[vsnet-alert 22964] MAXI J1348-630: a probable black-hole binary

[mkimura]

MAXI J1348-630: a probable black-hole binary

    Although this object was expected to be an X-ray pulsar
(a neutron-star X-ray binary), NICER team identified that
this is probably a black-hole X-ray binary.


===

ATEL #12447                                 ATEL #12447

Title:    NICER identification of MAXI J1348-630 as a probable black
         hole X-ray binary
Author:    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.)
Queries:    andrea.sanna at dsf.unica.it
Posted:    29 Jan 2019; 16:57 UT
Subjects:X-ray, Binary, Black Hole, Transient

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
<a href="http://maxi.riken.jp/star_data/J1348-632/J1348-632.html">link</a>
here) and Swift/BAT (see <a 
href="https://swift.gsfc.nasa.gov/results/transients/weak/MAXIJ1348-630/">link</a>
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 <a 
href="https://zenodo.org/record/2551805#.XE-Mcc_0mi4">link</a>
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

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