EXO 2030+375 restarts in reverse

ATel #9485; Peter Kretschmar (ESA/ESAC), Felix Fuerst (Caltech), Colleen A. Wilson-Hodge (NASA-MSFC), Peter Jenke (UAH/CSPAR), Jari Kajava (ESA/ESAC), Matthias Kuehnel, Joern Wilms (Remeis-Observatory & ECAP), Pere Blay (IAC-NOT), Katja Pottschmidt (NASA-GSFC & UMBC), Julia Alfonso-Garzon (CAB, CSIC-INTA)
on 10 Sep 2016; 18:22 UT
Credential Certification: Peter Kretschmar (Peter.Kretschmar@esa.int)

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The Be X-ray binary pulsar EXO 2030+375, known for its very regular outbursts has undergone an extended period of very low activity since late 2015 (ATel #8835). The regularity of the outbursts already decreased since early 2015, at the same time the previous spin-up trend maintained over many years first slowed down and then showed signs of reversal, as monitored by Fermi/GBM (ATel #8835, #9263). Following our Swift/XRT observations in 2016 March (ATel #8835) we observed EXO2030+375 again with XRT in April and with XRT and NuSTAR in 2016 July (MJD 57594.3), each time close to the predicted periastron passage of the 46d orbit. We observe fluxes between 2e-12 erg/s/cm^2 and 1e-10 erg/s/cm^2 (between 2-8 keV), but find a remarkably stable spectrum. The XRT data are well described by an absorbed power-law with Gamma ~1.05 and absorption column N_H ~ 4.3e22 cm^-2, while the NuSTAR data show a steeper photon index of Gamma ~1.6 and require an additional cutoff around 26keV. Using NuSTAR and XRT data taken in 2016 July we find a pulse frequency of 24.2207 +/- 0.0001 mHz (41.2870 +/- 0.0002 s). This is significantly slower than the highest frequency measured by Fermi/GBM between MJD 57634 and 57637 of 24.2201 +/- 0.0002 mHz (41.2879 +/- 0.0003 s). The most recent periastron passage has been followed by increased activity and a clear detection of pulsations with GBM at a frequency of 24.2201 +/- 0.0002 mHz (41.2879 +/- 0.0003 s) on MJD 57635.5 (4 September 2016), continuing the spin-down trend. EXO 2030+375 thus seems to have restarted its regular outburst activity. As noted previously (ATel #8835, #9263) the current torque reversal is remarkably similar in duration and magnitude to the one observed in 1994/1995 (Wilson, C.A. et al. 2002, ApJ, 570, 287). In addition, the new data seem to confirm that outbursts have shifted from peaking about 7 days after periastron to 1 day after periastron, as first noted in ATel #9263. This behavior is also very similar to an orbital phase shift observed in 1995. The repetition of these events along with the spacing of the two observed giant outbursts in 1985 (Parmar, A.N. et al. 1989, ApJ, 338, 359) and in 2006 (Wilson, C.A. et al. 2008, ApJ, 678, 1263) suggest a ~21-year quasi-period in this system, but the origin of these apparently recurrent events remains to be clarified. Swift/XRT will continue to monitor the source over the next 46d binary orbit. Observations at other wavelengths are encouraged.