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Growth rate of MWBH due to photon flux in W or fluence in W/m^2 at



 
 
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  #1  
Old September 26th 16, 07:55 AM posted to sci.astro.research
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Default Growth rate of MWBH due to photon flux in W or fluence in W/m^2 at

Hello,

I find a lot of papers dealing with black hole growth rate due to
accretion. But haven't been able to find anything on growth due
to photon consumption.

A paper 2013 by Victor Debattista, of the University of Central
Lancashire in England estimated the growth rate for MW BH at 1 M_sun
/ 3000 years. This works out to a consumption of E_dot = 1.89E36
W average. but this is total "accretion" rate which includes matter,
gas, stars, objects.... mass energy plus photon energy. I didn't
see explicit mention of photon mass gain.

I'm trying to determine the growth of the MW BH due solely to photon
consumption.

Alternately, knowing radius and area, I could compute the value if
I knew the fluence near the core of the MW, "What is the fluence
of photon energy near the center of the MW, in Watts per square
meter?"

Any assistance / direction appreciated,

rt

[[Mod. note -- I educated-guess that the photon flux will be many
orders of magnitude less than the E=mc^2 equivalent of the infalling
matter flux. But specific numbers would be welcome.
-- jt]]
  #2  
Old September 27th 16, 05:04 AM posted to sci.astro.research
Eric Flesch
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Default Growth rate of MWBH due to photon flux in W or fluence in W/m^2 at

Erm, photons are massless.

[[Mod. note -- I htink the question was asking about mass-energy, and
photons can certainly carry that.
-- jt]]
  #3  
Old October 4th 16, 05:04 PM posted to sci.astro.research
Martin Hardcastle
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Default Growth rate of MWBH due to photon flux in W or fluence in W/m^2 at

In article ,
wrote:
Alternately, knowing radius and area, I could compute the value if
I knew the fluence near the core of the MW, "What is the fluence
of photon energy near the center of the MW, in Watts per square
meter?"


[[Mod. note -- I educated-guess that the photon flux will be many
orders of magnitude less than the E=mc^2 equivalent of the infalling
matter flux. But specific numbers would be welcome.
-- jt]]


OK, back of the envelope: the energy density in starlight in the MW is
~ 10^-13 J/m^3: this is the dominant photon field (the CMB is about
half this). So the fluence is Uc/4 ~ 10^-5 W/m^2. For a 4 x 10^6 solar
mass black hole, R_S = 10^10 m, so the energy being added is ~ 1 x
10^16 W, or the equivalent of the accretion of 0.1 kg/s, or, in SMBH
terms, peanuts. All numbers to order of magnitude only. Assuming
I've not screwed up somewhere, the main error in this calculation is
taking a typical starlight energy density in the MW to represent the
energy density in the Galactic centre, but that won't get this number
more than ~ 1 order of magnitude higher.

Martin
--
Martin Hardcastle
School of Physics, Astronomy and Mathematics, University of Hertfordshire, UK
Please replace the xxx.xxx.xxx in the header with herts.ac.uk to mail me

  #4  
Old October 11th 16, 08:39 PM posted to sci.astro.research
Steve Willner
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Default Growth rate of MWBH due to photon flux in W or fluence in W/m^2 at

In article 20161004070749.GA495@sirius,
Martin Hardcastle writes:
OK, back of the envelope: the energy density in starlight in the MW is
~ 10^-13 J/m^3: this is the dominant photon field (the CMB is about
half this).


You could do a better job by looking up the stellar density near the
Galactic center and assuming a mass-to-light ratio. M/L will be
somewhere near 1 in solar units, but the stellar density is much
higher (maybe a couple or three orders of magnitude, but I'm just
guessing) than near the Sun's location in the disk.

So the fluence is Uc/4 ~ 10^-5 W/m^2.


That quantity is a "flux," which is in fact what you want. "Fluence"
is flux integrated over time, so its units would be J/m^2. That
would give the energy absorbed by the black hole over time rather
than the current accretion rate.

--
Help keep our newsgroup healthy; please don't feed the trolls.
Steve Willner Phone 617-495-7123
Cambridge, MA 02138 USA
  #5  
Old October 12th 16, 07:43 AM posted to sci.astro.research
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Default Growth rate of MWBH due to photon flux in W or fluence in W/m^2 at

On Tuesday, October 11, 2016 at 12:39:59 PM UTC-7, Steve Willner wrote:
In article 20161004070749.GA495@sirius,
Martin Hardcastle m.j.hardcastle writes:
OK, back of the envelope: the energy density in starlight in the MW is
~ 10^-13 J/m^3: this is the dominant photon field (the CMB is about
half this).


You could do a better job by looking up the stellar density near the
Galactic center and assuming a mass-to-light ratio. M/L will be
somewhere near 1 in solar units,


Thanks....
Best I found for stellar density is "several" hundred thousand stars per
cubic pc "very near" the center (so far anyway).

http://abyss.uoregon.edu/~js/ast122/lectures/lec26.html

using 0.1 pc for "very near" and 300,000 stars per pc^3 for "several
hundred thousand" as guesses, I get Energy Density = 1.22E-11 J/m^3

This is energy from stars L within sphere 0.1pc radius being emitted
outward (and in all directions) per second, divided by A at 0.1pc radius

This is about 100x Martin's previous envelope estimate, so pretty close,
and increases energy flow into MWBH to about 10kg/s equivalent mass flow
rate.

This is also 1.22E-11 Pa, which is close to values estimated for
cosmological dark energy.

Wiki gives two values, which when converted to energy yield 6E-10 and
9E-11, J/m^3 = Pa so the value I get (1.22E-11Pa) is about in between.

Sound about right?

Thanks for the help.

Ross

 




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