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).

Light Curve and General Information of This Object