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EP250207b is not a collapsar fast X-ray transient. Is it due to a compact object merger?

P. G. Jonker, A. J. Levan, Xing Liu, Dong Xu, Yuan Liu, Xinpeng Xu, An Li, N. Sarin, N. R. Tanvir, G. P. Lamb, M. E. Ravasio, J. Sánchez-Sierras, J. A. Quirola-Vásquez, B. C. Rayson, J. N. D. van Dalen, D. B. Malesani, A. P. C. van Hoof, F. E. Bauer, J. Chacón, S. J. Smartt, A. Martin-Carrillo, G. Corcoran, L. Cotter, A. Rossi, F. Onori, M. Fraser, P. T. O'Brien, R. A. J. Eyles-Ferris, J. Hjorth, T. -W. Chen, G. Leloudas, L. Tomasella, S. Schulze, M. De Pasquale, F. Carotenuto, J. Bright, Chenwei Wang, Shaolin Xiong, Jinpeng Zhang, Wangchen Xue, Jiacong Liu, Chengkui Li, D. Mata Sanchez, M. A. P. Torres

TL;DR

EP250207b is analyzed with a multi-wavelength campaign to test whether its FXT origin is a collapsar or a compact-object merger. The X-ray light curve shows a power-law decay $F_X \propto t^{m}$ with $m\approx-1.5$, while deep optical/NIR imaging and spectroscopy of the putative host at $z=0.082$ reveal an old stellar population in a lenticular galaxy and a spatially-offset transient consistent with merger scenarios. Afterglow modeling supports a mildly off-axis structured jet in a low-density medium, but late-time optical/NIR brightness requires extra emission components (e.g., a globular cluster or tidally disrupted dwarf galaxy core); kilonova models favor very low ejecta masses to match the HST data, challenging standard NS-NS merger expectations. Collectively, the results align with a compact-object merger origin for EP250207b and underscore parallels between FXTs and GRBs, while also leaving room for alternative host scenarios at higher redshift.

Abstract

Fast X-ray Transients (FXTs) are short-lived extra-galactic X-ray sources. Recent progress through multi-wavelength follow-up of Einstein Probe discovered FXTs has shown that several are related to collapsars, which can also produce gamma-ray bursts (GRBs). In this paper we investigate the nature of the FXT EP250207b. The VLT/MUSE spectra of a nearby (15.9 kpc in projection) lenticular galaxy reveal no signs of recent star formation. If this galaxy is indeed the host, EP250207b lies at a redshift of z=0.082, implying a peak observed absolute magnitude for the optical counterpart of M_r=-14.5. At the time when supernovae (SNe) would peak, it is substantially fainter than all SN types. These results are inconsistent with a collapsar origin for EP250207b. The properties favour a binary compact object merger driven origin. The X-ray, optical and radio observations are compared with predictions of several types of extra-galactic transients, including afterglow and kilonova models. The data can be fit with a slightly off-axis viewing angle afterglow. However, the late-time (~30 day) optical/NIR counterpart is too bright for the afterglow and also for conventional kilonova models. This could be remedied if that late emission is due to a globular cluster or the core of a (tidally disrupted) dwarf galaxy. If confirmed, this would be the first case where the multi-wavelength properties of an FXT are found to be consistent with a compact object merger origin, increasing the parallels between FXTs and GRBs. We finally discuss if the source could originate in a higher redshift host galaxy.

EP250207b is not a collapsar fast X-ray transient. Is it due to a compact object merger?

TL;DR

EP250207b is analyzed with a multi-wavelength campaign to test whether its FXT origin is a collapsar or a compact-object merger. The X-ray light curve shows a power-law decay with , while deep optical/NIR imaging and spectroscopy of the putative host at reveal an old stellar population in a lenticular galaxy and a spatially-offset transient consistent with merger scenarios. Afterglow modeling supports a mildly off-axis structured jet in a low-density medium, but late-time optical/NIR brightness requires extra emission components (e.g., a globular cluster or tidally disrupted dwarf galaxy core); kilonova models favor very low ejecta masses to match the HST data, challenging standard NS-NS merger expectations. Collectively, the results align with a compact-object merger origin for EP250207b and underscore parallels between FXTs and GRBs, while also leaving room for alternative host scenarios at higher redshift.

Abstract

Fast X-ray Transients (FXTs) are short-lived extra-galactic X-ray sources. Recent progress through multi-wavelength follow-up of Einstein Probe discovered FXTs has shown that several are related to collapsars, which can also produce gamma-ray bursts (GRBs). In this paper we investigate the nature of the FXT EP250207b. The VLT/MUSE spectra of a nearby (15.9 kpc in projection) lenticular galaxy reveal no signs of recent star formation. If this galaxy is indeed the host, EP250207b lies at a redshift of z=0.082, implying a peak observed absolute magnitude for the optical counterpart of M_r=-14.5. At the time when supernovae (SNe) would peak, it is substantially fainter than all SN types. These results are inconsistent with a collapsar origin for EP250207b. The properties favour a binary compact object merger driven origin. The X-ray, optical and radio observations are compared with predictions of several types of extra-galactic transients, including afterglow and kilonova models. The data can be fit with a slightly off-axis viewing angle afterglow. However, the late-time (~30 day) optical/NIR counterpart is too bright for the afterglow and also for conventional kilonova models. This could be remedied if that late emission is due to a globular cluster or the core of a (tidally disrupted) dwarf galaxy. If confirmed, this would be the first case where the multi-wavelength properties of an FXT are found to be consistent with a compact object merger origin, increasing the parallels between FXTs and GRBs. We finally discuss if the source could originate in a higher redshift host galaxy.

Paper Structure

This paper contains 16 sections, 13 figures, 2 tables.

Table of Contents

  1. Introduction
  2. Observations & Results
  3. Einstein Probe (EP) X-ray observations
  4. EP -- Wide-field X-ray Telescope (EP-WXT) observations
  5. EP-Follow-up X-ray Telescope (EP-FXT) observations
  6. GECAM limits on gamma-ray emission
  7. Optical and near-infrared ground based observations
  8. NOT/ALFOSC + NOT/NOTCam
  9. Gemini North and South Multi-Object Spectrograph observations (GMOS)
  10. Hubble Space Telescope observations
  11. Very Large Telescope/MUSE
  12. MeerKAT Radio observations
  13. Discussion
  14. Optical and NIR photometry
  15. Afterglow modelling posterior distribution
  16. ...and 1 more sections

Figures (13)

  • Figure 1: The background subtracted EP250207b discovery light curve obtained by the EP-WXT instrument. Time is in seconds after T$_0$ and the bin size is 6 s. A period of $\approx$200 s before the FXT start is shown to assess the number of events at the source location before the FXT onset.
  • Figure 2: Top panel: The EP250207b discovery spectrum (0.5--4 keV) obtained by the EP-WXT instrument averaged over 120 s. The best-fit power law model affected by Galactic extinction is shown. Bottom panel: The data, minus the best fit model, divided by the error in the data point is shown. No significant deviations with respect to the best-fit model are present.
  • Figure 3: Top panel: Shown is the EP-FXT spectrum for each of the three observations for each FXT telescope unit A and B separately. In addition, the best-fit power law model affected by Galactic extinction is shown using a matching-colour drawn line. Epoch 1 EP-FXT-A is shown in black and EP-FXT-B in red, while epoch 2 EP-FXT-A is shown in green and EP-FXT-B in blue, and finally, epoch 3 EP-FXT-A is shown in light blue and EP-FXT-B in purple. No large change in the source spectral shape is observed in between the three observing epochs while the flux decreased between the first and the second epoch. Bottom panel: The data, minus the best fit model, divided by the error in the data is shown for each of the EP-FXT-A and B unit telescope--detector system for each of the three observing epochs. No significant deviations with respect to the best-fit model are present. The colours represent the same data/epoch as in the top panel.
  • Figure 4: The EP 0.3--10 keV X-ray light curve of EP250207b. The first data point is the EP-WXT average flux and the next three data points are from the EP-FXT observations. The blue dashed line indicates the best fit power law function of $F_X=C\times (\frac{t}{1~\mathrm{d}})^m$. The location of the first data point is taken to be at $t=10^{-3}$ d ($\approx86$ s), reflecting that the first data point is an average over $\approx150$ s. The right hand y-axis shows the observed optical and NIR magnitudes. Clear fading is detected in the different filters (see Sections \ref{['groundbased']} & \ref{['hst']} ).
  • Figure 5: The NOT/ALFOSC discovery $r^\prime$-band image of the optical counterpart to the FXT EP250207b combining the two best-seeing images of the four. The white circle indicates the EP-FXT source localization uncertainty region (2025GCN.39266....1Z). The new faint optical source ($r^\prime=23.3\pm0.16$ mag) is indicated by the tick marks. It lies in projection close to the galaxy WISEA J111002.65$-$075211.9.
  • ...and 8 more figures