APOD: Fermi's Gamma-Ray Sky (2009 Mar 21)

[Karthik]

http://apod.nasa.gov/apod/ap090321.html

What do the blue areas represent? Lower energies of gamma ray? If so, what could be the sources? Gas? Globular clusters (like 47 Tuc which also appears blue)? Other galaxies?

Thanks

[neufer]

This is truly an amazing instrument:
http://www.nasa.gov/mission_pages/GLAST ... index.html

http://antwrp.gsfc.nasa.gov/apod/ap090321.html

What do the blue areas represent? Lower energies of gamma ray? If so, what could be the sources? Gas? Globular clusters (like 47 Tuc which also appears blue)? Other galaxies?

This seems like a reasonable assumption though 47 Tuc does have red in the high resolution image.

Assuming that it is not simply cosmic ray background noise...
the sources are probably the sources seen in X-ray as seen by Chandra:
http://chandra.harvard.edu/photo/3d_gallery/
http://chandra.harvard.edu/photo/2002/g ... ore.html#1

The X-Ray Sky

Explanation: Launched in 1990, the orbiting ROSAT observatory explored the Universe by viewing the entire sky in x-rays -- photons with about 1,000 times more energy than visible light. Both x-ray brightness and relative energy are represented with red, green, and blue colors indicating three x-ray energy ranges (from lowest to highest). Bright x-ray spots near the galactic plane are within our own Milky Way. The brightest region (right of center) is toward the Vela Pulsar and the Puppis supernova remnant. Bright sources beyond our Galaxy are also apparent, notably the Virgo cluster of galaxies (near top right) and the Large Magellanic Cloud (LMC). Over large areas of the sky a general diffuse background of x-rays dominates. Hot gas in our own Galaxy provides much of this background and gives rise to the grand looping structures visible in the direction of the galactic center. Unresolved extragalactic sources also add to this background, particularly above and below the plane.

[neufer]

http://antwrp.gsfc.nasa.gov/apod/ap090321.html

What do the blue areas represent? Lower energies of gamma ray? If so, what could be the sources? Gas? Globular clusters (like 47 Tuc which also appears blue)? Other galaxies?

<<The dominant process [for diffuse Galactic continuum Gamma-Ray emission] is inverse-Compton emission from scattering of cosmic-ray electrons on the interstellar radiation in a large halo around the Galaxy. However below 3 MeV much of the gamma-ray flux is more likely due to populations of unresolved point sources, such as supernova remnants. The energy spectrum of the diffuse Galactic continuum emission presents several puzzling features (Fig. 2.37). The main one is the excess above 1 GeV relative to the flux expected from interactions of cosmic-ray protons with the interstellar gas, which was expected to dominate in this range. One possibility is that the local directly measured proton spectrum is not typical of the large-scale Galactic environment; if the average spectrum is harder then the gamma-rays could be explained. But we have been able to show that such a scenario would produce a flux of secondary antiprotons and positrons which are in excess of recent measurements. Alternative theories have therefore been sought; most promising is inverse-Compton emission from a hard electron spectrum, the locally measured electron spectrum above 100 GeV (the relevant range for GeV inverse-Compton gamma rays) being strongly affected by energy losses and therefore very inhomogeneous so that local measurements are not necessarily typical. By postulating both a hard electron spectrum and a slightly modified proton spectrum, it is possible to reproduce the observed spectrum above 30 MeV.


Fig. 2.37: Gamma-ray spectrum of the inner Galaxy from COMPTEL, EGRET and OSSE, compared to a model with a hard electron spectrum and modified proton spectrum.

Below 30 MeV the observed emission is again more than expected using the best current estimates of the cosmic-ray electron spectrum including constraints from Galactic synchrotron radiation. The latter precludes the steep electron spectrum which would be required to explain the gamma-ray emission as bremsstrahlung or inverse Compton. For this reason we are led to suggest that this emission is actually dominated by unresolved point-sources, such as supernova remnants or black-hole binaries. Hard X-ray measurements by OSSE, GINGA and RXTE suggest that point-source populations produce the 'diffuse' emission from the Galactic ridge below a few 100 keV, so that the switchover from sources to truly diffuse emission must occur at some energy, and we suspect this to be in the few-MeV range.

Evidence has also accumulated for the existence of a large Galactic gamma-ray halo, which seems to support our recent studies of cosmic-ray composition which suggest that the halo extends between 4 and 10 kpc above the plane. Direct gamma-ray evidence for such a halo is shown in Fig. 2.38, where the EGRET 70-100 MeV latitude distribution is shown, compared to a model with a 4 kpc halo. The good agreement suggests that the halo is here visible via its inverse-Compton emission. This also would mean that the contribution of Galactic emission to the high-latitude intensity is large, so that estimates of the extragalactic diffuse emission will have to be revised accordingly.


Fig. 2.38: Latitude distribution of 70-100 MeV gamma rays measured by EGRET compared with predictions for a model with a halo of height 4 kpc. The dominant Galactic contribution at high latitudes is inverse-Compton emission. The horizontal line is the intensity of the extragalactic background.

[neufer]

3C 454.3 is a quasar/blazar located off the galactic plane. It lies some 7.1 billion light-years away in Pegasus and is currently undergoing a flaring episode that makes it especially bright, especially in the gamma-ray part of the spectrum.

--------------------------------------------
. Julius Caesar > Act II, scene II
.
CALPURNIA: When beggars die, there are no comets seen;
. The heavens themselves *BLAZE* forth the death of princes.

Blaze Starr (born 1932) is a former American stripper and burlesque star. Her vivacious presence and inventive use of stage props earned her the nickname "The Hottest Blaze in BurLesQUE". She was also notorious for her affair with Louisiana governor Earl Long.

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<<Many of the brighter blazars were first identified as irregular variable stars in our own galaxy. These blazars, like genuine irregular variable stars, changed in brightness on periods of days or years, but with no pattern.

The early development of radio astronomy had shown that there are numerous bright radio sources in the sky. By the end of the 1950s the resolution of radio telescopes was sufficient to be able to identify specific radio sources with optical counterparts, leading to the discovery of quasars. Blazars were highly represented among these early quasars, and indeed the first redshift was found for 3C 273 — a highly variable quasar which is also a blazar.

In 1968 a similar connection between the "variable star" BL LACertae and a powerful radio source VRO 42.22.01 was made. BL LACertae shows many of the characteristics of quasars, but the optical spectrum was devoid of the spectral lines used to determine redshift. Faint indications of an underlying galaxy — proof that BL LACertae was not a star — was found in 1974.

The extragalactic nature of BL LACertae was not a surprise. In 1972 a few variable optical and radio sources were grouped together and proposed as a new class of galaxy: BL LACertae-type objects. This terminology was soon shortened to "BL LACertae object," "BL LAC object," or simply "BL LAC."

As of 2003, a few hundred BL LAC objects are known.

--------------------------------------------
*BLAZE*, n. [OE. blase, AS. blæse, blase; akin to OHG. blass whitish, G. blass pale,
____ MHG. blas torch, Icel. blys torch; perh. fr. the same root as E. blast.]

1. A stream of gas or vapor emitting light and heat in the process of combustion; a bright flame. To heaven the *BLAZE* uprolled." Croly.

2. Intense, direct light accompanied with heat; as, to seek shelter from the *BLAZE* of the sun.

. O dark, dark, dark, amid the *BLAZE* of noon! Milton.

3. A bursting out, or active display of any quality; an outburst; a brilliant display. Fierce *BLAZE* of riot." His *BLAZE* of wrath." Shak.

. For what is glory but the *BLAZE* of fame? Milton.

4. A white spot on the forehead of a horse.

The Memoirs of Sherlock Holmes - Silver *BLAZE*

5. A spot made on trees by chipping off a piece of the bark, usually as a surveyor's mark.
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*BLAZER*: A boatman's jacket. Properly and originally applied to the Johnian crew (Camb.), whose boat jackets are the brightest possible scarlet.
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H.G. Wells (1866–1946). The War of the Worlds. 1898.
Chapter Fourteen
In London

One or two trains came in from Richmond, Putney, and Kingston, containing people who had gone out for a day’s boating and found the locks closed and a feeling of panic in the air. A man in a blue and white *BLAZER* addressed my brother, full of strange tidings.
“There’s hosts of people driving into Kingston in traps and carts and things, with boxes of valuables and all that,” he said. “They come from Molesey and Weybridge and Walton, and they say there’s been guns heard at Chertsey, heavy firing, and that mounted soldiers have told them to get off at once because the Martians are coming. We heard guns firing at Hampton Court station, but we thought it was thunder. What the dickens does it all mean? The Martians can’t get out of their pit, can they?”
-------------------------------------------------------------------
*BLAZER*, n. One who spreads reports or *BLAZES* matters abroad.
...........................................................

• _THE FAERY QUEEN_ bY EDMUND SPENSER.
  
  Utterers of secrets he from thence debarr'd.
  Babblers of folly, and *BLAZERS* of crime :

<<A blazar is a very compact and highly variable energy source associated with a presumed supermassive black hole at the center of a host galaxy. Blazars are among the most violent phenomena in the universe.

Blazars are members of a larger group of active galaxies, also termed active galactic nuclei (AGN). However, blazars are not a homogeneous group and can be divided into two: highly variable quasars, sometimes called Optically Violent Variable (OVV) quasars (these are a small subset of all quasars) and BL LACertae objects ("BL LAC objects" or simply "BL LACs"). A few rare objects may be "intermediate blazars" that appear to have a mixture of properties from both OVV quasars and BL LAC objects. The name "blazar" was originally coined in 1978 by astronomer Ed Spiegel to denote the combination of these two classes.

Blazars are AGN with a relativistic jet that is pointing in the general direction of the Earth. We observe "down" the jet, or nearly so, and this accounts for the rapid variability and compact features of both types of blazars. Many blazars have apparent superluminal features within the first few parsecs of their jets, probably due to relativistic shock fronts.

The generally accepted picture is that OVV quasars are intrinsically powerful radio galaxies while BL LAC objects are intrinsically weak radio galaxies. In both cases the host galaxies are giant ellipticals.

------------------------------------

[neufer]

http://antwrp.gsfc.nasa.gov/apod/ap080520.html
http://antwrp.gsfc.nasa.gov/apod/ap050725.html
http://antwrp.gsfc.nasa.gov/apod/ap051208.html
http://antwrp.gsfc.nasa.gov/apod/ap080822.html

NGC 1275, Perseus A, PGC 12429, UGC 2669

http://tinyurl.com/d5btxc
Distance 235 Mly
Apparent dimensions (V) 2′.2 × 1′.7
Apparent magnitude (V) 12.6

<<NGC 1275 (also known as Perseus A) is a type 1.5 Seyfert galaxy located around 235 million light-years away in the direction of the constellation Perseus. NGC 1275 corresponds to the radio galaxy Perseus A and is situated near the centre of the large Perseus Cluster of galaxies.

NGC 1275 consists of two galaxies, the central CD Galaxy in the Perseus Cluster, and the so-called "high velocity system" (HVS) which lies in front of it. The HVS is moving at 3000 km/s towards the dominant system, and is believed to be merging with the Perseus Cluster. The HVS is not affecting the CD Galaxy as it lies at least 200 thousand light years from it. The central cluster galaxy contains a massive network of spectral line emitting filaments, which appear to be being dragged out by rising bubbles of relativistic plasma generated by the central active galactic nucleus. Long gaseous filaments made up of threads of gas stretch out beyond the galaxy, into the multimillion-degree, X-ray–emitting gas that fills the cluster. The amount of gas contained in a typical thread is round one million times the mass of our own Sun. They are only 200 light-years wide, are often very straight, and extend for up to 20,000 light-years.

The existence of the filaments poses a problem. As they are much cooler than the surrounding intergalactic cloud, how have they persisted for perhaps 100 million years? Why haven’t they warmed, dissipated or collapsed to form stars? One possibility is that weak magnetic fields (about one-ten-thousandth the strength of Earth’s field) exert enough force on the ions within the threads to keep them together.>>

[iamlucky13]

What do the blue areas represent? Lower energies of gamma ray? If so, what could be the sources? Gas? Globular clusters (like 47 Tuc which also appears blue)? Other galaxies?

The short translation of neufer's answer is high energy cosmic rays, mostly electrons and protons from beyond the galaxy moving at very near the speed of light, interacting with radiation and probably gas and dust as well from within our galaxy.

[neufer]

http://apod.nasa.gov/apod/ap070213.html

The Vela supernova remnant is a supernova remnant in the southern constellation Vela. Its source supernova exploded approximately 11,000-12,300 years ago (and was about 800 light years away). The association of the Vela supernova remnant with the Vela pulsar, made by astronomers at the University of Sydney in 1968, was direct observational proof that supernovae form neutron stars.

The Vela supernova remnant includes NGC 2736. It also overlaps the Puppis Supernova Remnant, which is four times more distant. Both the Puppis and Vela remnants are among the largest and brightest features in the x-ray sky.

The Vela supernova remnant (SNR) is one of the closest known to us. The Geminga pulsar is closer (and also resulted from a supernova), and in 1998 another supernova remnant was discovered, RX_J0822-4300, which from our point of view appears to be contained in the southeastern part of the Vela remnant. One estimate of its distance puts it only 200 parsecs away (about 650 ly), closer than the Vela remnant, and surprisingly as well, it seems to have exploded much more recently (in the last thousand years or so) because it is still radiating gamma rays from the decay of titanium-44. This remnant was not seen earlier because in most wavelengths it is lost in the image of the Vela remnant.

------------------------------
GEMINGA : "it's not there"
GIN GAME

http://www.esa.int/esaSC/SEMLY9NQS7E_index_1.html
------------------------------

<<Geminga, is a neutron star approximately 552 light-years away from the Sun in the constellation Gemini. Its name is both a contraction of "Gemini gamma-ray source", and at the same time it also means "it's not there" in the Lombard dialect of Milan (generally spelt gh'è minga and pronounced [ɡ ɛ 'mĩŋɡa]).

The nature of Geminga was quite unknown for 20 years after its discovery by NASA's Second Small Astronomy Satellite (SAS-2). Finally, in March 1991 the ROSAT satellite detected a periodicity of 0.237 seconds in soft x-ray emission. Thus, it is supposed that Geminga is a sort of neutron star: the decaying core of a behemoth star that went supernova about 300,000 years ago.

This nearby explosion may be responsible for the low density of the interstellar medium in the immediate vicinity of the Solar System. This low-density area is known as the Local Bubble. Possible evidence for this includes findings by the Arecibo observatory that local micrometre-sized interstellar meteor particles appear to originate from its direction.

Geminga was the first example of an unidentified gamma-ray source, a source which could not be associated with any objects known at other wavelength. It was first detected as a significant excess of gamma-rays over the expected background of diffuse Galactic emission, by the SAS-2 satellite (Fictel et al. 1975) and subsequently by the COS-B satellite. The SAS-2 group reported a pulsation in the gamma-ray signal, with period approximately 59 s, although the limited number of detected gamma-rays (121 over a period of four months) led them to conclude that the pulsation was not statistically compelling. Due to the limited angular resolution of the instrument (approximately 2.5° at 100MeV) and the small number of gamma-rays detected, the exact location of the source was uncertain, constrained only to be within a relatively large "error region". At the time of detection, four weak radio sources were known within this region, two supernova remnants bordered it and a known satellite galaxy to the Milky Way lay nearby. None of these known sources were convincing associations to the gamma-ray source, and the SAS-2 team suggested that an undiscovered radio-pulsar was the most likely progenitor.

Despite the investment of a significant amount of observation time, the source remained unidentified through the COS-B era; their data did, however, rule out the claimed 59 s pulsation. Many claims were made about the source during this time, but its nature remained a mystery until the identification of a candidate source by the Einstein x-ray satellite, 1E 0630+178. The characteristics of the x-ray source were unique: large x-ray to optical luminosity, no radio emission detected by the sensitive VLA instrument, point-like emission in the Einstein imager and an estimated distance of approximately 100 pc, placing it within the Galaxy. An association between the gamma-ray and x-ray sources was not conclusively made until the ROSAT x-ray imager detected a 237 ms pulsation, which was also seen in gamma-rays by the EGRET instrument and retrospectively in the COS-B and SAS-2 data.

Geminga is the first example of a radio-quiet pulsar, and serves as an illustration of the difficulty of associating gamma-ray emission with objects known at other wavelengths: either no credible object is detected in the error region of the gamma-ray source, or a number are present and some characteristic of the gamma-ray source, such as periodicity or variability, must be identified in one of the prospective candidates (or vice-versa as in the case of Geminga).

In 1997, John Mattox et al. claimed to have discovered a planet orbiting Geminga by gamma-ray timing of Geminga. This hypothesized planet, Geminga b, was thought to orbit about 3.3 AU from Geminga in a 5.1 year orbit. With a mass of 1.7 earths, Geminga b would be a terrestrial planet. However, this discovery is now doubtful because recent analysis of the data indicates that the detected timing changes were due to signal noise, not a planet.>>

[DavidLeodis]

I thought the movement path of the Sun from August 4th 2008 to October 30th 2008 is fascinating. It (and thus we on Earth) moved a long way in that period! Well its looks a long way to me even though it may be a tiny movement in astronomical distances.

[BMAONE23]

I thought the movement path of the Sun from August 4th 2008 to October 30th 2008 is fascinating. It (and thus we on Earth) moved a long way in that period! Well its looks a long way to me even though it may be a tiny movement in astronomical distances.

Yes the solar procession against the annual sky is interesting. It would surely form an eventual figure eight analemma since, to our perspective, it crosses that galactic center in December.

[neufer]

I thought the movement path of the Sun from August 4th 2008 to October 30th 2008 is fascinating. It (and thus we on Earth) moved a long way in that period! Well its looks a long way to me even though it may be a tiny movement in astronomical distances.

Yes the solar procession against the annual sky is interesting. It would surely form an eventual figure eight analemma since, to our perspective, it crosses that galactic center in December.

A figure eight analemma plus the CONSTANT march of time to make a quasi-sinusoidal ecliptic.

[peter_from_nyc]

The picture is dazzling, but confusing. I wish I could see it in 3-D, or at least conceptualize it that way.

Like, it is puzzling to me that the sun moved, but the analemma
http://en.wikipedia.org/wiki/Analemma
seemed to bring a little more sense into it. I guess I'd like to see a box or a sphere, and be able to interact with it, so that I can see where the different elements are, including the Sun and the Milky Way. And then have the beautiful blue Fermi's Gamma-Ray Sky (21/March/2009) embedded in it, or more likely wrapped around it.

I have trouble conceptualizing 2-D maps (like finding my way around Wall Street is tough for me, but on a metaphorical level: I guess it's been tough for the country lately ). So a 3-D map in 2-D, or some sort of projection which this is (mapmaking is a strange art) seems a bit out of my league.

Any suggestions?

[BMAONE23]

What it (the analemma) really represents is the view of the the sun against the background sky and how it changes relative to the view from earth as we orbit the sun thruought the year. Consider this: relative to our perspective, in December, the sun would be located roughly over the "Unidentified" point at the galactic central band but in February, it would be located near "3C 454.3 and by April it will have reverse arced back toward "NGC 1275". But it is all relative to our perspective on Earth during our annual orbit.

[neufer]

What it (the analemma) really represents is the view of the the sun against the background sky and how it changes relative to the view from earth as we orbit the sun throughout the year.

The analemma represents is the view of the the sun against the background sky ONCE EVERY 24 HOURS.
http://antwrp.gsfc.nasa.gov/apod/ap081221.html

The ecliptic is the apparent path that the Sun traces out either against the celestial sphere or
against the background sky ONCE EVERY 23 hours 56 minutes & 4.090530833 seconds.
http://en.wikipedia.org/wiki/Sidereal_day
http://en.wikipedia.org/wiki/Ecliptic

[Frenchy]

Does this mean that the sun's motion is moving retrograde with respect to the galaxy?

[BMAONE23]

I believe I was incorrect in describing the completed path as a figure eight analemma. The path should actually form a sine wave of sorts. I forgot that the view presented was the entire sky and that the suns path should cross from one end of the image to the other to complete its path.

[neufer]

Fermi has detected more than 1,000 gamma-ray sources. Half are associated with active galaxies called blazars. This movie shows one year of blazar activity, starting on Aug. 4, 2008, around the galactic north pole. This region includes the constellations Ursa Major, Virgo, Leo, Boötes and Coma Berenices. Credit: NASA/DOE/Fermi LAT Collaboration>>