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Near-IR and Optical Observations of the Ongoing Outburst of SAX J1808.4-3658

ATEL # 1733; D. Maitra (Univ. of Amsterdam), M. Buxton, S. Tourtellotte (Yale), D. Altamirano, A. Patruno, P. Casella, M. Linares, V. Tudose, D. Russell, R. Wijnands, (Univ. of Amsterdam), C. Bailyn (Yale)
on 23 Sep 2008; 23:03 UT
Distributed as an Instant Email Notice (Transients)
Password Certification: Dipankar Maitra (

Subjects: Infra-Red, Optical, Binaries, Neutron Stars, Pulsars, Transients

Following the report of a new outburst of the accreting millisecond pulsar SAX J1808.4-3658 by Markwardt and Swank (ATel #1728), the source was observed using the CTIO/SMARTS 1.3m telescope on MJD 54732.0998, in I- and H-bands. We clearly detected the source at magnitudes significantly higher than its quiescent level, confirming that the source is indeed in outburst. The I-band magnitude of the source was estimated to be 16.93+/-0.05, by comparing against the reference stars in Greenhill et al. (2006 MNRAS, 370, 1303). This indicates that the source is already ~4.6 mag brighter than the average quiescent I-band magnitude (~21.5) reported by Campana et al. (2004, ApJ, 614, L49)! This increase in brightness is much larger than the expected variability in the light curve due to orbital modulation (~0.1-0.2 mag; Giles et al. 1999, MNRAS, 304, 47), and confirms the onset of a new outburst. The H-band magnitude was estimated to be 16.1 +/- 0.5 using three nearby stars from the 2MASS point source catalog as reference.

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