Astronomy

Is there any way to find out what sky surveys are *currently active?*

Is there any way to find out what sky surveys are *currently active?*


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I've asked a question about active sky surveys that might intercept a 2% of 4π patch of sky in the next six months and potentially record this dim (+22 to +26 magnitude) object.

I've added a bounty with about one more day on the grace period, the only thing helpful was a comment that suggested I ask for a "database of surveys".

Are there any pages that list at least major active surveys? That would be a start at least.


If you really just want a list, Wikipedia has a pretty good page on astronomical surveys.

For some comprehensive information, including some tabulated values (such as sky coverage for each survey), try this paper from S. G. Djorgovski et al.


Powerful new AI technique detects and classifies galaxies in astronomy image data

Researchers at UC Santa Cruz have developed a powerful new computer program called Morpheus that can analyze astronomical image data pixel by pixel to identify and classify all of the galaxies and stars in large data sets from astronomy surveys.

Morpheus is a deep-learning framework that incorporates a variety of artificial intelligence technologies developed for applications such as image and speech recognition. Brant Robertson, a professor of astronomy and astrophysics who leads the Computational Astrophysics Research Group at UC Santa Cruz, said the rapidly increasing size of astronomy data sets has made it essential to automate some of the tasks traditionally done by astronomers.

"There are some things we simply cannot do as humans, so we have to find ways to use computers to deal with the huge amount of data that will be coming in over the next few years from large astronomical survey projects," he said.

Robertson worked with Ryan Hausen, a computer science graduate student in UCSC's Baskin School of Engineering, who developed and tested Morpheus over the past two years. With the publication of their results May 12 in the Astrophysical Journal Supplement Series, Hausen and Robertson are also releasing the Morpheus code publicly and providing online demonstrations.

The morphologies of galaxies, from rotating disk galaxies like our own Milky Way to amorphous elliptical and spheroidal galaxies, can tell astronomers about how galaxies form and evolve over time. Large-scale surveys, such as the Legacy Survey of Space and Time (LSST) to be conducted at the Vera Rubin Observatory now under construction in Chile, will generate huge amounts of image data, and Robertson has been involved in planning how to use that data to understand the formation and evolution of galaxies. LSST will take more than 800 panoramic images each night with a 3.2-billion-pixel camera, recording the entire visible sky twice each week.

"Imagine if you went to astronomers and asked them to classify billions of objects -- how could they possibly do that? Now we'll be able to automatically classify those objects and use that information to learn about galaxy evolution," Robertson said.

Other astronomers have used deep-learning technology to classify galaxies, but previous efforts have typically involved adapting existing image recognition algorithms, and researchers have fed the algorithms curated images of galaxies to be classified. Hausen built Morpheus from the ground up specifically for astronomical image data, and the model uses as input the original image data in the standard digital file format used by astronomers.

Pixel-level classification is another important advantage of Morpheus, Robertson said. "With other models, you have to know something is there and feed the model an image, and it classifies the entire galaxy at once," he said. "Morpheus discovers the galaxies for you, and does it pixel by pixel, so it can handle very complicated images, where you might have a spheroidal right next to a disk. For a disk with a central bulge, it classifies the bulge separately. So it's very powerful."

To train the deep-learning algorithm, the researchers used information from a 2015 study in which dozens of astronomers classified about 10,000 galaxies in Hubble Space Telescope images from the CANDELS survey. They then applied Morpheus to image data from the Hubble Legacy Fields, which combines observations taken by several Hubble deep-field surveys.

When Morpheus processes an image of an area of the sky, it generates a new set of images of that part of the sky in which all objects are color-coded based on their morphology, separating astronomical objects from the background and identifying point sources (stars) and different types of galaxies. The output includes a confidence level for each classification. Running on UCSC's lux supercomputer, the program rapidly generates a pixel-by-pixel analysis for the entire data set.

"Morpheus provides detection and morphological classification of astronomical objects at a level of granularity that doesn't currently exist," Hausen said.

An interactive visualization of the Morpheus model results for GOODS South, a deep-field survey that imaged millions of galaxies, has been publicly released. This work was supported by NASA and the National Science Foundation.


The Expanding Universe

In two thousand years of astronomy, no one ever guessed that the universe might be expanding. To ancient Greek astronomers and philosophers, the universe was seen as the embodiment of perfection. The heavens were truly heavenly - unchanging, permanent, and geometrically perfect. In the early 1600s, Isaac Newton developed his law of gravity, showing that motion in the heavens could be explained using the same laws as motion on Earth.

However, Newton ran into trouble when he tried to apply his theory of gravity to the entire universe. Since gravity is always attractive, his law predicted that all the matter in the universe should eventually clump into one big ball. Newton knew this was not the case, and assumed that the universe had to be static, so he conjectured that the Creator placed the stars such that they were "at immense distances from one another."


Albert Einstein's Theory of Relativity is the basis for our cosmological models of space and time.

In 1916, Albert Einstein ran into the same problem that Newton did. Einstein had just completed his General Theory of Relativity, which explained gravity in a different way from Newton's law. Like Newton's theory, General Relativity predicted that the universe should be collapsing into a ball. Because Einstein assumed that the universe must be static, he added a constant term to his equations that counteracted gravity on very large distance scales. A few years later, someone pointed out that Einstein's equations had another solution in which the universe should be expanding, but Einstein continued to work with his constant term, believing the universe to be static.

Then, in 1924, Edwin Hubble of the Carnegie Observatories made a new map. He used a new telescope on California's Mount Wilson to observe a series of distant galaxies, and found that light from those galaxies was redshifted - that is, light waves were stretched out like sound waves from a passing siren. The further away the galaxy, Hubble found, the greater the redshift. Hubble's observation showed that the universe was expanding, meaning it had started at a single point called the big bang about fifteen billion years ago. When Einstein heard about Hubble's discovery, he realized that his equations predicted the expanding universe all along, and called his constant term his "biggest blunder." Today, the idea of the expanding universe is the basis for all of modern astronomy.


How to get Stellarium to show more DSOs?

As I scan the DSO forum for galaxies to observe, I'm finding there are some that are labeled in Stellarium on my laptop, but no image is shown. I can find them on stellarium-web.org, but I'd like to get them onto my laptop (i.e. the desktop version of Stellarium). Is there a way to do that?

Edited by orionic, 20 March 2020 - 10:05 AM.

#2 beggarly

Which version of Stellarium do you use? Latest version is 0.19.3.

Sky and Viewing Options - DSO?

#3 orionic

Yes, I use that panel and turned on all kinds of obscure catalogs today, but for many of them, all it does is show little red ovals to mark the location, showing black space (empty) where the DSO should be.

Could this be a bandwidth issue, i.e. does Stellarium (desktop) load the images dynamically?

Mind you, many of the DSOs are visible, it's just that as I explore more "advanced" ones I'm discovering many are not.

E.g. NGC 2485 in CMa - is this visible for you (as an actual galaxy image)?

#4 beggarly

There is no image of NGC 2485.

#5 Jii

You can get more galaxies (etc) visible to Stellarium by toggling DSS (Digitized Sky Survey) on. E.g. NGC 2485 is visible with it.

#6 orionic

Thanks for the tips! I went into Stellarium, into the Surveys tab, and selected both (identical-looking) items called "DSS colored". No change. I then restarted, but no change still (NGC2485 not visible). Is there another setting I should attend to?

#7 aeajr

Thanks for the tips! I went into Stellarium, into the Surveys tab, and selected both (identical-looking) items called "DSS colored". No change. I then restarted, but no change still (NGC2485 not visible). Is there another setting I should attend to?

First - as you zoom in you will get more and dimmer stars and more and dimmer DSOs. Those little red circles will start to get labels, though not all. If you click on them the details will be displayed in the upper left.

Go to the left menu, third icon down. Now go to the DSO tab. Have you clicked the "limit magnitude" button? If so, unclick it and you will see more DSOs.

#8 orionic

First - as you zoom in you will get more and dimmer stars and more and dimmer DSOs. Those little red circles will start to get labels, though not all. If you click on them the details will be displayed in the upper left.

Go to the left menu, third icon down. Now go to the DSO tab. Have you clicked the "limit magnitude" button? If so, unclick it and you will see more DSOs.

Thanks for your thoughts. No, I didn't have "limit magnitude" checked in the DSO tab. Interestingly, that checkbox is only check/uncheckable if "labels and markers" is checked on the DSO tab. So I checked "labels and markers" and then set "limit magnitude" to 16.0 just to be sure. Also, just to be sure, on the "sky" tab I set "limiting magnitude" for "stars" to 16. I also went to "Configure" -> "extras" tab and checked "show DSS button" to address what I suppose now that @Jii was suggesting then on the "main" tab, did "save settings". At this point, ngc2485 (actually it's in CMa) still not visible. I quit the app, restart and still not visible if I search.

Perhaps Virgo is more interesting as an example. Here, I see perhaps a few dozen galaxies visible, but several dozen more of the red ovals have no galaxy image. For example ngc4733, which Stellarium says has mag 11.81, has no image. Whereas ngc4302, with mag 11.61, is visible. However, ngc4206, with mag 12.15, is visible. So there is no clear logic for what is/isn't visible. Although, most of the visible ones seem to be brightish ones.

Edited by orionic, 21 March 2020 - 03:57 AM.

#9 lambermo

So you already have enabled the DSS button. But did you also actually click it when back in the main window ? It's a new button in the bottom bar.

You get to see this on NGC 2485 :

#10 aeajr

I have version 0.19.3.

Yes, I use that panel and turned on all kinds of obscure catalogs today, but for many of them, all it does is show little red ovals to mark the location, showing black space (empty) where the DSO should be.

Could this be a bandwidth issue, i.e. does Stellarium (desktop) load the images dynamically?

Mind you, many of the DSOs are visible, it's just that as I explore more "advanced" ones I'm discovering many are not.

E.g. NGC 2485 in CMa - is this visible for you (as an actual galaxy image)?

Using Stellium on my Windows 10 desktop, version 0.19.3, NGC2485 is visible to me.

Using the search function, the box came up on a blank area. So I zoomed in till I got the little red icon showing the exact location of the galaxy. Information in upper left corner is what I am looking for. Now I can plan a star hop if I want to try for it that way. Or I can use the real time AltAz coordinates to find it.

If you are looking for a pretty picture, well, there are not picture backgrounds for every item. In fact I have the picture backgrounds turned off as I find that feature useless since that is not what it will look like in the eyepiece. I don't care about a pretty pictures. If I want pictures I can look it up in Wikipedia or some other picture library.

I use Stellarium as a star chart to find things so I can plan an observing session.

I can also overlay the Telrad so I can use that for planning a star hop to find the galaxy.

Attached Thumbnails

Edited by aeajr, 21 March 2020 - 09:02 AM.

#11 orionic

@Jii - Whoops! It turns out that DSS button took care of it, but I didn't see it until now, must have been thinking it was the "DSO" button or something. Thanks again.

@Iambermo - Thank you, for for visualizing that mistake, that was the problem! To tell the truth overnight I started to think there was likely a bug in the Mac version (since most CN users are Windows-based), glad to find that is not the case.

@aeajr - Thanks for sharing your user scenario, which is quite different from mine.

For me, currently Stellarium is a way to quickly find a photo of a galaxy at a given FOV so that I can decide if it is a good candidate for my EAA sessions. I could search online but then I'd get those gorgeous color-sprinkled AP images that are so different from EAA results. Stellarium also puts everything in context, in a nice consistent way (you're right it doesn't have photo backgrounds for everything which isn't consistent, but now with DSS it is), so I can get a sense of how many stars surround it closely for plate-solving and livestack alignment.

The DSS imagery looks just perfect for my use. Galaxies I've already viewed using EAA look quite similar to what I see in Stellarium now. Well, Stellarium is still quite a bit better, but the shapes and structure are all quite similar so it gives me something semi-attainable to shoot for.

#12 brentknight

@Jii - Whoops! It turns out that DSS button took care of it, but I didn't see it until now, must have been thinking it was the "DSO" button or something. Thanks again.

@Iambermo - Thank you, for for visualizing that mistake, that was the problem! To tell the truth overnight I started to think there was likely a bug in the Mac version (since most CN users are Windows-based), glad to find that is not the case.

@aeajr - Thanks for sharing your user scenario, which is quite different from mine.

For me, currently Stellarium is a way to quickly find a photo of a galaxy at a given FOV so that I can decide if it is a good candidate for my EAA sessions. I could search online but then I'd get those gorgeous color-sprinkled AP images that are so different from EAA results. Stellarium also puts everything in context, in a nice consistent way (you're right it doesn't have photo backgrounds for everything which isn't consistent, but now with DSS it is), so I can get a sense of how many stars surround it closely for plate-solving and livestack alignment.

The DSS imagery looks just perfect for my use. Galaxies I've already viewed using EAA look quite similar to what I see in Stellarium now. Well, Stellarium is still quite a bit better, but the shapes and structure are all quite similar so it gives me something semi-attainable to shoot for.

If you haven't already, you might try Aladin Lite. It uses the same DSS data (and more), but I think the presentation is actually better. not as cluttered and much faster rendering. It lacks all the nice labels though.

#13 orionic

If you haven't already, you might try Aladin Lite. It uses the same DSS data (and more), but I think the presentation is actually better. not as cluttered and much faster rendering. It lacks all the nice labels though.

Wow, thanks!! In Aladin Lite I was able to identify tiny blurry objects that are not clickable in Stellarium found out one of the galaxies observed in my EAA session was a mag 16.36, by far my faintest yet (MCG+09-19-113)!

#14 markhardaker

Well, I have to admit I have tried and tried to find this famous DSS button on the lower bar, but it just isn't there. I have buttons for Deep-Sky Objects [D] and Deep-sky Objects Background Images [I] both of which are activated, but still no NGC 2485 (as an example). I am using version 0.20.4. What am I missing? Please help me find this DSS button.


Vera C. Rubin Observatory – Impact of Satellite Constellations

The Vera C. Rubin Observatory science community is concerned about the increasing deployment of communications satellite constellations which, if unchecked, could jeopardize the discoveries anticipated from Rubin Observatory when science operations begin in 2022. Because Rubin Observatory is uniquely impacted by these satellite constellations, its science team is taking an active role in pursuing mitigation strategies to reduce the impact of the satellites on Rubin Observatory science.

The Vera C. Rubin Observatory is nearing completion, and its Legacy Survey of Space and Time (LSST) will soon offer an unprecedented, detailed view of the changing sky. Starting in late 2022, Rubin Observatory will employ the 8.4-meter Simonyi Survey Telescope and the 3200 megapixel LSST Camera to capture about 1,000 images of the sky, every night, for ten years. Each image will cover a 9.6 square degree field of view, or about 40 times the area of the full Moon. Because of the telescope's large light-collecting area, each nominal 30-second exposure will reveal distant objects that are about 20 million times fainter than those visible with the unaided eye. This large combination of light-collecting area and field of view on the sky is unprecedented in the history of optical astronomy.

LSST survey images will contain data for about 20 billion galaxies and a similar number of stars, and will be used for investigations ranging from cosmological studies of the Universe to searches for potentially dangerous Earth-impacting asteroids. However, the revolutionary discoveries anticipated from the Rubin Observatory LSST could be significantly degraded by the fast deployment of Low Earth Orbiting (LEO) communications satellite constellations.

In late May 2019, SpaceX launched the first 60 of its planned Starlink constellation of 42,000 communications satellites to LEO orbits at altitudes of about 550 km. Since then, SpaceX has launched several more groups of 60 satellites, and plans to launch a similar group every 2-3 weeks in the near future. Other companies, including Amazon and Samsung have also entered the race, and the number of satellites launched may exceed 50,000 over the next decade. The negative impact of these satellites on optical astronomy depends on the number and brightness of satellites. According to Patrick Seitzer, an astronomer at the University of Michigan who studies orbital debris, “Satellites launched by SpaceX and others will be brighter than 99 percent of the population of objects of all types currently in Earth orbit.”

Rubin Observatory is an extreme case for the sensitivity of astronomical observations to satellite constellations because of its unprecedented ability to repeatedly monitor the sky widely and deeply. During the nominal 30 second visit to a sky patch, SpaceX satellites in LEO orbits typically move about 15 degrees across the sky (about four times the diameter of Rubin Observatory’s field of view), and are visible a few hours after sunset and before sunrise. With 42,000 satellites orbiting Earth, well over a thousand satellites would be visible above horizon and it would be difficult to find a circle of 9.6 square degrees anywhere on the sky that does not contain satellite streaks. Simulations of the LSST observing cadence and the full SpaceX satellite constellation show that as many as 30% of all LSST images would contain at least one satellite trail. Measurements of the brightness of the current LEO satellites in their final orbits indicate that these trails would cause residual artifacts in the reduced data. If these LEO satellites can be darkened to 7th magnitude, then a new instrument signature removal algorithm can remove the residual artifacts. The bright main satellite trail would still be present, potentially creating systematics at low surface brightness. This is a challenge for science data analysis, adding potentially significant effort. LEO satellites at 550 km are slightly out of focus–given the large 8.4 m mirror, this effect makes the trail significantly wider and lessens the peak surface brightness.

Strategies to lessen the impact of satellite constellations are currently being studied. Below are two mitigation plans currently in discussion:

Taking multiple exposures: When the nominal LSST visit time of 30 seconds is split into two back-to-back exposures of 15 seconds, one of the exposures with a satellite trail in it can be rejected if the other exposure didn't contain any satellite trails. This mitigation scenario would cost 8% of LSST observing time in order to accommodate the additional read-out time and shutter motion, and assumes a negligible cost due to rejected pixels. This only mitigates in certain science cases.

Decreasing satellite brightness: If satellites were darkened to 7th magnitude, they would be far below saturation in LSST images. In this case, it is likely that only small fractions of pixels in the affected images—probably in the 1% to 10% range—would be rendered scientifically useless. If this estimate proves correct, the net fraction of lost LSST pixels would be in the range of 0.3%-3%, which corresponds to several months of observing time.

Nevertheless, of additional concern are various systematic effects that do not simply scale with the number of lost pixels—in other words, the effect these mitigation strategies would have on the science cases for which LSST was designed. For example, the LSST ability to detect asteroids approaching from directions interior to the Earth's orbit would be severely impacted because those directions are visible only during twilight when LEO satellites are brightest—nearly every LSST image taken at this time would be affected by at least one satellite trail. Precision cosmological studies are another example they are very sensitive to small systematic effects, and might suffer from artifacts due to the removal or masking of elongated rectangular regions around the satellite tracks. At the low surface brightness of many LSST science programs, the trail is several hundred pixels wide.

The Rubin Observatory team is working closely with SpaceX engineers to jointly find ways to lessen the impact of the satellite trails. Efforts such as designing fainter satellites, improving image processing algorithms so they are capable of dealing with satellite streaks at the exquisite fidelity required for LSST science, and improving scheduling algorithms based on knowledge of the satellites' orbital motions, may provide additional mitigation strategies. Current efforts are centered on satellite darkening one satellite currently in orbit, ‘DarkSat,’ has been partially darkened as an initial experiment, and appears 6.1 g magnitude. Further experiments, such as ‘VisorSat,’ are planned, and results will be assessed via ground-based calibrated imaging in the months ahead. Once sufficient data are collected and analyzed, the Rubin Observatory team will share the results with the rest of the astronomical community and the public.


An Apertif to the Next Radio Astronomy Entrée

To aid in the digestion of a new era in radio astronomy, a new technique for improving the is unfolding at the Westerbork Synthesis Radio Telescope (WSRT) in the Netherlands. By adding a plate of detectors to the focal plane of just one of the 14 radio antennas at the WSRT, astronomers at the Netherlands Institute for Radio Astronomy (ASTRON) have been able to image two pulsars separated by over 3.5 degrees of arc, which is about 7 times the size of the full Moon as seen from Earth.

The new project – called Apertif – uses an array of detectors in the focal plane of the radio telescope. This ‘phased array feed’ – made of 121 separate detectors – increases the field of view of the radio telescope by over 30 times. In doing so, astronomers are able to see a larger portion of the sky in the radio spectrum. Why is this important? Well, in keeping with our food course analogy, imagine trying to eat a bowl of soup with a thimble – you can only get a small portion of the soup into your mouth at a time. Then imagine trying to eat it with a ladle.

This same analogy of surveying and observing the sky for radio sources holds true. Dr. Tom Oosterloo, the Principle Investigator of the Apertif project, explains the meat of the new technique:

“The phased array feed consists of 121 small antennas, closely packed together. This matrix covers about 1 square meter. Each WSRT will have such a antenna matrix in its focus. This matrix fully samples the radiation field in the focal plane. By combining the signals of all 121 elements, a ‘compound beams'[sic] can be formed which can be steered to be pointing at any location inside a region of 3ࡩ degrees on the sky. By combining the signals of all 121 elements, the response of the telescope can be optimised, i.e. all optical distortions can be removed (because the radiation field is fully measured). This process is done in parallel 37 times, i.e. 37 compound beams are formed. Each compound beam basically functions as a separate telescope. If we do this in all WSRT dishes, we have 37 WSRTs in parallel. By steering all the beams to different locations within the 3ࡩ degree region, we can observe this region entirely.”

In other words, traditional radio telescopes use only a single detector in the focal plane of the telescope (where all of the radiation is focused by the telescope). The new detectors are somewhat like the CCD chip in your camera, or those in use in modern optical telescopes like Hubble. Each separate detector in the array receives data, and by combining the data into a composite image a high-quality image can be captured.

The new array will also widen the field of view of the radio telescope, which allowed for this most recent observation of widely separated pulsars in the sky, a milestone test for the project. As an added bonus, the new detector will increase the efficiency of the “aperture” to around 75%, up from 55% with the traditional antennas.

Dr. Oosterloo explained, “The aperture efficiency is higher because we have much more control over the radiation field in the focal plane. With the classic single antenna systems (as in the old WSRT or as in the eVLA), one measures the radiation field in a single point only. By measuring the radiation field over the entire focal plane, and by cleverly combining the signals of all elements, optical distortion effects can be minimised and a larger fraction of the incoming radiation can be used to image the sky.”
This image illustrates the larger field of view afforded by the new instrument. Image Credit: ASTRON

For now, there is only one of the 14 radio antennas equipped with Apertif. Dr. Joeri Van Leeuwen, a researcher at ASTRON, said in an email interview that in 2011, 12 of the antennas will be outfitted with the new detector array.

Sky surveys have been a boon for astronomers in recent years. By taking enormous amounts of data and making it available to the scientific community, astronomers have been able to make many more discoveries than they would have been able to by applying for time on disparate instruments.

Though there are some sky surveys in the radio spectrum that have been completed so far – the VLA FIRST Survey being the most prominent – the field has a long way to go. Apertif is the first step in the direction of surveying the whole sky in the radio spectrum with great detail, and many discoveries are expected to be made by using the new technique.

Apertif is expected to discover over 1,000 pulsars, based on current modeling of the Galactic pulsar population. It will also be a useful tool in studying neutral hydrogen in the Universe on large scales.

Dr. Oosterloo et. al. wrote in a paper published on Arxiv in July, 2010, “One of the main scientific applications of wide-field radio telescopes operating at GHz frequencies is to observe large volumes of space in order to make an inventory of the neutral hydrogen in the Universe. With such information, the properties of the neutral hydrogen in galaxies as function of mass, type and environment can be studied in great detail, and, importantly, for the first time the evolution of these properties with redshift can be addressed.”

Adding the radio spectrum to the visible and infrared sky surveys would help to fine-tune current theories about the Universe, as well as make new discoveries. The more eyes on the sky we have in different spectra, the better.

Though Apertif is the first such detector in use, there are plans to update other radio telescopes with the technology. Dr. Oosterloo said of other such projects, “Phased array feeds are also being built by ASKAP, the Australia SKA Pathfinder. This is an instrument of similar characteristics as Apertif. It is our main competitor, although we also collaborate on many things. I am also aware of a prototype being tested at Arecibo currently. In Canada, DRAO [Dominion Radio Astrophysical Observatory] is doing work on phased array feed development. However, only Apertif and ASKAP will construct an actual radio telescope with working phased array feeds in the short term.”

On November 22nd and 23rd, a science coordination meeting was held about the Apertif project in Dwingeloo, Drenthe, Netherlands. Dr. Oosterloo said that the meeting was attended by 40 astronomers, from Europe, the US, Australia and South Africa to discuss the future of the project, and that there has been much interest in the potential of the technique.

Sources: ASTRON press release, Arxiv, email interview with Dr. Tom Oosterloo and Dr. Joeri Van Leeuwen


A new view of the X-ray sky

This projection shows the sky distribution of 2RXS sources in Galactic coordinates. The size of each dot scales with the source count rate (brightness) and the colour represents the X-ray colour. Credit: © MPE

Scientists at the Max Planck Institute for Extraterrestrial Physics (MPE) have revisited the all-sky survey carried out by the ROSAT satellite, to create a new image of the sky at X-ray wavelengths. Along with this a revised and extended version of the catalogue of bright and faint point-like sources will be released. The now published "2RXS catalogue" provides the deepest and cleanest X-ray all-sky survey to date, which will only be superseded with the launch of the next generation X-ray survey satellite, eROSITA, currently being completed at MPE.

In the 1990s, the ROSAT X-ray satellite performed the first deep all-sky survey with an imaging telescope in the 0.1-2.4 keV energy band, increasing the number of known X-ray sources by a factor of approximately 100. The intention of the new analysis was to improve the reliability of the catalogue, by re-analysing the original photon event files, using an advanced detection algorithm and a complete screening process.

An important feature of the new catalogue is a statistical assessment of the reliability of the sources. Because of the extreme sensitivity and low background of the ROSAT PSPC instrument, cosmic X-ray sources can be identified with the detection of just a few photons. These are sometimes difficult to distinguish from random fluctuations, and the new catalogue provides an assessment of this effect, based on simulated data.

The catalogue contains more than just a list of sources, for example X-ray images and overlaid X-ray contour lines for each of the detections are provided. For many sources, X-ray light curves were created to shows how the sources vary in brightness on intra-day timescales. For the brightest sources X-ray spectral fits were performed based on three basic spectral models, a power law, a thermal-plasma and a black-body emission model. This is important because it can distinguish what kind of cosmic source produces the X-rays. These include powerful accreting black holes, giant clusters of galaxies, active stars and the remnants of stars that exploded in supernova explosions.

With the new catalogue, the astrophysical community will now be able to explore these objects in the X-ray sky with more confidence, and with considerably more information.

Additionally, the experience gained by the high-energy group at MPE in creating the new ROSAT all-sky survey X-ray source catalogue will be integrated in the data reduction analysis and scientific exploration of the forthcoming eROSITA all-sky survey. The eROSITA X-ray survey telescope currently built by MPE will be launched in 2017 to scan the whole sky with even higher precision than ROSAT, reaching 30 times deeper into the universe. One of its main goals is to measure the distribution of about 100,000 galaxy clusters, containing thousands of galaxies each. The 2RXS catalogue is the deepest and most reliable X-ray all-sky survey before eROSITA.


Is there any way to find out what sky surveys are *currently active?* - Astronomy

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This is the most comprehensive X-ray map of the sky ever made

A new X-ray map of the entire sky reveals all kinds of energetic objects and phenomena in the Milky Way and beyond.

Jeremy Sanders, Hermann Brunner and the eSASS team/MPE, Eugene Churazov, Marat Gilfanov (on behalf of IKI)

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A new map of the entire sky, as seen in X-rays, looks deeper into space than any other of its kind.

The map, released June 19, is based on data from the first full scan of the sky made by the eROSITA X-ray telescope onboard the Russian-German SRG spacecraft, which launched in July 2019. The six-month, all-sky survey, which began in December and wrapped up in June, is only the first of eight total sky surveys that eROSITA will perform over the next few years. But this sweep alone cataloged some 1.1 million X-ray sources across the cosmos — just about doubling the number of known X-ray emitters in the universe.

These hot and energetic objects include Milky Way stars and supermassive black holes at the centers of other galaxies, some of which are billions of light-years away and date back to when the universe was just one-tenth of its current age.

eROSITA’s new map reveals objects about four times as faint as could be seen in the last survey of the whole X-ray sky, conducted by the ROSAT space telescope in the 1990s (SN: 6/29/91). The new images “are just spectacular to look at,” says Harvey Tananbaum, an astrophysicist at the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass., not involved in the mission. “You have this tremendous capability of looking at the near and the far … and then, of course, delving in detail to the parts of the images that you’re most interested in.”

Mapping the sky in X-rays

In this map of X-rays emitted by celestial objects across the sky, X-rays are color-coded by energy: Blue signifies the highest energies, followed by green, yellow and red. The red foreground glow comes from hot gas near the solar system, whereas the Milky Way plane appears blue, because gas and dust in the disk absorb all but the most energetic X-rays. The map includes many of the Milky Way’s supernova remnants, including Cassiopeia A and Vela, and a star system called Scorpius X-1, which was the first X-ray source discovered beyond the sun. It also showcases the Milky Way’s star-forming regions, such as the Orion Nebula and the Cygnus Superbubble (SN: 6/29/12), and a mysterious arc of X-rays called the North Polar Spur. Beyond the Milky Way are nearby galaxies, such as the Large Magellanic Cloud, and distant galaxy clusters, such as those contained in the Shapley Supercluster.

Jeremy Sanders, Hermann Brunner and the eSASS team/MPE, Eugene Churazov, Marat Gilfanov (on behalf of IKI)

Jeremy Sanders, Hermann Brunner and the eSASS team/MPE, Eugene Churazov, Marat Gilfanov (on behalf of IKI)

eROSITA can flag potentially interesting X-ray phenomena, such as flares from stars getting shredded by black holes, which other telescopes with narrower fields of view but better vision can then investigate in detail, Tananbaum says. The new map also allows astronomers to probe enigmatic X-ray features, such as a giant arc of radiation above the plane of the Milky Way called the North Polar Spur.

This X-ray feature may be left over from a nearby supernova explosion, or it might be related to the huge blobs of gas on either side of the Milky Way disk, known as Fermi Bubbles, says eROSITA team member Peter Predehl, an X-ray astronomer at the Max Planck Institute for Extraterrestrial Physics in Garching, Germany (SN:6/8/20). eROSITA observations could help the Russian and German teams involved in the mission figure out what the spur is.

About 20 percent of the marks on eROSITA’s map are stars in the Milky Way with intense magnetic fields and hot coronae. Scattered among these are star systems containing neutron stars, black holes and white dwarfs, and remnants of supernova explosions. eROSITA also caught several fleeting bursts from events like stellar collisions.

A supernova remnant called Vela (center, reddish green) is one of the most prominent X-ray sources in the sky. The supernova exploded about 12,000 years ago, about 800 light-years away, and overlaps with two other known supernova remnants: Vela Junior (faint purple ring at bottom left) and Puppis A (blue cloud at top right). All three explosions left behind neutron stars, but only the stars at the centers of Vela and Vela Junior are visible to eROSITA. Peter Predehl and Werner Becker/MPE, Davide Mella

Beyond the Milky Way, most of the X-ray emitters that eROSITA found are supermassive black holes gobbling up matter at the centers of other galaxies (SN: 6/18/20). Such active galactic nuclei comprise 77 percent of the catalog.

Distant clusters of galaxies made up another 2 percent of eROSITA’s haul. These clusters were visible to the telescope thanks to the piping hot gas that fills the space between galaxies in each cluster, which emits an X-ray glow.

The Shapley Supercluster (pictured at right) is composed of many smaller clumps of galaxies about 650 million light-years away. Each blob of X-rays in this picture — which spans 180 million light-years across — is a galaxy cluster that contains hundreds to thousands of galaxies. Zoomed-in images on the left showcase a few of the most massive clusters in the bunch. Esra Bulbul and Jeremy Sanders/MPE

“At present, we probably know about a little less than 8,000 clusters of galaxies,” says Craig Sarazin, an astronomer at the University of Virginia in Charlottesville not involved in the work. But over its four-year mission, eROSITA is expected to find a total of 50,000 to 100,000 clusters. In the first sweep alone, it picked up about 20,000.

That census could give astronomers a much better sense of the sizes and distributions of galaxy clusters over cosmic history, Sarazin says. And this, in turn, may give new insight into features of the universe that govern cluster formation and evolution. That includes the precise amount of invisible, gravitationally binding dark matter out there, and how fast the universe is expanding.

  • The Large Magellanic Cloud is a small galaxy that orbits the Milky Way. This little galactic neighbor emits a diffuse X-ray glow from its interstellar material (red) as well as brighter pinpoints of X-rays from supernova remnants (circles) and binary star systems. Frank Haberl and Chandreyee Maitra/MPE
  • This glowing ring is thought to have originated from a burst of X-rays emitted by the black hole and companion star at its center, which was detected by other telescopes in early 2019. Some of the X-rays in the outburst were scattered into a ring by a dust cloud that the radiation encountered. That scattering delayed the X-rays in the ring on their way to Earth, allowing the eROSITA telescope to detect them during its sky survey in February 2020, one year after the outburst was originally detected. Georg Lamer/Leibniz-Institut für Astrophysik Potsdam, Davide Mella
  • The Carina Nebula, about 7,500 light-years away is one of the largest diffuse nebulae our galaxy. It’s home to many massive, young stars, as well as a smaller nebula called Homunculus. That plume formed when one of the stars in the Eta Carinae system blew up — a celestial explosion famously observed in 1843 (SN: 1/4/10). Manami Sasaki/Dr. Karl Remeis Observatory/FAU, Davide Mella

Closer to home, observations of supernova remnants could help clear up some confusion about the life cycles of big stars, says eROSITA team member Andrea Merloni, an astronomer also at the Max Planck Institute for Extraterrestrial Physics. Past X-ray surveys have found fewer supernova remnants than theorists expect to see, based on how many massive stars they think have blown up over the course of the galaxy’s history. But eROSITA observations are now revealing plumes of debris that could be previously overlooked stellar graves. “Maybe we’ll start balancing this budget between the expected number of supernovae and the ones that we are detecting,” Merloni says.

eROSITA is now beginning its second six-month, all-sky survey. When combined, the telescope’s eight total maps will be able to reveal objects one-fifth as bright as those that could be seen on a single map. That not only allows astronomers to see more X-ray sources in more detail, but track how objects in the X-ray sky are changing over time.

Questions or comments on this article? E-mail us at [email protected]

A version of this article appears in the August 15, 2020 issue of Science News.

Citations

Max Planck Institute for Extraterrestrial Physics. Our deepest view of the X-ray sky. June 19, 2020.

About Maria Temming

Maria Temming is the staff reporter for physical sciences, covering everything from chemistry to computer science and cosmology. She has bachelor's degrees in physics and English, and a master's in science writing.


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