NuSTAR detection of a hot stellar superflare with a temperature of 95 MK in hard X-rays (2501.16710v1)

Abstract: A search of the hard X-ray archive data of NuSTAR found a transient source, NuSTAR J230059+5857.4, during an observation of 1E 2259+586 on 2013 April 25. A multi-wavelength analysis using X-ray, optical, and IR data, mostly taken in its quiescent phase, was conducted to identify the origin of NuSTAR J230059+5857.4 and elucidate the phenomena associated with the flare activity. The results indicated that NuSTAR J230059+5857.4 was a stellar flare that occurred on a single M-dwarf, M-dwarf binary, or pre-main-sequence star. NuSTAR J230059+5857.4 exhibited the higher emission measure and higher temperature, 8.60+2.15/-1.73x10^54 cm^-3 and 8.21+2.71/-1.86 keV, respectively, on average than the nominal values of stellar flares reported in the past. The flare loop size estimated on the basis of the model to balance the plasma and magnetic pressures was larger than the stellar radius by a factor of several. Since based on solar flare loops, this flare loop scale is excessively large, we conjecture that the observed large emission measure is possible to be attributed to the observation of mutually-associated multiple flares simultaneously occurring on the stellar surface, known as sympathetic flares. Thanks to the large effective area of NuSTAR in the hard X-ray band, we can conduct detailed discussion about a temperature variation associated with the flare. Investigation of the temperature variation during the flare revealed that the temperature remained significantly higher than during the quiescent phase even after the count rate dropped to around 5% of the peak. The sustained high temperature over the long duration is consistent with the idea of sympathetic flares. We found that it is essential to use theoretical models to evaluate loops and assess temporal changes in temperature as done in this study to determine whether there are multiple flares or not when analyzing flare observation data.

• The paper identifies a stellar superflare with a peak temperature of 95 MK using NuSTAR’s hard X-ray observations.
• It integrates multi-wavelength data from Swift, Chandra, XMM-Newton, and GAIA to determine flare parameters and host star characteristics.
• The study reveals a double exponential decay and prolonged energy release, challenging conventional models of magnetic reconnection in stellar flares.

NuSTAR Detection of a Hot Stellar Superflare: Unveiling a 95 MK Temperature in Hard X-rays

The presented paper discusses the detection and comprehensive analysis of a stellar superflare observed by the Nuclear Spectroscopic Telescope Array (NuSTAR). The source, designated as NuSTAR J230059+5857.4, was discovered on April 25, 2013, during routine observations of the magnetar 1E 2259+586. This transient event was attributed to a stellar flare occurring on potential candidates such as a single M-dwarf, an M-dwarf binary, or a pre-main-sequence star.

Observation and Analysis

The NuSTAR data analysis revealed a flare with notably high emission measures and temperatures, 8.60$^{+2.15}_{-1.73} \times 10^{54}$ cm$^{-3}$ and 8.21$^{+2.71}_{-1.86}$ keV, respectively. These values exceed typical parameters for stellar flares, posing questions about the processes enhancing these measures. The flare was interpreted to have originated from optically-thin thermal plasma, notably observable due to NuSTAR's capabilities in the hard X-ray regime (3-79 keV).

Central to the paper is the flare's duration and temperature evolution. The event exhibited a double exponential decay with fast and slow decay times of approximately 70.1 and 1055.7 seconds, respectively. Notably, the temperature remained elevated long after the peak emission, suggesting prolonged energy release mechanisms beyond initial reconnection events, possibly indicating the occurrence of sympathetic multiple flares.

Multi-Wavelength Observations

Additional data from X-ray facilities such as Swift, Chandra, and XMM-Newton, along with observations in optical and infrared bands from observatories like GAIA and WISE, were integrated to build a holistic profile of NuSTAR J230059+5857.4. This multi-wavelength analysis was crucial for pinpointing the potential stellar hosts. The GAIA observations provided a distance estimate of 281 pc, integral for inferring the flare's absolute luminosity.

Moreover, the paper’s utilization of spectral energy distribution (SED) modeling found congruence with a BT-NextGen stellar model, estimating the host star's temperature between 3400 and 4000 K, with radii from 0.73 to 1.04 R$_{\odot}$.

Implications and Future Directions

By leveraging NuSTAR's sensitivity, this paper illuminates the extreme nature of stellar flares, with findings that invite a reevaluation of flare loop dynamics and energy release processes. The flare's emission measure and sustained high temperature hint at complexities in magnetic field interactions and plasma behavior on magnetically active stars, potentially differing from solar analogs.

Future endeavors could explore the regularity and conditions favoring such large flares, helping to inform our understanding of magnetic activity across stellar types. High cadence, prolonged monitoring in hard X-rays and other observational domains may help to accurately differentiate between single and multiple flaring phenomenon by resolving alterations in emission measures over time.

The implications of sympathetic flares, if validated, could expand our perspective on magnetic connectivity and energy distribution in stellar coronae, impacting models of stellar activity cycles. This paper underscores the vital role of comprehensive, multi-faceted analyses in advancing stellar astrophysics. The integration of observational data across the electromagnetic spectrum paints a robust picture of the phenomena, vital for theoretical modeling and simulation work aimed at unraveling the nuances of stellar flare mechanics.