XTE J1118+480: Black hole candidate X-ray nova at high galactic latitude
Discovery of X-ray and optical outburst
The RXTE All-Sky Monitor has detected X-ray emission from a new
source, XTE J1118+480 on March 29. The object located at high
galactic latitude. X-ray spectrum is similar to the black hole candidate
Cyg X-1 in its hard state
(vsnet-alert 4501).
M. Uemura and T. Kato discovered the optical counterpart at Kyoto
(vsnet-alert 4503,
4511).
D. Buczynski reported the magnitude of 12.91mag on March 30
(vsnet-alert 4505).
H. Yamaoka reported the object corresponds to a USNO star of r=18.8
(vsnet-alert 4508,
4509).
M. Garcia performed spectroscopic observations. The optical counterpart
to XTE J1118+48 show weak (~2 Angstrom equivalent width) H-alpha emission,
which has a double peaked structure with a FWZI of ~70 Angstroms
(vsnet-alert 4512,
4527,
4603).
K. Takamizawa revealed the other outburst in January from his previous
patrol photographs
(vsnet-alert 4515,
4520,
4521).
B. H. Granslo reported the object has shown no clear proper motion since 1953
using DSS plates
(vsnet-alert 4555).
An extensive multi-wavelength campaign with EUVE, HST, RXTE, and UKIRT
is performed on April 8, and the other campaigns have been performed.
(vsnet-alert 4582,
vsnet-alert 4588,
vsnet-campaign 10).
B. Skiff recommended to use Henden's sequence for this field so that
additional comparison stars can be included:
ftp://ftp.nofs.navy.mil/pub/outgoing/aah/sequence/sumner/j1118.seq
(vsnet-alert 4628).
(CCD image by Denis Buczynski, taken on 2000 Mar. 30.83)
Long plateau with short-term variations
L. Cook presented a light curve at:
http://www.geocities.com/lcoo/xte1118.htm,
http://www.lewcook.com/xte1118.htm,
and reported the presence of sinusoidal short-term
variation with a period of about 0.17 - 0.18 day and with a amplitude
of 0.08mag
(vsnet-alert 4542,
vsnet-alert 4554).
The Kyoto team reported that an unprecedented 6-hour monotonous
rise was detected on April 8, and the reported 4.1-hour
periodicity (IACU 7397) was hardly detectable. This appeared only
on April 8 data and the other Kyoto data shows 4.1-hour modulation
(vsnet-alert 4599,
4683).
J. Patterson reported a periodicity of 0.17082 day with a large-amplitude
(>0.3mag) flickering
(vsnet-alert 4690).
R. Novak reported P=0.1706 day using 1006 data points
(vsnet-alert 4693).
L. Cook pointed out that the shape of humps was changing during the
outburst
(vsnet-alert 4733).
In early May, the about 4-hour periodic variation is still seen in the light
curve, however, the profile has been changed from sinusoidal curve
to superhump-like shape
(vsnet-campaign 5,
vsnet-campaign 21).
The object showed a slight decline from the end of April
(vsnet-alert 4796).
The ASCA data on May 11 (ASCA - VSNET
campaign) revealed a QPO at 0.115(2) (Hz), indicating the frequency
shifted from the previous value
(vsnet-campaign 100).
The data obtained by VSNET collaboration team in the end of May
showed a relatively sharp humps on the sinusoidal curve
(vsnet-campaign 135).
In June, the averaged light curve shows a flat peak and a flat bottom.
The amplitude of the humps does not change from the previous one
(about 0.08 mag)
(vsnet-campaign 210).
In July, the light curve shows possible
multiple peaks within one period calculated to be shorter; 0.1696(2)d.
It keeps general decline, but the decline speed became more rapid.
(vsnet-campaign-xray 8).
(4.1-hour periodic modulation observed by VSNET collaboration team)
Rapid fading; the end of the main outburst
And then, the fading trend has become more apparent
(vsnet-campaign-xray 14).The light curve observed at Kyoto
on July 18 is covered about three hours under good condition, but no
prominent humps were detected.
A dramatic fading from the long outburst was reported by M. Verdenet
(14.3mag) on July 31, and then, the light curve of Kyoto team confirmed
the trend
(vsnet-campaign-xray 28,
29).
The decline rate is about 0.07mag/d,
which is faster than previously reported one
(vsnet-campaign-xray 29,
25,
34).
In August this X-ray nova had shown a precipitous fading. On August 5, the
magnitude was 14.7 (by Nyrola Observatory team), and then, S. O'Connor
found the magnitude to be dimmer than 16.0 on August 13
(vsnet-campaign-xray 37).
A. Oksanen and M. Moilanen (Nyrola team) reported
that the object was 16.9 magnitudes on August 13
(vsnet-campaign-xray 38,
39).
In the end of August, the decline trend became more gradual
(vsnet-campaign-xray 42).
The magnitude was near that of quiescence (18.8mag) on August 31
(vsnet-campaign-xray 44).
Black hole candidate
M. Garcia and his collaborators performed spectroscopic observations
of X-ray nova XTE J1118+480 with the new 6.5-m MMT on Dec. 1 & 4
and reported a velocity amplitude of 698 +/- 11 km/s and an orbital
period of 0.1701 +/- 0.0003 days, which give a mass function of
6.0 +/- 0.3 solar masses. The large value of the mass function is rivaled
only by that of V404 Cyg
(vsnet-campaign-xray 58,
vsnet-campaign-xray 59).