REM Optical/NIR observations of SAX J1808.4-3658 during outburst

ATel #7469; Maria Cristina Baglio (Universita' degli studi dell'insubria), Sergio Campana (INAF-OAB), Paolo D'Avanzo (INAF-OAB), Stefano Covino (INAF-OAB), Paolo Goldoni (APC/Univ. of Paris 7, CEA/Irfu), Nicola Masetti (INAF-IASFBo), Eliana Palazzi (INAF-IASFBo), Teo Munoz-Darias (IAC)
on 4 May 2015; 14:54 UT
Credential Certification: Maria Cristina Baglio (cristina.baglio@brera.inaf.it)

Tweet

We report on optical and NIR observations of the accreting millisecond pulsar SAX J1808.4-3658 (hereafter SAX J1808), in outburst since 2015, April 9 (Sanna et al. 2015, ATel #7364, #7371; Li et al. 2015, ATel #7376; Campana et al. 2015, ATel #7379).
We performed optical (g, r, i, z- band) and NIR (H-band) observations using the REM telescope (La Silla, Chile) on Apr 24, 2015 between 6:36 UT and 7:32 UT. We obtained twelve 180 s integration images in the optical griz bands and fifteen H-band images, 30 seconds integration each. The optical and NIR counterpart of SAX J1808 is clearly detected.
Starting from the averaged images for each filter, the following AB (griz) and Vega (H) magnitudes have been measured:

g=17.12 +/- 0.01
r=16.51 +/- 0.02
i=16.57 +/- 0.02
z=16.68 +/- 0.03

H=15.04 +/- 0.04

The magnitudes were not corrected for the Galactic extinction.
The H-band magnitude was calibrated against the 2MASS catalogue; optical calibration was performed using the Landolt standard star RU 149F.
The above values suggest that the outburst is still ongoing.
Once corrected for the reddening (E(B-V)=0.22 +/- 0.02), the optical-NIR spectral energy distribution of SAX J1808 displays a clear NIR excess, that is suggestive for the emission of synchrotron radiation from a jet.
Further observations at shorter wavelengths are encouraged in order to confirm this result.