Optical photometry and spectroscopy of HETE J1900.1-2544

ATel #543; D. Steeghs(CfA), M. A.P. Torres (CfA), M. R.Garcia (CfA), J. E.McClintock (CfA), J. M.Miller (CfA), P. G.Jonker (CfA/SRON), P. J.Callanan (Cork University), P. Zhao (CfA), P. Berlind (CfA), R. Hutchins (CfA), C. Watson (Sheffield)
on 30 Jun 2005; 22:38 UT
Credential Certification: Danny Steeghs (dsteeghs@cfa.harvard.edu)

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We report optical photometry and spectroscopy of the candidate optical/NIR counterpart (ATEL #526, #533) to the millisecond X-ray pulsar HETE J1900.1-2455 (ATEL #516, #523, #538). Its consistency with the recent SWIFT localisation (ATEL #541) and our ground-based observations confirm its association with the X-ray source.

Photometry was acquired with the FLWO 1.2m telescope (Mt. Hopkins, AZ) and the Swope 1.0m telescope (LCO, Chile) during June 20,25,26,27,30 2005 UT. Our observations typically consist of multiple 240/300s exposures in different bandpasses. The observations took place under variable seeing ranging 0.7-1.5 arcsec at Las Campanas and 2.3-2.7 arcsec at FLWO. The LCO images were calibrated using Landolt standard stars and the extinction coefficients from Landolt (1992) to estimate the photometric zero points with an accuracy of <0.1 mag.

We measure the following mean magnitudes for the optical counterpart suggested in ATEL#526; V=18.09 +/- 0.03, R=18.02 +/- 0.03, I=17.88 +/- 0.02 (PSF photometry). De-reddening the colors using the N_H from ATEL #535 indicates an intrinsically blue spectrum for this object with (V-R)_0 = -0.16, (R-I)_0 = -0.08. Our monitoring data show no signs for a steady brightness trend across the June 20-30 baseline, with variations less than 0.1 mag between epochs.

Our LCO frames resolve a nearby fainter point source, 2-2.5" NW of the counterpart (see linked finding chart). This source is present in archival POSS-II plates and is 2 magnitudes fainter in the R-band compared to the pulsar counterpart. Given the lack of an archival source at the position of the pulsar counterpart, the outburst amplitude is thus at least 2 mags compared to the plate epochs.

In addition, a 45-min spectrum of the tentative optical counterpart was acquired on June 28.25 2005 (UT) with the FAST spectrograph attached to the 1.5-m telescope at the FLWO, achieving a S/N of ~30 per pixel and covering 3700-7500 Angstrom. The only significant emission feature detected was a broad emission line at 4686 Angstrom corresponding to HeII. The spectrum is contaminated by a nearby field star that was also in the wide slit, limiting our ability to determine Balmer emission components. However it is clear that such Balmer emission is at most very weak.

The positional coincidence with the SWIFT X-ray source position, its large brightness increase relative to archival plates, its blue colour typical of accreting sources and the detection of a broad emission line all favour this source to be the optical counterpart to HETE J1900.1-2455.

Optical and near-infrared finding charts for HETE J1900.1-2455