APOD: NGC 4993: The Galactic Home of an... (2017 Oct 23)

[APOD Robot]

NGC 4993: The Galactic Home of an Historic Explosion

Explanation: That reddish dot -- it wasn't there before. It's the dot to the upper left of galaxy NGC 4993's center, do you see it? When scanning the large field of possible locations of an optical counterpart to the unprecedented gravitational wave event GW170817 in August, the appearance of this fading dot quickly became of historic importance. It pinpointed GW170817's exact location, thereby enabling humanity's major telescopes to examine the first ever electromagnetic wave counterpart to a gravitational wave event, an event giving strong evidence of being a short gamma-ray burst kilonova, the element-forming explosion that occurs after two neutron stars merge. The featured image of lenticular galaxy NGC 4993 by Hubble shows the fading dot several days after it was discovered. Analyses, continuing, include the physics of the explosion, what heavy elements formed, the similarity of the speeds of gravitational radiation and light, and calibrating a new method for determining the distance scale of our universe.

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[Ann]

Click to view full size image

Early image of the kilonova.
Photo: ESO/MUSE.

This whole thing is fantastically interesting.

The kilonova was blue when it exploded, like many supernovas, and then faded to red.

Ann

[neufer]

The kilonova was blue when it exploded, like many supernovas, and then faded to red.

As I understand it kilonovas only emit gamma ray light. The emitted gamma rays then had to impinge upon NGC 4993 gas/dust to produce the X-ray, UV, light, IR & radio cascade observed. While the X-rays occurred too quickly to be observed in time the radio may still be observable for months to come. If the kilonova remnant had been a hot heavy new neutron star (rather than a cold light black hole) X-rays should have been observable for some time.

[Guest]

the similarity of the speeds of gravitational radiation and light

I thought the propagation speed (velocity) of both, esp light, were the same regardless of circumstances. Is the speed of light not a constant?

[Boomer12k]

Good to see a regular light image, and shows where it happened.

I wonder what, if any, effect the gravitational wave will/would have on the galaxy...

:---[===] *

[heehaw]

the similarity of the speeds of gravitational radiation and light

I thought the propagation speed (velocity) of both, esp light, were the same regardless of circumstances. Is the speed of light not a constant?

I sure think so, but you never know, and it is good to test, and make sure! Some have doubts about c : http://backreaction.blogspot.com

[neufer]

the similarity of the speeds of gravitational radiation and light

I thought the propagation speed (velocity) of both, esp light, were the same regardless of circumstances.
Is the speed of light not a constant?

I sure think so, but you never know, and it is good to test, and make sure! Some have doubts about c : http://backreaction.blogspot.com

Other than string theory we don't have a quantum theory of gravity.

This measurement puts limits on the possible mass (positive or negative) of a graviton.

[Guest3]

The Galactic Home of a Historic Explosion. NOT AN HISTORIC! An is only used when preceding a word that begins with a vowel sound. For example: An abuse of the English language.

[Wadsworth]

The Galactic Home of a Historic Explosion. NOT AN HISTORIC! An is only used when preceding a word that begins with a vowel sound. For example: An abuse of the English language.

Funny, I read it as 'a' when I read the title, but noticed the author used 'an' shortly thereafter. 'A' is probably best, but either will work.

See:

http://www.betterwritingskills.com/tip-w005.html

[alcor]

A truly marvellous thing to happen. And a big congratulation to all the pro's , who co-operated on all the observations while keeping it secret until last week [though there has been some rumours since late August].

Speaking of dots connecting to make a picture (like the constellation Cassiopeia), here we have *a* dot connecting electromagnetic waves with gravitational waves. As neutron stars are the smallest kind of stars, they if anyone can be considered as dots. You can argue about when a star is truly a star, but I define them as stars so long they emit electromagnetic radiation themselves without any help from the outside.

[dirt farmer]

Information about the system under observation indicates it is 130 million light years away. Are we just now seeing something that happened that long ago?

[neufer]

The Galactic Home of a Historic Explosion. NOT AN HISTORIC! An is only used when preceding a word that begins with a vowel sound. For example: An abuse of the English language.

Funny, I read it as 'a' when I read the title, but noticed the author used 'an' shortly thereafter. 'A' is probably best, but either will work.

See: http://www.betterwritingskills.com/tip-w005.html

<<Historically speaking, according to The American Heritage Dictionary of the English Language (Houghton Mifflin, 1996), "an was once a common variant before words beginning with h in which the first syllable was unstressed; thus, 18th-century authors wrote either a historical or an historical but a history, not an history." By 1926, H. W. Fowler (Modern English Usage) regarded the continued use of an before such words as pedantic. Nowadays it survives primarily before the word "historical"; one rarely encounters a reference to "an hysterectomy" or "an hereditary trait."

Apparently using a or an before the h- of the unstressed syllable of a few words is at the discretion of the speaker or writer. Some people say a historic time, others say an historic time; some say a hotel, others say an hotel; some say a hysterical child, others an hysterical child.>>

A Historical? An Historical?
by Tina Blue

<<The problem is that the h is a bit of a wuss as a consonant. When it occurs in an unaccented syllable and is followed by a vowel, it tends to soften to a vowel-like mushiness.

[N]ow say these words out loud: historian, historical, hysterical, heredity, habitual.

Do you notice how much less, well, pronounced the h is in these words? Now, put a or an before each one (the adjectives should be paired with nouns so you can get the full effect):

• a historian / an historian
  a historical reference / an historical reference
  a historic occasion / an historic occasion
  a hysterical display / an hysterical display
  a hereditary disease / an hereditary disease
  a habitual liar / an habitual liar

Notice that when you use a before the words, you fully aspirate the h, but when you use an, you do not--and the h sound very nearly disappears into the following vowel.

So here's the general rule.

If you speak and write British English, you can probably keep using an before historical, hysterical, habitual, etc. I doubt that you will be challenged by your own countrymen, and if Americans challenge you, just point out that British usage and American usage often differ.

If you are American, you probably should use a rather than an, even in a historic occasion or a historical reference. Most of us are comfortable with a historic occasion, because the word historic has fewer syllables than historical, so the h is more fully pronounced. But if, like me, you are old enough to find a historical reference a tad uncomfortable, then go ahead and say an historical reference. >>

[neufer]

Click to play embedded YouTube video.

Information about the system under observation indicates it is 130 million light years away. Are we just now seeing something that happened that long ago?

We are just now seeing and hearing something
that happened that long ago & far away.

[Jim Leff]

The Galactic Home of a Historic Explosion. NOT AN HISTORIC! An is only used when preceding a word that begins with a vowel sound.

You're clearly wrong on that. It was just so used.

This Yogi Berra-ish oxymoron is the perennial undoing of language pedantry. No one owns language; you may attempt to set rules but no one's compelled to obey. And any rules you'd concoct would formalize freely unfettered discourse, anyway. It's silly (and utterly pointless) to try to freeze it into a snapshot you point to as "correct".

See this definitive article: https://www.theatlantic.com/magazine/ar ... at/376744/ and this Stephen Fry video: https://m.youtube.com/watch?v=J7E-aoXLZGY.

[Jim Leff]

You can argue about when a star is truly a star, but I define them as stars so long they emit electromagnetic radiation themselves without any help from the outside.

By that strict definition, I believe you'd need to consider Jupiter a star.....

[Chris Peterson]

You can argue about when a star is truly a star, but I define them as stars so long they emit electromagnetic radiation themselves without any help from the outside.

By that strict definition, I believe you'd need to consider Jupiter a star.....

Even the Earth would qualify. Not all of its thermal radiation is re-radiated energy from the Sun.

[Chris Peterson]

the similarity of the speeds of gravitational radiation and light

I thought the propagation speed (velocity) of both, esp light, were the same regardless of circumstances. Is the speed of light not a constant?

Theory tells us that they should have the same propagation speed. This is the first time it's been tested, and the two speeds were "similar", as in yielding values that are very close and have overlapping error bars. This supports, but obviously does not prove that the propagation speeds are the same.

[neufer]

I thought the propagation speed (velocity) of both, esp light, were the same regardless of circumstances.

Theory tells us that they should have the same propagation speed. This is the first time it's been tested, and the two speeds were "similar", as in yielding values that are very close and have overlapping error bars. This supports, but obviously does not prove that the propagation speeds are the same.

One could argue that the first indirect observation of gravitational radiation by Hulse & Taylor in 1974
was also a crude indirect measurement of the propagation speed of gravitational waves (c_g)
since the radiation loss itself depends on (c_g)^-6.

[b][color=#0000FF]Orbital decay of PSR B1913+16[/color][/b]

---

<<PSR B1913+16 (also known as PSR J1915+1606, PSR 1913+16, and the Hulse–Taylor binary after its discoverers) is a pulsar (a radiating neutron star) which together with another neutron star is in orbit around a common center of mass, thus forming a binary star system. PSR 1913+16 was the first binary pulsar to be discovered. It was discovered by Russell Alan Hulse and Joseph Hooton Taylor, Jr., of the University of Massachusetts Amherst in 1974.

Using the Arecibo 305m antenna, Hulse and Taylor detected pulsed radio emissions and thus identified the source as a pulsar, a rapidly rotating, highly magnetized neutron star. The neutron star rotates on its axis 17 times per second; thus the pulse period is 59 milliseconds. After timing the radio pulses for some time, Hulse and Taylor noticed that there was a systematic variation in the arrival time of the pulses. Sometimes, the pulses were received a little sooner than expected; sometimes, later than expected. These variations changed in a smooth and repetitive manner, with a period of 7.75 hours. They realized that such behavior is predicted if the pulsar were in a binary orbit with another star, later confirmed to be another neutron star. Pulses from the companion neutron star have not been detected, but this might only be the result of an unfavorable viewing angle.

The pulses from the pulsar arrive 3 seconds earlier at some times relative to others, showing that the pulsar’s orbit is 3 light-seconds across, approximately two-thirds of the diameter of the Sun. Since this is a binary system, the masses of the two neutron stars can be determined, and they are each around 1.4 times the mass of the Sun. Observations have shown that the pulsar’s orbit is gradually contracting, which is generally understood to be evidence for the emission of energy in the form of gravitational waves, as described by Einstein’s theory of general relativity, causing the pulsar to reach periastron slightly early. Also, periastron advances 4° per year in longitude due to the gravitational field (thus the pulsar’s periastron moves as far in a day as Mercury’s moves in a century).>>

[Chris Peterson]

I thought the propagation speed (velocity) of both, esp light, were the same regardless of circumstances.

Theory tells us that they should have the same propagation speed. This is the first time it's been tested, and the two speeds were "similar", as in yielding values that are very close and have overlapping error bars. This supports, but obviously does not prove that the propagation speeds are the same.

One could argue that the first indirect observation of gravitational radiation by Hulse & Taylor in 1974
was also a crude indirect measurement of the propagation speed of gravitational waves (c_g)
since the radiation loss itself depends on (c_g)^-6.

Yeah, and I think there are other indirect measurements, as well. But AFAIK this was the first direct comparison that's been made.

[Jean-Loup Bertaux]

it seems that the link activated when clicking on "what heavy elements formed, " is not appropriate. (identify an obsject in the sky). You may rectify it. There was an APOD image some days ago about the origin of gold element, I assume that it should rather be this one.
JL Bertaux

[sillyworm2]

Would the Explosion have been as easily noticed had this Lenticular Galaxy NOT appear to us FACE on?

[neufer]

Would the Explosion have been as easily noticed had this Lenticular Galaxy NOT appear to us FACE on?

The gravitational waves and most of the gamma rays would have been visible regardless.

The other EM radiation actually required interaction with galactic material
and might well have been hard to notice if it sat in front of the galaxy... or even too far off to the side.

[Boomer12k]

How many more of them are out there....waiting????

:---[===] *

[MarkBour]

Don't blink! It took the waves 130 million years to arrive, and we had only 1.7 seconds in which to observe the matching gamma ray burst?

[neufer]

it seems that the link activated when clicking on "what heavy elements formed, " is not appropriate. (identify an obsject in the sky). You may rectify it. There was an APOD image some days ago about the origin of gold element, I assume that it should rather be this one.

• Blame it on the kilonova with its magic spell
  Blame it on the kilonova that they did so well
  Oh, it all began with just one little dance
  But then it ended up a big romance
  Blame it on the kilonova
  The dance of love
  
  (Now was it the moon?)
  No, no, the kilonova
  (Or the stars above?)
  No, no, the kilonova
  (Now was it the tune?)
  Yeah, yeah, the kilonova
  (The dance of love)