V838 Mon : Mysterious Eruption in 2002

(Left: Extremely brightened V838 Mon!! Ondrej Pejcha et al., Brno. Right: VSNET lightcurve of V838 Mon in 2002)

V838 Mon is a peculiar, unprecedented variable which appeared at the beginning of 2002. The nature of this object is totally unknown, while several interesting scenarios have been proposed; a kind of nova, a phenomenon related to a flash in hydrogen-poor environment, and a merger of two stars. Anyway, we can say V838 Mon is one of the most mysterious and puzzling objects which were reported to VSNET in 2002.

Discovery

The object was originally discovered by N. J. Brown as a possible nova. T. Iijima relayed the discovery report to VSNET, and the brightening object was confirmed on January 9 by some observers (vsnet-campaign-nova 668, 713. Pre-discovery observations show that no other brightening was recorded between 1997 September 29 - 2001 November 24, and that it was still faint until 2001 December 26 (vsnet-campaign-nova 672, 681, 690, 691, 730, 732, vsnet-campaign-unknown 31,33). At the position of the reported object, there is GSC4822.39, a red object which is identical to IRAS 07015-0346, or 2MASS source (vsnet-campaign-nova 686,696,697, 701,705, 706, vsnet-campaign-unknown 14, 20) (Summary of astrometry, see [vsnet-campaign-nova 748]. The spectrum at the beginning of the outburst was dominated by a very red continuum with a very weak H-alpha emission having a P Cyg profile. Several weak metal lines were also seen in emission. (vsnet-campaign 1272, vsnet-campaign-nova 670, 678, 680, 685, 692, vsnet-campaign-unknown 8 15, 17, 19). S. Kiyota reported magnitudes of B=12.16, V=10.10, Ic=8.28 on January 9 (vsnet-campaign-nova 671, vsnet-campaign-unknown 9). A. Henden reported magnitudes and colors of V=9.975, B-V=1.726, U-B=1.916, V-R=0.943, R-I=0.931 on January 11 (vsnet-campaign-nova 704). These red colors are quite atypical for ordinary novae.

(CCD image taken by S. Kiyota; the bright star near the center of the lower image (Ic-band) is the object)

(Sepctrum by M. Fujii)

The IR color excludes the possibility of a long-period variable. The IRAS color (of the likely far IR counterpart) is very unusual for a stellar object (vsnet-campaign 1270, vsnet-campaign-nova 668, vsnet-campaign-unknown 6). T. Kato mentioned that the color seems to be on a smooth extension of late-type Barium stars and the expanded photosphere may somehow mimic a late-type Barium star both in spectrum and colors (vsnet-campaign-nova 704). Regarding its K-type spectrum, T. Kato further commented on the similarity between the object and the classical nova V1148 Sgr (vsnet-campaign-nova 757, vsnet-campaign-unknown 48).

The VLA radio observations were performed on January 11 and 12 as reported by M. Rupen, and detecting a possible radio counterpart (vsnet-campaign-nova 709, vsnet-campaign-unknown 23).

A few days after the discovery, the object began gradual fading. The fading rate seems to have been slightly larger in bluer bands (vsnet-campaign-nova 729, 741, 754, 759, vsnet-campaign-unknown 30, 38, 50). No major change of the spectral feature was observed during the gradual fading (vsnet-campaign-nova 765, 766, vsnet-campaign-unknown 52, 53). This new object was given the permanent GCVS designation, "V838 Mon" as announced in IAUC 7812 (vsnet-campaign-nova 811, vsnet-campaign-unknown 62).

Reflare

The slow fading continued until February 2 when L. Brat found the object suddenly brightened from 11 mag to 8.8mag (vsnet-campaign-nova 803, 814, vsnet-campaign-unknown 61, 63, vsnet-campaign-v838mon 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 17, 18, 19, 20, 21, 22, 23, 24, 31). The rising rate became more gradual in the late January 3 when the object was V~7.5mag (vsnet-campaign-v838mon 26, 27, 28, 30, 33, 37, 38, 39, 40, 41, 42, 44, 45, 47, 48, 49, 50, 51). K. Ayani performed spectroscopy at Bisei Astronomical Observatory and reported the strong H-alpha emission with FWHM of 220 km/s. It has a broad wing with FWZI of about 3000 km/s, and a P-Cyg absorption with a blueshift of about 230 km/s relative to the emission peak (vsnet-campaign-v838mon 25). M. Fujii also performed spectroscopy and reported that there seems to be likely Fe II emission line series, accompanied with a P Cyg profile. The P Cyg profile was getting stronger with time (vsnet-campaign-v838mon 36). Based on the spectroscopic monitoring by M. Fujii and C. Buil, the H-alpha emission started to be stronger on January 29 (vsnet-campaign-v838mon 93, vsnet-campaign-v838mon 116). A. Henden compared its colors with those before the current eruption and reported that VRI colors are pretty consistent with earlier in the outburst, but the U and B colors have changed dramatically (vsnet-campaign-v838mon 52). This outburst, mainly in bluer bands, with strong H-alpha emission is rather reminiscent an ordinary nova eruption. But in this case, what is the nature of the first, earlier outburst?? Is the second outburst is really a kind of nova eruption??

(Extremely brightened V838 Mon!! Ondrej Pejcha et al., Brno)

(Incredible outburst on 2002 Feb. 2!: from VNSET data)

(H-alpha spectrum by K. Ayani, Bisei Astronomical Observatory, during the rising stage of the great eruption)

Then, since around February 8, the object had entered a fading stage (vsnet-campaign-v838mon 119, 120, 121, 122, 130, 131, 133, 134, 135, 136). As the continuum fading, the H-alpha peak intensity also weakened as reported by C. Buil and M. Fujii, which is contrary to the behavior seen in most novae (vsnet-campaign-v838mon 153, 169).

Y. Efimov, Crimean astrophysical observatory, performed polarimetry and photometry on February 11-18. Y. Efimov calculated the weighted average of wavelength dependence of polarization Pobs(%) which is very well fit by the curve of interstellar polarization (vsnet-campaign-v838mon 226).

The gradual fading once stopped around February 22-23 (vsnet-campaign-v838mon 218). During this period, the object was relatively calm at around 8.4mag and then, again started gradual fading on February 26-27 (vsnet-campaign-v838mon 229, 230, 231). The fading trend again stopped on March 1-2 (vsnet-campaign-v838mon 239), and then.....

The third brightening

The object, unexpectedly, started brightening again on March 3 (vsnet-campaign-v838mon 240, 241, 242, 243, 244, 245, 246, 247). In the spectrum on March 3, C. Buil reported that the H-alpha emission component was continuously weakening (vsnet-campaign-v838mon 256). M. Fujii took a new spectrum, which show that Na I absorption became weaker (vsnet-campaign-v838mon 268). The relatively rapid brightening continued until about March 4 (vsnet-campaign-v838mon 248, 249, 250, 251, 252, 253, 254, 255, 257, 260, 266). On March 5, the object was relatively calm at about 7.7mag (vsnet-campaign-v838mon 258), and then, it again started brightening and reached 7.1mag (vsnet-campaign-v838mon 259, 261, 262, 263, 264, 265, 267, 269, 270, 271, 272, 273, 274). This brightening finished around March 11 and the object finally started fading (vsnet-campaign-v838mon 275, 276, 277, 278, 279, 281, 282, 283, 284, 285, 286, 287, 288, 289, 290, 291). C. Buil reported that the spectrum was very complex with the fading H-alpha emission at this phase (vsnet-campaign-v838mon 280). The fading rate became more gradual from March 21-22, and remained at about 8mag for a while (vsnet-campaign-v838mon 301, 302, 303, 304, 305). V. P. Goranskij reported that Nova 1988 V1006/7 in M31 showed a spectrum which is quite similar to that of V838 Mon (vsnet-campaign-v838mon 309).

Light Echo and final fading

According to IAUC 7859, A. Henden found a light echo around V838 Mon, which was first detected on images taken on February 17. He further reported that the object faded a bit in the blue, so the nebular shell is becoming more visible (vsnet-campaign-v838mon 311, 312, 313, 314). The object was relatively calm at about 7.8mag until April 5-6 when the object started rapider, final fading (vsnet-campaign-v838mon 320, 321, 322, 323, 324, 325, 326, 327, 328, 329). M. Fujii reported that it showed an M-type spectrum in this fading phase (vsnet-campaign-v838mon 338). The fading speed significantly increased around the end of April (vsnet-campaign-v838mon 345, 347, 348). A. Henden reported that the light echo remained about the same diameter during April (vsnet-campaign-v838mon 354). M. Orio performed R, V, and narrow filters imaging and reported that the contour of the light echo is irregular, and it is a reflection rather than an ionization nebula (vsnet-campaign-v838mon 353, 364). In the echo, HST detected approximately circular concentric rings or arcs, and several highly structured, non-concentric filamentary features (vsnet-campaign-v838mon 363). An emission line of H-alpha appeared in the spectrum taken by M. Fujii on May 1 (vsnet-campaign-v838mon 356). D. West performed multi-color photometry on November 11 and reported that V=15.9+/-0.2 and V-Ic=6.1+/-0.2 for V838 Mon and V-Ic=1.8+/-0.2 for the light echo (vsnet-campaign-v838mon 403, 404).

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