Mosaikbillede fra JEM-X instrumenterne på ESA INTEGRAL satellit viser området tæt på Mælkevejens centrum med to nye røntgen kilder

Busy astrophysicist discovers two new x-ray sources

Wednesday 26 Nov 14


Jérôme Chenevez
Associate Professor
DTU Space
+45 45 25 97 03


To help in the observation and identification of transient astronomical sources, a web-based alert service has been set up. The Astronomer's Telegram allows astronomers who think they may have found a  new source (after a careful search in the catalogues of known objects), to alert other observatories to it, as quickly as possible, giving a strength and position. The ATEL is sent along to all the subscribing astronomers, who can point their telescopes in the stated direction and try to confirm that there is a new source there. Corroborating follow-up observations are essential for a new source's credibility.

September and October were very busy months for Jérôme Chenevez, an astrophysicist at DTU Space.Hidden in recent data from ESA's INTEGRAL satellite he discovered a signal from an unknown source, which behaves as though it might be a neutron star - and just over a month later he repeated his success by finding a new black hole candidate in the same region, near the galactic centre

 Jérôme Chenevez and five of his colleagues at the Danish National Space Institute are instrument specialists who monitor and calibrate the JEM-X instruments on ESA's INTEGRAL satellite. In return for having built the instruments for ESA, DTU Space's astrophysicists have access to the satellite's data. Hidden amongst JEM-X data from observations of the galactic centre, Jerome and his student, Louise Vandbæk Kroer, found the signal from an unknown source on 24th August, which behaves like a neutron star. On 27th September he found yet another unknown source, this one giving every indication of being a black hole candidate.

Discovery image of presumed new neutron star  IGR J1745-3022, as seen by  JEM-X

 A neutron star in a compact binary system

 The first new source, discovered in data collected between 22nd and 23rd August, emits mostly low energy (soft) x-rays. It has been assigned the name IGR J17451-3022, where the first three letters indicate that the source was discovered by the INTEGRAL satellite, and the digits indicate the source's position in the sky.  Twelve days later the source was strong enough for the SWIFT satellite to see, so that the satellite could confirm its position within 26 arcseconds of the position measured by JEM-X - which is a very precise confirmation of the source's position and it's existence. (NB: 1 arcsecond equals 0.0072 degrees.)

JEM-X discovery image of  IGR J17454-2919, a black hole candidate

 A black hole with a big appetite

The second source, with the name  IGR J17454-2919, has also been seen by Swift and NASA's NuSTAR satellite observatory, in follow-up observations that indicate the source is a black hole. The reason the source can be seen at all, is that it is in the process of gobbling up matter from an orbiting star - one that was so unfortunate as to evolve in a binary system with a massive star that rapidly came to the end of its life, exploded and became a black hole. The x-ray radiation that streams out of the system is the last signal from the stellar matter before it disappears into the black hole.

"It's completely incredible that two unknown sources should pop up so quickly after each other. We're working hard to find out what sort of objects we're dealing with here. Together with the followup observations from other satellites, we've got a load of data to process before with have a complete picture of the sources' identities. Quite honestly, I'll be able to tell you something certain in a month!"
Jérôme Chenevez

In the discovery image above, the black hole appears quite weak compared to the other, known, sources, while the source that was found just a month previously is still strong. As with all transient sources of this sort, their energy output can change very rapidly.

The newest image of the region (at the top of the page), it is the black hole (yellow arrow) that is the stronger of the two new sources, while the neutron star has faded and the known source 1E 1740.7-2942 has nearly become invisible.

X-rays from the centre of the Milky Way

The JEM-X instruments built by DTU Space detect only x-rays and were launched into space on ESA's INTEGRAL satellite in 2009, together with two gamma-ray telescopes. The mission's purpose was, and still is 12 years later, to observe the most violent obejects and events in the universe: gamma-ray bursts,  black holes, pulsars, neutron stars and active galaxies.

Transient signals

It is extremely difficult to find and observe new sources of this kind because their visible outbursts are usually unexpected and without a pattern. Therefore you have to be extremely lucky to be pointing your instruments in the right direction while the object is in its active phase. This is why the INTEGRAL satellite has an extensive observing programme covering the centre of the Milky Way, which is home to the largests population of massive stars and their explosive remains.

ATELs galore

The two new sources have aroused great interest in the astronomical community, and have become the subject of numerous Astronomer's Telegrams (ATELs, see infobox). The first two of these were posted by Jerome and his colleagues to alert other astonomers, but more recent ATELs describe follow-up observations by other satellite-based observatories. Swift, Chandra and NuSTAR have all been able to confirm the existence of the sources, and even make detailed observations that can determine their nature.

An extreme laboratory

The processes and behaviour exhibited by these violent, energetic sources, display the sort of extreme high energy physics only found in conditions of immense temperature, pressure and gravitation. These conditions can never be repeated in any laboratory and so observing these distant, massive objects is the only way to understand physics at its most extreme limits.




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