MAXI J1807+132: COATLI observations of fading in the optical
ATel #13123; Margarita Pereyra (UNAM), Alan M. Watson (UNAM), Nat Butler (ASU), Simone Dichiara (GSFC/UMD), Rosa L. Becerra (UNAM), Diego Gonzalez (UNAM), Alexander Kutyrev (GSFC/UMD), William H. Lee (UNAM), Eleonora Troja (GSFC/UMD), Gabriele Minervini (INAF/IAPS-Rome), and Tanner Wolfram (ASU)
on 23 Sep 2019; 19:00 UT
Credential Certification: Alan Watson (alan@astro.unam.mx)
Following the SWIFT/BAT trigger No. 924641 (GCN #25702), confirmed as a new outburst from MAXI J1807+132 by subsequent X-rays and radio observations (ATel #13907, ATel #13107), we started a follow up campaign of the field centered on the source using the COATLI 50-cm robotic telescope at the Observatorio Astronomico Nacional on Sierra San Pedro Martir (http://coatli.astroscu.unam.mx/).
An optical re-brightening of this source was detected with Gaia on September 6 (Denisenko et al. 2019, VSNET-alert 23544), when the source exhibited a 15.60G magnitude brightness. We have been following MAXI J1807+132 for about 13 consecutive nights after the Swift/BAT trigger. We observed ~ 2.9hrs on average, consecutively with w, R and B COATLI filters, every night from September 10 to September 23 (MJDs 58736 to 58749). Assuming that the outburst has started between 3th and 5th (as suggested by Denisenko et al. 2019, VSNET-alert 23544), our observations shows that the source remain on outburst for approximately 14 days and is now fading in optical wavelengths:
Observing date MJD (Start time - End time) w filter (mag, AB system)
Sep 10th 58736.339 - 58736.370 15.78 +/- 0.01
Sep 11th 58737.101 - 58737.333 15.65 +/- 0.01
Sep 12th 58738.138 - 58738.260 15.94 +/- 0.01
Sep 13th 58739.142 - 58739.294 15.70 +/- 0.01
Sep 14th 58740.130 - 58740.281 15.83 +/- 0.01
Sep 15th 58741.129 - 58741.283 15.90 +/- 0.01
Sep 16th 58742.131 - 58742.255 16.05 +/- 0.01
Sep 17th 58743.127 - 58743.270 16.15 +/- 0.01
Sep 18th 58744.127 - 58744.179 16.22 +/- 0.01
Sep 19th ----- (Bad weather conditions) -----
Sep 20th 58746.125 - 58746.264 16.37 +/- 0.01
Sep 21st ----- (Bad weather conditions) -----
Sep 22nd 58748.121 - 58748.248 16.52 +/- 0.01
Sep 23rd 58749.143 - 58749.154 16.78 +/- 0.01
We calibrated our images against the PanSTARRS catalog. Our 10-sigma limiting magnitudes are typically 20.92 in the w filter.
We note that the magnitudes reported at the beginning of our observations are consistent with those observed at the outburst onset on September 6. The duration of this new outburst of MAXI J1807+132 is consistent with the ~17 days that the source remain on outburst in 2017 (Shidatsu et al. 2017) before it gradually faded by more than one order of magnitude.
In the observations performed on Sep. 12 and Sep. 14 we found significant fading of the source in all filters. Within 30-60 min, the source exhibited a sharp decay of about 0.8mags followed by a smooth recovery in both observations. The results of the ongoing analysis to investigate the possible origin of this fading will be presented in future communications.
Since the presence of several rebrightenings at the decay period of MAXI J1807+132 discovery outburst have been reported by Jimenez-Ibarra et al. 2019, we plan to continue with COATLI/OAN optical observations. Multi-wavelength coverage is highly encourage to monitor the decay period and investigate further the behaviour of this source.
We thank the staff of the Observatorio Astronomico Nacional in San Pedro Martir.
