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Craig Markwardt wrote:
> "sean" <jaymoseley@xxxxxxxxxxx> writes:
> > Craig Markwardt wrote:
> > > "sean" <jaymoseley@xxxxxxxxxxx> writes:
> > > ...
> > > > THe other question is would it be the case that where lets say ,as
> > > > above ,the V filter was on the redward side of the L apha break and
> > > > visible at 19mag is it still possible that if one were to go much
> > > > deeper in U would there eventually be a detection but just at a much
> > > > weaker mag? The reason I ask this is theres a gcn example where the
> > > > Lyman break is 484nm and redward of B(almost) and V( definitely) yet
> > > > there are detections in B and U bands which shouldnt be the case if the
> > > > break were at 484. Thats why I was wondering if there are certain
> > > > circumstances where the rule is broken.
> > >
> > >
> > > The absorption is not necessarily 100%, but rather the overall
> > > continuum level changes quite dramatically (and often Ly-alpha is seen
> > > in emission from the emitting galaxy).
> > > Example: Steidel et al. 2003, ApJ, 592, 728 (astro-ph/0305378)
> > Thanks for the info. Unfortunately Im unable to download the above
> > paper to my pc for some reason. Ill try again from an outside pc.
> > Heres the gcn example Im citing from that Im trying to figure out...
> >
> > TITLE: GCN GRB OBSERVATION REPORT
> > NUMBER: 5243
> > SUBJECT: GCN 060607: Swift-UVOT obswervations.
> > DATE: 06/06/07 23:43:44 GMT
> > FROM: Samantha Oates at MSSL <sro@xxxxxxxxxxxxxx>
> >
> > S. R. Oates (UCL-MSSL), A. J. Blustin (UCL-MSSL), H. Z. Ziaeepour
> > (UCL-MSSL) report on behalf of the Swift/UVOT team:
> >
> > The Swift/UVOT began observing the field of GRB 060607 at 05:15:12 on
> > 2006-06-06, 179 s after the BAT trigger (Ziaeepour et al., GCN 5233).
> > An
> > optical counterpart was detected in the V-band at a position (RA,DEC)
> > (21h
> > 58m 50.4s, -22d 29' 47.4'') (J2000) to within 0.5 arcsec. The optical
> > afterglow was detected in white, V, B and U filters. The non-detections
> > in
> > the UV bands is consistent with reported redshift z=3 (Ledoux et al.,
> > GCN 5237).
> >
> > The photometry results are given for the 7 filters below:
> >
> > Filter T_Start(s) Exposure(s) Magnitude/3sig_UL
> >
> > V 179 486.9 15.35 +/-0.09
> > V 7880 127.1 18.16 +/-0.33
> > B 5833 194.9 19.13 +/-0.18
> > B 7265 192.5 19.48 +/-0.28
> > U 7061 187.8 18.87 +/-0.34
> > W1 5429 1257 18.83 (3 sigma UL)
> > M2 6651 1075 20.47 (3 sigma UL)
> > W2 1843 383.1 19.97 (3 sigma UL)
> > WHITE 74 97.4 15.98 +/-0.15
> > WHITE 6037 188.4 18.89 +/-0.17
> >
> > The values quoted above are not corrected for the expected Galactic
> > extinction E(B-V)=0.03
> >
> >
> > Theres still some things that I dont seem clear about this example
> > as opposed to others that follow the rule.
> > (I assume its not because of galactic contamination as there
> > is no mention in the gcns of that burst having any visible
> > host galaxy.)
> >
> > I take it from your reply that the reason it is visible in
> > U (even though bandwidth of U at 300nm-400nm is still at best
> > 84 nm blueward of the Lyman break in this case), is because
> > the continuum flux level of the optical afterglow
> > on the blueward side of the break is still so bright that in
> > U the afterglow is still visible?
>
> Possibly. The attenuation is not 100% (e.g. Steidel et al). At still
> shorter wavelengths one runs into Ly-beta absorption (rest frame ~1060
> AA), Ly-gamma, and so on. Also, depending on the emission spectrum,
> there may be a Lyman alpha emission line in the emitter's rest frame.
>
> > How is this calculated? Is it overall flux of the
> > afterglow in all other filters maybe.
>
> How is it calculated? It's not. It's measured. One would need to
> know the underlying spectrum to "calculate" the individual filter
> response. If you had read the Ledoux et al circular, GCN #5237, you
> would find,
>
> ... "A preliminary analysis shows the Lya forest up to a rather weak
> Lya line, as well as a CIV doublet at a redshift of z=3.082, the
> presumed GRB redshift. We also detect two probable intervening
> Damped Lya systems (DLAs) at z=3.050 and z=2.937. ...
>
> So yes, there is a Ly-alpha emission line, and yes there is a Ly-alpha
> forest which attenuates the continuum. To predict more about the
> broadband response, one would require the detailed spectrum.
>
> > Even still there are discrepencies that dont
> > seem to fit. For instance if a U detection blueward of
> > the L break were still possible as you say because the continuum
> > flux level is still so high then how does
> > that account for the fact that U is brighter than B? Shouldnt
> > B being closer to the L break be the brighter detection with the
> > steep falloff being evident in a fainter U detection?
>
> It is not legitimate to compare magnitudes in different broadband
> filters. The filter bandwidths and system zero points can be
> different, thus it is not possible to simple compare numerical
> magnitudes and say which is "brighter." [ One would need to convert
> to flux, probably assuming a template spectrum. ]
>
>
> > Also this burst seems similar to others where the roughly the same
> > levels of mag in all filters are observed. What is it
> > about 060607 that makes a detection in U , blueward of
> > the break possible where in other similar bursts a non
> > detection in U gives the same redshift?
>
> Claims based on "seeming" are suspect. It would be more appropriate
> to make claims based on real numbers.
I think you misunderstand me . I wasnt making any claim. I was
just asking how it was possible to get a U and B detection where
normally one expects one wouldnt, due to these filterband
detections being on the blueward side of the L-a break. And Joe
concurred that this is the usual assumption.
However I agree that, although you havent explained how this is
possible in the example of 060607, you *have* given a general
indication of how it is possible to interpret a detection in U,B
and V as being consistent with any redshift >z=1.7 which I wasnt
aware of until now.Thanks. So although I question the validity
of a z=3 interpretation until such time that they supply
the detail of the calculations, I can understand how its within
the realm of possibilty now.
Unfortunately as you know already, Im not familar with the
calculations needed to double check the photomatric interpretation
for 060607 nor do I have access to the spectral info needed .
(Ive tried and its not available which makes me wonder how the
Swift team got access to it only 12 hours after the spectral
observation was made seeing as I cant get it more than a week
later).
As you have suggested, I can see now that gcn5237 mentions that
the L break is weak which must also interpret as weak suppression
of the flux blueward of that break which in turn could allow some
detection blueward, hence the Swift photometric redshift gcn of z=3.
But if I can be the devils advocate ,..Looking at it from my
knowledge so far of filters etc it does seem a close run
interpretation as B is very close in mag to U at roughly the
same time
yet W which has an upper limit at 18.8 doesnt supply a detection,
even though in fact it was observed much earlier when the afterglow
in all bands should be even brighter.
If I could present it in the only way I know... at ~7000 U-B is
18.87-19.48, and if center points of B-U-W filter ranges are
430-330 and 220 then using these parameters I would interpolate
that at 7000 sec W could be ~ 18 mag? Thats assuming the increase
from B-U of .4 mag over 100nm would give a increase of .4 mag
over the next 100nm (from U-W) .Yet at 5000 seconds where B is
even brighter again by.35mag, W should be brighter still than
my calculated 18mag or at the very least >the U mag of 18.87
3000 seconds later. Yet we get no detection in W up to 18.83.
In other words, shouldnt W have given a detection >18.83 at
5000 sec?
Although Im sure Ive got the general trend correct Im sure youll
disagree with all the details of my calculations above but at
least it must be true that assuming a straight line power law
continuum,if Umag > Bmag at 7000, then Wmag has to be >Umag at
7000 sec?
Sean
V 179 486.9 15.35 +/-0.09
V 7880 127.1 18.16 +/-0.33
B 5833 194.9 19.13 +/-0.18
B 7265 192.5 19.48 +/-0.28
U 7061 187.8 18.87 +/-0.34
W1 5429 1257 18.83 (3 sigma UL)
M2 6651 1075 20.47 (3 sigma UL)
W2 1843 383.1 19.97 (3 sigma UL)
WHITE 74 97.4 15.98 +/-0.15
WHITE 6037 188.4 18.89 +/-0.17
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