Low Frequency (11 mHz) Oscillations in H1743-322: A New Class of Black Hole QPOs?

We report the discovery of quasi-periodic oscillations (QPO) at ~11 mHz in two RXTE observations and one Chandra observation of the black hole candidate H1743-322. The QPO is observed only at the beginning of the 2010 and 2011 outbursts at similar hard colour and intensity, suggestive of an accretion state dependence for the QPO. Although its frequency appears to be correlated with X-ray intensity on timescales of a day, in successive outbursts eight months apart we measure a QPO frequency that differs by less than ~2.2 mHz while the intensity had changed significantly. We show that this ~11 mHz QPO is different from the so-called Type-C QPOs seen in black holes and that the mechanisms that produce the two flavours of variability are most probably independent. After comparing this QPO with other variability phenomena seen in accreting black holes and neutron stars, we conclude that it best resembles the so-called “1 Hz” QPOs seen in dipping neutron star systems, although having a significantly lower (1-2 orders of magnitude) frequency. If confirmed, H1743-322 is the first black hole showing this type of variability. Given the unusual characteristics and the hard-state dependence of the ~11 mHz QPO, we also speculate whether these oscillations could instead be related to the radio jets observed in H1743-322. A systematic search for this type of low-frequency QPOs in similar systems is needed to test this speculation. In any case, it remains unexplained why these QPOs have only been seen in the last two outbursts of H1743-322.

💡 Research Summary

In this paper the authors report the discovery of a new, low‑frequency quasi‑periodic oscillation (QPO) at approximately 11 mHz in the black‑hole candidate X‑ray binary H1743‑322. The signal was detected in two separate Rossi X‑ray Timing Explorer (RXTE) observations (one in August 2010, one in April 2011) and in a contemporaneous Chandra/HETG observation (August 2010). The QPO appears only during the early rise of the 2010 and 2011 outbursts, when the source is in a hard spectral state characterized by a high hard‑color ratio (16–20 keV / 2–6 keV) and relatively low luminosity (L_X ≲ 3 × 10^37 erg s⁻¹).

The oscillation is narrow, with quality factors (Q) ranging from ~10 to ~100, and fractional rms amplitudes of 2–4 % in the 2–60 keV band. Its energy dependence is modest: in the 2010 RXTE data the rms is essentially constant with energy, while in the 2011 data the rms rises modestly at intermediate energies before decreasing at the highest energies. The frequency is remarkably stable: across the two outbursts separated by ~800 days the centroid frequency differs by less than 0.4 mHz, despite a change in source intensity of ~10 mCrab. Within a single Chandra observation the centroid drifts upward on a timescale of a few hours, apparently tracking a modest increase in X‑ray flux.

Simultaneously, the power spectra show the usual Type‑C QPOs typical of the low‑hard state, with frequencies around 0.4–0.8 Hz and rms amplitudes of ~30 %. Importantly, the frequency of the new 11 mHz QPO does not track the Type‑C QPO frequency, which varies by a factor of ~2 between the two RXTE observations, indicating that the two phenomena arise from distinct physical mechanisms. Phase‑resolved spectroscopy shows no significant change in the broadband noise or the Type‑C QPO properties when the 11 mHz QPO is at different phases, further supporting independence.

The authors compare this oscillation with several known classes of low‑frequency variability. “Heart‑beat” oscillations in GRS 1915+105 and IGR J17091‑3624 occur at similar mHz frequencies but only in soft, high‑luminosity states, unlike the hard‑state, low‑luminosity occurrence here. mHz QPOs in neutron‑star systems associated with marginally stable nuclear burning are soft‑spectrum, burst‑related, and thus unlikely to be related. The only comparable phenomenon is the so‑called “1 Hz” QPO seen in dipping neutron‑star binaries, which also appears only in high‑inclination systems and is thought to be linked to periodic obscuration by the outer disc. H1743‑322 is believed to have a high inclination (> 70°) based on occasional dipping, making this analogy plausible, though the frequency is an order of magnitude lower (11 mHz versus 0.5–2 Hz).

Given the unusual characteristics—hard‑state dependence, stability across outbursts, and similarity to dipping‑source QPOs—the authors speculate that the oscillation might be related to the radio jets that are known to be active in H1743‑322 during the hard state. Jet‑disc coupling could produce a modulation on timescales of tens of seconds, but current data are insufficient to confirm this scenario.

The paper concludes that the 11 mHz QPO represents a new class of black‑hole variability, distinct from the canonical Type‑A/B/C QPOs, and possibly the first black‑hole analogue of the dipping‑neutron‑star “1 Hz” QPO. A systematic search for similar low‑frequency QPOs in other high‑inclination black‑hole transients is required to test the universality of this phenomenon and to clarify its physical origin.