Visual brightness measurements of SN 1987A are being obtained with the IUE acquisition camera and star tracker Fine Error Sensor, (FES) as part of each ultraviolet observation. Due to IUE's around-the-clock operation and lack of clouds, the FES record of SN 1987A provides the most complete set of visual photometry of the supernova made by any single instrument. The data illustrate the photometric limitations of the FES (+ or - 0.03 mag). Use of differential photometric methods are recommended for IUE observers desiring accurate photometry from FES measurements made during their observing shifts

Sonneborn, G.

English

NASA Technical Reports Server

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N89- 1069 8
The Complete Visual Light Curve of SN 1987A:
Thirteen Months of FES Observations
G. Sonneholli
kstloiIonly Plogrluns, Coniputcr Sciences Corporation:
and IUE Observatory, (',od<l<lld Space Fhghl. (._enl, or
A BSTRACT
Visual brightness measurements of SN 1987A arc" hc'hlg
oht_ined with the IUE acquisition cairleret and star tr<tcker
{Fine Error Sensor, or Fh]S) as part of each ultravioh.'t ob-
servation. Due to IUE's around-the-clock operation and
lack of clouds, the FES record of SN 1987A is perhaps the
most conlplete sel, of visual photometry of the supeinow."
made by any single instrument. Tllcse daA.,t ilhlstr_g.c the
photoinetric limitations of the FES (:I: 0.03 lliag). Use of
differential ptiotometric methods are reconmlended for IUE
observers desiring accurate photomc'try from I:ES measure-
ments nlade during their ohserviHg shifts.
l(eywords: Supernovae, visual light curves, differential pho-
tOllletly
1. 1N'I'II.OI)UCTION
The IIJE Fine Error Sensor plays an integral role in
the target acquisition for each spectral observation made by
the statellite's sl)ectrograph/can-,zra systems (Refs. I and
2). Prior to each exposure of the supernova, as for virtu
all 3" all exposures of objects 14 nlag < V < -1 mag, the
target's center of light is positioned at a standard position
(the "l{efel'ence Point") ill the telescope field of view. From
the Reference Point canned slews are used to acculately
position the target ill the spectrograph aperture. The' pro-
cess of positioning the supnrnowt's center of light produces
mean brightness nleasul'ement. Tllese IcES counts have
been recorded for each observation of SN 1987A at CSFC
I,t
t%
_- 5
7
C9
<
2;
t.d
D
1987 1988
xMAR JAN 1 MAR 1
J
I
1 MAY I JUL 1 SEP 1 NOV 1
/'_r/¢_ i i i i
SN 1987A
FES Visual Light Curve (4000 - 7000 A)
__ -_-__+_.
%+
++
+1-%
++ %
+%
MAY
+
8 I I I I I I I I
0 50 100 150 200 250 ,300 350 400 450
DAYS AFTER OUTBURST
Figure 1 IUE"V" magnitude hghl curve for the SN 1987A out-
burst throligl_ 18 April 1{)88
A Decade of UV Astronomy with IUE, Prec. Celebratory. Sympostum, GSFC, Greenbelt, USA, 12-15 Aprd 1988, ESA SP-281, Vol. 1 (June 1988)
https://ntrs.nasa.gov/search.jsp?R=19890001327 2020-03-20T05:58:11+00:00Z
112 G SONNEBORN
to produce a database with over 520 entries as of 18 April
1988. The complete IUE light curve for SN 1987A is shown
in Figure 1, which includes all corrections discussed below.
The FES is an image disector with an unfiltered $20
photocathode, and therefore has a very broad spectral re-
sponse (~ 4000-7000 /_, ). Despite its moderate photometric
precision it has proved to be very useful because it is avail-
able 24 hours per day and is not vulnerable to bad weather
or seeing conditions.
2. OBSERVATIONS
The first FES measurements of SN 1987A were made
on 24.812 February 1987, about 1.5 days after the outburst
began. The time of outburst is taken to coincide with the
Kamiokande-II and IMB neutrino detections (23.316 Febru-
ary 1987, Refs. 3, 4). SN 1987A was observed daily by IUE
from 24 February through late May 1987, approximately
every other day during June, July, and August, and with
decreasing frequency since then. Observations are now ob-
tained every 7-10 days. During each observing period FES
counts for SN 1987A were measured with the supernova ac-
curately positioned at the Reference Point as part of the
normal process of obtaining UV spectra.
The FEB was used in the Underlap Mode (used for
my < 4.8) for all observations of SN 1987A prior to mid-
July. Measurements were made in both Underlap and Over-
lap modes through March 1988, and only in Overlap there-
after. Collection of FES data in both scan modes was con-
tinued as long as possible so as to build up a unique data
base of FES measurements which will improve the FES pho-
tometric calibration. All data to date have been in the Fast
Track scan rate.
The FES calibration of Imhoff (Ref. 5) was used to
derive FES V magnitudes, which includes a correction for
time-dependent changes in the FEB sensitivity. No correc-
tion has been made for the small (B-V) dependence in the
calibration. A significant part of the loss of sensitivity ap-
pears to be due to a "fatigue spot" on the FES aperture
plate located precisely at the Reference Point. The origin
of this fatigue spot is unknown, but might plausibly arise
from long-term damage to the Reference Point's location of
the photocathode due to 10 years of accumulated target ac-
qusition activity. This effect accounts for tile majority of
the sensitivity change (Ref. 6)
Tile Underlap data for SN 1987A was found to have low
scatter (+ 0.03 mag). However, the Overlap data obtained
at tile same time had much larger scatter, even though every
precaution was taken to ensure that the target was properly
centered at the Reference Point. Beginning m early Octo-
ber 1987 a star of similar brightness and color close to the
supernova was observed with the FES within an hour of
the SN 1987A measurements so that &fferential methods
could be used. The comparison star was HD 45669 (1{5 11I,
V=5.56, (B-V)=1.51). A moderately red star was chosen
as an approximation to the general spectral dmtribution of
SN 1987A , even though there are significant differences be-
tween a K star and the supernova's spectrurn (most notably
the strong emission hnes).
Figure 2 shows the variation in FES counts for liD 45669
from October 1987 to April 1988. Two points should be
noted: First, there is a long-term decline m mean brightness
of about 7-8%. During this time period, the supernova's
FEB counts decreased from about 18000 to 4000. Second,
there is a large short-term variability. The latter appears to
be instrumental because the FES counts of SN 1987A vary
in exactly the same manner. Figure 3 shows the uncorrected
FEB magnitudes for SN 1987A . In Figure 4 is shown the
same data when the FEB counts for SN 1987A are increased
or decreased for the day to day fluctuations about the mean
for HD 45669 (the solid line in Figure 3). This procedure
results in a significant improvement m the scatter, compa-
rable to that in Underlap mode. The corrected magnitudes
shown in Figure 4 are an expanded version of the data in
Figure 1.
Tim long-term decline in counts for HD 45669 is believed
to be due to the FEB fatigue spot. The origin of the day
to day variations is unknown, but instrumental effects are
suspected. The short-term changes actually occur on a time
scale of hours (Ref. 7).
16000
15000
14000
13000
12000
HO 45669
+
+
+ ÷ + + +
+ + + + +
++ + + ++ +
+ +
++
+
11000 I ' '2o0 25o 3d0 3so 4_o 45o
DAYS AFTER OUTBURST
l'hgure 2 FEB counts (f_ust track, overlap mode) for the SN 1987A
comparison star, lid 45669 The sohd hne _s a hnear least-squares tlt
to the data, winch defines the long-term change m mean Imghtness
due to FEB senslt_vaD (faUgue spot) losses
4
45
5
55
6
65
7
140
Figure 3
AUG I OCT : DECI F_B I _R l
I I I I i
++%
"q+#+++ *
FEScounts uncorrected
for componson stor
i I P ,
174 209 243 278
the calibration from Ref 5
+++
+ T+
++
+t
++
+%
I i t I
,3 2 547 581 416 45O
DAYS Ar'-fER OUTBURST
IUI" "V'" magmtudes for SN 1987A as computed with
No correctaon for (B-V) has been made
VISUAL LIGHT CURVE OF SN 1987A 113
I_B7
4 _c 1 ocl o[¢ _ rob I _R i -02
,
45
5
B
5
55
u
%9
+t
+ ÷
+.
++.
+_
+
s 5 FES measurements corrected +%_-
+
wdh HD 45669 data l+
÷
_,,o ,', 2_9 2;3 2;B _42 3;_ 3;, & ,_o
DAYS AFIER OUTBURST
lqgurc 4 lUg "V" nmgmtudes for SN 1987A corrected for day-
to-day fluctuahons of tile comparison slar 111) 45669 about lt_ mean
Imghtness (see lrlg 2) Note the slgmflcant improvement° m scatter,
compared to Fig 3
c_
0
O2
04
o6
o8
1140
i i _ l l
%_+++ +
++\_:_**+,
+++ +
SN 1987A
Oewabon from exponent,G decoy
++4-
+_"
+
_+
++
I 4 209 243 276 3 2 347 381 4 6
BAYS A_3"ER OUIBURSr
45O
t'hgure 5 Dc'vlatlon of the lull; SN 1987A FES hght curve from
the exponential decay measured bet.v, een days 140-210
3. I)ISCUSSION
The SN 1987A light cutw: has 1)con wMely discussed
(cg. [h'fs. 8-10). The 1UE FI:;S dal,_ agrees well with
giound based mcasurelnent, s The data clcal ly shows t.}lc ex-
ponential decay portion between days 1,t0-210 After l,hat,
however, there is a gradual deviation from a linear (in mag-
nil, udes) decay rate as the supernova I)ecomes fainter at a
slightly faster rate. This first became evident in the IUI';
da_a in early ,Janllary 1988 ([{ef 111. ],ooking back al.
the data, the non-exponential dccrease was underway 1)y
lat, e Novcmbcl (_ (lay 250), cor[esl)OiKlirlg closely to t.he
emclgence of gamma rays flora l,he SUlmino\'a ejccl, a tilers.
12 15)
The deviation from an cxpouc-lltial dechne i% now quit, c
large (over 0.5 mag) Figure 5 shows the devl,d, ion from the
cxp<mcJH, ia] decay hght curve p]edictcd from days 140-210.
If the FI::S b,mdpass (:lose])' approximates the I,olomct,lm
light curve, then this implies t,llat over 30% of the gamma-
ray C]lergy is (_cal)lng , either as gamlna rays oi at, slightly
lower encigies m tile X-ray ,_pcct_urrl, and is not contribut-
iI]g 1,O |,hc ViStl&l SDCCI,I lilt/.
References
1. lloggess, A., el al. 1978. Nature 275, 372
2. Sonneborn, (', , el al. 1987. NA,5'A IUE Ncw_,lellcr No.
32, I.
3. l(oshiba, M. 1987. IAU C,'cular No. 4338.
4 Svoboda, R 1987. IAU C_rctdar No. 4340.
5 lmhoff, C. 1986 NASA IUI'7 Newsletter No. 29, 45.
6. lmhoff, C. 1988. Private COmlmmicat, ion.
7 A_qmll,h R., 1987. IUE Observato W internal memoran-
(hllll
S. l(,'sh,c'r, R. c_ ,/. 1!)87. /11). J., a20, 602
!1. l'hilhps, M. 1988. in S_pernova 1987:1 en Ihe Larqe Mag-
etga._c 6'/o:,d, ed. M b:afatos am{ A Michahtslat_os,
C,mlbtidge, Calnbrldgeljniv Plcss, 16
10 I[,mnly, M. c/ al. 1988. :1 J , 95, 63.
I I. Somlcborn, (I., and Ku'shncr, R 1987. /3.A A.,5', 19,
1050.
12 Malz, S. ct al 1987. 1AU Czrcular No. 4510
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The complete visual light curve of SN 1987A: Thirteen months of FES observations

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