NICER identification of MAXI J1348-630 as a probable black hole X-ray binary

ATel #12447; A. Sanna (Univ. of Cagliari), P. Uttley (Univ. of Amsterdam), D. Altamirano (Univ. of Southampton), J. Homan (Eureka Scientific and SRON), G. K. Jaisawal (DTU Space), K. Gendreau, Z. Arzoumanian (NASA/GSFC), T. Guver (Istanbul Univ.), E. Bozzo, C. Ferrigno (ISDC-Switzerland), A. Papitto (INAF-OAR), L. Burderi, A. Riggio (Univ. of Cagliari), T. Di Salvo (Univ. of Palermo), J. M. Miller (Univ. of Michigan), S. Guillot (IRAP, CNES), J. Neilsen (Villanova Univ.)
on 29 Jan 2019; 16:57 UT
Credential Certification: Andrea Sanna (andrea.sanna@dsf.unica.it)

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On 2019 January 26 at 03:16 UT, MAXI/GSC detected the new X-ray transient MAXI J1348-630 (ATel #12425), which was then quickly also observed by Swift/BAT (GCN #23795, #23796, #23797, #23801), INTEGRAL (GCN #23799, ATel #12441), iTelescope.Net T31 (ATel #12430), and the 1-m LCO telescope at Cerro Tololo (ATel #12439). Based on a 1 ks Swift/XRT observation, a coherent signal was reported around 9.8 s (or perhaps at 4.9 s), suggesting that MAXI J1348-630 may harbor an X-ray pulsar (ATel #12434). Here we report on NICER observations of MAXI J1348-630.

NICER observed MAXI J1348-630 for a total of ~8.1 ksec between 2019 January 26 20:40 UT and January 28 20:34 UT. The source flux is seen to increase systematically over this time interval, and in an observation performed on January 28 NICER detected an average of 5398 cts/sec (0.2-12 keV). The fast increase is consistent with the flux increase detected by MAXI (see link here) and Swift/BAT (see link here).

The 2-10 keV power spectrum is dominated by strong broadband noise (37% rms in the 0.1-64 Hz range on Jan 26, decreasing to 32% rms on Jan 28), characteristic of black hole X-ray binaries in the hard state. No periodic signal is detected at the frequencies of the reported 9.8 s or 4.9 s periods, but there is marginal (<3σ) evidence for a pair of weak (5-10% rms) non-harmonically-related QPOs on similar timescales, just above the low-frequency break of the broadband noise, at frequencies beginning at ~0.15 Hz in the first NICER observations on Jan 26 and moving to > 0.3 Hz in the most recent observations. We can compare the broadband power spectral shape with those obtained for a large sample of RXTE observations of accreting black holes and neutron stars, using ‘power color’ ratios of integrated power from different frequency bands (see link here for the diagram including MAXI J1348-630, and for more details see Gardenier & Uttley 2018, MNRAS 481, 3761). The source power colors clearly lie along the track followed by black holes.

The continuum X-ray spectrum (0.6-10 keV), after applying the Crab correction to deal with unmodelled systematics (see, e.g., Ludlam et al. 2018, ApJL 858, L5), could be approximated with an absorbed power-law plus disk-blackbody model. The absorption column density was measured at n_H = 0.66(1)E22 cm^-2, comparable to the integrated H column density of 1.5E22 cm^-2 from the Leiden/Argentine/Bonn maps. We find that during January 26-28 the power-law photon index Gamma steepens from 1.66(1) to 1.84(2) while the disk blackbody temperature kT increases from 0.27(2) keV to 0.37(3) keV. The absorbed 0.6-10 keV flux increased from 6.2(2)E-9 erg cm^-2 s^-1 up to 2.26(2)E-8 erg cm^-2 s^-1. We observed residuals in the energy range 6-7 keV, likely associated with a weak iron reflection feature.

The timing and spectral properties of MAXI J1348-630 strongly suggest that it is a black hole X-ray binary in a rapidly evolving hard state. Due to the rapid rise in flux, we advise urgency in scheduling follow-up observations at other wavelengths. Further NICER observations of this source are underway; additional multi-wavelength observations are strongly encouraged.

NICER is a 0.2-12 keV X-ray telescope operating on the International Space Station. The NICER mission and portions of the NICER science team activities are funded by NASA.

References: Gardenier & Uttley 2018, MNRAS 481, 3761; Ludlam et al. 2018, ApJL 858, L5