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Our deepest view of the X-ray sky (mpg.de)
53 points by xioxox on June 20, 2020 | hide | past | favorite | 19 comments

"But wait, how can you perform telescopic observations in X-rays? They tend to penetrate matter (including telescopes), right?"

Correct, normal optics would not work. X-ray telescopes use grazing incidence mirror arrangement (Wolter telescope [1] [2]).

[1] https://en.wikipedia.org/wiki/Wolter_telescope

[2] https://en.wikipedia.org/wiki/EROSITA

I'm a scientist on the team who did this (I actually helped put this picture together), so I can try to answer questions...

I helped build and calibrate XMM, earning my PhD https://www.researchgate.net/publication/252418991_The_X-ray... in the 90s, before leaving academia to start my first company.

I haven't kept track of the field. Has eROSITA reproduced most of XMM's data? I expect XMM will keep collecting data, but how useful/obsolete has it become?

XMM is still extremely useful. There was a recent report that using clever techniques ESA are going to keep it going for another 10 years: https://www.esa.int/Enabling_Support/Operations/Teaching_an_...

XMM has a smaller field of view than eROSITA, but more light gathering capability (effective area), particularly at higher energies. It also has different types of instruments, such as the X-ray gratings.

eROSITA is really designed for doing surveys with its wide unobstructed field of view, and it will be doing just that for the next 3.5 yrs. We really need XMM to follow up in detail the most interesting sources we discover during the survey.

How does this align with expectations, meaning more or fewer X-ray sources? What's the biggest "Holy -----!" moment after viewing the results? (Other than the fact that after 182 days of data collection you produced this astonishing map!)

I'm interpreting this as a picture of all massive X-ray sources in the visible universe. And I'm kinda ... sad? that it looks so empty....? But at the same time I'm also thrilled by each and every point.

I believe it's detecting more sources than expected. This can be to do with how efficient observations are (very good so far), how low the background is (higher than expected, but stable), the performance of the telescope in space, and how good the simulations of the data are (we're still working on them!).

I was very impressed with the picture of the Shapley supercluster (see article), which is very overdense part of the local universe, containing dozens of clusters of galaxies, each of those with 100s to 1000s of galaxies. In addition, we took some performance verification observations at the start, and these were really nice: http://www.mpe.mpg.de/7362095/news20191022

The picture is a bit misleading about the number of sources. It's really dominated by our galaxy, as most sources are point-like. You cannot really see the majority of them individually in the full-sky map given its spatial resolution, particularly as we applied some smoothing to the data to help emphasise the galactic structure.

We're also going to get another 3.5 years more data in the survey, which is going to find a lot more sources.

Here's a nice video showing how SRG/eROSITA scans the sky: https://www.youtube.com/watch?v=z8M-80aMDO0

Has the eROSITA mission discovered any previously unknown black holes?

Lots! It has discovered about a million X-ray sources, about twice as many as previously known about. Most of these sources will be active galactic nuclei (supermassive black holes that are accreting material) in distant galaxies. There's still lots of work to do in characterising these objects though and understanding the population.

The Fermi bubbles are super-prominent, is eROSITA the first x-ray telescope sensitive to detect them?

Some of the structure can be seen in the old ROSAT maps from the 90s: http://www.xray.mpe.mpg.de/rosat/survey/sxrb/12/ass.html

Recently XMM looked at the centre of the galaxy, finding chimney-like structures which may link to the bubbles. This is on a much smaller region, however: http://www.esa.int/Science_Exploration/Space_Science/Giant_c...

Nothing since ROSAT has had the ability to map large areas like eROSITA/SRG. ABRIXAS was supposed to be the successor to ROSAT, but it failed shortly after launch.

And what is the angular resolution of the real thing?

The half energy width (angle over which half the energy is deposited) is around 16 arcsec on the axis of the telescopes (3600 arcsec=1 deg). Due to the Wolter geometry, it gets worse as you go off-axis. In the survey sources scan over the field of view. The average survey performance is around 28 arcsec.

The eROSITA telescope onboard the SRG observatory has completed its first survey of the sky in X-rays, producing the deepest ever map in X-rays and finding over a million sources. Most of these sources are active black holes, clusters of galaxies, supernova remnants and stars.

Annotated and high res images can be found here: http://www.mpe.mpg.de/7461950/erass1-presskit

This just blew my mind, from the fifth image:

"Vela Junior was discovered just 20 years ago, although this object is so close to Earth that remains of this explosion were found in polar ice cores."

Aside from being some serious Hurcule Poirot meets Kip Thorne crossover stuff, I want to ask "How do we know?" but I'm pretty sure I wouldn't understand the answer.

Imagine a whole article about high-energy astronomy never even mentioning plasma or synchrotron radiation.

Trotted out "hot gas", though.

I am mistaken: "plasma" appears once in a hidden caption I missed first time through.

I'm not sure why you're bringing up synchrotron radiation in particular. Although it's important for some sources, it's not driving the majority of what is being seen here. Comptonization is important for active black holes. The radiation from the milky way has strong emission lines - it's not a synchrotron spectrum. Mostly thermal emission is emitted from clusters of galaxies. There are a lot of different emission processes to consider in astrophysics.

If, as you say, there are a lot of different emission processes, why only ever mention the, by far, least interesting? Elsewhere in science exceptions are interesting. The unexplained is interesting.

Lev Landau is reputed to have said, "Cosmologists are often in error, never in doubt." Doubt is productive.

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