes life an ice volcano at a time

September 17, 2018

Every year throughout its 4.5-billion-year life, ice volcanoes on the dwarf planet Ceres generate enough material on average to fill a movie theater, according to a new study led by the University of Arizona.

TESS shares first science image in hunt to find new worlds

September 17, 2018

NASA's newest planet hunter, the Transiting Exoplanet Survey Satellite (TESS), is now providing valuable data to help scientists discover and study exciting new exoplanets, or planets beyond our solar system. Part of the ...

0 comments

Please sign in to add a comment. Registration is free, and takes less than a minute. Read more

Click here to reset your password.
Sign in to get notified via email when new comments are made.